Incorporation of A2Q into HgQ and dimensional reduction to A2Hg3Q4 and A2Hg6Q7 (A = K, Rb, Cs; Q = S, Se). Access of Li ions in A2Hg6Q7 through topotactic ion-exchange

Article abstract of DOI:10.1021/ja9713486

The synthesis of the one-dimensional K₂Hg₃Q₄ (Q = S, Se) and Cs₂Hg₃Se₄ and the three-dimensional A₂Hg₆S₇ (A = K, Rb, Cs), and A₂Hg₆Se₇ (A = Rb, Cs) in reactive A₂Q(x) fluxes is reported. Pale yellow, hexagonal plates of K₂Hg₃S₄ crystallize in space group Pbcn, with a = 10.561(5) ?, b = 6.534(3) ?, and c = 13.706(2) ?, V = 945.8(7) ?,³ d(calc) = 5.68 g/cm³, and final R = 5.7%, R(w) = 6.3%. Red, hexagonal plates of K₂Hg₃Se₄ crystallize in space group Pbcn, with a = 10.820(2) ?, b = 6.783(1) ?, and c 14.042(2) ?, v = 1030.6(5) ?,³ d(calc) = 6.42 g/cm³, and final R = 7.7%, R(w) = 8.4%. Orange yellow, hexagonal plates of Cs₂Hg₃Se₄ crystallize in space group Pbcn, with a = 12.047(4) ?, b = 6.465(2) ?, and c = 14.771(6) ?, V = 1150.4(7) ?, ³ d(calc) = 6.83 g/cm³, and final R = 5.5%, R(w) = 6.2%. Black needles of K₂Hg₆S₇ crystallize in space group P42₁m, with a = 13.805(8) ? and c = 4.080(3) ?, V = 778(1) ?, ³ d(calc) = 6.43 g/cm³, and final R = 3.1%, R(w) 3.6%. Black needles of Rb₂Hg₆S₇ crystallize in space group P4₂nm, with a = 13.9221(8) ? and c = 4.1204(2) ?, V = 798.6(1) ?, ³ d(calc) = 6.65 g/cm³, and final R = 4.3%, R(w) = 5.0%. Black needles of Cs₂Hg₆S₇ crystallize in space group P4₂nm, with a = 13.958(4) ? and c = 4.159(2) ?, V = 810.2(8) ?, ³ d(calc) = 6.94 g/cm³, and final R = 4.3%, R(w) = 4.4%. Black needles of Cs₂Hg₆Se₇ crystallize in space group P4₂nm, with a = 14.505(7) ? and c = 4.308(2) ?, V = 906(1) ?, ³ d(calc) = 7.41 g/cm³, and final R = 3.6%, R(w) = 4.0%. The A₂Hg₃Q₄ compounds Contain linear chains. The A₂Hg₆Q₇ compounds display noncentrosymmetric frameworks with A⁺ cations residing in tunnels formed by both tetrahedral and linear Hg atoms. K₂Hg₆S₇, Rb₂Hg₆S₇, Cs₂Hg₆S₇, Rb₂Hg₆Se₇, and Cs₂Hg₆Se₇ display room-temperature bandgaps of 1.51, 1.55, 1.61, 1.13, and 1.17 eV, respectively. Bandgap engineering through S/Se solid solutions of the type Rb₂Hg₆Se(7-x)S(x) and Cs₂Hg₆Se(7-x)S(x) is possible in-these materials. All A₂Hg₆Q₇ melt congruently, with melting points of 556 ± 10 °C, except for Rb₂Hg₆Se₇ which degrades. Rb₂Hg₆S₇ can undergo ion exchange reactions with LiI to give Li_1.8Rb_0.2Hg₆S₇.

Full text of DOI:10.1021/ja9713486

124
J. Am. Chem. Soc. 1998, 120, 124-136
Incorporation of A2Q into HgQ and Dimensional Reduction to
A2Hg3Q4 and A2Hg6Q7 (A ) K, Rb, Cs; Q ) S, Se). Access of Li
Ions in A2Hg6Q7 through Topotactic Ion-Exchange
Enos A. Axtell, III, Younbong Park, Konstantinos Chondroudis, and
Mercouri G. Kanatzidis*
Contribution from the Department of Chemistry and Center for Fundamental Materials Research,
Michigan State UniVersity, East Lansing, Michigan 48824-1322
ReceiVed April 28, 1997X
Abstract: The synthesis of the one-dimensional K2Hg3Q4 (Q ) S, Se) and Cs2Hg3Se4 and the three-dimensional
A2Hg6S7 (A ) K, Rb, Cs), and A2Hg6Se7 (A ) Rb, Cs) in reactive A2Qx fluxes is reported. Pale yellow,
hexagonal plates of K2Hg3S4 crystallize in space group Pbcn, with a ) 10.561(5) Å, b ) 6.534(3) Å, and c
) 13.706(2) Å, V ) 945.8(7) Å,3 dcalc ) 5.68 g/cm3, and final R ) 5.7%, Rw ) 6.3%. Red, hexagonal plates
of K2Hg3Se4 crystallize in space group Pbcn, with a ) 10.820(2) Å, b ) 6.783(1) Å, and c ) 14.042(2) Å,
V ) 1030.6(5) Å,3 dcalc ) 6.42 g/cm3, and final R ) 7.7%, Rw ) 8.4%. Orange yellow, hexagonal plates of
Cs2Hg3Se4 crystallize in space group Pbcn, with a ) 12.047(4) Å, b ) 6.465(2) Å, and c ) 14.771(6) Å, V
) 1150.4(7) Å,3 dcalc ) 6.83 g/cm3, and final R ) 5.5%, Rw ) 6.2%. Black needles of K2Hg6S7 crystallize
in space group P4h21m, with a ) 13.805(8) Å and c ) 4.080(3) Å, V ) 778(1) Å,3 dcalc ) 6.43 g/cm3, and final
R ) 3.1%, Rw ) 3.6%. Black needles of Rb2Hg6S7 crystallize in space group P42nm, with a ) 13.9221(8)
Å and c ) 4.1204(2) Å, V ) 798.6(1) Å,3 dcalc ) 6.65 g/cm3, and final R ) 4.3%, Rw ) 5.0%. Black needles
of Cs2Hg6S7 crystallize in space group P42nm, with a ) 13.958(4) Å and c ) 4.159(2) Å, V ) 810.2(8) Å,3
dcalc ) 6.94 g/cm3, and final R ) 4.3%, Rw ) 4.4%. Black needles of Cs2Hg6Se7 crystallize in space group
P42nm, with a ) 14.505(7) Å and c ) 4.308(2) Å, V ) 906(1) Å,3 dcalc ) 7.41 g/cm3, and final R ) 3.6%,
Rw ) 4.0%. The A2Hg3Q4 compounds contain linear chains. The A2Hg6Q7 compounds display noncen-
trosymmetric frameworks with A+ cations residing in tunnels formed by both tetrahedral and linear Hg atoms.
K2Hg6S7, Rb2Hg6S7, Cs2Hg6S7, Rb2Hg6Se7, and Cs2Hg6Se7 display room-temperature bandgaps of 1.51, 1.55,
1.61, 1.13, and 1.17 eV, respectively. Bandgap engineering through S/Se solid solutions of the type
Rb2Hg6Se7-xSx and Cs2Hg6Se7-xSx is possible in these materials. All A2Hg6Q7 melt congruently, with melting
points of 556 ( 10 °C, except for Rb2Hg6Se7 which degrades. Rb2Hg6S7 can undergo ion exchange reactions
with LiI to give Li1.8Rb0.2Hg6S7.
Introduction
tions in photovoltaic devices for solar energy conversion and
infrared detection, respectively.4 From a chemical point of view
it would be interesting to see how the insertion of alkali
chalcogenide equivalents into such three-dimensional structures
affects the structural organization of the resulting metal-
chalcogenide framework, particularly in terms of dimensionality
and how the latter correlates with observed properties. In this
context, recently we have demonstrated the ability of the molten
salt method to generate “break-up” or low-dimensional structures
such as K2Cd2S3,5 K2Cd3S4,6 K2Cd3Se4,6 Rb2Cd3Se4,6 and K2-
Cd3Te4.7 These compounds, whose Cd/Q connectivity is
reduced relative to that of the parent CdQ structure, display
bandgaps that range from 0.31 to 0.76 eV higher than the CdQ
The utility of alkali metal polychalcogenide fluxes as solvents
and reagents at intermediate temperatures has been amply
noted.1 An enormous number of interesting new compounds
with low-dimensional structures occur at these temperatures
(150∼500 °C). Solid-state chalcogenides are of great interest
due to their useful electronic2 and catalytic3 properties. For
example, the chalcogenides with zinc blende or chalcopyrite
structure such as CuInSe2, CdTe, and Hg1-xCdxTe find applica-
X Abstract published in AdVance ACS Abstracts, December 15, 1997.
(1) (a) Kanatzidis, M. G. Chem. Mater. 1990, 2, 353-363. (b) Kanatzidis,
M. G.; Sutorik, A. C. Prog. Inorg. Chem. 1995, 43, 151-265.
(2) (a) Meerschaut, A.; Rouxel, J. In Crystal Chemistry and Properties
of Materials with Quasi One-Dimensional Structures; Rouxel, J., Ed.; D.
Reidel Publishing Co.: 1986, 205-279. (b) SuperconductiVity in Ternary
Compounds; Bischer, O., Maple, M. B., Eds., Springer: Berlin, 1982; Vols
I and II. (c) Chevral, R. In Superconductor Materials Science: Metallurgy,
Fabrication and Applications; Foner, S.; Schwart, B. B., Eds.; Plenum
Press: New York, 1981, Chapter 10. d) Whittingham, M. S. Prog. Solid
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K. J. Am. Chem. Soc. 1987, 111, 3251-3257. (b) Weisser, O.; Landa, S.
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R. R. J. Catal. 1981, 67, 430-445.
(4) (a) Zweibel, K.; Michell, R. CuInSe2 and CdTe: Scale-up for
Manufacturing; Solar Energy Research Institute, Dec 1989, and references
therein. (b) Mickelsen, R. A.; Chen, W. S. In Ternary and Multinary
Compounds; Deb, S. K., Zunger, Z., Eds.; Proceedings of the 7th
Conference, Materials Research Society, 1987; pp 39-47. (c) Steward, J.
M.; Chen, W. S.; Devaney, W. E.; Mickelsen, R. A. In Ternary and
Multinary Compounds; Deb, S. K., Zunger, Z., Eds.; Proceedings of the
7th Conference, Materials Research Society, 1987; pp 59-64.
(5) Axtell, E. A.; Liao, J-H.; Pikramenou, Z.; Park, Y.; Kanatzidis, M.
G. J. Am. Chem. Soc. 1993, 115, 12191-2.
(6) Axtell, E. A.; Liao, J-H.; Pikramenou, Z.; Kanatzidis, M. G. Chem.
Eur. J. 1996, 2, 656-666.
(7) Axtell, E. A. Ph.D. Dissertation, Michigan State University, 1995.
S0002-7863(97)01348-6 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/14/1998

Incorporation of A2Q into HgQ
J. Am. Chem. Soc., Vol. 120, No. 1, 1998 125
added through a side neck. The stoichiometric amount of elemental
sulfur or selenium needed was added with stirring. Subsequently, the
material was treated as above, resulting in a pale yellow or orange
powder for the sulfides and the selenides, respectively. Caution: Alkali
metals are highly reactive! The flask residue was rinsed carefully with
isopropyl alcohol to destroy any remaining alkali metal.
binaries. An interesting illustration of dimensional reduction
has been recently shown in the Cat/Re6S8/X system where Cat
is a counterion and X is a halide.8
Dimensional reduction can be exploited to generate materials
with desired bandgaps from corresponding binary compounds
with smaller bandgaps. For example, the cubic forms of HgS
and HgSe have bandgaps with Eg ∼ 0.0 eV and are two possible
candidates from which to generate new “break-up” structures
with higher bandgaps. This would result in ternary A/Hg/Q
type compounds (A ) alkali metal) of which few are known.
These include A6HgQ4 (A ) K, Rb; Q ) S, Se),9a Na2HgS2, 9b
K2HgS2,9b Na2Hg3S4,10 Na2Hg6S7‚H2O,11 and Rb2Hg3Te4.12 We
focused our investigation of ternary A2Q/HgQ systems on the
alkali metal polychalcogenide fluxes (A2Qx) at intermediate
temperatures. In this contribution, we report a full account of
the preparation and properties of a family of new ternary
homologous compounds, A2Hg3Q4 (A ) K, Cs; Q ) S, Se)
and A2Hg6Q7 (A ) K, Rb, Cs; Q ) S, Se), which feature mixed
mercury coordination and unusual low-dimensional structure
types. These materials and their properties are discussed in the
context of dimensional reduction of the parent compound HgQ
brought about by the incorporation into it of A2Q equivalents.
Of these, K2Hg3S4 and K2Hg6S7 have been reported in prelimi-
nary form.13 We demonstrate that the tunneled framework in
A2Hg6Q7 can act as a host structure by undergoing topotatcic
ion-exchange reactions with LiI.
(c) HgSe. HgSe was prepared by pipeting approximately 1.0 g of
liquid mercury into a 9 mm o.d. × 7 mm i.d. fused silica tube and
then adding the stoichiometric amount of elemental selenium. The tube
was sealed under a dynamic vacuum of 2.0 × 10-4 mbar and placed in
a computer-controlled furnace. The furnace was heated to 700 °C over
48 h, held at isotherm for 12 h, then cooled at 20 °C/h to 50 °C. The
ingot was removed from the tube and ground to a powder before use.
Caution: Liquid mercury is volatile and toxic! The temperature was
raised slowly to allow the mercury to react without evaporating. Ideally,
the reaction should be run in a fume hood.
(d) K2Hg3S4 (I). 0.165 g (1.5 mmol) amount of K2S, 0.116 g (0.5
mmol) of HgS, and 0.128 g (4.0 mmol) of S were mixed together and
loaded in a Pyrex tube that was then flame-sealed under vacuum (∼10-3
mbar). The tube was placed in a computer-controlled furnace and
heated at 220 °C for 99 h, then cooled slowly to 50 °C at a rate of 2
°C/h. Pale yellow transparent hexagonal shaped crystals were obtained
with a small contamination of HgS by removing excess potassium
polysulfides with degassed dimethylformamide (DMF) under a N2
atmosphere. A yield of 46%, based on HgS, is typical. The product
is not stable in water and decomposes rapidly. The presence of K,
Hg, and S atoms in a large number of crystals was confirmed by using
the EDS/SEM system.
(e) K2Hg3Se4 (II). A of 0.118 g (1.75 mmol) amount of K2Se, 0.070
g (0.25 mmol) of HgSe, and 0.158 g (2.0 mmol) of Se were mixed
together and loaded in a Pyrex tube that was then flame-sealed under
vacuum (∼10-3 mbar). The tube was placed in a computer-controlled
furnace and heated at 250 °C for 99 h, then cooled slowly to 50 °C at
a rate of 2 °C/h Red hexagonal shaped crystals were obtained with a
small contamination of HgSe by removing excess potassium polyse-
lenides with degassed DMF under a N2 atmosphere. A yield of 53%,
based on HgSe, is typical. The product is not stable in water and
decomposes rapidly.
Experimental Section
Materials. Chemicals in this work were used as obtained: mercury
sulfide (HgS) powder, analytical reagent, J. T. Baker Chemical Co.,
Phillipsburg, NJ; mercury selenide (HgSe) powder, -100 mesh, 99.9%
purity, Cerac, Milwaukee, WI; sulfur powder, Spectrum Chemical Mfg.
Corp. (Lot No. EE597); selenium powder, -100 mesh, 99.95% purity,
Aldrich Chemical Co., Milwaukee, WI; potassium metal (98%), Aldrich
Chemical Co., Milwaukee, WI; rubidium metal, Cerac; and cesium
metal, 99.98% purity, AESAR, Johnson Matthey, Seabrook, NH.
Synthesis. (a) K2S. Potassium sulfide was produced in liquid
ammonia. In a nitrogen-filled glovebox, the stoichiometric amounts
of potassium metal chunks and sulfur powder necessary to produce 20
g of starting material were loaded into a 250 mL round-bottom flask.
A Teflon stir bar was added and the flask closed to air with a glass
adapter and valve. This apparatus was removed to a Schlenk line, where
approximately 150 mL of liquid ammonia was condensed under
nitrogen, with stirring, into the dry ice/acetone cooled round-bottom
flask. After the flask was allowed to warm to room temperature, the
apparatus was put under vacuum for several hours, followed by heating
with a hot air gun to drive off any remaining ammonia. The apparatus
was returned to the glovebox, and the product was ground to a fine
powder before use. The powder was pale yellow in color.
(f) Cs2Hg3Se4 (III). A 0.115 g (0.33 mmol) amount of Cs2Se, 0.047
g (0.17 mmol) of HgSe, and 0.105 g (1.33 mmol) of Se were mixed
together and loaded in a Pyrex tube that was then flame-sealed under
vacuum (∼10-3 mbar). The tube was placed in a computer-controlled
furnace and heated at 250 °C for 99 h, then cooled slowly to 50 °C at
a rate of 2 °C/h Orange-yellow hexagonal shaped crystals were
obtained, with a small contamination of HgSe, by removing excess
cesium polyselenides with degassed DMF under a N2 atmosphere. A
yield of 58%, based on HgSe, is typical. The product is relatively
stable in water for a short period of time, but decomposes in an hour.
A quantitative microprobe analysis performed on a large number of
crystals with an EDS/SEM microanalysis system gave an average
composition of Cs1.9Hg3.0Se3.8
.
(g) K2Hg6S7 (IV). A 0.055 g (0.5 mmol) amount of K2S, 0.203 g
(0.87 mmol) of HgS, and 0.064 g (2.0 mmol) of S were mixed together
and loaded in a Pyrex tube that was then flame-sealed under vacuum
(∼10-3 mbar). The tube was placed in a computer-controlled furnace
and heated at 370 °C for 99 h, then cooled slowly to 50 °C at a rate of
2 °C/h. Black needlelike crystals were obtained, with little contamina-
tion of red HgS crystals, by removing excess molten potassium
polysulfides with water under a N2 atmosphere. A yield of 72%, based
on HgS, is typical. The product was washed with ethanol and ether
and vacuum dried. The product is insoluble in water and common
organic solvents. A quantitative microprobe analysis performed on a
large number of crystals with the EDS/SEM system gave an average
(b) Rb2S, Cs2S, Rb2Se, and Cs2Se. In a nitrogen filled glovebox,
rubidium or cesium metal was gently heated until molten. Ap-
proximately 10 g of the alkali metal was subsequently weighed into a
three-necked, 500-mL round-bottom flask. The center neck was closed
with a glass adapter and valve, while the two outer necks were closed
with ground glass stoppers. The apparatus was then moved to a Schlenk
line. Liquid ammonia was condensed, under nitrogen, into the dry ice/
acetone-cooled round-bottom flask. After the flask was filled half full
and swirled to dissolve the alkali metal, a Teflon-coated stir bar was
(8) Long, J. R.; McCarty, L. S.; Holm, R. H. J. Am. Chem. Soc. 1996,
118, 4603-4616.
composition of K2.0Hg5.8S7.1
.
(9) (a) Sommer, H.; Hoppe, R. Z. Anorg. Allg. Chem. 1978, 443, 201-
211. (b) Klepp, K. O.; Prager, K. Z. Naturforsch. 1992, 47B, 491-496.
(10) Klepp, K. O. J. Alloys Compds. 1992, 182, 281-288.
(11) Herath Banda, R. M.; Craig, D.; Dance, I. G.; Scudder M.
Polyhedron 1991, 10, 41-45.
This compound can also be prepared by direct synthesis: 0.110 g
(1.0 mmol) of K2S and 1.396 g (6 mmol) of HgS were mixed together
and loaded in a Pyrex tube that was then flame-sealed under vacuum
(∼10-3 mbar). The tube was placed in a computer-controlled furnace
and heated at 375 °C for 7 days and cooled slowly to 50 °C at a rate
of 2 °C/h. A black powder of K2Hg6S7 was obtained with little
(12) Li, J.; Chen, Z.; Lam, K-C.; Mulley, S.; Proserpio, D. M. Inorg.
Chem. 1997, 36, 684-687.
(13) Kanatzidis, M. G.; Park, Y. Chem. Mater. 1990, 2, 99-101.

126 J. Am. Chem. Soc., Vol. 120, No. 1, 1998
Axtell et al.
contamination of red HgS and was identified by its X-ray powder
diffraction pattern.
and Cs2Hg6Se7 were collected on a Nicolet P3 four-circle diffractometer
with graphite monochromated Mo-KR radiation, using the θ-2θ scan
mode. The data for K2Hg3Se4 were collected on an Enraf-Nonius CAD4
diffractometer with Mo KR radiation, using the ω-2θ scan mode by
Dr. M. Sabat at Northwestern University. A Siemens SMART Platform
CCD diffractometer was used to collect data from Rb2Hg6S7 at 25 °C,
using 80 s exposure time (crystal-detector distance 5 cm). An empirical
absorption correction was applied to the data of Rb2Hg6S7 (Blessing,
R. H. Acta Crystallogr. 1995, A51, 33-38). Accurate unit cell
parameters for all compounds were obtained from the least-squares
refinement of the angles of 20-25 machine-centered reflections with
2θ values between 10 and 30°. The stability of the experimental setup
and crystal integrity were monitored by measuring three standard
reflections periodically (every 100-150 reflections) during the data
collection period. The data of K2Hg3S4 showed appreciable decay
(34.3% decay) due to decomposition of the crystal. The color of the
crystal changed from yellow to gray during the data collection period.
The other data sets did not show any appreciable decay. Two
absorption corrections were applied to all data: an empirical absorption
correction based on ψ scans for three reflections (with ø ∼ 90°)
followed by a DIFABS correction according to the recommended
protocol.14 The structures of K2Hg3S4, K2Hg3Se4, Cs2Hg3Se4, K2Hg6S7,
and Cs2Hg6Se7 were solved with the direct methods of the SHELXS-
8615 software package and were refined with the SDP16 package of
crystallographic programs on a VAX station 2000 computer. The
structures of Rb2Hg6S7 and Cs2Hg6S7 were solved with the direct
methods of the SHELXS-8615 software package and were refined with
the TEXSAN17 package of crystallographic programs on a VAX station
3100/76 computer. The effect of secondary extinction was considered
in the least-squares refinement as an additional parameter due to the
heavy atomic constituents of the crystals. All atoms except the sulfur
atoms in K2Hg3S4 and Cs2Hg6S7 were refined anisotropically. The
sulfur atoms in these two structures become nonpositive definite when
refined anisotropically, and this is attributed to the crystal decay for
K2Hg3S4 and to the larger than average size for Cs2Hg6S7 which resulted
in insufficient correction for absorption. For K2Hg6S7, Rb2Hg6S7, Cs2-
Hg7S7, and Cs2Hg6Se7, the correct enantiomorph was assigned by
determining which one resulted in the best refinement. Long exposure
(1-2 h) axial photographs for all crystals showed no evidence of
superstructure. The complete data collection parameters and details
of the structure solution and refinement for all compounds are given
in Tables 1-3. The final atomic coordinates, temperature factors, and
their estimated standard deviations are given in Tables 4-10.
(h) Rb2Hg6S7 (V). Black needles of Rb2Hg6S7 were obtained in bulk
amounts by the reaction of 1.486 g (7.3 mmol) of Rb2S and 10.221 g
(44 mmol) of HgS. This charge was sealed inside an evacuated quartz
tube. The tube was placed inside a computer-controlled furnace and
heated at 400 °C for 1.5 days, followed by heating at 500 °C for 1
day, slow cooling to 415 °C, and quenching to room temperature. The
product was isolated with methanol under a nitrogen atmosphere. This
compound is air and water stable, and is insoluble in common organic
solvents. Semiquantitative microprobe analysis on a number of these
crystals by SEM/EDS gave an average composition of Rb1.0Hg5.8S7.5
A yield of 92%, based on HgS, is typical.
.
(i) Cs2Hg6S7 (VI). Black needles of Cs2Hg6S7 were obtained by
the reaction of 2.161 g (7.3 mmol) of Cs2S and 10.221 g (44 mmol) of
HgS. This charge was sealed inside an evacuated quartz tube. The
tube was placed inside a computer-controlled furnace and heated at
400 °C for 1.5 days, followed by heating at 500 °C for 1 day, slow
cooling to 50 °C, and quenching to room temperature. The product
was isolated with methanol under a nitrogen atmosphere. This
compound is air and water stable, and is insoluble in common organic
solvents. Semiquantitative microprobe analysis on a number of these
crystals by SEM/EDS gave an average composition of Cs1.5Hg6.0S7.6
.
A yield of 90%, based on HgS, is typical.
(j) Rb2Hg6Se7 (VII). Black needles of Rb2Hg6Se7 were obtained
by the reaction of 0.816 g (3.3 mmol) of Rb2Se and 5.5480 g (20 mmol)
of HgSe. This charge was sealed inside an evacuated quartz tube. The
tube was placed inside a computer-controlled furnace and heated at
550 °C for 2 days, followed by slow cooling to 150 °C and quenching
to room temperature. The product was isolated with methanol under
a nitrogen atmosphere. This compound is air and water stable, and is
insoluble in common organic solvents. Semiquantitative microprobe
analysis on a number of these crystals by SEM/EDS indicated the
presence of Rb, Hg, and S. A yield of 85%, based on HgS, is typical.
(k) Cs2Hg6Se7 (VIII). This compound could be prepared only by
direct synthesis: 0.086 g (0.25 mmol) of Cs2Se and 0.420 g (1.50 mmol)
of HgSe were mixed together and loaded in a Pyrex tube that was then
flame-sealed under vacuum (∼10-3 mbar). The tube was placed in a
computer-controlled furnace and heated at 375 °C for 72 h and cooled
slowly to 50 °C at a rate of 3 °C/h. Black needle-shaped crystals were
obtained. The product is not soluble in water and any common organic
solvents. A quantitative microprobe analysis performed on a large
number of crystals with the EDS/SEM system gave an average
composition of Cs2.1Hg6.0Se7.3
.
The compounds were examined by X-ray powder diffraction to check
for phase purity and for identification. Powder patterns obtained on a
Philips XRG-3000 powder diffractometer and a Rigaku rotating anode
(Cu KR) X-ray powder diffractometer, Rigaku-Denki/Rw400F2 (Ro-
taFlex), provided accurate dhkl spacings and showed that the set K2-
Hg3S4, K2Hg3Se4, and Cs2Hg3Se4 and the set Rb2Hg6S7, Cs2Hg6S7,
Rb2Hg6Se7, and Cs2Hg6Se7 are X-ray isomorphous. The calculated18
and observed powder patterns of K2Hg3S4, K2Hg6S7, and Cs2Hg6Se7
are given in Tables 11-13, respectively, while the rest have been
deposited as Supporting Information.
(l) Rb2Hg6S3.5Se3.5 (IX). Black needles of Rb2Hg6S3.5Se3.5 were
obtained by the reaction of 0.203 g (1.0 mmol) of Rb2S, 0.249 g (1.0
mmol) of Rb2Se, 1.396 g (6.0 mmol) of HgS, and 1.887 g (6.0 mmol)
of HgSe. This charge was sealed inside an evacuated quartz tube. The
tube was placed inside a computer-controlled furnace and heated at
650 °C for 2 days, followed by slow cooling to 50 °C and quenching
to room temperature. The product was isolated with methanol under
a nitrogen atmosphere. This compound is air and water stable, and is
insoluble in common organic solvents. Semiquantitative microprobe
analyses on a number of these crystals from different locations in the
sample were consistent and verified the presence of Rb, Hg, Se, and
S. A yield of 90%, based on HgSe and HgS, is typical.
(m) Cs2Hg6S5.25Se1.75 (X). Black needles of Cs2Hg6S5.25Se1.75 were
obtained by the reaction of 0.222 g (0.75 mmol) of Cs2S, 0.086 g (0.25
mmol) of Cs2Se, 1.047 g (4.5 mmol) of HgS, and 0.472 g (1.5 mmol)
of HgSe. This charge was sealed inside an evacuated fused silica tube.
The tube was placed inside a computer-controlled furnace and heated
at 650 °C for 2 days, followed by slow cooling to 50 °C and quenching
to room temperature. The product was isolated with methanol under
a nitrogen atmosphere. This compound is air and water stable, and is
insoluble in common organic solvents. Semiquantitative microprobe
analyses on a number of these crystals from different locations in the
sample were consistent and verified the presence of Cs, Hg, Se, and S.
A yield of 90%, based on HgSe and HgS, is typical. Other solid
solutions were prepared similarly.
Physical Measurements. (a) Semiquantitative Microprobe Analy-
sis. Semiquantitative microprobe analyses was performed on a JEOL
JSM-35C scanning electron microscope (SEM) equipped with a Noran
Energy Dispersive Spectroscopy (EDS) system. Data acquisition was
performed with an accelerating voltage of 20 kV. Compositions are
the average of several 30 s acquisitions.
(14) Walker, N.; Stuart, D. Acta Crystallogr. 1983, 39A, 158.
(15) Sheldrick, G. M. In Crystallographic Computing, 3; Ed. Sheldrick,
G. M., Kruger, C., Doddard, R., Eds.; Oxford University Press: Oxford,
England, 1985; pp 175-189.
(16) Frenz, B. A. The Enraf-Nonius CAD4 SDP System. In Computing
in Crystallography; Delft University Press: Delft, Holland, 1978; pp 64-
71.
(17) TEXSAN: Single-Crystal Structure Analysis Software, Version 5.0;
Molecular Structure Corp.: The Woodlands, TX 77381, 1981.
(18) Smith, D. K.; Nichols, M. C.; Zolensky, M. E. POWD10: A Fortran
IV Program for Calculating X-ray Powder Diffraction Patterns, Version
10; Pennsylvania State University, 1983.
Crystallographic Studies. Single-Crystal X-ray Diffraction. The
X-ray single-crystal data of K2Hg3S4, Cs2Hg3Se4, K2Hg6S7, Cs2Hg6S7,

Incorporation of A2Q into HgQ
J. Am. Chem. Soc., Vol. 120, No. 1, 1998 127
Table 1. Summary of Crystallographic Data for K2Hg3S4, K2Hg3Se4, and Cs2Hg3Se4
formula
K2Hg3S4
K2Hg3Se4
Cs2Hg3Se4
1183.42
FW, g/mol
color/habit
dimensions, (mm)
a, Å
808.23
995.81
yellow/ hex. plate
0.15 × 0.10 × 0.01
10.561(5)
6.534(3)
red/hex. plate
0.17 × 0.12 × 0.02
10.820(2)
6.783(1)
orange/hex. plate
0.22 × 0.18 × 0.02
12.047(4)
6.465(2)
b, Å
c, Å
V, Å; Z
13.706(2)
945.8(7); 4
Pbcn (no. 60)
5.68
14.042(3)
1030.6(5); 4
Pbcn (no. 60)
6.42
14.771(6)
1150.4(7); 4
Pbcn (no. 60)
6.83
space group
d
µ
calc, g/cm3
(MoKR), cm-1
502.3
593.2
586.1
radiation
Mo KR (λ ) 0.71069 Å)
23
θ/2θ
50
Mo KR(λ ) 0.71069 Å)
Mo KR(λ ) 0.71069 Å)
23
θ/2θ
48
temp, °C
scan type
-120
θ/2θ
2θmax, deg
reflections:
measd/unique
50
2829/831
326
33
1088/903
567
43
1058/900
319
43
no. of reflcns with Fo2 > 3σ(Fo )
2
no. of variables
R/Rw
extinction coeff
a
5.7%/6.3%
7.7%/8.4%
5.5%/6.2%
1.58 × 10-8
2.06 × 10-8
3.15 × 10-8
a R ) ∑(|Fo| - |Fc|)/∑|Fo|. Rw ) {∑w(|Fo| - |Fc|)2/∑w|Fo| }1/2. w ) 1/σ2(F).
2
Table 2. Summary of Crystallographic Data for K2Hg6S7 and Rb2Hg6S7
formula
K2Hg6S7
1506.19
black/needle
Rb2Hg6S7
FW, g/mol
color/habit
1598.90
black/needle
0.26 × 0.01 × 0.01
13.9221(8) Å
13.9221(8) Å
4.1204(2) Å
798.64(6); 2
P4h2nm (no. 102)
6.65
dimensions, mm
a, Å
0.04 × 0.04 × 0.46
13.805(8)
b, Å
c, Å
V, Å; Z
13.805(8)
4.080(3)
778(1); 2
P4h21m (no. 113)
6.43
604.2
space group
d
µ
calc, g/cm3
(MoKR), cm-1
645.32
radiation
Mo KR (λ ) 0.71069 Å)
23
Mo KR (λ ) 0.71069 Å)
temp, °C
23
scan type
2θmax, deg
θ/2θ
50
55
0.42/1.00
min/max abs coef for ψ scan corr
0.48/1.00
reflections:
measured/unique
1614/453
402 (on 2σ)
42
2594/697
441
41
2
no. of reflcns with Fo2 > 3σ(Fo )
no. of variables
a
R/Rw
3.1%/3.6%
4.3%/4.0%
extinction coeff
1.50 × 10-7
1.57 × 10-7
a R ) ∑(|Fo| - |Fc|)/∑|Fo|. Rw ) {∑w(|Fo| - |Fc|)2/∑w|Fo| }1/2. w ) 1/σ2(F).
2
(b) Optical Spectroscopy. A Shimadzu UV-3101PC double beam,
double-monochromator spectrophotometer was used to measure the
room temperature, optical diffuse reflectance spectra of the five
compounds. The methods used to collect the reflectance data and
convert it to absorbance values have been described elsewhere.19 We
have obtained the bandgap values by extrapolating the linear regions
of each (a/S)2 versus energy plot to (a/S)2 ) 0.20 These values are
estimated to be accurate to (0.03 eV.
(c) Differential Thermal Analysis. Thermal analysis was performed
on a Shimadzu DTA-50 Differential Thermal Analyzer. High-quality
crystals were selected from the reaction product and ground. Ap-
proximately 15 mg of each material was loaded into a 2.0 mm i.d. ×
3.0 mm o.d. quartz tube with a flattened bottom and sealed under a
vacuum of less than 1 × 10-4 mbar. Heating and cooling rates of 10
°C/min were used. Typically, the samples showed an endothermic peak
upon heating and an exothermic peak upon cooling. The temperature
associated with each thermal event was assigned to the melting and
recrystallization, respectively, of each compound. Comparisons be-
tween powder patterns taken before and after heating were made to
establish whether each compound melted with or without decomposi-
tion.
Results and Discussion
Dimensional Reduction of the Covalent HgQ Framework.
The incorporation of A2Q into the binary HgQ amounts to the
introduction of Q2- atoms in the diamond lattice of HgQ and
the generation of anionic [HgxQy]n- frameworks. In essence
the dense packed lattice of the binary compound is “diluted”
with Q2- atoms which progressively dismantle it. The A+ acts
as a noninterfering counterion. The chemical homology created
in this way is of the type (A2Q)m(HgQ)n with the two end
members being HgQ (m ) 0, n ) 1) and A6HgQ4 (m ) 3, n )
1). The latter contains discrete tetrahedral [HgQ4]6- anions and
represents the maximum possible A2Q equivalents around a Hg
atom. As the Q2-/Hg2+ ratio increases the dismantling of the
original structure increases as well. This also leads to an
attendant reduction in the coordination number of the chalcogen,
which of course results in the dimensional reduction of the
(19) McCarthy, T. J.; Ngeyi, S-P.; Liao, J-H.; DeGroot, D. C.;
Schindler, J.; Kannewurf, C. R.; Kanatzidis, M. G. Chem. Mater. 1993, 5,
331-340.
(20) Pankove, J. I. In Optical Processes in Semiconductors; Dover
Publications: New York, 1975.

128 J. Am. Chem. Soc., Vol. 120, No. 1, 1998
Axtell et al.
Table 3. Summary of Crystallographic Data for Cs2Hg6S7 and Cs2Hg6Se7
formula
Cs2Hg6S7
Cs2Hg6Se7
2022.07
FW, g/mol
color/habit
dimensions (mm)
a, Å
b, Å
c, Å
1693.77
black/needle
0.05 × 0.05 × 0.8
13.958(4)
13.958(4)
4.159(2)
black/needle
0.02 × 0.04 × 0.76
14.505(7)
14.505(7)
4.308(2)
V, Å; Z
space group
810.2(8); 2
P42nm (no. 102)
6.94
906(1); 2
P42nm (no. 102)
7.41
d
µ
calc, g/cm3
(MoKR), cm-1
604.2
684.7
radiation
Mo KR (λ ) 0.71069 Å)
Mo KR (λ ) 0.71069 Å)
23
ω/2θ
44
temp, °C
scan type
2θmax, deg
min/max abs coef for ψ scan corr
reflections:
-80
ω/2θ
60
0.78/1.00
0.536/1.00
measured/unique
2962/753
1752
49
1440/604
530
41
2
reflections with Fo2 > 3σ(Fo )
no. of variables
a
R/Rw
4.3%/4.4%
3.6/4.0
extinction coeff
5.31 × 10-8
1.11 × 10-7
a R ) ∑(|Fo| - |Fc|)/∑|Fo|. Rw ) {∑w(|Fo| - |Fc|)2/∑w|Fo| }1/2. w ) 1/σ2(F).
2
a
Table 4. Fractional Atomic Coordinates and Beq Values for
Table 7. Fractional Atomic Coordinates and Beq Values for
K2Hg3S4 with Their Estimated Standard Deviations in Parentheses
K2Hg6S7 with Estimated Standard Deviations in Parentheses
atom
x
y
z
Beq,a Å2
atom
x
y
z
Beq
Hg(1)
Hg(2)
K
S(1)
S(2)
0.34148(9)
0
0.3788(6)
0.3674(7)
0.3710(7)
0.0852(3)
0.0837(5)
0.766(3)
0.175(2)
0.362(2)
0.2582(1)
1/4
0.5018(8)
0.6416(8)
0.3678(8)
1.48(3)
1.77(5)
1.8(2)
0.8(2)*
1.1(2)*
Hg1
Hg2
K1
S1
S2
S3
0.56782(6)
0.90144(6)
0.6713(4)
0.6965(4)
0.0
0.34115(7)
0.59856
0.8287
0.4650(4)
0.5
0.7608(4)
0.7238(4)
0.764(3)
0.743(2)
0.061(4)
0.330(2)
2.65(2)
2.41(2)
3.5(1)
1.8(1)
2.8(2)
2.0(1)
0.8139(4)
0.6861
a B values for anisotropically refined atoms are given in the form of
the isotropic equivalent displacement parameter defined as Beq ) (4/
3)[a2B11 + b2B22 + c2B33 + ab(cos γ)B12 + ac(cos â)B13 + bc(cos
R)B23]. Starred atoms were refined isotropically.
a B values for anisotropically refined atoms are given in the form of
the isotropic equivalent displacement parameters defined as Beq ) (4/
3)[a2B11 + b2B22 + c2B33 + ab(cos γ)B12 + ac(cos â)B13 + bc(cos
R)B23].
Table 5. Fractional Atomic Coordinates and Beq Values for
K2Hg3Se4 with Their Estimated Standard Deviations in Parentheses
a
Table 8. Fractional Atomic Coordinates and Beq Values for
Rb2Hg6S7 with Estimated Standard Deviations in Parentheses
atom
x
y
z
Beq,a Å2
atom
x
y
z
Beq
Hg(1)
Hg(2)
Se(1)
Se(2)
K
0.3383(2)
0
0.1338(5)
0.3723(6)
0.118(1)
0.0889(3)
0.0874(6)
0.3239(9)
0.3649(8)
0.270(2)
0 .2586(2)
1/4
0 .1382(4)
0 .3707(4)
0 .501(1)
2.74(4)
2.77(6)
2.2(1)
2.3(1)
2.6(2)
Hg1
Hg2
Rb1
S1
S2
S3
0.92937(3)
0.90098(3)
0.67018(8)
1.0283(2)
0.5
0.65555(3)
0.90098
0.67018
0.6947(2)
0.5
0.7894(4)
1.2597(7)
1.296(1)
1.310(7)
1.044(2)
0.8189
2.35(1)
4.16(1)
2.57(2)
1.65(4)
2.83(9)
2.11(4)
a B values for anisotropically refined atoms are given in the form of
the isotropic equivalent displacement parameter defined as Beq ) (4/
3)[a2B11 + b2B22 + c2B33 + ab(cos γ)B12 + ac(cos â)B13 + bc(cos
R)B23].
0.8151(2)
0.8151
a B values for anisotropically refined atoms are given in the form of
the isotropic equivalent displacement parameters defined as Beq ) (4/
3)[a2B(1,1) + b2B(2,2) + c2B(3,3) + ab(cos γ)B(1,2) + ac(cos â)B(1,3)
+ bc(cos R)B(2,3)].
Table 6. Fractional Atomic Coordinates and Beq Values for
Cs2Hg3Se4 with Their Estimated Standard Deviations in Parentheses
atom
x
y
z
Beq,a Å2
no single family of compounds in which all members are known,
but a glance can be obtained from the partial collections which
are available, see for example the Cat/Re6S8/X,8 (A2Q)m(CdQ)n,6
and (A2Q)m(Bi2Q3)n19,21 systems. The decreasing dimensionality
with increasing m leads to an increasingly inhibited ability to
extend orbital overlap in various directions in the lattice of the
[HgxQy]n- frameworks which leads to narrowing of the bands
and widening of the bandgaps. Therefore one may obtain
materials in which quantum confinement may be present in
certain crystallographic directions.22 A simple diagram that
illustrates the structural evolution from bulk HgQ to [HgQ4]6-
Hg(1)
Hg(2)
Cs
Se(1)
Se(2)
0.3309(2)
0
0.1203(3)
0.1234(5)
0.3604(4)
0.0051(6)
-0.0123(9)
0.2656(4)
0.2316(7)
0.2918(7)
0.2416(3)
1/4
0.5105(3)
0.1353(4)
0.3541(5)
3.66(4)
3.40(6)
2.94(6)
2.5(1)
2.2(1)
a B values for anisotropically refined atoms are given in the form of
the isotropic equivalent displacement parameter defined as Beq ) (4/
3)[a2B11 + b2B22 + c2B33 + ab(cos γ)B12 + ac(cos â)B13 + bc(cos
R)B23].
structure. If many members of the (A2Q)m(HgQ)n homology
are known one can observe the successive dimensional changes
in the system from dense three-dimensional motif to “porous”
three-dimensional, to lamellar, and finally to discrete chains,
clusters, and mononuclear complexes. Unfortunately there is
(21) (a) McCarthy, T. J.; Tanzer, T. A.; Kanatzidis, M. G. J. Am. Chem.
Soc. 1995, 117, 1294-1301. (b) Kanatzidis, M. G.; McCarthy, T. J.; Tanzer,
T. A.; Chen, L.-H.; Iordanidis, L.; Hogan, T.; Kannewurf, C. R.; Uher, C.;
Chen, B. Chem. Mater. 1996, 8, 1465-1474.

Incorporation of A2Q into HgQ
J. Am. Chem. Soc., Vol. 120, No. 1, 1998 129
a
Table 9. Fractional Atomic Coordinates and Beq Values for
Table 12. Calculated and Observed Powder Patterns for K2Hg6S7
Cs2Hg6S7 with Estimated Standard Deviations in Parentheses
hkl
200
dcalc (Å)
dobs (Å)
I/Io,obs (%)
atom
x
y
z
Beq
6.90
6.95
8
43
17
45
55
100
43
88
13
14
22
68
18
36
16
12
16
24
18
47
15
16
14
51
18
15
9
Hg1
Hg2
Cs1
S1
S2
S3
0.0739(1)
0.0993(1)
0.3311(2)
0.1926(7)
0.5
0.3474(1)
0.0993
0.3311
0.4788(7)
0.5
0.0580
0.4640(9)
-0.430(2)
0.071(4)
-0.719(6)
0.066(6)
1.10(7)
0.88(5)
1.07(8)
0.9(2)b
0.5(3)b
1.0(2)b
201
301
400
211
410
330
221
420
301
321
430
510
520
421
440
441
531
002
611
550
640
613
711, 551
810
731
432
830
522
6.17
4.36
3.45
3.40
3.34
3.25
3.13
3.08
3.05
2.79
2.76
2.70
6.24
4.40
3.47
3.43
3.37
3.27
3.14
3.10
3.06
2.80
2.77
2.72
0.1821(7)
0.1821
a B values for anisotropically refined atoms are given in the form of
the isotropic equivalent displacement parameters defined as Beq ) (4/
3)[a2B(1,1) + b2B(2,2) + c2B(3,3) + ab(cos γ)B(1,2) + ac(cos â)B(1,3)
+ bc(cos R)B(2,3)]. b Isotropic temperature factors.
a
Table 10. Fractional Atomic Coordinates and Beq Values for
Cs2Hg6Se7 with Estimated Standard Deviations in Parentheses
2.56
2.57
2.461
2.440
2.094
2.047
2.040
1.983
1.952
1.914
1.837
1.761
1.712
1.657
1.641
1.616
1.596
2.470
2.448
2.099
2.048
2.044
1.988
1.957
1.919
1.841
1.765
1.716
1.660
1.644
1.619
1.597
atom
x
y
z
Beq
Hg1
Hg2
Cs1
Se1
Se2
Se3
0.07503(9)
0.10077(9)
-0.1686(2)
0.0200(7)
0.5
0.3488(1)
0.10077
0.1686
0.6935(2)
0.5
0.058
2.71(2)
2.42(2)
2.68(4)
1.68(6)
2.28(9)
1.66(5)
0.4687(5)
0.063(1)
0.556(1)
0.285(2)
0.548(1)
0.3168(2)
0.6832
a B values for anisotropically refined atoms are given in the form of
the isotropic equivalent displacement parameters defined as Beq ) (4/
3)[a2B(1,1) + b2B(2,2) + c2B(3,3) + ab(cos γ)B(1,2) + ac(cos â)B(1,3)
+ bc(cos R)B(2,3)].
21
4
Table 11. Calculated and Observed Powder Patterns of K2Hg3S4
hkl
dcalc (Å)
dobs (Å)
I/Io,obs (%)
Table 13. Calculated and Observed Powder Patterns for
Cs2Hg6Se7
002
004
021
310
213
311
022
312
023
313
024
314
006
025
315
026
316
331
333
008
041
042
604
6.85
3.42
3.17
3.09
3.05
3.02
2.94
2.82
6.90
3.43
3.18
3.10
3.06
3.03
2.95
2.82
33
100
6
7
5
4
3
13
10
4
hkl
dcalc (Å)
dobs (Å)
I/Io,obs (%)
210
400
211
410
330
221
311
321
430
520
421
440
530
6.48
3.62
3.58
3.51
3.41
3.29
3.14
2.94
2.90
2.69
2.59
2.56
2.487
2.406
2.265
2.154
2.086
1.933
1.867
1.852
1.823
1.799
1.742
1.729
1.697
1.682
1.628
6.48
3.66
3.61
3.54
3.44
3.32
3.16
2.96
2.92
2.71
2.60
2.58
10
21
16
43
44
100
12
29
27
10
11
15
8
2.65
2.56
2.67
2.57
2.364
2.298
2.284
2.099
2.053
1.872
1.839
1.835
1.716
1.713
1.622
1.589
1.566
2.366
2.301
2.285
2.104
2.057
1.875
1.843
1.838
1.717
1.717
1.627
1.585
1.569
3
13
30
8
5
2
13
12
40
22
3
2
2
2.50
501, 431
540
002
611
631
2.419
2.295
2.162
2.097
1.941
1.877
1.860
1.843
1.808
1.749
1.736
1.705
1.690
1.635
17
8
42
42
31
14
29
21
24
8
701
711, 551, 402
641, 332
810
731
432
830
522
532
molecules is shown in Figure 1. The colors and optical
spectroscopic properties of A2Hg3Q4 and A2Hg6Q7 can be
rationalized within this framework.
Structure Description of the A2Hg3Q4 Compounds. K2-
Hg3S4, K2Hg3Se4, and Cs2Hg3Se4 are isostructural. They
11
13
8
11
2n-
possess centrosymmetric one-dimensional [Hg3Q4]n
chains.
Two views of the unit cell of A2Hg3Q4 are shown in Figure 2.
spiro-polymer of eight-membered Hg4Q4 rings. There are two
crystallographically distinct Hg atoms in the asymmetric unit.
The Hg1 atom has linear geometry, while the Hg2 atom, situated
on a crystallographic 2-fold axis, has distorted tetrahedral
geometry. The Q-Hg-Q angles around the linear Hg1 atoms
are 165.3(3)°, 164.2(2),° and 158.8(2)° for K2Hg3S4, K2Hg3-
Se4, and Cs2Hg3Se4, respectively. These angles are smaller than
those about the Hg atoms in HgS, at 172°.22 These smaller
angles result from a small distortion toward trigonal coordination
2n-
The [Hg3Q4]n
chains run parallel to the crystallographic
b-axis. The makeup of this chain can be regarded as a one-
dimensional assembly of distorted tetrahedral [HgQ4]6- building
blocks connected by two-coordinate Hg2+ ions as shown in
Figure 3. Alternatively, it can be viewed as a one-dimensional
(22) (a) Ozin, G. A. AdV. Mater. 1992, 4, 612-649. (b) Reed, M. A.
Sci. Am. 1993, 118-123. (c) Ozin, G. A. Supramol. Chem. 1995, 6, 125-
134.

130 J. Am. Chem. Soc., Vol. 120, No. 1, 1998
Axtell et al.
Figure 1. Schematic representation of progressive dimensional reduction in the structure of a dense binary semiconducting solid through “dilution”
with A2Q equivalents. The evolution of the energy bandgap is toward widening gaps with decreasing dimensionality.
Figure 2. Two views of the unit cell of A2Hg3Q4: (A) looking down the crystallographic b direction and (B) looking down the crystallographic
c direction.
average Hg-Q bond distances for the tetrahedral Hg2 atoms
are 2.57(1), 2.66(1), and 2.68(8) Å for K2Hg3S4, K2Hg3Se4, and
Cs2Hg3Se4, respectively. The value observed in K2Hg3S4 is
comparable to the distances of 2.542 and 2.592 Å reported for
K6HgS4.7 The Hg-S bonds for the quasilinear Hg1 atoms are
2.37(1), 2.48(1), and 2.47(6) Å for K2Hg3S4, K2Hg3Se4, and
Cs2Hg3Se4, respectively. The value observed in K2Hg3S4 is
slightly longer than the distances of 2.308(5) and 2.312(5) Å
reported for K2HgS2.9 Selected bond distances and angles for
the three compounds are given in Tables 14and 15, respectively.
2n-
There is one crystallographically distinct alkali metal in the
asymmetric unit of each compound. The charge balancing alkali
metal cations, A+ ions (A ) K, Cs), are distributed between
the chains. They participate in ionic interactions with the
chalcogenides.
Figure 3. Two views of the one-dimensional (Hg3S4)n
A2Hg3Q4.
chains in
geometry due to short Hg1-Q2 contacts between the chains.
These contacts occur along the (001) plane, at distances of 3.08-
(1), 3.159(6), and 3.158(6) Å for K2Hg3S4, K2Hg3Se4, and Cs2-
Hg3Se4, respectively. In this structure type, long Hg-Q bonds
are associated with the tetrahedral Hg2+ centers, while shorter
Hg-Q bonds are associated quasi-linear Hg2+ centers. The
It is interesting to comment on the structural difference
between Cs2Hg3Se4 and Rb2Hg3Te4.12 The latter compound was
found to be layered but the structural makeup of the layers is

Incorporation of A2Q into HgQ
J. Am. Chem. Soc., Vol. 120, No. 1, 1998 131
2n-
2n-
Figure 4. The transition from infinite (Hg3Se4)n
chains to (Hg3Te4)n
layers is straightforward through interchain Hg-Q interactions. This
associative transformation converts the linearly coordinated Hg atoms to trigonal planar.
Table 14. Selected Bond Distances (Å) or K2Hg3S4, K2Hg3Se4,
the countercation, can cause similar structural changes. The
counterion effect also seems to explain the transition to a two-
dimensional structure in going from K2Hg3S4 to Na2Hg3S4.10
and Cs2Hg3Se4 with Standard Deviations in Parenthesesa
distance
K2Hg3S4
K2Hg3Se4
Cs2Hg3Se4
The K+ ions are surrounded by six chalcogen atoms in the
range of 3.22(2)-3.35(1) and 3.30(2)-3.37(1) Å in K2Hg3S4
and K2Hg3Se4, respectively, while Cs+ ions in Cs2Hg3Se4 are
surrounded by seven selenium atoms in the range of 3.596(7)-
3.844(7) Å.
Hg1-Q1
Hg1-Q2
Hg2-Q1
Hg2-Q2
Hg1-Q1
Hg1-Q1
A1-Q1
A1-Q1
A1-Q1
A1-Q1
A1-Q2
A1-Q2
A1-Q2
2.36(1)
2.38(1)
2.58(1)
2.57(1)
3.08(1)
3.15(2)
3.29(2)
3.35(1)
3.28(1)
3.34(1)
3.22(2)
3.30(1)
3.27(1)
2.486(6)
2.473(6)
2.671(6)
2.657(6)
3.159(6)
3.210(6)
3.37(1)
3.37(1)
3.37(1)
3.38(1)
3.30(2)
3.36(1)
3.35(1)
2.429(7)
2.515(7)
2.751(6)
2.608(6)
3.158(6)
3.295(6)
3.596(7)
3.647(7)
3.706(6)
3.737(6)
3.706(5)
3.734(7)
3.844(7)
Structure Description of the A2Hg6S7 Compounds. The
A2Hg6Q7 feature unique structure types with three-dimensional
(3-D) frameworks and two kinds of parallel tunnels, see Figure
5. In fact the compounds adopt two different variants of what
is essentially the same architectural motif. So K2Hg6S7 crystal-
lizes in the space group P4h21m while Rb2Hg6S7, Cs2Hg6S7, and
Cs2Hg6Se7 belong to the space group P42nm. These two groups
differ in the bonding of the corresponding Hg2 and S3 atoms.
The K compound has linear and tetrahedral Hg2+ ions bridged
by three crystallographically unique Q2- ions, while the other
three compounds have trigonal planar and tetrahedral Hg2+
centers also bridged by three crystallographically unique Q2-
ions. Every atom in the asymmetric unit has the same point
symmetry in both space groups.
Table 15. Selected Bond Angles (deg) for K2Hg3S4, K2Hg3Se4,
and Cs2Hg3Se4 with Standard Deviations in Parenthesesa
angle
K2Hg3S4
K2Hg3Se4
Cs2Hg3Se4
Q1-Hg1-Q2
Q1-Hg2-Q1
Q1-Hg2-Q2
Q1-Hg2-Q2
Q2-Hg2-Q2
Hg1-Q1-Hg2
Hg1-Q2-Hg2
165.3(3)
104.7(3)
105.7(3)
114.9(9)
111.0(4)
96.5(4)
164.2(2)
115.7(2)
104.4(2)
110.7(2)
106.2(2)
95.7(2)
158.8(2)
110.1(2)
105.3(2)
107.0(2)
121.9(3)
98.2(2)
There are two kinds of easily recognizable one-dimensional
tunnels running parallel to the crystallographic c axis. A set of
empty, narrow tunnels with an octagonal cross section is
composed of distorted tetrahedral Hg1 and trigonal pyramidal
Q1 ions. The diameters of these tunnels are approximately 4.8-
4.9 Å, corresponding to the Hg1-Hg1 distance. A second set
of “stuffed”, wider tunnels, of dimensions 6.14 × 5.26 Å, has
a 12-member ring cross section in which both tetrahedral Hg1
and linear Hg2 atoms are held together by triply and doubly
bridging Q2- ions. The major difference between the two
structures is the symmetry element passing through the center
of the empty tunnels: a 4-fold inversion axis (4h) in K2Hg6S7
versus a 4-fold screw axis (42) in the rest of the compounds.
The two structures are related by an origin shift in the a-axis
direction by 1/2. The geometries of the doubly bridging Q2 and
the triply bridging Q1 atoms are chevron-shaped and trigonal
pyramidal, respectively. The other chalcogen atom Q3 differs
in its binding between the K and the Rb compound.
95.8(4)
95.8(2)
93.5(2)
the result of associative bonding interactions of the parallel
chains of [Hg3Q4]n
2n-
in which Q atoms from one chain bind
to linear Hg atoms of a neighboring chain, as illustrated in Figure
4. Given that the [Hg3Te4]n2n- framework is larger in volume
than the [Hg3Se4]n2n-, it is tempting to attribute the origin of
this structural transition to the counterion effect. While a certain
counterion may have the ability to screen properly anionic
species of a given size and shape, a smaller set of counterions
of equal charge may not be able to do so, thus causing the
anionic species to associate. These associative interactions can
lead to higher dimensionalities in the anionic part of the structure
even though the stoichiometry of atoms in it remains constant.
Conversely, the increase in size and volume of the anionic part
of a structure without a concomitant increase in the volume of

132 J. Am. Chem. Soc., Vol. 120, No. 1, 1998
Axtell et al.
see Table 16. Both space groups P42nm and P4h21m are
derivatives of the same minimal nonisomorphic super group P42/
mnm.
The Hg1-S3-Hg1 and Hg1-S3-Hg2 angles in K2Hg6S7
are 158.1(3)° and 100.9(2)°, respectively. The Hg1-Q3-Hg1
and Hg1-Q3-Hg2 angles in Rb2Hg6S7, Cs2Hg6S7, Rb2Hg6-
Se7, and Cs2Hg6Se7 are generally both smaller: for example,
156.2(1)° and 97.5(1)° respectively for Cs2Hg6Se7. The Hg1-
Q3 bonds are unusually long, varying from 2.718(4) to 2.879-
(2) Å in K2Hg6S7 and Cs2Hg6Se7, respectively, while the Hg2-
Q3 bonds are normal (for a linear and trigonal planar coordinated
Hg2+ ion), ranging from 2.345(8) to 2.477(5) Å for K2Hg6S7
and Cs2Hg6Se7, respectively. Selected bond distances and
angles are given in Tables 16 and 17, respectively.
There is one crystallographically distinct alkali metal in each
of the asymmetric units of the compounds. The alkali cations
are found inserted in the center of the large 12-member tunnels,
interacting with the chalcogenide lone pairs on electrons that
are directed toward the tunnel center. The alkali ions are
surrounded by seven chalcogen atoms in the range of 3.30(1)
to 3.764(5) Å.
A view of K2Hg6S7 and Rb2Hg6S7 down their respective
crystallographic [110] direction displays dramatically how all
the Q2 atoms point in the same direction, see Figure 8. The
acentric nature of these new compounds suggests the possibility
that they may be useful as nonlinear optical materials, for second
harmonic generation (SHG). Investigations are in progress to
determine their properties in this regard.
Optical Spectroscopy. All of the A2Hg6Q7 compounds
possess larger bandgaps than their corresponding cubic HgQ
parent compounds, see Figure 9. The absorption spectra of K2-
Hg6S7, Rb2Hg6S7, Cs2Hg6S7, Rb2Hg6Se7, Cs2Hg6Se7, and their
solid solutions display steep absorption edges that are the result
of charge-transfer excitations between a Q p-like valence band
and a Hg 6s-like conduction band, similar to those occurring in
red HgS. The bandgaps have increased from 0 eV in cubic
HgS20 to 1.50, 1.55, and 1.61 eV in K2Hg6S7,23 Rb2Hg6S7, and
Cs2Hg6S7, respectively, and from -0.15 eV in HgSe20 to 1.13
and 1.17 eV in Rb2Hg6Se7 and Cs2Hg6Se7, respectively. To
get sharp absorption spectra high purity HgS and HgSe must
be used in the synthesis, otherwise “tailing” is observed below
the bandgap arising from impurity mid-gap states.
The bandgaps mentioned above are consistent with the colors
of the materials. The sulfides are dark red to black in color,
while the selenides are black. This dramatic change from near
zero gap in the HgQ to the intermediate gap semiconductor in
the ternaries is attributed to the breakup of the parent structure.
The result is more open 3-D framework with reduced covalent
bonding because of lower coordination numbers in Hg and Q
atoms in the structure and the fact that the high energy s and p
orbitals of the alkali metal ions do not contribute to the band
dispersion near the Fermi level. Band dispersion in these
materials is, therefore, expected to be narrower than that in the
Figure 5. The unit cells of (A) K2Hg6S7 and (B) Rb2Hg6S7. Both views
are down the crystallographic c direction. Hg, crossed ellipsoids; S,
bonded, open ellipsoids; and K, isolated, open ellipsoids.
In K2Hg6S7 the Q3 has a T-type coordination, while in Rb2-
Hg6S7 the same atom has a “seesaw” geometry bonding to two
Hg(2) and two Hg(1) atoms, see Figure 6. The seesaw geometry
is obtained by displacing the Q3 atom in the structure of K2-
Hg6S7 away from the plane defined by the atoms Hg(2)-Hg-
(1)-Hg(1). It is this displacement along the c axis that imposes
lower symmetry on the frameworks of Rb2Hg6S7, Cs2Hg6S7,
Rb2Hg6Se7, and Cs2Hg6Se7, by destroying the 21 screw-axis in
the cell, causing a departure from the P4h21m space group, see
Figure 7. Notice that a mere destruction of the 21 screw-axis
leads to the nonisomorphic space group P4h. The n-glide plane,
required by the P4h2nm space group, is obtained when the
translation of the Q3 atom along the z axis reaches a point where
all Q3 have the same z coordinate. At this point the Q3 atom
still does not have equal Hg(2)-Q(3) distances in the structure,
5
binary HgQ compounds. This effect has been noted in K2Cd2S3
6
the A2Cd3Q4 compounds, and in the Cat+/Pb/I system.24
The A2Hg6Q7 compounds form solid solution compounds
between the S and Se end members. Several members of these
compounds were prepared and their lattice parameters were
found to obey Vegard’s law. In the solid solution compound
Rb2Hg6Se3.5S3.5, a bandgap of 1.3 eV was observed. It is at a
value halfway between those of the end members, Rb2Hg6Se7
(1.13 eV) and Rb2Hg6S7 (1.55 eV), and suggests that the
Rb2Hg6Se7-xSx system possesses a tunable bandgap. The
Cs2Hg6Se7-xSx system also demonstrates the same character-
(23) An extended Hu¨ckel band structure calculation on K2Hg6S7 shows
that bandgap in this material is direct. J. Li and M. G. Kanatzidis,
unpublished work.

Incorporation of A2Q into HgQ
J. Am. Chem. Soc., Vol. 120, No. 1, 1998 133
Figure 6. The T-type coordination of Q3 in the A2Hg6Q7 compounds.
Figure 7. Space group comparison of the structurally similar but crystallographically different K2Hg6S7 and Rb2Hg6S7 looking down the c axis.
The n-glide plane in P42nm is generated by moving the Q3 atoms in the P4h21m structure along the z axis so that they all attain the same z coordinate.
At this point the 4h axis evolves into a 42 axis.
Table 16. Selected Bond Distances (Å) or K2Hg6S7, Rb2Hg6S7,
value, the Cs2Hg6Se7 structure is expected to become unstable,
since we have been unsuccessful in isolating “Cs2Hg6Te7”.25
The A2Hg3Q4 compounds are also semiconductors. Their
great air sensitivity has inhibited an accurate determination of
their bandgaps. The yellow color of K2Hg3S4 suggests a
bandgap of 2.5-2.6 eV, while the red and orange colors of K2-
Hg3Se4 and Cs2Hg3Se4 suggest bandgaps of about 2.1 and 2.2-
2.3 eV, respectively. These materials definitely possess higher
bandgaps than the A2Hg6Q7 compounds which can be rational-
Cs2Hg6S7, and Cs2Hg6Se7 with Standard Deviations in Parenthesesa
distance
K2Hg6S7
Rb2Hg6S7
Cs2Hg6S7
Cs2Hg6Se7
Hg1-Q1
Hg1-Q1
Hg1-Q1
Hg1-Q3
Hg1-Q3
Hg2-Q2
Hg2-Q3
Hg2-Q3
A1-Q1
A1-Q1
A1-Q2
A1-Q3
A1-Q3
2.467(5)
2.552(8)
2.527(8)
2.718(4)
2.718(5)
2.366(9)
2.345(8)
3.007(9)
3.34(2)
2.518(9)
2.564(1)
2.475(4)
2.7322(6)
2.7322(6)
2.275(7)
2.57(1)
2.472(9)
2.574(7)
2.485(8)
2.757(7)
2.757(7)
2.362(9)
2.327(9)
2.988(9)
3.51(1)
2.637(4)
2.622(4)
2.564(3)
2.870(2)
2.870(2)
2.476(4)
2.477(5)
2.477(5)
3.664(4)
3.626(4)
3.660(3)
3.764(5)
3.690(5)
2.76(1)
(24) (a) Ishihara, T.; Takahashi, J.; Goto, T. Phys. ReV. B 1990, 42,
11099-11107. (b) Papavassiliou, G. C.; Patsis, A. P.; Lagouvardos, D. J.;
Koutselas, L. B. Synth. Met. 1993, 55-57, 3889-3894. (c) Hong, X.;
Ishihara, T.; Nurmikko, A. V. Phys. ReV. B 1992, 45, 6961-6964. (d)
Ishihara, T.; Takahashi, J.; Goto, T. Solid State Commun. 1989, 69, 933-
936. (e) Papavassiliou, G. C.; Koutselas, J. B.; Lagouvardos, D. J. Z.
Naturforsch. 1993, 48b, 1013-1014. (f) Vincent, B. R.; Robertson, K. N.;
Camaron, T. S.; Knop, O. Can. J. Chem. 1987, 65, 1042-1046. (g)
Nagapetyan, S. S.; Dolzhenko, Yu. I.; Arakelova, E. R.; Koshkin, V. M.;
Struchkov, Yu. T.; Shklover, V. E. Russ. J. Inorg. Chem. 1988, 33, 1614-
1618. (h) Poglitsch, A.; Weber, D. J. Chem. Phys. 1987, 87, 6373-6378.
(i) Xu, C-Q.; Sakakura, H.; Kondo, T.; Takeyama, S.; Miura, N.; Takahashi,
Y.; Kumata, K.; Ito, R. Solid State Commun. 1991, 79, 249-253. (j) Xu,
C-Q.; Kondo, T.; Sakakura, H.; Kumata, K.; Takahashi, Y.; Ito, R. Solid
State Commun. 1991, 79, 245-248. (k) Era, M.; Morimoto, S.; Tsutsui,
T.; Saito, S. Synth. Met. 1995, 71, 2013-2014.
3.450(8)
3.450(8)
3.508(5)
3.50(1)
3.30(1)
3.420(6)
3.30(1)
3.51(1)
3.544(9)
3.59(2)
3.62(1)
3.54(1)
3.59(2)
istics. A sample with the composition Cs2Hg6Se1.75S5.25 pos-
sesses a bandgap of 1.4 eV. The absorption spectra of some of
these solid solutions are displayed in Figure 9. In fact, all the
members examined in the Rb2Hg6S7-xSex series obey Vegard’s
law. These characteristics make these systems interesting for
examination in cascade solar cells, waveguides, and other
optoelectronic devices. Solid solutions of the type
Cs2Hg6Te7-xSex should also be possible, although at some x
(25) Instead we have obtained Cs2Hg3Te4, which has a different structure
from that of Cs2Hg3Se4 and is more closely related to that of K2Zn3S4. See
refs 6 and 7.

134 J. Am. Chem. Soc., Vol. 120, No. 1, 1998
Axtell et al.
Figure 9. Solid-state absorption spectra of (A) Rb2Hg6Se7, Rb2Hg6-
Se3.5S3.5, and Rb2Hg6S7 and (B) Cs2Hg6Se7, Cs2Hg6Se1.75S5.25, and Cs2-
Hg6S7.
Figure 8. The (Hg6Q7)n2n- frameworks of (A) K2Hg6Q7 and (B) Rb2-
Hg6S7. Both views are down the respective crystallographic [1, 1, 0]
directions.
are in two-coordinate, chevron-shaped coordination, while in
the latter, only 1/7 of the Q atoms are two coordinate, with others
in trigonal pyramidal or T-shaped geometries, as described
above. Therefore, the generally lower connectivity of the A2-
Hg3Q4 compounds at the atomic level can also be seen to result
in the higher bandgaps. This trend is expected to continue, with
the A6HgQ4 and the A2HgQ2 compounds possessing higher
bandgaps than the A2Hg3Q4 compounds. The former two
structure types contain isolated HgQ46- tetrahedra and isolated,
Table 17. Selected Bond Angles (deg) for K2Hg6S7, Rb2Hg6S7,
Cs2Hg6S7, and Cs2Hg6Se7 with Standard Deviations in Parenthesesa
angle
K2Hg6S7 Rb2Hg6S7 Cs2Hg6S7 Cs2Hg6Se7
Q1-Hg1-Q1
Q1-Hg1-Q1
Q1-Hg1-Q1
Q1-Hg1-Q3
Q1-Hg1-Q3
Q1-Hg1-Q3
Q2-Hg2-Q3
Hg1-Q1-Hg1 105.2(2) 108.6(1)
Hg1-Q1-Hg1 106.9(2) 108.6(1)
Hg1-Q1-Hg1 102.6(3) 103.2(3)
Hg2-Q2-Hg2 108.8(6) 117.9(5)
Hg1-Q3-Hg1 158.1(3) 161.22(3)
Hg1-Q3-Hg2 100.9(2)
Hg2-Q3-Hg2 108.8(6) 101.13(5)
121.2(2) 121.4(4)
124.8(3) 123.0(4)
106.9(2) 108.6(1)
96.3(2) 100.11(9)
119.2(1)
122.8(6)
110.6(6)
96.5(1)
121.3(1)
121.9(1)
110.0(1)
94.5(1)
2-
linear HgQ2 “dumbbell” anions, respectively.
93.7(2)
98.1(3)
95.3(6)
95.9(1)
Thermal Properties. Differential thermal analysis (DTA)
was used to evaluate the thermal properties of these materials.
Upon heating, K2Hg6S7, Rb2Hg6S7, Cs2Hg6S7, Rb2Hg6Se7, and
Cs2Hg6Se7 showed endothermic peaks centered at 556, 587, 586,
554, and 567 °C, respectively. These peaks correspond to
melting transitions and are followed by exothermic crystalliza-
tion peaks upon cooling at 472, 562, 542, 528, and 547 °C,
respectively. In Figure 10, a representative DTA thermogram
for Cs2Hg6S7 is displayed, with the temperature and interpreta-
tion of each peak included. After the analysis, the presence of
a tiny polycrystalline ingot instead of a loose powder provided
assurance that the material had indeed melted. X-ray powder
patterns were used for compound identification. All compounds
but Rb2Hg6Se7 melted congruently. The X-ray powder pattern
of this compound becomes much simpler after the DTA
experiment, suggesting degradation to HgSe.
106.5(2) 100.11(9)
172.4(4) 162.2(4)
104.7(7)
168.5(6)
105.3(7)
110.6(8)
102.7(5)
112.2(9)
156.4(8)
98.7(8)
104.8(1)
166.4(2)
110.0(1)
103.7(1)
103.2(1)
113.2(3)
156.2(1)
97.5(1)
96.2(3)
102.5(8)
102.8(1)
ized on the basis of both bulk dimensionality and framework
connectivity at the atomic level. The A2Hg6Q7 compounds
possess tunneled, 3-D structures, while the A2Hg3Q4 compounds
have linear chain anions. The more interrupted the covalent
bonding in HgQ becomes, the larger the bandgap is expected
to be.6 The higher bandgaps in the A2Hg3Q4 compounds can
also be rationalized by considering the coordination numbers
of the mercury atoms in the two structural classes. In the A2-
Hg3Q4 compounds, there are two linear mercury atoms for every
one tetrahedral mercury atom, while these numbers are reversed
in the A2Hg6Q7 family. All of the Q atoms in the former family
The thermal properties of these new mercury chalcogenides
appear to depend on the amount of sample and container size.
The small amount of sample and the tube in the DTA experiment
does not yield detectable byproducts after melting and recrys-

Incorporation of A2Q into HgQ
J. Am. Chem. Soc., Vol. 120, No. 1, 1998 135
Figure 10. Differential thermogram of Cs2Hg6S7. Heating/cooling rate
of 10 °C/min.
tallization. Scale-up to bulk size samples, however, has been
inhibited by the formation of HgQ or elemental Hg itself at
temperatures 50-100 °C above the melting points of the
materials. Preliminary attempts to grow large single crystals
of Cs2Hg6S7 in a vertical Bridgman type furnace resulted in
directional solidification, which formed a dense agglomeration
of parallel thin needles with their long axis (c axis) parallel to
the growth direction.
Figure 11. Comparison of the X-ray powder diffraction patterns of
Rb2Hg6S7 and Li1.8Rb0.2Hg6S7.
Ion-Exchange Properties of A2Hg6Q7. The alkali ion-filled
infinite tunnels in the 3-D structure of A2Hg6Q7 prompted us
to ask whether they would be amenable to ion-exchange
chemistry. This was particularly interesting to us because the
lithium compound Li2Hg6S7 could not be prepared under our
synthetic flux conditions. At first, attempts were made using
the rubidium sulfide analogue with aqueous and nonaqueous
solutions of Li salts to obtain Li2Hg6S7. These reactions were
unsuccessful mainly because the large hydration (or solvation)
sphere of Li renders it too large for the tunnel and kinetically
inert. Recently, we reported a new convenient, low-temperature
method, which serves to exchange, topotactically, large alkali
ions for smaller ones by using alkali iodide salts.26 This method
involves the solid-state reaction at T < 120 °C of the host
material with great excess LiI for several days, according to eq
1.
Figure 12. The optical absorption spectrum of Li1.8Rb0.2Hg6S7.
effects are observed for the Se analogues. Further characteriza-
tion of the ion-exchanged material is in progress.
Rb2Hg6S7 + LiI f Li2Hg6S7 + RbI
(1)
Concluding Remarks
The exceptional advantage of this reaction is (a) the low
temperature, which prevents the destruction of the host material
(should the material be unstable at higher temperatures), and
(b) the absence of solvent, which avoids cation solvation. With
this reaction 90% Rb exchange can be achieved, giving rise to
Li1.8Rb0.2Hg6S7. Figure 11 shows a comparison of the X-ray
powder diffraction patterns of parent and the ion-exchanged
material and suggests a topotactic ion-exchange. Given that
the tunnels in Rb2Hg6S7 are relatively small and completely
filled, and are only accessible by their two open ends (front
and back of tunnel) and not via side openings, that ion-exchange
The introduction of A2Q into HgQ breaks up its three-
dimensional dense diamond-like structure and produces the two
new families of ternary compounds, with the formulas A2Hg3Q4
and A2Hg6Q7, which have Hg/Q anions with low dimensional
structures. The A2Hg3Q4 compounds consist of linear chain
anions separated by alkali metal cations. The A2Hg6Q7
compounds display unique, tunneled 3-D structures with A+
cations residing in parallel tunnels. The anisotropic framework
structure of A2Hg6Q7 can sustain topotactic ion-exchange
reactions with LiI. The presence of Li ions in the structure
causes a large bathochromic shift in the optical spectrum of
Li2Hg6Q7 because of stronger interaction with the Hg/Q
framework. The A2Hg6Q7 group compounds are medium
bandgap semiconductors with optical gaps in the optimum
region for efficient solar energy absorption (i.e. 1.0 eV-1.5 eV)
and are (a) amenable to bandgap engineering through S/Se solid
solution and (b) melt processing. These new compounds have
larger bandgaps than HgQ, because of the reduced dimensional-
ity caused by the incorporation of A2Q into the HgQ lattice. In
happens at all is quite remarkable. The new compound Li1.8
-
Rb0.2Hg6S7 has a dramatically decreased optical bandgap of 0.78
eV with respect to the 1.55 eV of Rb2Hg6S7, see Figure 12.
This decrease is attributed to considerably greater covalent
bonding interactions of the Li ion with the sulfur atoms of the
framework, in sharp contrast with the heavier alkali analogues
in which the same interactions are primarily ionic. Similar
(26) Chondroudis, K.; Kanatzidis, M. G. Submitted for publication.

136 J. Am. Chem. Soc., Vol. 120, No. 1, 1998
Axtell et al.
turn the bandgaps of the linear chain A2Hg3Q4 compounds are
higher than those of the A2Hg6Q7 compounds, consistent with
their higher Q2-/Hg2+ ratio, lower Hg-Q connectivity, and
lower dimensionality. The noncentrosymmetric structure of A2-
Hg6Q7 suggests possible nonlinear optical properties.
Fellow and a Camille and Henry Dreyfus Teacher Scholar.
Supporting Information Available: Tables of calculated
and observed powder patterns for K2Hg3Se4, Cs2Hg3Se4, Rb2-
Hg6S7, Cs2Hg6S7, Rb2Hg6Se7, and Cs2Hg6Se1.75S3.25, anisotropic
thermal parameters for K2Hg3S4, K2Hg3Se4, Cs2Hg3Se4, K2-
Hg6S7, Rb2Hg6S7, Cs2Hg6S7, and Cs2Hg6Se7, and bond distances
and angles for K2Hg3S4, K2Hg3Se4, Cs2Hg3Se4, K2Hg6S7, Rb2-
Hg6S7, Cs2Hg6S7, and Cs2Hg6Se7 (36 pages). See any current
masthead for ordering and Internet access instructions.
Acknowledgment. This research was made possible with
funding from NSF Grant DMR-9527347. This work made use
of the SEM facilities of the Center for Electron Optics at
Michigan State University. M.G.K. is an A. P. Sloan Foundation
JA9713486