RbCu1.2Ag3.8Se3 and Cs2Cu2Sb2Se5: Novel Quaternary Intermetallics Synthesized from Superheated Organic Media

Article abstract of DOI:10.1006/jssc.1999.8187

Reactions in superheated ethylenediamine (en) solutions at 160°C resulted in two novel quaternary intermetallic copper selenides, RbCu_1.2Ag_3.8Se₃(I) and Cs₂Cu₂Sb₂Se₅(II). Both I and II are metal rich and represent new layered structure types. Compound I crystallizes in the tetragonal crystal system, space group P4/nbm (No. 125) with a=5.991(1) A, c=10.918(2) A, Z=2, V=391.9(1) A³, R1/wR2=0.0373/0.0458 for all reflections. Compound II belongs to the triclinic crystal system, space group P1 (No. 2), a=7.645(1) A, b=8.768(2) A, c=10.264(1) A, α=91.97(2)°, β=92.07(2)°, γ=103.05(1)°, Z=2, V=669.2(3) A³, R1/wR2=0.0685/0.0740 for all reflections. I consists of ²_∞[(Cu_1.2Ag_3.8Se ₃)^-] layers and Rb⁺ counterions located between these layers. There are two types of metal-to-selenium coordination, a square planar (Ag) and a trigonal pyramidal (Cu/Ag). The Se(1) atom displays an unusual eight coordination with Ag and Cu. II contains alternating ²_∞[(Cu₂Sb₂Se₅) ^2-] anionic and Cs⁺ cationic layers. Each copper atom has a distorted tetrahedral coordination to four Se atoms, and each antimony atom bonds to three Se atoms to result in a trigonal pyramidal geometry. Both I and II are semiconductors with estimated band gaps of 0.7-0.8 and 1.2-1.3 eV, respectively.

Full text of DOI:10.1006/jssc.1999.8187

Journal of Solid State Chemistry 147, 132}139 (1999)
Article ID jssc.1999.8187, available online at http://www.idealibrary.com on
RbCu1.2Ag3.8Se3 and Cs2Cu2Sb2Se5: Novel Quaternary Intermetallics
Synthesized from Superheated Organic Media1
Zhen Chen, Ru-Ji Wang, Kieran J. Dilks, and Jing Liꢀ
Department of Chemistry, Rutgers University, Camden, New Jersey 08102
Received November 9, 1998; in revised form January 28, 1999; accepted January 29, 1999
case of ammonia, a number of sul"des and polysul"des have
been synthesized from reactions at 160}1703C (11).
Reactions in superheated ethylenediamine (en) solutions at
1603C resulted in two novel quaternary intermetallic copper
selenides, RbCu1.2Ag3.8Se3(I) and Cs2Cu2Sb2Se5(II). Both I and II
are metal rich and represent new layered structure types. Com-
pound I crystallizes in the tetragonal crystal system, space group
P4/nbm (No. 125) with a ؍ 5.991(1) A, c ؍ 10.918(2) A, Z ؍ 2,
V ؍ 391.9(1) A3, R1/wR2 ؍ 0.0373/0.0458 for all re6ections.
Compound II belongs to the triclinic crystal system, space group
P1 (No. 2), a ؍ 7.645(1) A, b ؍ 8.768(2) A, c ؍ 10.264(1) A, a ؍
91.97(2)3, b ؍ 92.07(2)3, c ؍ 103.05(1)3, Z ؍ 2, V ؍ 669.2(3) A3,
J. W. Kolis and coworkers have reported the synthesis of
several Group-11 metal sul"des and selenides in supercriti-
cal ethylenediamine or en (12). While these reactions were
conducted under relatively vigorous conditions (300}3503C),
we have focused primarily on a softer synthetic route in
which reactions are carried out in en medium below 1803C.
Our synthesis has thus far led to the formation of a variety
of new compounds with diverse and rich structural proper-
ties. The organic solvent may function di!erently in these
compounds. In some cases, it incorporates into the products
by chelating to a metal center to form a complex cation,
R1/wR2 ؍ 0.0685/0.0740 for all re6ections.
I consists of
2 [(Cu1.2Ag3.8Se3)؊] layers and Rb؉ counterions located between
ꢁ
these layers. There are two types of metal-to-selenium coordina-
tion, a square planar (Ag) and a trigonal pyramidal (Cu/Ag). The such as in [M(en) ] (Hg Te ), M"Fe, Mn and +Mn(en) ]
ꢃ ꢀ
Cl ,Hg Te (13), [M(en) ] SnQ , M"Mn, Zn, Q"Se
ꢃ ꢀ
(14), Te (15), [M(en) ]In Te (M"Fe, Zn), [Mo (en) (Te )
ꢀ ꢃ
(O)(Te)]In Te (16), [La(en) Cl]In Te (17), [Fe(en) [enH]
ꢀ
ꢄ
ꢃ ꢀ
Se(1) atom displays an unusual eight coordination with Ag and
ꢀ
ꢀ
ꢅ
ꢆ
2
Cu. II contains alternating [(Cu2Sb2Se5)2؊] anionic and Cs؉
ꢁ
ꢃ
ꢀ
ꢆ
ꢅ
ꢃ
ꢃ
ꢃ
cationic layers. Each copper atom has a distorted tetrahedral
ꢀ
ꢆ
ꢀ
ꢅ
coordination to four Se atoms, and each antimony atom bonds to
three Se atoms to result in a trigonal pyramidal geometry. Both
I and II are semiconductors with estimated band gaps of 0.7+0.8
and 1.2+1.3 eV, respectively. ( 1999 Academic Press
SbSe (18), [Ga(en) ]In Te (19), and [Mn(en) ]CdSnTe
ꢅ
ꢃ
ꢃ
ꢇ
ꢃ
ꢅ
(20). It can also enter the structures as free molecules, as in
Cu SbSe 0.5en and Cu SbSe en (21), where en molecules
)
)
ꢀ
ꢃ
ꢀ
ꢃ
"ll the spaces between the covalent layers. In other cases, en
acts as a solvent solely, in which synthesis produces inter-
metallic phases such as Rb Hg Te (22), KCuSe (23),
INTRODUCTION
ꢀ
ꢃ
ꢅ
ꢅ
ꢀ
RbHgSbTe (24), AHgSbSe (25), (A"Rb, Cs), Cs PdSe
ꢃ
ꢃ
ꢈ
(26), Rb Hg Se , and K Cu Te (27). Here we report the
solvothermal synthesis, crystal structures, and optical prop-
Much of the e!ort and progress in the development of soft
hydro(solvo)thermal synthesis has been made by solid-state
synthetic chemists during the past decade. In the area of
metal chalcogenides, several solvent systems have been ex-
tensively investigated and have shown great promise in
crystal growth of these materials. Use of water (an inorganic
solvent) in the temperature range 110}2203C has produced
many Group 14}16 metal sul"des and selenides (1}4), as
well as transition-metal-containing chalcogenides such as
Mo (5), Pd (6), and Cu (7). Reactions in methanol in the
same temperature range have also resulted in a variety of
new compounds, including binary alkali-metal polychal-
cogenides (8) and ternary Group 14}15 phases (9}10). In the
ꢀ
ꢆ
ꢇ
ꢅ
ꢈ
ꢂꢂ
erties of two new types of intermetallic selenides, RbCu
ꢂꢉꢀ
Ag Se (I) and Cs Cu Sb Se (II), both are Cu-based
ꢃꢉꢈ
ꢃ
ꢀ
ꢀ
ꢀ ꢊ
quaternary compounds and belong to the third group
described above.
EXPERIMENTAL
Materials
CuCl (99.9%, Strem Co.), AgCl (99.8%, Strem Co.), SbCl
ꢃ
(99#%, Strem Co.), and Se (99.5%, Strem Co.) were used as
purchased without further treatment. Rb Se and Cs Se
ꢀ
ꢀ
were prepared by reactions of alkali metal and elemental
selenium in a 2 : 1 ratio in liquid ammonia. Ethylenediamine
(99%, anhydrous, Fisher Scienti"c) was used as a solvent in
all reactions.
ꢂIn memory of Professor Jean Rouxel.
ꢀTo whom correspondence should be addressed.
132
0022-4596/99 $30.00
Copyright ( 1999 by Academic Press
All rights of reproduction in any form reserved.

RbCu Ag Se AND Cs Cu Sb Se
133
ꢂꢉꢀ ꢃꢉꢈ
ꢃ
ꢀ
ꢀ
ꢀ
ꢊ
TABLE 1
Crystallographic Data for RbCu1.2Ag3.8Se3 (I) and Cs2Cu2Sb2Se5(II)
Compound
I
II
Formula
Formula weight
Crystal system
RbCu Ag Se
808.50
Tetragonal
Cs Cu Sb Se
1031.20
triclinic
ꢂꢉꢀ ꢃꢉꢈ
ꢃ
ꢀ
ꢀ
ꢀ ꢊ
Space group
a (A)
b (A)
c (A)
a (3)
b (3)
c (3)
< (Aꢃ)
P4/nbm (No. 125)
5.991(1)
5.991(1)
10.918(2)
90.00
90.00
90.00
391.9(1)
P1 (No. 2)
7.645(1)
8.768(2)
10.264(1)
91.97(2)
92.07(2)
103.05(1)
669.2(3)
2
Z
2
Density (calculated, g cm\ꢃ)
Temperature (K)
Di!ractometer
6.852
294
5.118
294
Enraf-Nonius CAD4
Enraf-Nonius CAD4
Radiation
Monochromator
MoKa radiation (j"0.71073 A)
Oriented graphite
Crystal color
Morphology
dark brown
plate
black
block
Crystal size (mmꢃ)
Linear absorption coe$cient (mm\ꢂ)
Scan mode
Recording range 2h (3)
hkl range
0.03;0.2;0.4
32.528
u-scan
0.1;0.1;0.2
26.035
u-scan
4-50
!9/9 !10/10 0/12
2495
2350
1807
0.0282
t-scan
4-50
!7/7 0/7 !12/12
1261
202
171
0.0651
Number of measure re#ections
Number of independent re#ections
Number of observed re#ections [I'2p(I)]
R
*ꢀꢁ
Absorption correction
t-scan
Transmission (min}max)
Re"nement
0.7784}0.9931
Fꢀ
0.7671}0.9999
Fꢀ
Calculated weights
R [I'2p(I)]
R (all data)
w"1/pꢀ(Fꢀ)
w"1/pꢀ(Fꢀ)
ꢂ
ꢂ
R1 0.0273, wR2 0.0445
R1 0.0437, wR2 0.0701
R1 0.0373, wR2 0.0458
R1 0.0685, wR2 0.0740
S (all data on Fꢀ)
Number of re"ned parameters
1.673
22
1.317
101
Synthesis
TABLE 3
Single crystals of I were obtained from a solvothermal
Atomic Coordinates and Equivalent Isotropic Displacement
reaction in en. A sample containing 0.25 mmol of Rb Se
Parameters (A2) for Cs2Cu2Sb2Se5 (II)
ꢀ
TABLE 2
Atoms
x
y
z
;
?
%ꢃ
Atmoic Coordinates and Equivalent Isotropic Displacement
Parameters (A2) for RuCu1.2Ag3.8Se3 (I)
Cs(1)
Cs(2)
Sb(1)
Sb(2)
Cu(1)
Cu(2)
Se(1)
Se(2)
Se(3)
Se(4)
Se(5)
0.2632(1)
!0.2641(1)
0.2002(1)
0.1857(1)
0.4031(3)
0.1278(3)
0.0805(2)
0.1440(2)
0.5128(2)
0.2555(2)
0.4270(2)
0.1953(1)
0.9433(1)
0.6408(1)
0.5804(1)
0.5070(2)
0.6405(2)
0.3408(2)
0.8434(2)
0.6180(2)
0.8160(2)
0.7008(2)
0.0936(1)
0.3871(1)
0.1162(1)
0.0327(3)
0.0347(3)
0.0208(3)
0.0193(3)
0.0310(5)
0.0336(5)
0.0198(3)
0.0341(4)
0.0245(4)
0.0257(4)
0.0192(3)
Atoms
x
y
z
;
?
%ꢃ
!0.2614(1)
0.3859(2)
Ag(1)
M(1)@
Rb(1)
Se(1)
Se(2)
0.2500
0.0500(1)
0.2500
0.2500
0.2500
0.2500
0.5500(1)
0.7500
0.0000
0.18848(8)
0.5000
0.0000
0.3162(1)
0.0280(5)
0.047(4)
0.0247(5)
0.0334(6)
0.0282(5)
0.4324(2)
0.3853(1)
0.2778(2)
0.1831(2)
!0.2500
0.2500
!0.0882(1)
!0.4202(1)
?; de"ned as one-third of the trace of the orthogonalized U tensor.
%ꢃ
@M(1)"0.7 Ag#0.3 Cu, and the ; is the average of those for Ag and
%ꢃ
?; is de"ned as one-third of the trace of the orthogonalized U tensor.
Cu.
%ꢃ

134
CHEN ET AL.
and 0.75 mmol of Se (0.059 g). The approximate composi-
tions of I (Rb : Cu : Ag : Se+1 : 1 : 4 :3) and II (Cs : Cu : Sb :
Se+2 : 2 : 2 : 6) were established from microprobe analysis
on a JEOL JXA-8600 Superprobe.
TABLE 4
Selected Bond Lengths (A) and Angles (3) for RuCu1.2Ag3.8Se3 (I)
Ag(1)}M(1);8
2.9834(7)
2.9955(5)
3.0548(6)
180.0
90.0
121.56(3)
110.95(6)
90.0
M(1)}Se(1)
2.666(1)
2.571(1)
3.606(1)
63.29(1)
Ag(1)}Se(1);4
M(1)}Se(2);2
Rb(1)}Se(2);4
Ag(1)}Se(1)}M(1);8
Ag(1)}Se(1)}M(1);8 116.71(1)
M(1)}Se(1)}M(1);4 126.58(3)
M(1)}Se(1)}M(1);2
M(1)}Se(2)}M(1);4
M(1)}Se(2)}M(1);2 114.32(6)
M(1)}M(1);4
Se(1)}Ag(1)}Se(1);2
Se(1)}Ag(1)}Se(1);4
Se(2)}M(1)}Se(1);2
Se(2)}M(1)}Se(2)
Ag(1)}Se(1)}Ag(1);4
Crystal Structure Determination
A dark-brown plate crystal of I (0.03;0.2;0.4 mm) and
a black block crystal of II (0.1;0.1;0.2 mm) were mounted
on glass "bers in air on an Enraf-Nonius CAD4 automated
di!ractometer equipped with graphite monochromatized
MoKa radiation. Room temperature (294$13K) single-
crystal X-ray experiments were performed on both crystals.
Unit cells were obtained and re"ned by 25 well-centered
re#ections with 5.53(h(11.63 (I) and 6.23(h(13.53
(II). Data collections were monitored by three standards
78.94(5)
72.90(3)
Ag(1)}Se(1)}Ag(1);2 180.0
(0.060 g), CuCl (0.025 g), and AgCl (0.036 g), respectively,
and 0.75 mmol of Se (0.059 g) was weighed and mixed in
a glove box under argon atmosphere. The sample was then
transferred to thick-walled Pyrex tubes and approximately
0.4 ml of solvent was added to the tube. After the liquid was
condensed by liquid nitrogen, the tube was sealed with
a torch under vacuum (&10\ꢃ Torr). The samples were
then heated at 1603C for 5 days. After being cooled to room
temperature, the sample was washed with water and ethanol
and dried with anhydrous diethyl ether. Single crystals of II
were grown from a reaction using 0.25 mmol of Cs Se
ꢀ
(0.088 g), 0.25 mmol of SbCl (0.057 g) and CuCl (0.025 g),
ꢃ
TABLE 5
Selected Bond Lengths (A) and Angles (3) for Cs2Cu2Sb2Se5 (II)
Sb(1)}Se(2)
Sb(1)}Se(3)
Sb(1)}Se(4)
Sb(2)}Se(1a)
Sb(2)}Se(4)
Sb(2)}Se(5)
Cu(1)}Se(1)
2.507(2)
2.515(2)
2.633(2)
2.597(2)
2.625(2)
2.575(2)
2.565(3)
Cu(1)}Se(3)
Cu(1)}Se(5b)
Cu(1)}Se(5c)
Cu(2)}Se(1)
Cu(2)}Se(1d)
Cu(2)}Se(2)
Cu(2)}Se(5c)
2.425(3)
2.493(2)
2.546(3)
2.596(2)
2.525(3)
2.410(3)
2.637(3)
Se(2)}Sb(1)}Se(3)
Se(2)}Sb(1)}Se(4)
Se(3)}Sb(1)}Se(4)
Se(1a)}Sb(2)}Se(5)
Se(1a)}Sb(2)}Se(4)
Se(4)}Sb(2)}Se(5)
Se(1)}Cu(1)}Se(3)
Se(1)}Cu(1)}Se(5b)
Se(1)}Cu(1)}Se(5c)
Se(3)}Cu(1)}Se(5b)
Se(3)}Cu(1)}Se(5c)
102.68(7)
98.47(6)
102.44(7)
96.05(6)
97.19(6)
96.40(6)
118.3(1)
99.79(9)
104.67(9)
103.51(9)
115.84(9)
Se(2)}Cu(2)}Se(5c)
Cu(1)}Se(1)}Cu(2)
Cu(1)}Se(1)}Sb(2a)
Cu(1)}Se(1)}Cu(2d)
Cu(2)}Se(1)}Sb(2a)
Cu(2)}Se(1)}Cu(2d)
Sb(2a)}Se(1)}Cu(2d)
Sb(1)}Se(2)}Cu(2)
Sb(1)}Se(3)}Cu(1)
Sb(1)}Se(4)}Sb(2)
Cu(1b)}Se(5)}Cu(1e)
Cu(1b)}Se(5)}Sb(2)
Cu(1b)}Se(5)}Cu(2e)
Cu(1e)}Se(5)}Sb(2)
Cu(1e)}Se(5)}Cu(2e)
Sb(2)}Se(5)}Cu(2e)
108.8(1)
62.52(7)
122.27(8)
122.76(9)
74.77(7)
76.58(9)
78.36(7)
84.68(7)
89.86(8)
95.35(6)
66.08(9)
92.56(7)
120.29(9)
107.43(7)
62.20(7)
76.75(7)
Se(5b)}Cu(1)}Se(5c) 113.92(9)
Se(1)}Cu(2)}Se(1d)
Se(1)}Cu(2)}Se(2)
Se(1)}Cu(2)}Se(5c)
Se(1d)}Cu(2)}Se(2)
Se(1d)}Cu(2)}Se(5c)
103.42(9)
128.2(1)
101.30(9)
113.71(9)
96.29(8)
FIG. 1. Perspective views of RbCu Ag Se (I) along the a direction.
ꢂꢉꢀ ꢃꢉꢈ
ꢃ
Note. Symmetry transformation: a (!x, 1!y,!z); b (1!x, 1!y,!z); Open circles are Se atoms, shaded circles are Cu, black circles are Cu/Ag,
c (x, y, 1#z); d (!x, 1!y, 1!z); e (x, y, !1#z); f (1!x, 1!y, 1!z). and cross shaded circles are Rb.

RbCu Ag Se AND Cs Cu Sb Se
ꢂꢉꢀ ꢃꢉꢈ ꢊ
135
ꢃ
ꢀ
ꢀ
ꢀ
every two hours. No decay was observed in either case
except the statistic #uctuations in the range of $2.1% (I)
and $1.2% (II). Raw intensities were corrected for Lorentz
and polarization e!ects, and for absorption by empirical
method based on t-scan data. Direct phase determination
yielded the positions of all atoms. There are two indepen-
dent metal positions in I, one (Wycko! position 2a) is
entirely occupied by Ag, while the other (Wycko! position
8m) is shared randomly by Cu and Ag. The s.o.f. of the two
atoms were assigned 0.3 and 0.7 for copper and silver,
respectively, in the "nal re"nement, which agrees with the
results from the re"nement for their s.o.f. and from the
microprobe analysis. All atoms were subjected to anisot-
ropic re"nement. For compound I, the "nal full-matrix
least-square re"nement on Fꢀ converged with R1"0.0273
and wR2"0.0445 for observed re#ections, and with
R1/wR2"0.0373/0.0458 for all re#ections. R1 and wR2 for
compound II were re"ned to be 0.0437 and 0.0701 for
observed re#ections, and 0.0685 and 0.0740 for all re#ec-
tions. The "nal di!erence electron density map showed no
features in both cases. Details of crystal parameters, data
collection and structure re"nement are given in Table 1.
Anisotropic thermal displacements were assigned to all
atoms. All calculations were performed using SHELX-97
(28). Crystal structure drawings were produced with
SCHAKAL 92 (29). Final atomic coordinates and average
temperature factors are listed in Tables 2 and 3 for I and II,
respectively. Bond lengths and angles are reported in
Tables 4 (I) and 5 (II), respectively.
FIG. 3. ORTEP drawings with displacement ellipsoids at 50% prob-
ability level showing coordination of each atom in RbCu Ag Se (I).
ꢂꢉꢀ ꢃꢉꢈ
ꢃ
Diwuse Reyectance Measurements
Optical di!use re#ectance measurements of powder sam-
ples of I and II were performed on a Shimadzu UV-3101PC
double-beam, double-monochromator spectrophotometer
FIG. 2. View of a single layer in RbCu Ag Se (I) along the c axis.
The labeling scheme is the same as in Fig. 1. The 8m positions are labeled as
M ("Cu/Ag).
FIG. 4. View of the anionic layer along the b axis in Cs Cu Sb Se (II).
Open circles are Se atoms, shaded circles are Cu, and black circles are Sb.
The alternating M}Se 12-membered rings are shown clearly in the "gure.
ꢂꢉꢀ ꢃꢉꢈ
ꢃ
ꢀ
ꢀ
ꢀ ꢊ

136
CHEN ET AL.
at room temperature. Data were collected in the wavelength large channels. Some relations may be drawn between com-
range 250}1800 nm. BaSO powder was used as a standard pound I and the ThCr Si type structure (31), such as
ꢅ
ꢀ ꢀ
(100% re#ectance). A similar procedure as previously de- CsFe
Ag
Te (32) and KCuZnTe (33), although the
ꢋꢉꢇꢀ ꢂꢉꢀꢈ
ꢀ
ꢀ
scribed (24) was used to collect and convert the data using di!erences in the anionic layers are signi"cant. There are
the Kubelka}Munk function. The scattering coe$cient (S) two independent metal positions within a single layer of I
was treated as a constant since the average particle size of (Fig. 2). Position 2a is occupied by Ag only, while position
the samples used in the measurements was signi"cantly 8m is occupied randomly by both Ag and Cu, indicated as
larger than 5 lm ('20 lm).
M in the "gure. The coordination of each atom is shown in
Fig. 3. The Ag(1) to Se(1) coordination is a square-planar
with a single Ag(1)}Se(1) bond distance of 2.996(1) A. This is
signi"cantly longer than those found in Rb NbAgSe (34),
RESULTS AND DISCUSSION
ꢀ
ꢅ
Structure of RbCu Ag Se
ꢂꢉꢀ ꢃꢉꢈ
ꢃ
in which Ag has a tetrahedral coordination to Se with
Compound I is a quaternary compound rich in metal a Ag}Se bond length of 2.640 A, in K Ag As Se (av.
ꢊ
ꢀ
ꢃ ꢄ
content (metal : chalcogen ratio, 6 : 3). As shown in Fig. 1, it 2.676 A) (35) and in KAg Se (av. 2.794 A) (36), where Ag
ꢃ
ꢀ
has a layered structure composed of in"nite two-dimen- has a trigonal coordination with Se. The square-planar
ꢀ
sional anionic [(Cu Ag Se )\] slabs separated by Rb> geometry of Ag is scarce, and there are no known examples
ꢁ
ꢂꢉꢀ ꢃꢉꢈ ꢃ
counterions. While the composition of I is the same as of such a geometry previously reported for the A}Ag}Se
CsAg Te (30), its crystal structure has no relation to the systems. The M (Ag/Cu) atoms bond to three Se to give
latter, which is a three-dimensional network containing a trigonal pyramidal coordination [M}Se(1)"2.666(1) A
ꢊ
ꢃ
FIG. 5. Perspective view of Cs Cu Sb Se (II) along the c direction. The labeling scheme is the same as in Fig. 4. The cross shaded circles are Rb.
ꢀ
ꢀ
ꢀ ꢊ

RbCu Ag Se AND Cs Cu Sb Se
ꢂꢉꢀ ꢃꢉꢈ ꢊ
137
ꢃ
ꢀ
ꢀ
ꢀ
and M}Se(2)"2.571(1) A;2]. The M}Se distances are
slightly longer than those found in RbCuSe (2.400, 2.425,
ꢅ
2.433, and 2.572 A) (37) and are comparable with those in
the aforementioned silver compounds. The Se(1) atoms in
the title compound display a very unusual coordination
with the metals. Instead of taking the usual capping posi-
tions, they reside in the interior of the layers and are eight-
fold coordinated to four Ag(1) (in the center of the Ag
ꢅ
square) and four M (in the center of the M tetrahedron).
ꢅ
While a high coordination number is quite common for
chalcogen elements, encapsulation of these atoms by metals
is very unusual. To the best of our knowledge, I represents
the "rst example in which the chalcogens are encapsulated
by the metal elements.
Structure of Cs Cu Sb Se
ꢀ
ꢀ ꢀ ꢊ
Compound II is also a quaternary intermetallic phase
having a new type of layered structure. Figure 4 gives
a perspective view of its crystal structure along the b axis.
ꢀ
The [(Cu Sb Se )ꢀ\] layers stack on top of each other
ꢁ
ꢀ
ꢀ ꢊ
along the b axis with Cs> cations located between them. The
cations themselves form double layers that extend in the ac
plane in a zigzag fashion (see Fig. 5). Each antimony atom is
surrounded by three selenium atoms to give a trigonal
pyramidal geometry and each copper atom by four selenium
atoms to form a tetrahedral coordination. Of the "ve inde-
FIG. 6. ORTEP drawing with displacement ellipsoids at 50% prob-
ability level showing the coordination of Cu(1), Cu(2), Sb(1), Sb(2), Se(1),
and Se(5) in Cs Cu Sb Se (II).
ꢀ
ꢀ
ꢀ ꢊ
pendent sleneium atoms, Se(1) and Se(5) and k -bridging
ꢅ
ligands, each bonds to three copper and an antimony atom,
converted from the di!use re#ectance data (F"(1!R )ꢀ/
while Se(2), Se(3), and Se(4) are k -bridging ligands bonded
ꢁ
ꢀ
2R , where R is the relative di!use re#ectance of an
to two metal (M"Cu, Sb) atoms, as shown in Fig. 6. The
ꢁ
ꢁ
in"nitely thick layer. For practical purposes, this can be
achieved at a layer depth of a few millimeters). The re-
mission functions are plotted in Fig. 7 versus the wavelength
anionic layer may be constructed by connecting the edge-
sharing CuSe tetrahedral chains with bridging Sb Se
ꢅ
ꢀ ꢃ
units, or it may be described as composed of fused 12-
membered, alternating M}Se rings linked by interring
Cu}Se and/or Sb}Se bonds. The approximate dimension of
these rings is 4.6;7.0 A. The Cu}Se bond distances range
from 2.410 to 2.596 A, slightly shorter than those of I (2.571,
2666 A) and compare well with those of RbCuSe (37) and
ꢅ
RbCuSb Se H O (38). The Sb}Se distances are between
)
ꢀ
ꢅ
ꢀ
2.515 and 2.625 A, comparable with those observed in
AHgSbSe , 2.520, 2.594 A (A"Rb), 2.780, 2.851 A (A"Cs)
ꢃ
(25), and in RbCuSb Se H O, 2.556}2.726 A (38). There
)
ꢀ
ꢅ
ꢀ
are also short}metal contacts between the copper atoms,
giving 2.748 A for Cu(1)}Cu(1) and 2.678 A for Cu(1)}Cu(2).
These are similar to 2.674 A in Cu SbSe 1/2en (21),
)
ꢀ
ꢃ
2.649 A in Cu SbSe en (21), 2.665 A in Cu SbSe (39), and
)
ꢀ
ꢃ
ꢃ
ꢅ
ꢃ
ꢃ
2.830 A in Cu SbSe (40).
Optical Properties
The optical properties of I and II were examined by
analyzing the di!use re#ectance data. The Kubelka}Munk
(or remission, F) functions (41) for both I and II were over the energy range of 0.5}4.0 eV.
FIG. 7. Optical absorption spectra (Kubelka}Munk Function) for
RbCu Ag Se (I) (solid line) and Cs Cu Sb Se (II) (dotted line) plotted
ꢂꢉꢀ ꢃꢉꢈ
ꢃ
ꢀ
ꢀ
ꢀ ꢊ

138
CHEN ET AL.
(in the unit of electron volts). The absorption coe$cient (a)
can be computed from the remission function (F"a/S)
where S is the scattering factor (24, 41). The band gaps can
be estimated from the sharp absorption edges. The values
obtained for I and II are 0.7}0.8 and 1.2}1.3 eV, respectively,
suggesting that both compounds are semiconductors with
relatively small band gaps. The results from tight-binding
band structure calculations performed on compound I are
consistent with those of optical experiments (42).
9. W. S. Sheldrick and H.-J. Hausler, Z. Anorg. Allg. Chem. 561, 139
(1988); 561, 149 (1988); W. S. Sheldrick and H. G. Braunbeck, Z.
Naturforsch. B 44, 851 (1989); W. S. Sheldrick and B. Schaaf, Z.
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(1988); W. S. Sheldrick and H. Schaaf, Z. Naturforsch. 49, (1994); G.
Dittmar, and H. Schafer, Z. Naturforsch. B 32, 1346 (1977); W. S.
Sheldrick, and G. Z. Braunbeck, Anorg. Allg. Chem. 619, 1300 (1993);
W. S. Sheldrick, and H. G. Braunbeck, Z. Naturforsch. B. 47, 151
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Schwerer, and H. Schafer, Mater. Res. Bull. 18, 383 (1983); C. Brink-
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T. Jiang, A. J. Lough, G. A. Ozin, D. Young, and R. L. Bedard, Chem.
Mater. 7, 245 (1995); H. Ahari, C. L. Bowes, T. Jiang, A. Lough, G. A.
Orzin, R. L. Bedard, S. Petrov, and D. Young, Adv. Mater. 7, 375
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Adv. Mater. 7, 370 (1995).
Financial support from the National Science Foundation (Grant DMR-
9553066 and supplementary funds) is greatly appreciated. J.L. is grateful to
the Camille and Henry Dreyfus Foundation for a Henry Dreyfus
Teacher}Scholar Award.
11. G. L. Schimek, W. T. Pennington, and J. W. Kolis, J. Solid State Chem.
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RbCu Ag Se AND Cs Cu Sb Se
ꢂꢉꢀ ꢃꢉꢈ ꢊ
139
ꢃ
ꢀ
ꢀ
ꢀ
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