One-Dimensional Zinc Selenophosphates: A2ZnP2Se6(A = K, Rb, Cs)

Article abstract of DOI:10.1002/zaac.201600231

The new compounds A₂ZnP₂Se₆(A = K, Rb, Cs) were synthesized via molten salt flux syntheses. The crystals feature one-dimensional¹/_∞[ZnP₂Se₆]^2–chains charge balanced by alkali metal ions between the chains. K₂ZnP₂Se₆crystallizes in the monoclinic space group P2₁/c; cell parameters a = 12.537(3) ?, b = 7.2742(14) ?, c = 14.164(3) ?, β = 109.63(3)°, Z = 4, and V = 1216.7(4) ?³. Rb₂ZnP₂Se₆and Cs₂ZnP₂Se₆are isotypic, crystallizing in the triclinic space group P2₁/c. Rb₂ZnP₂Se₆has cell parameters of a = 7.4944(15) ?, b = 7.6013(15) ?, c = 12.729(3) ?, α = 96.57(3)°, β = 105.52(3)°, γ = 110.54(3)°, Z = 2, and V = 636.6(2) ?³. Cs₂ZnP₂Se₆has cell parameters of a = 7.6543(6) ?, b = 7.7006(6) ?, c = 12.7373(11) ?, α = 97.007(7)°, β = 104.335(7)°, γ = 109.241(6)°, Z = 2, and V = 669.54(10) ?³.

Full text of DOI:10.1002/zaac.201600231

Journal of Inorganic and General Chemistry
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DOI: 10.1002/zaac.201600231
One-Dimensional Zinc Selenophosphates: A2ZnP2Se6 (A = K, Rb, Cs)
Alyssa S. Haynes,[a][‡] Katherine Lee,[a][‡] and Mercouri G. Kanatzidis*[a]
Keywords: Polychalcogenides; Zinc selenide; Chalcogenides; Semiconductors
¯
Abstract. The new compounds A2ZnP2Se6 (A = K, Rb, Cs) were syn-
P1. Rb2ZnP2Se6 has cell parameters of a = 7.4944(15) Å, b =
thesized via molten salt flux syntheses. The crystals feature one-dimen-
7.6013(15) Å, c = 12.729(3) Å, α = 96.57(3)°, β = 105.52(3)°, γ =
110.54(3)°, Z = 2, and V = 636.6(2) Å3. Cs2ZnP2Se6 has cell param-
1
sional /ϱ[ZnP2Se6]2– chains charge balanced by alkali metal ions be-
tween the chains. K2ZnP2Se6 crystallizes in the monoclinic space eters of a = 7.6543(6) Å, b = 7.7006(6) Å, c = 12.7373(11) Å, α =
group P21/c; cell parameters a = 12.537(3) Å, b = 7.2742(14) Å, c = 97.007(7)°, β = 104.335(7)°, γ = 109.241(6)°, Z = 2, and V =
14.164(3) Å, β = 109.63(3)°, Z = 4, and V = 1216.7(4) Å3. Rb2ZnP2Se6 669.54(10) Å3.
and Cs2ZnP2Se6 are isotypic, crystallizing in the triclinic space group
kali metal selenophosphates. Most discovered quaternary chal-
Introduction
cophosphates with [P2Se6]4– also have 1D chain structures
Selenophosphates are compounds containing selenium and
composed of [P2Se6]4– units coordinated to a metal in an alter-
phosphorus, typically conjoined molecular units of [PxSey]z– as
nating pattern. Depending on the metal identity, [P2Se6]4– has
molecular species or coordinated ligands to metals. Seleno-
been shown to accommodate tetrahedral, octahedral, seesaw,
phosphates have promising applications in nonlinear optics,[1]
or square planar metal coordination. For example, A2HgP2Se6
ferroelectricity,[2] magnetism,[3] reversible phase-change tran-
(A = K, Rb)[20] have tetrahedral metal coordination, A2PdP2Se6
sitions,[4] γ-ray detection,[5] and photoluminescence.[4b,6] Gen-
(A = K, Cs)[20,21] have square planar metal coordination,
erally, [PxSey]z– units are made up of tetrahedral or pyramidal
Cs2SnP2Se6 has see saw metal coordination,[22] and A2MP2Se6
phosphorus and some have complex structures such as
1/ϱ[P3Se4]–,[7]
(A = K, Rb, Cs; M = Mn, Fe, Sn, Cd, Mg)[3a,18c,20] have octa-
2 – [8]
1/ϱ[P2Se6 ] ,
1/ϱ[PSe6]–,[4a]
P2Se5,[9]
hedral metal coordination. A2MP2Se6 (A = K, Rb, Cs; M = Mn,
Fe)[3a] are interesting in that they have magnetic properties
resulting from the high-spin Fe and Mn atoms coupling antifer-
romagnetically from 10–20 K. There are also many unique 1D
structures containing [P2Se6]4–, including A2M2P2Se6
[P8Se18]6–,[10] [P2Se10]4–,[11] [P2Se9]4–,[12] [P5Se12]5–,[6a]
[P6Se12]4–,[6a,13] and [P3Se7]3–.[14] The most prominent seleno-
phosphate building block is [P2Se6]4–.[15] Its structure is analo-
gous to ethane, where the phosphorus atoms are located where
the carbon would be and the Se atoms are situated at the
hydrogen positions.
(A
= K, Cs; M =
Cu, Ag, Au),[3a,23] KInP2Se6,[24]
K4Sc2(PSe4)2(P2Se6),[25] K4In2(PSe5)2(P2Se6),[26] Rb3Sn(PSe5)
(P2Se6),[26] K5In3P6Se19,[27] K4In4P6Se20,[27] and Rb2MP2Se7
(M = Ce, Gd).[28] 2D structures with [P2Se6]4– include
KMP2Se6 (M = Sb, Bi),[29] NaCeP2Se6,[30] K2LaP2Se7,[31]
A broad variety of ternary chalcophosphates containing
[P2Se6]4– units have been discovered including the zero-di-
mensional (0D) M2P2Se6 (M = Ba, Ca, Eu, Pb, Sn, Sr)[16] and
MЈ4P2Se6 (MЈ = Ag, K, Na, Tl),[12,17] all of which are com-
posed of [P2Se6]4– molecular anions charge balanced by metal
cations. There are also the two dimensional (2D), layered
MЈЈ2P2Se6 (MЈЈ = Fe, Hg, Mg, Zn).[18] Finally there are the
one-dimensional (1D) A2P2Se6 (A = K, Rb),[19] with P2Se6
A2ThP3Se9 (A
=
K, Rb),[32] Cs4ThP5Se17,[32] and
Rb4Sn5P4Se20.[33] Three-dimensional structures include
[22]
K10Sn3(P2Se6)4
and K(RE)P2Se6 (RE = Y, La, Ce, Pr,
Gd).[3b] Many of these compounds have been isolated via the
molten salt flux synthesis method,[34] in which high melting
reagents are dissolved in a lower-melting salt. In these condi-
tions, products are able to form at lower temperatures than
traditional solid state methods. Additionally, the acidity/basic-
ity of the flux is easily tunable by varying the ratio of A2Se:Se
(A = alkali metal). Therefore within the same system of ele-
ments, different products are stabilized with different flux ra-
tios.
units condensing into /ϱ[P2Se6]2– helical chains with strong
1
SHG responses and high transparency in the mid-IR region.[19]
There is a plethora of quaternary materials containing the
[P2Se6]4– unit. We will focus our discussion on quaternary al-
* Dr. M. G. Kanatzidis
E-Mail: m-kanatzidis@northwestern.edu
[a] Department of Chemistry
Northwestern University
Evanston, Illinois, 60208, USA
There are no known solved structures for quaternary seleno-
phosphates containing both an alkali metal and zinc; hence, we
performed exploratory synthesis experiments using the molten
salt flux technique to uncover them. Herein we discuss the
[‡] Both authors equally contributed to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201600231 or from the au-
thor.
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a 2:1:2:10 ratio of Cs2Se:Zn:P2Se5:Se resulted in yellow rod crystals
of Cs2ZnP2Se6 (50%) and orange rod crystals of β-CsPSe6 (50%).
discovery of three new quaternary selenophosphates,
A2ZnP2Se6 (A = K, Rb, Cs). These compounds resemble some
of the aforementioned quaternary compounds in that the struc-
tures are comprised of /ϱ[MP2Se6]2– chains. The zinc ions in
1
Direct Combination Synthesis
A2ZnP2Se6 are tetrahedrally coordinated to the [P2Se6]4– units,
and the alternation of Zn2+ and [P2Se6]4– yields 1D chains.
The potassium compound is monoclinic, whereas the rubidium
and cesium compounds are triclinic, and all three compounds
are centrosymmetric. The compounds were synthesized via the
molten salt flux technique with stoichiometric and nonstoichio-
metric ratios of the starting materials. We describe the syn-
thetic pathways utilized as well as the difficulties we encoun-
tered in attempts to synthesize pure samples of the compounds.
Stoichiometric amounts of the starting material were placed in 9 mm
fused silica tubes in an inert nitrogen atmosphere contained within a
glovebox. The tubes were evacuated to 10–4 mbar, flame sealed, and
shaken to mix the contents. The tubes were heated to 500 °C in 5 h,
dwelled for 10 h, and cooled to room temperature in 48 h. The products
from these experiments were air and water sensitive and therefore
stored in an evacuated desiccator.
K2ZnP2Se6: A mixture of K2Se (0.116 g, 0.736 mmol), Zn (0.048 g,
0.736 mmol), P (0.046 g, 1.47 mmol), and Se (0.291 g, 3.68 mmol) in
a 1:1:2:5 molar ratio of K2Se:Zn:P:Se resulted in a mixture of yellow
rod crystals of K2ZnP2Se6 (80%), orange rod crystals of K2P2Se6
(10%), and yellow block crystals of ZnSe (10%).
Experimental Section
Reagents
Rb2ZnP2Se6: A mixture of Rb2Se2 (0.223 g, 0.648 mmol), P (0.040 g,
1.30 mmol), Zn (0.042 g, 0.648 mmol), and Se (0.194 g, 2.46 mmol)
in a 1:1:2:3.8 molar ratio of Rb2Se2:Zn:P:Se resulted in a mixture of
yellow rod crystals of Rb2ZnP2Se6 (95%) and yellow block crystals
All chemicals were used as obtained: potassium metal (98%, Sigma
Aldrich, St. Louis, MO); rubidium metal (99.9+%, Strem Chemicals,
Inc., Newburyport, MA); cesium metal (99.9+%, Strem Chemicals,
Inc., Newburyport, MA); red phosphorus powder (99%, Sigma Ald- of ZnSe (5%).
rich, St. Louis, MO); selenium pellets (99.99%, Sigma Aldrich, St.
Cs2ZnP2Se6: A mixture of Cs2Se (0.199 g, 0.577 mmol), Zn (0.038 g,
Louis, MO); granular zinc (30 mesh, 99.8%, J.T. Baker Chemical Co.,
Phillipsburg, NJ); N,N-dimethylformamide (DMF) (99.8%, BDH,
Radnor, PA); anhydrous ethyl ether (99%, Mallinckrodt Baker, Inc.,
Phillisburg, NJ); triethylphosphine (99%, Sigma Aldrich, St. Louis,
MO). K2Se, Rb2Se2 and Cs2Se were synthesized by reacting stoichio-
metric amounts of the elements in liquid ammonia as described else-
where.[3a,35] P2Se5 was synthesized by reacting stoichiometric ratios of
phosphorus and selenium by traditional solid state methods; the syn-
thesis is described elsewhere.[3a]
0.577 mmol), P (0.036 g, 1.15 mmol), and Se (0.228 g, 2.88 mmol) in
a 1:1:2:5 molar ratio of Cs2Se:Zn:P:Se resulted in a mixture of yellow
rod crystals of Cs2ZnP2Se6 (80%) and orange rod crystals of β-CsPSe6
(20%).
Powder X-ray Diffraction (PXRD)
PXRD patterns were collected with a computer-controlled Rigaku
Miniflex 600 powder diffractometer with Cu-Kα radiation operating at
40 kV and 15 mA and a high-speed silicon strip detector. Calculated
PXRD patterns for A2ZnP2Se6 (A = K, Rb, Cs) were generated with
CIFs through the Visualizer program in FINDIT.
Molten Salt Flux Synthesis
Starting materials were placed in 9 mm fused silica tubes in an inert
nitrogen atmosphere contained within a glovebox. The tubes were
evacuated to 10–4 mbar, flame sealed, and shaken to mix the contents.
The reactions were heated to 500 °C, dwelled for 72 h, and cooled to
room temperature in 48 h. The resulting material was placed in de-
gassed, room-temperature DMF (50 mL) to dissolve the flux, and the
DMF was changed after the solution had become a dark green or
brown color. After the DMF solution remained clear, the samples were
placed in diethyl ether (approx. 20 mL) and triethylphosphine (0.3 mL)
was added to dissolve the excess selenium present. The products from
these experiments were air and water sensitive and therefore stored in
an evacuated desiccator.
Single Crystal X-ray Diffraction
Single crystals of K2ZnP2Se6 (0.19ϫ0.07ϫ0.04 mm3), Rb2ZnP2Se6
(0.16ϫ0.10ϫ0.01 mm3), and Cs2ZnP2Se6 (0.04ϫ0.02ϫ0.01 mm3)
were glued to the tip of a glass fiber. Both the Rb2ZnP2Se6 and
Cs2ZnP2Se6 crystals were twinned, and numerous attempts to find un-
twined crystals were unsuccessful. Measurements were collected with
a STOE IPDS II single crystal diffractometer. The diffractometer oper-
ated at 50 kV and 40 mA with graphite-monochromatized Mo-Kα radi-
ation (λ = 0.71073 Å) at room temperature. The detector comprised of
a 34 cm diameter imaging plate, and the crystal had ω rotation during
collection. X-Area software was used for data collection, integration
was performed with X-RED and numerical absorption correction was
executed with X-SHAPE. The crystal structures were solved using
SHELXT in OLEX2.[36] The K and Rb analogues were refined in the
OLEX2 program and the Cs analogue was refined in Jana2006 due to
twinning.[37] Details on refining twinning are listed in the Supporting
Information.
K2ZnP2Se6: A mixture of K2Se (0.079 g, 0.5 mmol), Zn (0.016 g,
0.25 mmol), P2Se5 (0.228 g, 0.5 mmol), and Se (0.197 g, 2.5 mmol) in
a 2:1:2:10 molar ratio of K2Se:Zn:P2Se5:Se resulted in yellow rod crys-
tals of K2ZnP2Se6 (95%) and yellow block crystals of ZnSe (5%).
Rb2ZnP2Se6: A mixture of Rb2Se2 (0.164 g, 0.5 mmol), Zn (0.016 g,
0.25 mmol), P2Se5 (0.228 g, 0.5 mmol), and Se (0.197 g, 2.5 mmol) in
a 2:1:2:10 ratio of Rb2Se2:Zn:P2Se5:Se resulted in yellow rod crystals
of Rb2ZnP2Se6 (85%), orange rod crystals of RbPSe6 (10%) and yel-
low block crystals of ZnSe (5%).
Scanning Electron Microscopy
Cs2ZnP2Se6: A mixture of Cs2Se (0.172 g, 0.5 mmol), Zn (0.016 g, Semi-quantitative microprobe analyses and energy dispersive spec-
0.25 mmol), P2Se5 (0.228 g, 0.5 mmol), and Se (0.197 g, 2.5 mmol) in
troscopy (EDS) were conducted with a Hitachi S-3400 scanning elec-
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tron microscope with a PGT energy-dispersive X-ray analyzer. The
phase (Figure 1d). This demonstrates the benefits of molten
EDS spectra were collected at 25 kV accelerating voltage, 60 mA
probe current, and with 60 s acquisition time. SEM-EDS analysis
of multiple single crystals gave an average composition of
salt flux syntheses in its ability to preferentially stabilize
phases.
We also attempted to form the materials using stoichiometric
amounts of the starting materials, which we call the direct
combination method. These syntheses of the potassium ana-
logue yields K2ZnP2Se6 as the major phase, but it also contains
K2P2Se6[19] and ZnSe[38] as additional phases (Figure S5, Sup-
porting Information). Direct combination reaction of the rubid-
ium analogue results in Rb2ZnP2Se6 as the major phase with
K2.2Zn1.0P2.0Se6.2, Rb2.1Zn1.0P2.2Se6.2, and Cs2.0Zn1.0P2.0Se5.5.
Differential Thermal Analysis (DTA)
Approximately 50–80 mg of each sample was placed in a silica am-
poule and α-Al2O3, the reference, was placed in a separate ampoule.
The ampoules were evacuated to 10–4 mbar and flame sealed. A
Shimadzu DTA-50 thermal analyzer collected data with a rate
ofϮ5 K·min–1 from room temperature to 700 °C. The temperature at
the maxima of crystallizing peaks and minima of melting peaks are
the reported crystallization and melting temperatures, respectively.
ZnSe as a second phase (Figure S6). Direct combination of the
[39]
cesium analogue consistently yields β-CsPSe6
as a second
phase (Figure S7).
Attempts to make the glassy phases of the compounds were
conducted by heating the direct combination stoichiometry of
starting materials to temperatures ranging from 700–1100 °C
then water quenching the reactions. All samples were a melt
at the quenching temperatures used, but the resulting product
was always crystalline and contained the second phases seen
for other direct combination syntheses.
Supporting Information (see footnote on the first page of this article):
X-ray crystallographic data in CIF format, atomic coordinates and dis-
placement parameters, PXRD patterns of direct combination syntheses,
SEM images, and EDS spectra.
Results and Discussion
Crystal Structures
Synthesis
The new cesium and rubidium compounds crystallize in the
triclinic space group P1, and the potassium analogue crys-
The title compounds were synthesized via the molten salt
flux synthesis technique using excess A2Se (A = K, Rb, Cs),
P2Se5, and Se to form the flux. The potassium and rubidium
analogues yield a persistent ZnSe phase regardless of the flux
composition or alkali metal (Figure 1a and b). Most flux syn-
theses of the cesium analogue additionally resulted in β-
CsPSe6 as a second phase (Figure 1c), but one particular flux
synthesis, in which the ratios of the reagents Cs:Zn:P:Se were
2:1:3:8, yielded only very little amounts of ZnSe as a second
¯
tallizes in the monoclinic space group P21/c (Table 1). Even
though the space groups are different, all three structures are
similar. They contain parallel 1/ϱ[ZnP2Se6]2– chains comprised
of alternating units of tetrahedrally coordinated Zn2+ ions to
the ethane-like [P2Se6]4– units (Figure 2). The chains are sepa-
rated by charge-balancing alkali metal ions to make up the unit
cell. In all three compounds, there is an inversion center, which
causes adjacent ZnSe4 tetrahedra within the same chain to face
opposite directions, giving centrosymmetric chain conforma-
tion. In the cesium and rubidium structures, this is the only
symmetry and therefore there are two asymmetric units in the
unit cell (Figure 3).
Figure 1. PXRD patterns of A2ZnP2Se6 (A = K, Rb, Cs) made with
molten salt flux syntheses: (a) K2ZnP2Se6 showing ZnSe as a second
phase; (b) Rb2ZnP2Se6 showing RbPSe6 and ZnSe as additional
phases; (c) Cs2ZnP2Se6 showing β-CsPSe6 as a second phase; (d)
Cs2ZnP2Se6 synthesized via the 2:1:3:8 flux composition showing
ZnSe as a slight second phase. Zoomed-in versions of these PXRD
patterns in Figures S1–S4 (Supporting Information).
1
Figure 2. Structure of /ϱ[ZnP2Se6]2– chains in A2ZnP2Se6 with ellip-
soids set to 90% probability; (a) along the chain; (b) down the chain.
Color scheme: selenium (red), zinc (cyan), and phosphorus (black).
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Table 1. Crystal data and structure refinement for A2ZnP2Se6.
K2ZnP2Se6
Rb2ZnP2Se6
Cs2ZnP2Se6
Formula weight
Temperature /K
Wavelength /Å
Crystal system
679.27
293(2)
0.71073
monoclinic
P21/c
772.01
293(2)
0.71073
triclinic
¯
P1
866.9
293(2)
0.71073
triclinic
¯
P1
Space group
Unit cell lengths and angles
a /Å
b /Å
c /Å
α /°
12.537(3)
7.2742(15)
14.164(3)
90.00
109.63(3)
90.00
1216.7(4)
4
3.708
7.4944(15)
7.6013(15)
12.729(3)
96.57(3)
105.52(3)
110.54(3)
636.6(2)
7.6543(6)
7.7006(6)
12.7373(11)
97.007(7)
104.335(7)
109.241(6)
669.54(10)
2
β /°
γ /°
Volume /Å3
Z
2
Calculated ρ /g·cm–3
4.028
26.874
4.2986
23.688
μ /mm–1
20.857
F(000)
1208
676
748
θ range for data collection
Index ranges
1.72 to 29.14°
–17 Յ h Յ 17
–9 Յ k Յ 9
–19 Յ l Յ 18
11290
1.71 to 25.00°
–8 Յ h Յ 8
–9 Յ k Յ 9
–15 Յ l Յ 13
8129
3.39 to 34.34°
–12 Յ h Յ 11
–12 Յ k Յ 12
–20 Յ l Յ 20
18911
Reflections collected
Independent reflections
Rint
3265
0.0589
2232
0.0724
2410
0.0575
Completeness to θ = 25°
Refinement method
Data / restraints / parameters
Goodness-of-fit
99.90%
99.90%
99.90%
Full-matrix least-squares on F2
2232 / 0 / 100
1.333
3265 / 0 / 100
0.988
Robs = 0.0356
wRobs = 0.0565
Rall = 0.0603
wRall = 0.0612
2410 / 0 / 101
5.61
Robs = 0.0590
wRobs = 0.1792
Rall = 0.0658
wRall = 0.1817
Final R indicesa) [Ͼ 2σ(I)]
Robs = 0.0397
wRobs = 0.0831
Rall = 0.0447
wRall = 0.0845
R indicesa) [all data]
2
2
2
a) R = Σ||Fo|–|Fc||/Σ|Fo|; wR = {Σ[w(|Fo|2–|Fc|2)2]/Σ[w(|Fo|4)]}1/2 and calcd w = 1/[σ2(Fo )+(0.0234P)2+0.0000P], where P = (Fo +2Fc )/3.
Figure 4. Crystal structure of K2ZnP2Se6 as viewed down the c axis
with ellipsoids set to 90% probability. Color scheme: potassium (blue),
selenium (red), zinc (cyan), and phosphorus (black).
Figure 3. Crystal structure of Cs2ZnP2Se6 and Rb2ZnP2Se6 as viewed
down the b axis with ellipsoids set to 90% probability. Color scheme:
cesium or rubidium (green), selenium (red), zinc (cyan), and phos-
phorus (black).
ume that is about twice that of the rubidium and cesium com-
pound unit cells: 1216.7(4) Å3 for K2ZnP2Se6, and 636.6(2) Å3
and 669.54(10) Å3 for Rb2ZnP2Se6 and Cs2ZnP2Se6, respec-
The potassium phase has other symmetry in addition to the tively. The compositional phase transition from monoclinic to
inversion center, namely a 21 screw axis about the b axis nor- triclinic going from K to Rb and Cs also reflects on the coordi-
mal to a c-glide plane (Figure 4). This creates two additional nation of the alkali metal. In monoclinic K2ZnP2Se6, there are
asymmetric units in the potassium unit cell, resulting in a vol- 4 and 6 contacts, respectively, by the two crystallographically
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Table 2. Selected bond lengths /Å and angles /° in A2ZnP2Se6.
K2ZnP2Se6
Rb2ZnP2Se6
Cs2ZnP2Se6
Zn–Se
P–P
P–Se(bridging)
P–Se(terminal)
Se–Zn–Se
2.383(1)–2.489(1)
2.239(2)–2.245(2)
2.206(2)–2.215(2)
2.139(2)–2.141(2)
103.99(3)–114.25(3)
2.473(1)–2.492(1)
2.253(1)–2.255(1)
2.201(1)–2.205(1)
2.128(1)–2.145(1)
100.99(1)–121.68(2)
2.468(3)–2.498(3)
2.247(6)–2.260(7)
2.199(7)–2.210(7)
2.134(6)–2.145(6)
100.79(11)–123.13(12)
1
distinct K atoms to Se within the sum of their ionic radii.[40] by alkali metal ions. /ϱ[ZnP2Se6]2– has tetrahedrally coordi-
On the other hand, in triclinic Rb2ZnP2Se6 and Cs2ZnP2Se6, nated Zn2+ to the adaptable [P2Se6]4– ligand. Thus far, direct
there are 6 contacts between each Rb or Cs atom to Se within combination syntheses led to K2P2Se6, RbPSe6, β-CsPSe6, and
the sum of their ionic radii. This shows that the Rb and Cs ZnSe as additional phases while nonstoichiometric flux synthe-
analogues tend to adopt a higher coordination number than the ses successfully eliminated all additional phases except for
K analogue. This behavior follows the general trend of increas- ZnSe. Further work should be conducted on this system to
ing coordination number with increasing atomic radii and is obtain the pure phases so additional characterization can be
responsible for the observed structural transition.
performed.
In all three materials, all of the Zn–Se bond lengths are nor-
mal, and Zn has slightly distorted tetrahedral coordination, as
evidenced by the Se–Zn–Se bond angles (Table 2). The P–P
bonds are normal for [P2Se6]4– units, and as expected, the P–
Se bridging bonds are slightly longer than the P–Se terminal
bonds (Table 2).
Acknowledgements
We gratefully acknowledge support from the National Science Founda-
tion Grant DMR-1410169 and a Graduate Research Fellowship for
A. S. H. under Grant No DGE-1324585. K. L. was supported by an
undergraduate summer research grant funded by Weinberg College of
Arts and Sciences at Northwestern University. This work made use of
the EPIC facility of the NUANCE Center at Northwestern University,
which was funded by the Soft and Hybrid Nanotechnology Experimen-
tal (SHyNE) Resource (NSF NNCI-1542205); the MRSEC program
(NSF DMR-1121262) at the Materials Research Center; the Inter-
national Institute for Nanotechnology (IIN); the Keck Foundation; and
the State of Illinois, through the IIN. A. S. H. and K. L. acknowledge
Dr. Amy Sarjeant, Ms. Charlotte Stern, and Dr. Christos Malliakas for
their crystallography mentorship.
The A2ZnP2Se6 structure is analogous to other materials
1
with 1D chains of /ϱ[MP2Se6]2–. The metal coordination has
been shown to be octahedral [A2MP2Se6 (A = K, Rb, Cs; M =
Mn, Fe, Sn, Cd, Mg)],[3a,18c,20,22] square planar [A2PdP2Se6 (A
= K, Cs)],[20,21] or tetrahedral [A2HgP2Se6 (A = K, Rb)],[20]
depending on the coordination preferences of the metal. This
shows the versatility of the [P2Se6]4– unit in exploratory syn-
thesis. In the title compounds, we found the Zn2+ is tetrahe-
drally coordinated to [P2Se6]4–, as expected. This is a differ-
ence from the prediction of octahedral coordination made in a
previous article from our group,[20] where it was reported that
microcrystals were found of K2ZnP2Se6 but were too small for
single-crystal X-ray diffraction. K2ZnP2Se6 was expected to be
isostructural with A2CdP2Se6 (A = K, Rb, Cs)[20] and therefore
have octahedrally coordinated Zn. Due to similar PXRD pat-
terns between the Zn2+ and Cd2+ quaternaries, we initially as-
sumed that Zn2+ also had octahedral coordination in the quater-
nary compound. Our work shows this assumption was incor-
rect, and the Zn2+ is actually tetrahedrally coordinated.
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Received: June 30, 2016
Published Online: ᭿
Z. Anorg. Allg. Chem. 0000, 0–0
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Journal of Inorganic and General Chemistry
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ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
A. S. Haynes, K. Lee, M. G. Kanatzidis* ......................... 1–7
One-Dimensional Zinc Selenophosphates: A2ZnP2Se6 (A = K,
Rb, Cs)
Z. Anorg. Allg. Chem. 0000, 0–0
7
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