The noncentrosymmetric chain compounds, A3M2AsSe 11 (A = K, Rb, Cs; M = Nb, Ta)

Article abstract of DOI:10.1016/j.jallcom.2004.03.088

The noncentrosymmetric niobium and tantalum selenoarsenates, A ₃Nb₂AsSe₁₁ (A=K, Rb, Cs) and K ₃Ta₂AsSe₁₁, were synthesized in a polyselenoarsenate flux. All compounds crystallize in the polar monoclinic space group Cc. The structures are comprised of the same type of infinite chain anions, [M₂Se₂(Se₂)₃(AsSe ₃)]^3- (M=Nb, Ta) separated by alkali metal cations. The As³⁺ centers with nonbonded electron pairs play an important role in stabilizing the noncentrosymmetric structures. UV-Vis spectroscopy, Raman spectroscopy and differential thermal analysis data are reported. The energy gaps of these compounds vary between 1.35 and 1.53eV.

Full text of DOI:10.1016/j.jallcom.2004.03.088

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Journal of Alloys and Compounds 381 (2004) 41–49
The noncentrosymmetric chain compounds,
A3M2AsSe11 (A = K, Rb, Cs; M = Nb, Ta)
Junghwan Do, Mercouri G. Kanatzidis∗
Department of Chemistry, Michigan State University, East Lansing, MI 48824, USA
Received 15 January 2004; received in revised form 1 March 2004; accepted 8 March 2004
Abstract
The noncentrosymmetric niobium and tantalum selenoarsenates, A3Nb2AsSe11 (A = K, Rb, Cs) and K3Ta2AsSe11, were synthesized in a
polyselenoarsenate flux. All compounds crystallize in the polar monoclinic space group Cc. The structures are comprised of the same type
of infinite chain anions, [M2Se2(Se2)3(AsSe3)]3− (M = Nb, Ta) separated by alkali metal cations. The As3+ centers with nonbonded electron
pairs play an important role in stabilizing the noncentrosymmetric structures. UV-Vis spectroscopy, Raman spectroscopy and differential
thermal analysis data are reported. The energy gaps of these compounds vary between 1.35 and 1.53 eV.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Flux synthesis; Non linear optical piezoelectricity; Non-oxide materials
1. Introduction
nary alkali metal incorporated group 5 metal chalcoarsen-
ates. Their structure represents the insertion of As atoms into
that of K6Nb4Se22. All compounds contain infinite chain
anions [M2AsSe11]3− (M = Nb, Ta) and adopt the polar
space group, Cc which arises from a parallel alignment of
the [AsSe3]3− pyramids.
Recently, we have reported several quaternary tin chal-
coarsenates, KSnAsS5, K2SnAs2S6, [1] and Cs2SnAs2Q9
(Q = S, Se) [2] prepared in polyselenoarsenate fluxes. One
important feature that becomes evident in this type of chem-
istry is that the reactivity of arsenic differs significantly
from that of phosphorus. The oxidation of the heavier pnico-
gen favors predominantly the 3+ state whereas the 4+ and
5+ states are more stable with phosphorus. This divergence
lies at the foundation of the bifurcation in chemical reactiv-
ity of the two elements and gives rise to the different types
of compounds encountered between the chalco-phosphate
and the chalco-arsenate phases. The overall symmetry in the
structure of the compounds is determined by the arrange-
ment of As3+ cations with its lone pair of electrons. In all
the cases an antiferroelectric arrangement is found along
with a centrosymmetric lattice. Here, we report results of
the polyselenoarsenate flux synthesis using early transition
metals and describe the niobium/tantalum selenoarsenates,
A3Nb2AsSe11 (A = K, Rb, Cs) and K3Ta2AsSe11. To the
best of our knowledge, these compounds are the first quater-
2. Experimental
2.1. Reagents
The following reagents were used as obtained: Nb (99.9
%; Aldrich Chemical Co, Milwaukee, WI), Ta (99.8 %;
Cerac, Milwaukee, WI), As2Se3 (99.9 %; Strem Chemi-
cals, Newburyport, MA), As (99.9%; Aldrich Chemical Co,
Milwaukee, WI), Se (99.999 %; Noranda Advanced Mate-
rials, Quebec, Canada). N,N-dimethylformamide (Spectrum
Chemicals, ACS reagent grade); diethyl ether. K2Se, Rb2Se
and Cs2Se starting materials were prepared by stoichiomet-
ric reactions of potassium, rubidium, cesium metal and se-
lenium in liquid NH3.
2.2. Synthesis
∗
Corresponding author. Tel.: +1-517-355-9715-174;
Compound K3Nb2AsSe11 (1) was synthesized from a
mixture of 0.157 g (1.0 mmol) K2Se, 0.092 g (1.0 mmol)
fax: +1-517-353-1793.
E-mail address: kanatzid@cem.msu.edu (M.G. Kanatzidis).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.03.088

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J. Do, M.G. Kanatzidis / Journal of Alloys and Compounds 381 (2004) 41–49
Nb, 0.193 g (0.5 mmol) As2Se3, and 0.396 g (5.0 mmol)
Se. The reagents were mixed, sealed in an evacuated sil-
ica tube, and heated at 550 ◦C for 3 days, then cooled
at a rate of 5 ◦C/h to 250 ◦C followed by rapid cooling
to room temperature. The solid products were washed
with N,N-dimethylformamide (DMF) to remove the flux
and dried with ether. Black rod-shaped single crystals of
K3Nb2AsSe11 were obtained together with big black poly-
hedra, Nb2Se9 as indicated by X-ray powder diffraction
and energy dispersive spectroscopy (EDS). Quantitative
synthesis of pure K3Nb2AsSe11 was achieved by a reaction
of stoichiometric amounts of K2Se/Nb/As/Se at 550 ◦C.
The product was stable in air and water. EDS analysis of
a number of crystals gave an average nominal formula of
“K3Nb2AsSe11”. The errors in these analyses are to within
4%.
Rb3Nb2AsSe11 (2) was synthesized from a mixture of
0.250 g (1.0 mmol) Rb2Se, 0.092 g (1.0 mmol) Nb, 0.193 g
(0.5 mmol) As2Se3, and 0.396 g (5.0 mmol) Se. The reagents
were mixed, sealed in an evacuated silica tube, and heated
at 550 ◦C for 3 days, then cooled at a rate of 5–250 ◦C/h
followed by rapid cooling to room temperature. The solid
products were washed with DMF to remove the flux
and dried with ether. Black rod-shaped single crystals of
Rb3Nb2AsSe11 were obtained in pure form (yield 92% based
Nb). Quantitative synthesis of pure Rb3Nb2AsSe11 was
also achieved by a reaction of the stoichiometric amounts
of Rb2Se/Nb/As/Se at 550 ◦C. The product was stable in
air and water in the order of several days. EDS analysis of
the crystals gave an nominal formula of “Rb3Nb2AsSe11”.
Cs3Nb2AsSe11 (3) was synthesized from a mixture of
0.345 g (1.0 mmol) Cs2Se, 0.092 g (1.0 mmol) Nb, 0.193 g
(0.5 mmol) As2Se3, and 0.396 g (5.0 mmol) Se. The reagents
were mixed, sealed in an evacuated silica tube, and heated
at 550 ◦C for 3 days, then cooled at a rate of 5–250 ◦C/h
followed by rapid cooling to room temperature. The solid
products were washed with DMF) and ether. Pure black
rod-shaped single crystals of Cs3Nb2AsSe11 were obtained
(yield 94% based on Nb). The product was stable in air and
water for several days. EDS analysis of the crystals gave an
average nominal formula of “Cs3Nb2AsSe11”.
All compounds 1–4 are insoluble in polar solvents, such as
DMF and dimethylsulfoxide.
3. Physical characterization techniques
3.1. X-ray powder diffraction
Analyses were performed using a calibrated CPS 120
INEL X-ray powder diffractometer (Cu K␣ radiation) oper-
ating at 40 kV/20 mA and equipped with a position-sensitive
detector with a flat sample geometry.
3.2. Electron microscopy
Semi quantitative analyses of the compounds were per-
formed with a JEOL JSM-35C scanning electron microscope
(SEM) equipped with a Tracor Northern energy dispersive
spectroscopy (EDS) detector.
3.3. Solid state UV-Vis
Optical diffuse reflectance measurements were per-
formed at room temperature using a Shimadzu UV-3101
PC double-beam, double-monochromator spectrophotome-
ter operating in the 200–2500 nm region. The bands gaps
were derived using a protocol reported elsewhere [3].
3.4. Raman spectroscopy
Raman spectra were recorded on a Holoprobe Raman
spectrograph equipped with a CCD camera detector using
633 nm radiation from a HeNe laser for excitation and a res-
olution of 4 cm−1. Laser power at the sample was estimated
to be about 5 mW and the focused laser beam diameter was
ca. 10 ␮m. Sixty-four scans were sufficient to obtain good
quality spectra.
3.5. Differential thermal analysis (DTA)
DTA experiments were performed on Shimadzu DTA-50
thermal analyzer. Typically a sample (∼20 mg) of ground
crystalline material was sealed in a silica ampoule under
vacuum. A similar ampoule of equal mass filled with Al2O3
was sealed and placed on the reference side of the detec-
tor. Sample 1 was heated to 650 ◦C at 10 ◦C/min, and after
3 min it was cooled at a rate of −10 ◦C/min to 50 ◦C. A
sample of 2 was heated to 600 ◦C using the same cycle as
above. Residues of the DTA experiments were examined by
X-ray powder diffraction. Reproducibility of the results was
checked by running multiple heating/cooling cycles.
K3Ta2AsSe11 (4) was synthesized from a mixture of
0.158 g (1.0 mmol) K2Se, 0.181 g (1.0 mmol) Ta, 0.193 g
(0.5 mmol) As2Se3, and 0.317 g (4.0 mmol) Se. The reagents
were mixed, sealed in an evacuated silica tube, and heated
at 550 ◦C for 3 days, then cooled at a rate of 5–250 ◦C/h
followed by rapid cooling to room temperature. The solid
products were washed with DMF and ether. Isolation pro-
duced a single phase of pure black rod-shaped single crys-
tals of K3Ta2AsSe11(yield 88% based on Ta). The product
was stable in air and water. Quantitative synthesis of pure
K3Ta2AsSe11 was also achieved by a reaction of stoichio-
metric amounts of K2Se/Ta/As/Se at 550 ◦C. The product
was stable in air and water. EDS analysis of the crystals gave
“K3Ta2AsSe11”. The structural details of the compounds
were determined by a single-crystal X-ray diffraction study.
3.6. X-ray crystallography
The crystal structures of 1–4 were determined by
single-crystal X-ray diffraction methods. Preliminary

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J. Do, M.G. Kanatzidis / Journal of Alloys and Compounds 381 (2004) 41–49
43
Table 1
Crystallographic details for 1–4
Empirical formula
K3Nb2AsSe11 (1)
Rb3Nb2AsSe11 (2)
Cs3Nb2AsSe11 (3)
K3Ta2AsSe11 (4)
fw
1246.60
Cc
1385.71
Cc
1528.03
Cc
1422.68
Cc
Space group
a ( Å)
b ( Å)
c ( Å)
β (◦)
V ( Å)
Z
Crystal size (mm3)
ρcalcd (g/cm3)
µ (cm−1
T (K)
␭ (Å)
θ range (◦)
Absolute structure parameter
R1 [I > 2␴(I)]a
wR2 (all data)b
14.944(6)
12.916(5)
9.945(4)
100.483(6)
1887.6(13)
4
15.111(2)
13.242(2)
10.041(2)
99.768(2)
1980.2(5)
4
15.254(3)
13.748(3)
10.187(2)
98.367(4)
2113.6(8)
4
14.924(3)
12.938(2)
9.943(2)
100.319(3)
1888.9(5)
4
0.12 × 0.13 × 0.34
0.09 × 0.12 × 0.30
0.11 × 0.14 × 0.40
0.10 × 0.12 × 0.42
4.387
4.648
4.802
5.003
)
248.02
296(2)
0.71073
2.10–28.25
0.050(8)
0.0281
303.38
296(2)
0.71073
2.06–28.29
0.03(1)
266.52
296(2)
0.71073
2.00–28.44
0.05(1)
351.49
296(2)
0.71073
2.10 to 28.30
0.22(1)
0.0392
0.1028
0.0298
0.0712
0.0226
0.0570
0.0697
a
Rw = ꢀ||F0| − |F0||/|F0|.
b
2
1/2
wR2 = {ꢀ[w(F0 − Fc2)2]/ꢀ[w(F02)2]}
.
examination and data collection were performed on a
SMART platform diffractometer equipped with a 1 K CCD
area detector using graphite monochromatized Mo K␣ ra-
diation at room temperature. A hemisphere of data was
collected with narrow scan widths of 0.30◦ in ␻ and an
exposure time of 30 s per frame for 1–4. The data were inte-
grated using the SAINT program [3]. For 1–4 the program
SADABS was used for the absorption correction. Addi-
tional crystallographic data for 1–4 are given in Table 1.
In all cases satisfactory refinements were obtained with the
noncentrosymmetric space group, Cc.
phase-separates to produce K12Nb6Se35, (EDS analysis and
single crystal X-ray diffraction analysis confirmed it to be
isostructural to Rb12Nb6Se35) [5] and K3AsSe4. Decreas-
ing the ratio to 1:10 led to pure Nb2Se9. In the case of
rubidium, increasing the ratio to 3:10 and 4:10 led to the
formation of 2 along with Rb3AsSe4. Decreasing the ra-
tio to 1:10 again produced exclusively Nb2Se9. In the Cs
system, increasing the ratio to 3:10 formed the phase of
3 together with black rods. Electron microscope analysis
of the black rod crystals gave an average composition of
Cs2.9Nb2Se11.3 and upon single crystal X-ray diffraction
analysis proved to be Cs3Nb2Se11, which is isostructural to
K3Nb2Se11 [6] More basic conditions (4:10) produced 3 to-
gether with Cs3NbSe4 and Cs21(Nb4Se22)3(AsSe3) contain-
ing isolated [Nb4Se22]6− and [AsSe3]3− anions. [7] In the
less basic flux (1:10), again single phase Nb2Se9 was ob-
served. It is apparent in all systems that increased fractions
of A2Se (more basic) favor the formation of Nb5+ contain-
ing species such as [Nb2Se11]3− and [NbSe4]3− as found
in K12Nb6Se35, Cs3Nb2Se11, and Cs3NbSe4, whereas reac-
tions with lower A2Se content (less basic) give Nb4+ com-
The initial positions for all atoms were obtained with di-
rect methods and the structures were refined with full-matrix
least-squares techniques using SHELXTL [4]. The R values
2
for the final cycle of the refinements based on F0 are given
in Table 1. Final values of selected bond lengths and angles
are given in Table 2.
4. Results and discussion
pounds such as Nb2Se9: 2Nb4+2(Se2)2−(Se5)2−
.
4.1. Synthesis
All the compounds 1–4 were prepared by using a flux ratio
of 2:2:1:10 A2Se/Nb/As2Se3/Se (A = K, Rb, Cs) at 550 ◦C.
The influence of flux basicity on the reaction outcome was
explored in some detail in order to better appreciate its role
in phase formation in this system. Such a flux basicity in-
vestigation should always be performed where possible.
A change in the flux basicity was achieved by altering
the A2Se:Se (A = K, Rb, Cs) ratio from 1:10 to 4:10. In
the case of potassium, increasing this ratio to 3:10, gives
the phase of 1 forms without Nb2Se9, but mixed with the
water soluble K3AsSe4. Increasing the A2Se:Se ratio to
4:10, the phase of 1 no longer forms; instead, the system
4.2. Structures
The crystal structure type of A3Nb2AsSe11 (A = K, Rb,
Cs) 1–3 is polar and adopts the space group Cc. The struc-
ture of 1–3 is composed of chain anions 1/ [Nb2AsSe11]3−
∞
separated by alkali metal cations, Fig. 1. Each of the
two crystallographically independent niobium atoms in
the chain is coordinated by seven selenium atoms in a
distorted pentagonal bipyramidal arrangement. Two pen-
tagonal prisms one containing a Nb5+ ion coordinated
by 3Se2− and 3Se2
Nb5+ ion coordinated by 2Se2− and 3Se2 ligands share
ligands and the other containing a
2−
2−

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44
J. Do, M.G. Kanatzidis / Journal of Alloys and Compounds 381 (2004) 41–49
Table 2
Bond lengths (Å) and angles (◦) for 1–4
For 1
K(1)-Se(9)#1
K(1)-Se(10)#2
K(1)-Se(11)#1
K(1)-Se(5)
K(1)-Se(1)#4
K(1)-Se(1)
3.266(3)
3.345(3)
3.530(3)
3.722(3)
3.896(3)
3.935(3)
K(1)-Se(11)
K(1)-As#3
K(1)-Se(2)
K(1)-Se(5)#4
K(1)-Se(7)#4
3.269(3)
3.451(3)
3.604(3)
3.737(3)
3.918(3)
K(2)-Se(8)
3.311(2)
3.396(3)
3.443(2)
3.503(2)
3.672(3)
K(2)-Se(11)#5
K(2)-Se(9)#7
K(2)-Se(11)#6
K(2)-Se(10)
K(2)-Se(7)
3.355(3)
3.426(3)
3.491(3)
3.538(3)
3.904(3)
K(2)-Se(6)#6
K(2)-Se(3)#7
K(2)-Se(1)#6
K(2)-Se(10)#7
K(3)-Se(4)#8
K(3)-Se(10)#9
K(3)-Se(6)
K(3)-Se(5)#9
K(3)-Se(6)#7
3.368(3)
3.499(3)
3.601(3)
3.677(3)
3.905(3)
K(3)-Se(4)#7
K(3)-Se(8)
K(3)-Se(3)#7
K(3)-As#7
3.466(3)
3.510(3)
3.618(3)
3.863(3)
3.915(3)
K(3)-Se(1)
Nb(1)-Se(10)
Nb(1)-Se(3)
Nb(1)-Se(5)
Nb(1)-Se(1)
2.3244(13)
2.5979(11)
2.6939(14)
2.9269(14)
Nb(1)-Se(8)
Nb(1)-Se(7)
Nb(1)-Se(2)
2.5936(13)
2.6444(14)
2.7490(13)
Nb(2)-Se(11)
Nb(2)-Se(1)
Nb(2)-Se(4)
Nb(2)-Se(3)
2.3350(13)
2.6149(14)
2.6451(12)
2.8688(13)
Nb(2)-Se(6)
Nb(2)-Se(9)
Nb(2)-Se(2)
2.5937(12)
2.6252(13)
2.6655(14)
As-Se(4)
As-Se(5)#10
2.3780(15)
2.4338(14)
As-Se(7)#10
Se(2)-Se(9)
2.3796(15)
2.3602(13)
Se(1)-Se(6)
Se(3)-Se(8)
2.3714(13)
2.3315(13)
For 2
Rb(1)-Se(8)#1
Nb(1)-Se(3)vvRb(1)-Se(6)
Rb(1)-Se(3)#3
Nb(1)-Se(3)
3.4082(16)
3.4852(18)
3.5552(16)
3.5736(16)
3.6627(18)
Rb(1)-Se(11)#2
Rb(1)-Se(9)#3
Rb(1)-Se(11)
Rb(1)-Se(10)#1
Rb(1)-Se(7)#1
3.4293(18)
3.4905(18)
3.5706(18)
3.5803(17)
3.9305(19)
Rb(1)-Se(10)#3
Rb(2)-Se(4)#4
Rb(2)-Se(4)#5
Rb(2)-Se(6)
Rb(2)-Se(5)#3
Rb(2)-Se(2)#3
Rb(2)-Se(1)
3.4448(17)
3.584(2)
3.6575(18)
3.7288(19)
3.9484(19)
3.989(2)
Rb(2)-Se(8)
3.5752(18)
3.6276(19)
3.6997(18)
3.930(2)
Rb(2)-Se(10)#3
Rb(2)-Se(3)#5
Rb(2)-As#5
Rb(2)-Se(6)#5
3.978(2)
Rb(3)-Se(11)#6
Rb(3)-Se(10)#7
Rb(3)-Se(11)
Rb(3)-Se(5)#9
Rb(3)-Se(1)#9
Rb(3)-Se(1)#6
3.3296(19)
3.4729(19)
3.6013(19)
3.7528(19)
3.9672(19)
4.081(2)
Rb(3)-Se(9)
Rb(3)-As#8
Rb(3)-Se(2)#6
Rb(3)-Se(5)#6
Rb(3)-Se(7)#9
3.3572(17)
3.5358(19)
3.7327(18)
3.820(2)
4.028(2)
Nb(1)-Se(10)
Nb(1)-Se(3)
Nb(1)-Se(5)
Nb(1)-Se(1)
2.3293(15)
2.5985(13)
2.6994(15)
2.9347(14)
Nb(1)-Se(8)
Nb(1)-Se(7)
Nb(1)-Se(2)
2.5949(15)
2.6507(14)
2.7500(14)
Nb(2)-Se(11)
Nb(2)-Se(1)
Nb(2)-Se(4)
Nb(2)-Se(3)
2.3312(15)
2.6209(14)
2.6374(14)
2.8788(14)
Nb(2)-Se(6)
Nb(2)-Se(9)
Nb(2)-Se(2)
2.5923(14)
2.6253(15)
2.6659(15)
As-Se(4)
As-Se(5)#7
2.3767(18)
2.4300(17)
As-Se(7)#7
Se(2)-Se(9)
2.3816(18)
2.3587(16)
Se(1)-Se(6)
Se(3)-Se(8)
2.3719(16)
2.3321(14)