Helical polymer 1/∞[P2Se62-]: Strong second harmonic generation response and phase-change properties of its K and Rb salts

Article abstract of DOI:10.1021/ja075096c

The selenophosphates A₂P₂Se₆ (A = K, Rb) crystallize in the chiral trigonal space group P3₁21, with a = 7.2728(9) A, c = 18.872(4) A, and Z = 3 at 298(2) K and a = 14.4916(7) A, c = 18.7999(17) A, and Z = 12 at 173(2) K for K ⁺ salt and a = 7.2982(5) A, c = 19.0019(16) A, and Z = 3 at 100(2) K for Rb⁺ salt. The A₂P₂Se ₆ feature parallel one-dimensional helical chains of 1/∞[P ₂Se₆^2-] which depict an oxidative polymerization of the ethane-like [P₂Se₆]^4- anion. On cooling well below room temperature K₂P₂Se ₆ exhibits a displacive phase transition to a crystallographic subgroup and forms a superstructure with a cell doubling along the a- and b-axes. The Rb analogue does not exhibit the phase transition. The compounds are air stable and show reversible glass-crystal phase-change behavior with a band gap red shift of 0.11 and 0.22 eV for K⁺ and Rb⁺ salts, respectively. Raman spectroscopy, ³¹P magic angle spinning solid-state NMR, and pair distribution function (PDF) analysis for crystalline and glassy K₂P₂Se₆ give further understanding of the phase transition and the local structure of the amorphous state. K ₂P₂Se₆ exhibits excellent mid-IR transparency and a strong second harmonic generation (SHG) response. The SHG response is type-I phase-matchable and in the wavelength range of 1000-2000 nm was measured to be 50 times larger than that of the commercially used material AgGaSe ₂. Glassy K₂P₂Se₆ also exhibits an SHG response without the application of electric field poling. In connection with the NLO properties the thermal expansion coefficients for K ₂P₂Se₆ are reported.

Full text of DOI:10.1021/ja075096c

Published on Web 11/08/2007
1
Helical Polymer /∞[P2Se62-]: Strong Second Harmonic
Generation Response and Phase-Change Properties of Its K
and Rb Salts
In Chung,†,‡ Christos D. Malliakas,†,‡ Joon I. Jang,§ Christian G. Canlas,†
David P. Weliky,† and Mercouri G. Kanatzidis*,†,‡
Contribution from the Department of Chemistry, Michigan State UniVersity, East Lansing,
Michigan 48864, Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208,
and Department of Physics and Astronomy, Northwestern UniVersity, EVanston, Illinois 60208
Received July 10, 2007; E-mail: m-kanatzidis@northwestern.edu
Abstract: The selenophosphates A2P2Se6 (A ) K, Rb) crystallize in the chiral trigonal space group P3121,
with a ) 7.2728(9) Å, c ) 18.872(4) Å, and Z ) 3 at 298(2) K and a ) 14.4916(7) Å, c ) 18.7999(17) Å,
and Z ) 12 at 173(2) K for K+ salt and a ) 7.2982(5) Å, c ) 19.0019(16) Å, and Z ) 3 at 100(2) K for Rb+
salt. The A2P2Se6 feature parallel one-dimensional helical chains of 1/∞[P2Se62-] which depict an oxidative
polymerization of the ethane-like [P2Se6]4- anion. On cooling well below room temperature K2P2Se6 exhibits
a displacive phase transition to a crystallographic subgroup and forms a superstructure with a cell doubling
along the a- and b-axes. The Rb analogue does not exhibit the phase transition. The compounds are air
stable and show reversible glass-crystal phase-change behavior with a band gap red shift of 0.11 and
0.22 eV for K+ and Rb+ salts, respectively. Raman spectroscopy, 31P magic angle spinning solid-state
NMR, and pair distribution function (PDF) analysis for crystalline and glassy K2P2Se6 give further
understanding of the phase transition and the local structure of the amorphous state. K2P2Se6 exhibits
excellent mid-IR transparency and a strong second harmonic generation (SHG) response. The SHG
response is type-I phase-matchable and in the wavelength range of 1000-2000 nm was measured to be
50 times larger than that of the commercially used material AgGaSe2. Glassy K2P2Se6 also exhibits an
SHG response without the application of electric field poling. In connection with the NLO properties the
thermal expansion coefficients for K2P2Se6 are reported.
1. Introduction
genide fluxes of suitable composition. Our experimental inves-
tigations of alkali selenophosphates provided useful insights on
the relationship between structure and flux composition (A/P/
Se ratio in the composition) in this class.5 The diversity of these
anions seems greater than that of the oxo-phosphate anions
which are responsible for defining the enormous class of metal
phosphates. The chalcophosphate anions, for example, are
capable of condensing via Q-Q bonds, whereas the phosphates
are not known to do so. Therefore, the chalcogenide analogues
have the potential to produce an even greater family of
compounds with metal ions. One striking contrast between
phosphates and chalcophosphates is the ability of the latter to
couple through Q-Q bonds (Q ) S, Se) forming catenated
species. In principle, every simple molecular anion [PxQy]z-
could be oxidatively coupled through its terminal P-Q bonds
to form larger species linked with P-Q-Q-P moieties.
The chalcophosphate anions show great structural diversity
and are a predominantly discrete molecular species as for
example the [PSe4]3-,1 [P2Se6]4-,2 [P2Se9]4-,3 [P8Se18]6-,4 and
the trans-decalin-like [P6Se12]4-.5 The number of infinite anions
is very small and includes 1 [PSe6-]6 and 1 [P5Se105-]. The
latter is found bonded to transition metal atoms in A3MP5Se10
(A ) K, Rb; M ) Ru, Os).7 The polymeric anions show unusual
structural moieties and coordination chemistry compared to the
classical [PSe4]3- and [P2Se6]4-. All chalcophosphates are
potential ligands to metal ions for building finite or extended
structures.8 The anions can be easily stabilized in polychalco-
∞
∞
† Michigan State University.
‡ Department of Chemistry, Northwestern University.
§ Department of Physics and Astronomy, Northwestern University.
(1) Garin, J.; Parthe, E. Acta Crystallogr. B 1972, 28, 3672-3674.
(2) Francisco, R. H. P.; Tepe, T.; Eckert, H. J. Solid State Chem. 1993, 107,
452-459.
Here we describe the unique helical anion 1 [P2Se62-] in
∞
the form of K+ and Rb+ salts. Although it was not formed via
actual oxidative coupling chemistry, the chiral structure derives
from coupling of the ethane-like [P2Se6]4- anion. Both K2P2-
Se6 and Rb2P2Se6 can be quenched from the melt to a glassy
state and can undergo reversible phase-change crystal-glass
transitions. Interest in phase-change materials is growing, as a
result of emerging technologies including commercially avail-
able rewritable optical media and the development of nonvolatile
(3) (a) Chondroudis, K.; Kanatzidis, M. G. Inorg. Chem. 1995, 34, 5401-
5402. (b) Mccarthy, T. J.; Kanatzidis, M. G. Chem. Mat. 1993, 5, 1061-
1063. (c) Chondroudis, K.; Mccarthy, T. J.; Kanatzidis, M. G. Inorg. Chem.
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Chem. 2004, 43, 2762-2764.
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36, 1324-1326. (b) Chung, I.; Kanatzidis, M. G., unpublished work.
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9
14996
J. AM. CHEM. SOC. 2007, 129, 14996-15006
10.1021/ja075096c CCC: $37.00 © 2007 American Chemical Society

Helical Polymer 1/∞[P2Se6
]
A R T I C L E S
2-
phase-change memory.9,10 The most interesting materials are
those with stoichiometric composition because they can switch
between the two states without complications due to composi-
tional changes.
using a calibrated CPS 120 INEL X-ray powder diffractometer (Cu
KR graphite monochromatized radiation) operating at 40 kV/20 mA
and equipped with a position-sensitive detector with flat sample
geometry.
The powder X-ray diffraction data for PDF analysis of the crystalline
and glassy K2P2Se6 were collected at room temperature using an Inel
CPS 120 diffractometer. A graphite monochromatized and Rh filtered
Ag KR (0.560 80 Å) radiation was used for the crystalline and glassy
K2P2Se6, respectively. The samples were ground to a fine powder under
a nitrogen atmosphere, loaded into 0.5 mm capillaries, and flame sealed.
Fluorescence due to selenium atoms was successfully filtered out by
using a 0.150 mm thick bronze foil in front of the CPS detector. Data
reduction and the calculation of the PDF were performed as described
elsewhere11 with the PDFGetX2 software.12
2.3.2. Electron Microscopy. Semiquantitative analyses of the
compounds were performed with a JEOL JSM-35C scanning electron
microscope (SEM) equipped with a Tracor Northern energy dispersive
spectroscopy (EDS) detector.
2.3.3. Solid-State UV-vis Spectroscopy. Optical diffuse reflectance
measurements were performed at room temperature using a Shimadzu
UV-3101 PC double-beam, double-monochromator spectrophotometer
operating in the 200-2500 nm region using a procedure described in
detail elsewhere.13
2.3.4. 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 resolution
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 µm. A total of
128 scans was sufficient to obtain good quality spectra.
2.3.5. Infrared Spectroscopy. FT-IR spectra were recorded as solids
in a CsI or KBr matrix. The samples were ground with dry CsI or KBr
into a fine powder and pressed into translucent pellets. The spectra
were recorded in the far-IR region (600-100 cm-1, 4 cm-1 resolution)
and mid-IR region (500-4000 cm-1, 4 cm-1 resolution) with the use
of a Nicolet 740 FT-IR spectrometer equipped with a TGS/PE detector
and silicon beam splitter.
2.3.6. Differential Thermal Analysis (DTA). Experiments were
performed on a Shimadzu DTA-50 thermal analyzer. A sample (∼30
mg) of ground crystalline material was sealed in a silica ampule under
vacuum. A similar ampule of equal mass filled with Al2O3 was sealed
and placed on the reference side of the detector. The sample was heated
to 600 °C at 10 °C/min, and after 1 min it was cooled at a rate of -10
°C/min to 50 °C. The residues of the DTA experiments were examined
by X-ray powder diffraction. Reproducibility of the results was
confirmed by running multiple heating/cooling cycles. The melting and
crystallization points were measured at a minimum of the endothermic
peak and a maximum of the exothermic peak.
2.3.7. Solid-State Nuclear Magnetic Resonance (NMR) Spectros-
copy. The room temperature solid-state NMR measurement was taken
on a 9.4 T NMR spectrometer (Varian Infinity Plus) using a double
resonance magic angle spinning (MAS) probe. The sample was spun
at 8.0 kHz using zirconia rotors of 6 mm outer diameter. Bloch decay
spectra were taken with the excitation/detection channel tuned to 31P
at 161.39 MHz, a 4.5 µs 90° pulse, and a relaxation delay of 3000 s.
The spectrum was referenced using 85% H3PO4 at 0 ppm.
At low-temperature K2P2Se6 (but not Rb2P2Se6) undergoes a
phase transition to a structure of lower symmetry while retaining
the helical structure. Polar structure K2P2Se6 exhibits remarkably
strong second harmonic generation (SHG) intensity. In the
infrared region the material is highly transmissive over a wide
wavelength range. Compared to the SHG intensity of the
chalcopyrite compound AgGaSe2, which is the top infrared NLO
material used commercially, K2P2Se6 exhibited a 50-fold
stronger response as well as a type-I phase matching property
making it a potential contender material for applications in the
IR region. To our best knowledge this is one of the largest NLO
SHG responses ever reported for this near- and mid-infrared
region of the spectrum. Glassy K2P2Se6 also exhibited an SHG
response. Since homogeneous glasses generally do not show
SHG due to the macroscopically present inversion center, this
observation is not only a rare example of amorphous NLO
response with no specific treatment such as poling but also of
interest for further investigations in optical devices.
2. Experimental Section
2.1. Reagents. The reagents mentioned in this work were used as
obtained: K metal (analytical reagent, Aldrich Chemical Co., Milwau-
kee, WI); Rb metal (analytical reagent, Johnson Matthey/AESAR
Group, Seabrook, NH); red phosphorus powder (-100 mesh, Morton
Thiokol, Inc., Danvers, MA); Se (99.9999%; Noranda Advanced
Materials, Quebec, Canada); N,N-dimethylformamide (Spectrum Chemi-
cals, ACS reagent grade); diethyl ether (Columbus Chemical Industries,
Columbus WI, ACS reagent grade, anhydrous). A2Se (A ) K and Rb)
starting materials were prepared by reacting stoichiometric amounts
of the elements in liquid ammonia. P2Se5 was prepared by heating the
mixture of P and Se with a stoichiometric ratio sealing in an evacuated
silica tube at 460 °C for 24 h.
2.2 Synthesis. Pure K2P2Se6 and Rb2P2Se6 were obtained in
quantitative yield by heating a mixture of K2Se/P2Se5 ) 1:1 and Rb2-
Se/P/Se ) 1:2.4:5 in an evacuated and sealed silica tube at 450 °C for
3 d followed by cooling at a rate of 5°/h to 250 °C, respectively. After
washing with N,N-dimethylformamide (DMF) and ether, we obtained
pure red/orange thick plate-typed single crystals. Energy dispersive
spectroscopy (EDS) analysis of the crystals showed an average
composition of “K2P1.9Se5.8” and “Rb2P1.9Se5.9”. The glassy phases of
K2P2Se6 and Rb2P2Se6 were prepared from a mixture of K2Se/P2Se5 )
1:1 and Rb2P2Se6 crystals, respectively, placed in a silica tube and
melted at 800-900 °C for 1-2 min with subsequent quenching in ice
water. The crystalline compounds were air-stable for at least a week
and stable under polar solvents such as DMF, N-methylformamide,
methyl and ethyl alcohol, and H2O. Glassy K2P2Se6 and Rb2P2Se6 were
soluble in DMF and methyl alcohol without stirring to give clear light
orange solution.
2.3. Physical Measurements.
2.3.8. X-ray Crystallography. The crystal structure was determined
by single-crystal X-ray diffraction methods. For K2P2Se6, preliminary
examination and data collection were performed on a SMART platform
diffractometer equipped with a 1K CCD area detector using graphite
monochromatized Mo KR radiation at 173(2) and 298(2) K. A
hemisphere of data was collected at 298(2) K using a narrow-frame
2.3.1. X-ray Powder Diffraction and Pair Distribution Function
(PDF) Analysis. Phase purity X-ray diffraction analyses were performed
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9
J. AM. CHEM. SOC. VOL. 129, NO. 48, 2007 14997

A R T I C L E S
Chung et al.
Table 1. Crystallographic Refinement Details for K2P2Se6 and
method with scan widths of 0.30° in ω and an exposure time of 30
s/frame. For the data collected at 173(2) K, an exposure time of 60
s/frame was used. The data were integrated using the SAINT program.
An analytical absorption and empirical absorption correction using the
program SADABS were performed for both sets. The initial positions
for all atoms were obtained using direct methods, and the structures
were refined with the full-matrix least-squares techniques of the
SHELXTL14 crystallographic software package. Crystallographic data
are given in Table 1. Satisfactory refinement was obtained with the
chiral space group, P3121 at 298(2) K and P31 at 173(2) K (Tables
2-4). The crystals used for structure refinement were racemically and
merohedrally twinned, and the twinning fractions were refined to 49.5,
25.8, 23.1, and 1.6% at 173(2) K. The racemic and merohedral twinning
behavior was typical for K2P2Se6, and their relative ratios determined
for several crystals were very similar. Interestingly, flux synthesis using
the mixture of K2Se/P/Se ) 1:2.4:5 significantly increased the portion
of one enantiomorph compared to the crystals obtained with a direct
combination reaction of K2Se and P2Se5. The racemic ratio improved
from 1:1 to 3:1.
Rb2P2Se6
formula
space group
K2P2Se6
P3121
(no. 152)
7.2746(8)
18.870(4)
90
K2P2Se6
P31
(no. 144)
14.4916(7)
18.800(2)
90
Rb2P2Se6
P3121
(no. 152)
7.2982(5)
19.002(2)
90
a, b, Å
c, Å
R, â (deg)
γ (deg)
120
3
120
12
120
3
Z
V, Å3
864.8(2)
3.536
298(2)
0.710 73
19.974
816
28.27
7444
1382
3419.2(4)
3.578
173(2)
0.710 73
20.209
3264
876.5(1)
4.016
101(2)
0.710 73
27.272
924
30.99
9968
1866
d (calculated), gr/cm3
temperature, K
λ, Å
µ, mm-1
F(000)
θ
max (deg)
total reflections
28.29
21703
10294
365
total unique reflections
no. parameters
refinement method
47
49
full-matrix least-squares on F2
final R indices [I > 2σ(I)],
2.69/5.89
4.61/8.17
3.87/4.56
R1a/wR2b (%)
Intensity data for Rb2P2Se6 were collected at 100(2) K on a STOE
IPDS II diffractometer with Mo KR radiation operating at 50 kV and
40 mA with a 34 cm image plate. Individual frames were collected
with a 5 min exposure time and a 1.0 ω rotation. The X-AREA, X-RED,
R indices (all data), R1/wR2
goodness-of-fit on F2
absolute structure parameter
3.10/6.04
1.078
0.27(2)
4.92/11.43
0.748
0.25(1)
4.74/4.70
0.976
0.08(3)
2
2
2
b
a R1 ) Σ ||Fo| - |Fc||/Σ|Fo|. wR2 ) {Σ [w(Fo - Fc)2]/Σ [w(Fo)2]}1/2
.
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Table 2. Selected Bond Distances (Å) and Angles (deg) for
K2P2Se6 at 298(2) K
P-P
2.242(2)
2.329(1)
2.131(2)
2.128(1)
2.337(1)
Se(1)-P-Se(2)
Se(1)-P-Se(3)
Se(2)-P-Se(3)
P-Se(1)-Se(1)
P-P-Se(1)
111.88(6)
112.29(6)
117.95(6)
100.55(4)
93.85(7)
P-Se(1)
P-Se(2)
P-Se(3)
Se(1)-Se(1)
P-P-Se(2)
111.42(4)
106.69(5)
P-P-Se(3)
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Table 3. Selected Bond Distances (Å) and Angles (deg) for
K2P2Se6 at 173(2) K
P(1)-P(2)
2.227(17)
2.27(3)
2.236(18)
2.23(3)
P(7)-Se(21)
2.126(17)
2.113(16)
2.131(16)
2.368(17)
2.334(13)
2.330(8)
2.342(7)
2.347(12)
113.6(7)
111.1(7)
116.8(8)
120.0(7)
112.8(8)
109.4(7)
113.9(7)
112.9(7)
116.5(8)
P(3)-P(4)
P(8)-Se(22)
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P(5)-P(6)
P(7)-P(8)
P(8)-Se(23)
P(8)-Se(24)
P(1)-Se(1)
P(1)-Se(2)
P(1)-Se(3)
P(2)-Se(4)
P(2)-Se(5)
P(2)-Se(6)
P(3)-Se(7)
P(3)-Se(8)
P(3)-Se(9)
P(4)-Se(10)
P(4)-Se(11)
P(4)-Se(12)
P(5)-Se(13)
P(5)-Se(14)
P(5)-Se(15)
P(6)-Se(16)
P(6)-Se(17)
P(6)-Se(18)
P(7)-Se(19)
P(7)-Se(20)
2.297(17)
2.141(17)
2.154(17)
2.118(17)
2.151(17)
2.346(17)
2.305(17)
2.138(17)
2.119(18)
2.123(17)
2.133(18)
2.356(17)
2.286(17)
2.140(17)
2.155(16)
2.157(15)
2.129(16)
2.363(17)
2.309(17)
2.125(17)
Se(1)-Se(18)
Se(6)-Se(7)
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Se(12)-Se(13)
Se(19)-Se(24)
Se(1)-P(1)-Se(2)
Se(1)-P(1)-Se(3)
Se(2)-P(1)-Se(3)
Se(4)-P(2)-Se(5)
Se(4)-P(2)-Se(6)
Se(5)-P(2)-Se(6)
Se(7)-P(3)-Se(8)
Se(7)-P(3)-Se(9)
Se(8)-P(3)-Se(9)
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Lett. 2000, 85, 3436.
(27) Nikogosyan, D. N. Nonlinear optical crystals: a complete surVey; Springer-
Science: New York, 2005.
(28) Liao, J. H.; Marking, G. M.; Hsu, K. F.; Matsushita, Y.; Ewbank, M. D.;
Borwick, R.; Cunningham, P.; Rosker, M. J.; Kanatzidis, M. G. J. Am.
Chem. Soc. 2003, 125, 9484-9493.
(29) Hanko, J. A.; Kanatzidis, V. G. J. Solid State Chem. 2000, 151, 326-329.
(30) Aitken, J. A.; Marking, G. A.; Evain, M.; Iordanidis, L.; Kanatzidis, M.
G. J. Solid State Chem. 2000, 153, 158-169.
(31) Chondroudis, K.; Hanko, J. A.; Kanatzidis, M. G. Inorg. Chem. 1997, 36,
2623-2632.
(32) (a) Dougherty, J. P.; Kurtz, S. K. J. Appl. Crystallogr. 1976, 9, 145-158.
(b) Kurtz, S. K.; Perry, T. T. J. Appl. Phys. 1968, 39, 3798-3813.
(33) Choy, M. M.; Byer, R. L. Phys. ReV. B 1976, 14, 1693-1706.
(34) Antonyuk, B. P.; Antonyuk, V. B. Opt. Commun. 1998, 147, 143-147.
(35) (a) Fokine, M.; Saito, K.; Ikushima, A. J. Appl. Phys. Lett. 2005, 87,
171907. (b) Kityk, I. V. J. Phys. Chem. B 2003, 107, 10083-10087. (c)
Qiu, M. X.; Pi, F.; Orriols, G. Appl. Phys. Lett. 1998, 73, 3040-3042. (d)
Fujiwara, T.; Takahashi, M.; Ikushima, A. J. Appl. Phys. Lett. 1997, 71,
1032-1034.
and X-SHAPE software package was used for data extraction and
integration and to apply empirical and analytical absorption corrections.
The SHELXTL software package was used to solve and refine the
structure. The most satisfactory refinement was obtained with the chiral
space group, P3121 at 100(2) K rather than P3221. The refined Flack
x parameter was 0.901(96). After inversion to P3121, the Flack
(36) Iseler, G. W. J. Cryst. Growth 1977, 41, 146-150.
(37) Korczak, P.; Staff, C. B. J. Cryst. Growth 1974, 24, 386-389.
9
14998 J. AM. CHEM. SOC. VOL. 129, NO. 48, 2007

Helical Polymer 1/∞[P2Se6
]
A R T I C L E S
2-
Table 4. K-Se Distances in K2P2Se6 at 173(2) K with Standard
Deviation in Parenthesesa,b
K(1)-Se(17)
K(1)-Se(14)
K(1)-Se(8)#12
K(1)-Se(20)#7
K(1)-Se(23)
K(1)-Se(12)#9
K(1)-Se(10)
K(1)-Se(24)
3.345(14)
3.361(17)
3.363(17)
3.391(16)
3.456(16)
3.473(16)
3.554(14)
3.842(17)
K(5)-Se(5)#5
K(5)-Se(10)
K(5)-Se(20)#7
K(5)-Se(3)#5
K(5)-Se(23)
K(5)-Se(7)#12
K(5)-Se(2)#5
K(5)-Se(19)#5
3.321(15)
3.351(15)
3.352(15)
3.367(15)
3.481(15)
3.484(15)
3.606(15)
3.888(16)
K(2)-Se(20)#2
K(2)-Se(4)#3
K(2)-Se(11)
K(2)-Se(9)#5
K(2)-Se(21)
K(2)-Se(22)
K(2)-Se(24)
K(2)-Se(12)
3.310(17)
3.360(16)
3.433(17)
3.393(17)
3.358(17)
3.595(16)
3.462(16)
3.940(16)
K(6)-Se(11)#9
K(6)-Se(17)#11
K(6)-Se(21)#11
K(6)-Se(5)#11
K(6)-Se(2)#6
K(6)-Se(13)
3.307(15)
3.329(16)
3.339(15)
3.344(15)
3.423(16)
3.433(15)
3.547(14)
3.849(15)
K(6)-Se(15)
K(6)-Se(1)#9
K(3)-Se(22)
K(3)-Se(3)#5
K(3)-Se(9)
K(3)-Se(16)
K(3)-Se(14)#5
K(3)-Se(6)
K(3)-Se(23)
K(3)-Se(18)
3.317(16)
3.317(17)
3.348(17)
3.348(15)
3.505(16)
3.509(16)
3.582(15)
3.915(16)
K(7)-Se(10)
3.309(14)
3.325(15)
3.363(15)
3.398(15)
3.470(15)
3.473(14)
3.616(15)
3.811(14)
K(7)-Se(2)#6
K(7)-Se(15)#12
K(7)-Se(5)#6
K(7)-Se(2)#5
K(7)-Se(18)#9
K(7)-Se(11)#9
K(7)-Se(6)
Figure 1. Structure of K2P2Se6 at 298(2) K. (a) The unit cell viewed down
the c-axis. The thermal ellipsoids with 30% probability are shown. (b) View
of a 1 [P2Se62-] chain looking down the a-axis. A helix forms by three
K(4)-Se(23)
K(4)-Se(22)#4
K(4)-Se(15)
K(4)-Se(9)
K(4)-Se(19)
K(4)-Se(11)#4
K(4)-Se(14)
K(4)-Se(7)
3.341(14)
3.345(14)
3.359(15)
3.388(15)
3.483(15)
3.509(16)
3.646(15)
3.836(15)
K(8)-Se(8)#10
K(8)-Se(16)#10
K(8)-Se(4)#4
K(8)-Se(14)
K(8)-Se(16)#4
K(8)-Se(1)#10
K(8)-Se(3)
3.318(14)
3.325(14)
3.346(14)
3.380(15)
3.420(15)
3.478(14)
3.518(15)
∞
[P2Se6] units and repeats itself at every 18.872 Å.
and focused onto a fiber optic bundle. The output of the fiber optic
bundle was coupled to the entrance slit of a Spex Spec-One 500 M
spectrometer and detected using a nitrogen-cooled CCD camera. The
data collection time was 20 s.
a The maximum threshold for bond distances is 3.750 Å. The coordination
number of K atoms was determined to be seven. b Symmetry transformations
used to generate equivalent atoms: (#1) x + 1, y, z; (#2) x + 1, y + 1, z;
(#3) x, y + 1, z; (#4) -x + y + 1, -x + 2, z - 1/3; (#5) -y + 2, x - y +
3. Results and Discussion
3.1. Crystal Structure. Because at RT K2P2Se6 and Rb2P2-
Se6 are isostructural, the description that follows will be
concentrated on the K salt. The structure has infinitely extended
helical chains parallel to the c-axis; see Figure 1a. The chains
are composed of ethane-like [P2Se6] repeat units that are linked
via terminal Se-Se linkages to give infinite helices of
1
1
1, z + /3; (#6) x, y - 1, z; (#7) -y + 1, x - y + 1, z + /3; (#8) -x + y
1
+ 1, -x + 1, z - /3; (#9) x - 1, y - 1, z; (#10) x - 1, y, z; (#11) -x +
1
1
y, -x + 1, z - /3; (#12) -y + 1, x - y, z + /3.
parameter x was 0 and the BASF value was 0.89. Following refinement
for racemic and merohedral twinning done at the same time, the
twinning fractions were refined to 89.43, 5.83, 5.59, and 2.15% and x
parameter 0.0. Rb2P2Se6 did not show a phase transition with temper-
ature change, and its crystal structure is the same as that of K2P2Se6 at
room temperature (RT). The parameters for data collection and the
details of the structural refinement are given in Table 1. Fractional
atomic coordinates and displacement parameters for each structure are
given in the Supporting Information. In all cases the atoms were refined
to full occupancy.
1 [P2Se62-], Figure 1b. The chirality of K2P2Se6 is preserved
∞
in the crystal, and at RT the compound adopts the chiral trigonal
space group, P3121. Helical features have been described in
the quaternary thiophosphates ANb2P2S12 (A ) K, Rb, Cs)15
and A11U7(PS4)13 (A ) K, Rb),16 but K2P2Se6 is the first
example to display an infinite helix structure of free-standing
chalcophosphate anions.
The unit cell contains a single crystallographically unique
strand of the chain. Each helix runs with identical handedness
along the c-axis. Because the helix is generated by a crystal-
lographic 31 screw axis along the c-direction, each coil of the
helix in the unit cell includes three P2Se6 units. There is a single
crystallographic P site in the structure, and pitch of the helix is
18.872(4) Å, corresponding to the unit cell repeat distance along
the c-axis. The interspiral distance is equal to the a-lattice
constant of the unit cell. Projected along its axis, a chiral 1D
channel is seen with a cross-section diameter of ∼1.17 Å. The
P-P bond distance is normal at 2.242(2) Å. P-Se bond
distances range from 2.126(1) to 2.329(1) Å comparable to
known values. The P-Se-Se-P dihedral angle of 88.55(1)°,
2.3.9. Nonlinear Optical Property Measurements. We used the
frequency-tripled output of a passive-active mode-locked Nd:YAG laser
with a pulse width of about 15 ps and a repetition rate of 10 Hz to
pump an optical parametric amplifier (OPA). The OPA generates
vertically polarized pulses in the ranges 400-685 nm and 737-3156
nm. In order to check the SHG efficiency as a function of the excitation
energy, we tuned the wavelength of the incident light from 1000 to
2000 nm. In this range, the spectral bandwidth of the linearly polarized
light from the OPA is rather broad, about 2 meV full width at half-
maximum. However, the phase space compression phenomena ensure
effective SHG where lower energy portions are exactly compensated
by higher parts thereby satisfying both energy and momentum
conservation. The incident laser pulse of 300 µJ was focused onto a
spot 500 µm in diameter using a 3 cm focal-length lens. The
corresponding incident photon flux was about 10 GW/cm2. The SHG
signal was collected in a reflection geometry from the excitation surface
(38) Kozhina, I. I.; Borshchevskii, A. S. Vestnik Leningradskogo UniVersiteta,
Seriya 4: Fizika, Khimiya 1971, 87-92.
(39) Feigelson, R. S.; Route, R. K. Opt. Eng. 1987, 26, 113-19.
9
J. AM. CHEM. SOC. VOL. 129, NO. 48, 2007 14999

A R T I C L E S
Chung et al.
Figure 2. (a) Projection view of a 1 [P2Se62-] chain slightly slanted in
∞
order to show the staggered anti-conformation of [P2Se6]. The role of short
Se‚‚‚Se nonbonding interactions (dashed line) along a helix segment is
shown. Se(1)‚‚‚Se(2), 3.559(1) Å. (b) Projection through P-P bonding
showing the relationship between short Se‚‚‚Se nonbonding interaction and
chain propagation.
almost 90°, causes a relatively short Se-Se bond distance of
2.337(1) Å compared to those in KPSe6.6 The ethane-like P2-
Se6 repeat unit adopts a local anti-conformation to minimize
the steric hindrance around the P-P bond created by the helical
geometry, Figure 2a. The unit is also distorted from the ideal
geometry with the Se-P-Se angles ranging from 111.88(6)°
to 117.95(6)°.
Figure 3. Structure of K2P2Se6 at 173(2) K. (a) The unit cell viewed down
the c-axis showing the superstructure. Crystallographically unique chains
are differentiated as A and B. K atoms are labeled. (b) View of
It is noteworthy to point out the unusually short Se‚‚‚Se
nonbonding interaction that is observed at 3.559(1) Å for
Se1‚‚‚Se2, Figure 2a. This is shorter than the van der Waals
radii sum of 3.80 Å and probably meaningful in stabilizing the
crystal structure. Because the P2Se6 fragment is intrinsically
centrosymmetric, the helical architecture results from the
conformation of the selenophosphate chain. In this regard, the
short Se‚‚‚Se contacts likely play a critical role in forming the
coil by maximizing Se pπ orbital overlap through Se2‚‚‚Se1-
Se1‚‚‚Se2 connections, Figure 2. To minimize the repulsion
force between them, each [P2Se6] subunit rotates along the
Se2‚‚‚Se1-Se1‚‚‚Se2 connection as an axis and consequently
achieves the helical form. In fact, the Se3-P-P-Se3 torsion
angle is severely compressed to only 36.23(1)°. Similarly short
Se‚‚‚Se interactions are well-known in other chalcogenide
systems. In Rb4P6Se125 and APSe6,6 for example, these interac-
tions act to form pseudo one- or two-dimensional structures,
respectively. The low-dimensional charge density wave com-
1 [P2Se62-] chains with labeling down the a-axis. K atoms are omitted for
∞
clarity.
crystallographically unique and generated by a 31 screw and
2-fold rotation operation perpendicular to a- and b-axes. They
are surrounded by two A and one B strands.
Rb2P2Se6 did not show such a structural transition on cooling
down to 100 K. The different behavior can be understood by
the alkali metal size effect. The most notable change in going
from the P3121 to P31 for the K analogue is found in the
coordination environment of the K atoms. At room temperature
K atoms are coordinated by six closely lying Se atoms, whereas
this expands to seven at low temperature, Figure 4. This is
reminiscent of a pressure effect imposed on the structure as the
unit cell contracts with falling temperature. Therefore the driving
force for the transition seems to be the formation of the extra
K‚‚‚Se bonds. This effect seems to be absent in the Rb analogue
as the coordination environment of these atoms is already high
at room temperature involving 10 Se atoms. Tables 4-7 show
a comparison of the K‚‚‚Se and Rb‚‚‚Se distances in the two
structures for both room and low temperature.
3.3. Synthesis, Reaction Chemistry, and Characterization.
The alkali metal chalcophosphate ternary system is highly
attractive as a model system to probe how flux conditions
influence the synthetic outcome and the structure of a resulting
compound.5 A reaction medium forms by simple in situ fusion
of A2Q/P2Q5 or P/Q, and this flux is conceptually defined as
AxPyQz. The basicity of the flux is determined by the A/P/Q
ratio. Higher A/P ratios impart stronger basicity to the flux.
Lower A/Q ratios produce lower basicity, etc. Alternatively,
for the same A/P ratio the basicity can be increased in going
from the smaller to the larger alkali atoms. Therefore both the
A/P ratio and the nature of the A atoms could be conveniently
17
pound NbSe3 also has similar interactions.
3.2. Low-Temperature Structure. We discovered that when
cooled K2P2Se6 undergoes a displacive phase transition to lower
symmetry with no bond breaking at lower temperature. The P31-
21 space group at room temperature was lowered to P31 at 173
K by losing twofold symmetry perpendicular to a- and b-axis,
Figure 3a. The new cell was enlarged to a 2a × 2b × c supercell.
P31 is a maximal non-isomorphic subspace group of P3121. As
a result, the crystallographically unique K and P atoms and three
independent Se atoms in K2P2Se6 split to 8, 8, and 24
independent atoms, respectively. The resulting four helixes in
the unit cell are differentiated crystallographically into two
groups as denoted by A and B in Figure 3a and b with a ratio
of 3:1. It is noted that K7 and K8 still generate themselves
by a threefold screw axis surrounded by three identical strands
of 1 [P2Se62-] of type A. On the other hand, K1-K6 are
∞
9
15000 J. AM. CHEM. SOC. VOL. 129, NO. 48, 2007

Helical Polymer 1/∞[P2Se6
]
A R T I C L E S
2-
Table 6. Rb-Se Distances in Rb2P2Se6 at 100(2) K with
Standard Deviation in Parenthesesa,b
Rb-Se(3)
Rb-Se(2)
Rb-Se(1)
Rb-Se(3)#1
Rb-Se(2)#4
3.4778(12)
3.5715(13)
3.6011(13)
3.4248(12)
3.4357(11)
Rb-Se(2)#7
Rb-Se(2)#10
Rb-Se(2)#8
Rb-Se(1)#5
Rb-Se(3)#1
3.5101(12)
3.7393(13)
3.7754(13)
3.8599(12)
3.8706(13)
a The maximum threshold for bond distances is 4.000 Å. Rb-Se distance
was not found up to 4.500 Å over the threshold. The coordination number
of Rb atoms was determined to be 10. b Symmetry transformations used to
generate equivalent atoms: (#1) -x + 1, -x + y, -z + 1/3; (#2) y, x - 1,
-z; (#3) -x + 2, -x + y + 1, -z + 1/3; (#4) x - 1, y - 1, z; (#5) x, y +
1, z; (#6) y + 1, x, -z; (#7) x + 1, y, z; (#8) y + 1, x - 1, -z; (#9) x, y
- 1, z; (#10) x + 1, y + 1, z; (#11) x - 1, y, z; (#12) -x + 1, -x + y +
1
1, -z + /3.
Table 7. Rb-Se Distances in Rb2P2Se6 at 298(2) K with
Standard Deviation in Parenthesesa,b
Rb-Se(3)
3.4714(23)
3.4735(25)
3.5172(22)
3.5481(23)
3.6271(23)
Rb-Se(1)
3.6545(22)
3.7805(23)
3.8185(22)
3.9055(21)
3.9302(23)
Rb-Se(2)
Rb-Se(2)#6
Rb-Se(1)#7
Rb-Se(1)#5
Rb-Se(3)#3
Rb-Se(3)#11
Rb-Se(2)#9
Rb-Se(2)#5
a The maximum threshold for bond distances is 4.000 Å. Rb-Se distance
was not found up to 4.500 Å over the threshold. The coordination number
of Rb atoms was determined to be 10. b Symmetry transformations used to
generate equivalent atoms: (#1) -x + 1, -x + y, -z + 1/3; (#2) y, x - 1,
-z; (#3) -x + 2, -x + y + 1, -z + 1/3; (#4) x - 1, y - 1, z; (#5) x, y +
1, z; (#6) y + 1, x, -z; (#7) x + 1, y, z; (#8) y + 1, x - 1, -z; (#9) x, y
- 1, z; (#10) x + 1, y + 1, z; (#11) x - 1, y, z; (#12) -x + 1, -x + y +
1
1, -z + /3.
of Rb and Cs change the reaction path to a more oxidized
chalcophosphates (i.e., the P5+ species RbPSe6 and CsPSe6).
Rb2P2Se6 could be obtained however by adjusting/correcting
the flux basicity by adding 0.4 mol of P to the Rb2Se/P2Se5 )
1:1 ratio. This prevents the oxidation for P from 4+ to 5+.
Thus it would appear that extended structural motifs for
chalcophosphates are more likely to be produced by weak basic
fluxes with smaller less Lewis basic alkali metals such as
potassium, sodium, and lithium.
Figure 4. The coordination environment of K atoms in K2P2Se6 is six at
298(2) K (a) and expands to seven at 173(2) K (b). The coordination
environment of Rb atoms in Rb2P2Se6 at (c) 298(2) K and (d) 100(2) K. It
remains unchanged at both temperatures where Rb is coordinated to 10 Se
atoms. Large blue spheres are K or Rb atoms, and small black ones are Se
atoms. P atoms are omitted for clarity.
Conceptually, 1 [P2Se62-] can be regarded as deriving from
Table 5. K-Se Distances in K2P2Se6 at 273(2) K with Standard
∞
Deviation in Parenthesesa,b
oxidative polymerization of the [P2Se6]4- anion, see eq 1.
Although it was not synthesized in this fashion, we speculate
that an actual oxidation in solution, with, e.g., I2 as the oxidant,
may in fact be successful if run under proper conditions.
K-Se(2)
3.3367(18)
3.3388(18)
3.3695(19)
3.4263(18)
3.4537(17)
K-Se(1)#1
K-Se(2)#2
K-Se(1)#4
K-Se(3)#1
3.4834(17)
3.8395(19)
3.8731(18)
3.9244(19)
K-Se(3)#6
K-Se(3)#2
K-Se(2)#4
K-Se(2)#7
a The maximum threshold for bond distances is 3.750 Å. The coordination
number of K atoms was determined to be six. b Symmetry transformations
used to generate equivalent atoms: (#1) -x, -x + y, -z + 1/3; (#2) -x -
1
1, -x + y - 1, -z + 1/3; (#3) -x + y - 2, -x ,z - /3; (#4) -x - 1, -x
1
1
+ y, -z + /3; (#5) -x + y - 1, -x + 1, z - /3; (#6) -y + 1, x - y +
2, z + 1/3; (#7) -y, x - y + 2, z + 1/3; (#8) x - y + 1, -y + 2, -z + 2/3;
(#9) y - 1, x + 1, -z.
used as a key reaction parameter to control the formation of a
product from being a simple molecular species to being a more
complex discrete or extended structure. For example to obtain
the classical molecular salts A3PSe4 and A4P2Se6 (A ) alkali
metal), high ratios of A2Se/P2Se5 (>2:1) are required.
The polymeric K2P2Se6 is realized under less basic conditions,
namely with K2Se/P2Se5 ) 1:1, than the typical ternary alkali
selenophosphates with molecular structures. In fact, less basic
conditions or lower flux temperatures tend to generate longer
or extended fragments.18 This could explain why more basic
Rb or Cs analogues to K2P2Se6 could not be obtained from
identical reaction conditions. The slightly higher basic character
The polymeric chain is stabilized with no help of coordinating
to a metal and represents a rare chalcophosphate anion. The
only other example which is stabilized by alkali cations is
1 [PSe6-].6 The latter has PSe4 tetrahedra condensed with
∞
diselenide (Se22-) groups.
Finally, the existence of a single crystallographic P site in
the RT structure of K2P2Se6 is supported by solid state 31P NMR
9
J. AM. CHEM. SOC. VOL. 129, NO. 48, 2007 15001

A R T I C L E S
Chung et al.
Figure 5. Solid state 31P MAS NMR spectrum of crystalline K2P2Se6 at
room temperature. The asterisk (*) indicates the isotropic peak.
Figure 7. Solid-state UV-vis optical absorption spectra of crystalline and
glassy K2P2Se6 and Rb2P2Se6 showing the red shift in absorption edge in
the glass samples. The band gaps are 2.09, 1.98 eV for the K and 2.32,
2.10 eV for Rb analogues, respectively.
The thermal behavior of Rb2P2Se6 is different from that of
K2P2Se6. The DTA carried out at a rate of 10 °C/min revealed
the melting at 364 °C on heating and gave a dark red glass at
255 °C on cooling during a first cycle. On a second cycle, no
crystallization was observed as the glass could not recover the
crystal structure. The melting was observed again at 369 °C
followed by vitrification at 255 °C on cooling. The PXRD
confirmed the amorphous nature of the glassy sample after the
first and second cycles. The amorphous sample recrystallized
to a pristine structure only when the heating rate was lowered
to less than 5 °C/min. The DTA performed at a rate of 5 °C/
min showed melting at 366 °C on heating and subsequent
crystallization at 305 °C on cooling. The PXRD of pristine and
the sample obtained after each cycle matched perfectly. These
results are consistent with a greater glass forming ability of the
Rb salt compared to the K salt. This is in agreement with a
similar trend identified recently in the series K1-xRbxSb5S8.20
The solid-state optical absorption UV-vis spectra of crystal-
line and glassy A2P2Se6 show sharp absorption edges, Figure
7. The band gap of crystalline and glassy phases was measured
at 2.08 and 1.97 eV for K+ salt and 2.32 and 2.10 eV for Rb+
salt, respectively. The energy gaps are consistent with a dark
orange/dark red color for K+ salt and an orange/dark orange
color for Rb+ salt, respectively. A red shift in absorption edge
in the glassy phases is a common phenomenon as glass
formation generally induces substantial defects and mid-gap
states to give a lower band gap than the crystalline counterpart.21
This creates optical contrast and can be a useful feature for
optical storage systems based on phase-change materials.22
The results of Raman spectroscopy and PDF analysis (see
below) shed light on the local structure in the glass and how
these materials can readily recover their crystal structure from
the amorphous state. The Raman spectrum of crystalline K2P2-
Se6 at room temperature shows shifts at 154(bw), 221(s), 236-
(m), 259(m), 388(w), 496(w), and 514(vw) cm-1, Figure 8. The
shift at 221 cm-1 is unambiguously assigned to the P2Se6
stretching mode by comparison in the Ag stretching mode of
Figure 6. Differential thermal analysis diagram of a sample of K2P2Se6.
(a) Heating curve showing melting at 387 °C in the first heating cycle (black
solid line) with no crystallization upon cooling. (b) Exothermic crystal-
lization followed by melting upon heating in the second cycle (blue dotted
line). Asterisk indicates the vitrification event upon cooling.
spectroscopy. Under MAS at RT, crystalline K2P2Se6 gave a
single isotropic chemical shift (CS) at 54.6 ppm, Figure 5. The
result supports the crystal structure of K2P2Se6 at room
temperature which indicates only one crystallographically unique
P atom. The chemical shift value is close to that of A2CdP2Se6
(A ) K, Rb) and K2Cu2P4Se10, all of which include the P2Se6-
type ligand.19
3.4. Glass Formation, Phase-Change Behavior, and Local
Structure. Both title compounds described here exhibit revers-
ible phase change behavior. Differential thermal analysis of
K2P2Se6 performed at a rate of 10 °C/min indicates that the
compound melts congruently at 387 °C, Figure 6. Upon cooling
it forms a dark red glass at 270 °C. Crystallization is only
observed on heating where the glass recrystallizes exothermi-
cally at 297 °C. The vitrification is observed again on cooling
the melt to room temperature. The powder X-ray diffraction
patterns (PXRD) of the pristine and recrystallized sample from
DTA were identical confirming the structure recovering ability
of this material.
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15002 J. AM. CHEM. SOC. VOL. 129, NO. 48, 2007

Helical Polymer 1/∞[P2Se6
]
A R T I C L E S
2-
Figure 9. Pair distribution function G(r) of the crystalline and glassy K2P2-
Se6.
Figure 8. Raman spectra of crystalline (black line) and glassy (blue line)
K2P2Se6 at room temperature. The similar but broader features in the
spectrum of the glass suggest the local structure is preserved but long range
order is lost.
of the glass. Similar conclusions were reached from the PDF
analysis of KSb5S8, another interesting phase-change material,
which shows that its local structure within a radius of ∼5 Å in
the glass phase is clearly refined even though long range order
is not observed.20
D3d symmetry of [P2Se6]4- ligand.23 Other peaks at 154, 496,
and 514 cm-1 are also related with the P2Se6 fragment.24 By
analogy, shifts at 236 and 259 cm-1 can be assigned to
antisymmetric and symmetric Se-Se stretching vibration modes
of the diselenide group, respectively.25
The Raman spectrum of glassy K2P2Se6 shows that broader
and weaker peaks at 162 (bw), 218 (bm), and 258 (bw) are
observed, whereas the overall peak pattern is similar to that of
the crystals. This suggests that the [P2Se6] unit and Se-Se bonds
are still intact and the local structural motifs are largely
preserved in the glass but crystallographic long-range order is
lost. This situation makes it easy to restore the crystal structure
from the amorphous state for the reversible crystal-glass phase
transition.
To probe the local structure of both the glassy and crystalline
forms of K2P2Se6 we performed a pair distribution function
(PDF) analysis. This technique is emerging as a useful tool for
the analysis of the local structure of crystalline and noncrystal-
line compounds.26 The PDFs are shown in Figure 9. Also shown
in blue is the calculated PDF based on the room-temperature
single-crystal structure model of K2P2Se6. There appears to be
good agreement between the crystal structure model and the
experimentally determined PDF of K2P2Se6. This in fact
validates the correctness of the crystal structure.
The PDF of the glass shows well-defined correlations up to
∼8 Å with the first two at 2.2 and 3.6 Å being very similar to
those of the crystalline form. The interatomic correlations in
the structure disappear above ∼8 Å indicating the lack of long
range order periodicity. The first strong correlation at 2.2 Å is
assigned to Se-Se, P-Se, and P-P bonds in the structure. The
second strong peak at 3.6 Å is assigned to K‚‚‚Se and second
neighbor Se‚‚‚Se distances. The PDF data suggest that the
crystalline and glassy forms of K2P2Se6 are structurally similar
which implies that the [P2Se6] units remain intact in the glass
form. This is also in agreement with the Raman spectroscopic
data discussed above. The close structural relationship between
crystal and glass, coupled with the stoichiometric nature of the
composition, accounts for the facile congruent crystallization
3.5. Infrared Transmission and Nonlinear Optical Proper-
ties. Materials with large NLO susceptibilities for IR application
are highly sought. AgGaQ2 (Q ) S, Se), ZnGeP2, and GaSe
are main infrared materials,27 and we recently introduced the
promising long wave IR NLO compound â-K2Hg3Ge2S8.28 The
chalcophosphates are an attractive class in which to search for
new NLO materials in that basic building units of [PQ4]3- or
[P2Q6]4- frequently form noncentrosymmetric arrangements by
coordination to central metals or polychalcogenide fragments
[Qn]2-. Examples include APSe6 (A ) K, Rb),6 ANb2P2S12 (A
) K, Rb, Cs),15 A11U7(PS4)13 (A ) K, Rb),16 Cs2CuP3S9,20 Na0.5-
Pb1.75PS4,30 and A3AuP2Se8 (A ) K, Rb, Cs).31 In addition,
chalcogenide compounds demonstrate better polarizability than
oxide compounds, which have been predominantly studied for
NLO application and wide IR transmission.
K2P2Se6 exhibits wide optical transparency ranging from long
wave IR (LWIR) to near-IR (NIR)/visible light, Figure 10. The
mid-IR transmittance spectrum showed little absorption from
505 cm-1 (19.8 µm) to 4000 cm-1 (2 µm). There is no light
absorption below the band gap transition suggesting uninter-
rupted light transmission in the compound. The optical transpar-
ency extends over to its absorption edge of 2.08 eV (596 nm)
in the visible region. Above 19.8 µm in the far-IR region, the
compound exhibited a complex set of absorptions, consistent
with its Raman and far-IR spectra. Optical transparency is a
key feature for materials aimed at NLO applications. For
example, the important NLO material for IR applications,
AgGaSe2,27 shows an LWIR transmission up to 17 µm.
The polar, noncentrosymmetric helical chain structure
of 1 [P2Se62-] composed of easily polarizable P and Se atoms
∞
linked by covalent bonding can produce large optical nonlin-
earity. SHG measurements were performed using a modified
Kurtz powder method32 with an IR light source ranging from
1000 to 2000 nm. SHG intensities of crystalline K2P2Se6 were
directly compared with that of AgGaSe2 powder. All samples
were prepared in a similar fashion, and the same particle size
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A R T I C L E S
Chung et al.
Figure 10. Far-IR (red line)/mid-IR (green line)/vis (blue line) absorption
spectra of crystalline K2P2Se6. Wide transparent range of crystalline K2P2-
Se6 above the absorption band at 19.8 µm at far-IR region through mid-IR
to 0.596 µm at visible region is shown.
range of 45.5 ( 7.5 µm was measured and compared. Crystalline
K2P2Se6 generated strong double frequency signals from the
fundamental idler beam. The SHG intensity of K2P2Se6 showed
a maximum at 789 nm which is ∼50 times larger than that at
the same wavelength. Under the same experimental conditions
AgGaSe2 showed an SHG maximum at 890 nm, and at this
wavelength the corresponding response of K2P2Se6 was 20-fold
higher. At shorter wavelengths the K2P2Se6 outperforms the
chalcopyrite material by over 100-fold, Figure 11a. These results
demonstrate that the crystalline K2P2Se6 is very promising in
IR NLO applications. For comparison, the absolute nonlinear
optical susceptibility at 2.12 µm of AgGaSe2 and LiNbO3 is
67.7 ( 13 and 29.1 ( 5.2 pm/V, respectively.33
The SHG intensity of the crystalline phase increased with
the particle size and reached a plateau, Figure 11b. In principle,
phase-matchable samples reach maximum intensity, and then
for larger average particle sizes the intensity is size-independent
because of the existence of a phase-matching direction in the
sample.32 In this regard, crystalline K2P2Se6 is type I phase-
matchable and eminently suitable for consideration in applica-
tions.
Figure 11. (a) SHG response of K2P2Se6 relative to AgGaSe2 over a wide
range of wavelengths. (b) Particle size to SHG intensities diagram of
crystalline K2P2Se6 showing type-I phase-matching.
Because of great optical transparency and potential formability,
there have been tremendous efforts to induce SHG in glasses34
for use in optical fibers in telecommunications. The observation
of significant NLO activity in K2P2Se6 glass could be a rare
example of this property found in an amorphous material with
no specific treatment such as thermal poling, electron beam
irradiation, and so on.35 Since K2P2Se6 is a phase-change
material and retains its local structural motif in the glassy phase,
we expect the noncentrosymmetric arrangement to be partially
preserved and consequently to exhibit some SHG response.
Indeed, the SHG response of the K2P2Se6 glass started to be
observed at a longer wavelength than that of the crystal. This
suggests that the SHG signal was generated from the glass the
band gap of which is red-shifted relative to the crystal, and
probably did not originate from traces of the recrystallized phase
embedded in the glass matrix. X-ray powder diffraction patterns
after the SHG measurements did not show evidence of crystal-
lization. It cannot be completely ruled out, however, that idler
beam-induced crystallization of glass may be occurring. Ad-
ditional work will be necessary to better understand the NLO
properties of the glass.
These results suggest that K2P2Se6 is of special interest for
the middle and deep infrared (IR) applications due to its large
nonlinear optical coefficients and high transmission in the IR
region. The phase matching and transmission characteristics of
K2P2Se6 should allow 3-wave interactions in the mid- and near-
IR, particularly for optical parametric oscillator (OPO) devices
pumped with a Nd:YAG laser, a frequency mixing of OPO
outputs pumped by a Ti:Sapphire or Nd:YAG laser, as well as
frequency mixing Nd:YAG laser with dye and Ti:Sapphire or
other laser sources. K2P2Se6 is also promising as an efficient
frequency doubling crystal for infrared radiation such as 10.6
µm output of CO2 lasers.
3.7. Thermal Properties of K2P2Se6. The thermal properties,
such as thermal expansion, are important properties relevant to
crystal growing for NLO applications. The thermal expansion
of K2P2Se6 crystal was determined by a single-crystal X-ray
3.6. Second Harmonic Generation Response of Glassy
K2P2Se6. Surprisingly, the glassy K2P2Se6 powder exhibited an
SHG response, and its intensity is 38% that of AgGaSe2.
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Helical Polymer 1/∞[P2Se6
]
A R T I C L E S
2-
unit cell volume. Because K2P2Se6 exhibited 2a × 2b × c
supercell, the a-axis distance was divided by 2 for the calcula-
tion. The thermal expansion coefficients of K2P2Se6 were
calculated to be Ra ) 1.46 × 10-5/K and Rc ) 2.41 × 10-5/K,
indicating somewhat isotropic expansion. For comparison, cor-
responding values for other relevant infrared NLO materials are
provided: AgGaSe2 (R|c ) -0.81 × 10-5/K, R ) 1.98 ×
c
10-5/K, 298-423 K),36 AgGaS2 (R|c ) -1.32 × 10-5/K, R
c
) 1.27 × 10-5/K, 298-523 K),37 and ZnGeP2 (R|c ) 1.59 ×
10-5/K, R c ) 1.75 × 10-5/K, 293-573 K).38 The R of stainless
steel is 1.73 × 10-5/K at 293 K. Most importantly, K2P2Se6
does not exhibit the anomalous behavior of AgGaQ2 (Q ) S,
Se) which contracts along the a- and b-axes and expands along
the c-direction. The latter is an anomaly which can cause
thermomechanical stress in high-power applications and fail-
ure.39
The volumetric thermal expansion coefficient of K2P2Se6 at
â ) 5.35 × 10-5/K also indicates its moderate thermal
expansion. It is noteworthy that crystals of K2P2Se6 did not show
any signs of cracking or deformation when exposed to abrupt
temperature change (200 K/h) or soaked at a high temperature
of 500 K for several days.
4. Concluding Remarks
The compounds A2P2Se6 (A ) K, Rb) have novel chiral
structures composed of helices of 1 [P2Se62-]. The chirality
∞
combined with its phase-change behavior could enable A2P2-
Se6 to be explored as a multifunctional switchable material. On
cooling K2P2Se6 is susceptible to a displacive phase transition
to a lower space group that seems to be driven by the
coordination sphere expansion of the alkali ions. If correct, the
phase transition could also be effected by the application of
high pressure as a means of achieving coordination sphere
expansion. When molten the compounds can be quenched to a
glassy state and exhibit reversible crystal-glass phase change
behavior. The optical absorption edge for the glassy phase is
red-shifted, and on the basis of Raman spectroscopy and PDF
analysis it appears that the main building motifs are largely intact
in the glassy form. This accounts for the facile restoration of
the crystal structure from the corresponding amorphous phase.
A2P2Se6 (A ) K, Rb) and the previously described APSe6
(A ) K, Rb, Cs) are the only reported extended structure
compounds in the alkali chalcophosphate family to exhibit easy
glass-formation and phase-change properties. This suggests that
polymeric chalcophosphates have broad glass-forming tenden-
cies and may constitute a fertile source for stoichiometric
chalcogenide glasses. The glass forming property of these
materials makes them potentially valuable for producing IR
optical glass fibers. Finally, K2P2Se6 widely transparent in the
mid-IR up to 19.2 µm that is coupled with a very large SHG
response. The material is type-I phase-matchable with a response
that is over 50 times larger than that of top performing NLO
material AgGaSe2. The remarkably stronger response is at-
tributed to the helical structure of the selenophosphate which
is highly polar, in contrast to the weakly polar chalcopyrite
structure of AgGaSe2. This makes K2P2Se6 one of the best NLO
materials known for nonresonant SHG. Finally, the glassy form
of K2P2Se6 exhibited a surprising SHG response with no poling
treatment.
Figure 12. Temperature variation of the lattice parameters and cell volume
for K2P2Se6 from 100 to 400 K.
diffraction study in the range of 100-400 K, Figure 12. The
linear (eq 2) and thermal expansion coefficients (eq 3) are
defined as
dLt
1
Rt )
(2)
Lt dT
and
1 dV
V dT
â )
(3)
where Lt is the unit cell dimension along t-axis and V is the
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A R T I C L E S
Chung et al.
Acknowledgment. Financial support from the National Sci-
ence Foundation (Grant DMR-0702911) and the National
Science Foundation under US/Ireland cooperation (Grant
0306731) and the Northwestern Materials Research Center under
the NSF (Grant DMR-0520513) for J.I.J. are gratefully ac-
knowledged.
Supporting Information Available: X-ray crystallographic
file (in CIF format) as well as tables for structural information.
This material is avalable free of charge via the Internet at
http://pubs.acs.org.
JA075096C
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15006 J. AM. CHEM. SOC. VOL. 129, NO. 48, 2007