Journal of Solid State Chemistry 153, 158}169 (2000) doi:10.1006/jssc.2000.8767, available online at http://www.idealibrary.com on
Flux Synthesis and Isostructural Relationship of Cubic Na1.5Pb0.75PSe4, Na0.5Pb1.75GeS4, and Li0.5Pb1.75GeS4
Jennifer A. Aitken,* Gregory A. Marking,* Michel Evain,- Lykourgos Iordanidis,* and Mercouri G. Kanatzidis*
*Department of Chemistry and Center for Fundamental Materials Research, Michigan State University, East Lansing, Michigan 48824; and -IMN, UMR CNRS C6502, Institut des MateH riaux Jean Rouxel, Laboratoire de Chimie des Solides, 2 rue de la Houssinie` re, BP 32229, 44322 Nantes Cedex 03, France Received February 3, 2000; in revised form May 8, 2000; accepted May 11, 2000; published online July 7, 2000
Na1.5Pb0.75PSe4 was synthesized by the reaction of Pb with a molten mixture of Na2Se/P2Se5/ Se at 4953C. Na0.5Pb1.75GeS4 was synthesized by reacting Pb and Ge in molten Na2Sx at 5303C. Likewise, Li0.5Pb1.75GeS4 can be synthesized in a Li2Sx 6ux at 5003C. Na0.5Pb1.75GeS4 and Li0.5Pb1.75GeS4 are relatively air- and water-stable, while Na1.5Pb0.75PSe4 is only stable in air and water for less than 1 day. The structures of all three compounds were determined by single-crystal X-ray di4raction. The compounds crystallize in the cubic, noncentrosymmetric space group I43d with a )2(9743.41 A, Z ,61 R1 ,6220.0 and s wR2 7150.0 for Na1.5Pb0.75PSe4, a )1(511.41 A, Z ,61 s R1 ,4820.0 and wR2 4460.0 for Na0.5Pb1.75GeS4, and a )6(3610.41 A, Z ,61 R1 ,3720.0 and wR2 7360.0 for s Li0.5Pb1.75GeS4. The compounds adopt a structure that is similar to that of Ba3CdSn2S8 and feature [PSe4]3 or [GeS4]4 tetrahedral building blocks. In this three-dimensional structure, there are two types of metal sites. In each structure, these sites are occupied di4erently because of a disorder between the alkali and lead cations. All three compounds are semiconductors with band gaps around 2 eV. The observation of a large second harmonic generation (SHG) signal for Na0.5Pb1.75GeS4 indicates that it may be a potential nonlinear optical (NLO) material. Infrared and Raman spectroscopic characterization is also re2000 Academic Press ported. Key Words: chalcogenides; second harmonic generation; molten salt 6uxes; selenophosphate; thiogermanate.

INTRODUCTION

The application of molten alkali metal polychalcogenide #uxes in the past decade has contributed a great deal to the synthetic chemistry of ternary and quaternary chalcogenides (1}4). More recently, this #ux method has been modi"ed and extended to address the solid state chemistry of multinary thio- and selenophosphates (5). The use of a polychalcophosphate #ux involves the in-situ generation of wellde"ned, discrete [P Q ]L\ anionic fragments, which are the V W
0022-4596/00 $35.00 Copyright 2000 by Academic Press All rights of reproduction in any form reserved.
building blocks required to form chalcophosphate solids. These anionic species can become solubilized in an excess of polychalcogenide #ux and then coordinate to metal cations to form molecular, (6}9), one-dimensional (6, 8, 10}14), twodimensional (6, 11, 15}19), and three-dimensional compounds (7, 19}21). Prior to the use of #uxes to prepare new thio- and selenophosphates, the state of development in this class of materials was limited mostly to the M P Q (Q"S, Se) family and some other ternary solids. A few of these compounds have been investigated for their nonlinear optical properties (22), ferroelectric applications (23), ionexchange capacity (24), and luminescent behavior (25). The structures and properties of these materials underscore a great potential with respect to learning new chemistry and physics from investigating these types of materials. We have now extended this work from chalcophosphates to chalcogermanates and chalcostannates. Because the [M Q ]X\ (M"Ge, Sn; Q"S, Se) anions are more highly V W charged than the [P Q ]L\ anions, the structures are exV W pected to be di!erent because of di!erent charge-balancing requirements. Further investigations into these compounds promise to expose a comparably rich chemistry (26}28). In combination with selenophosphate and thiogermanate anions we have explored the reactivity of Pb which gave rise to APbPSe (A"Rb, Cs), A Pb(PSe ) (A"Rb, Cs) (11), Rb PbGe S , K PbGe S , K Pb Ge S , and Cs Pb Ge S (29). Later, we examined these systems using the corresponding sodium and lithium chalcogenide #uxes. These #uxes are more challenging because as the size of the alkali metal decreases, the basicity decreases along with the probability that the alkali metal will be incorporated into the "nal product (2). Yet, this proved successful in synthesizing Na Pb [Ge S ] (26) and recently Na Pb PSe , Na Pb GeS , and Li Pb GeS , which are isostruc tural and reported here. Na Pb PSe , Na Pb GeS , and Li Pb GeS could not be made in pure form via direct combination

CUBIC SELENOPHOSPHATES AND THIOGERMANATES
reactions of the elements or the binary chalcogenides; they are products which seem to be stabilized only in the #ux. These compounds adopt a cubic structure, which is similar to that of Ba CdSn S and Ba CdAg Sn S (30); however, they exhibit some disorder between the lead and alkali cations. Spectroscopic characterization is also reported.

EXPERIMENTAL SECTION

scope (SEM) on a number of the red plates and red polyhedra indicated the presence of all four elements. The powder di!raction pattern of the red plates and red polyhedra indicated a new phase. The powder di!raction pattern of the gray polyhedra was indexed to PbSe. Preparation of Na0.5Pb1.75GeS4. In a nitrogen-"lled glovebox, 0.052 g (0.25 mmol) of Pb, 0.054 g (0.75 mmol) of Ge, 0.039 g (0.50 mmol) of Na S, and 0.064 g (2 mmol) of S were loaded into a Pyrex tube. The Pyrex tube was #ame-sealed under vacuum (approximately 2;10\ mbar) and inserted into a programmable furnace. The temperature was raised from 503C to 5303C in 20 h. It was kept at 5303C for 34 h, and then cooled 53C/h to 503C. N,N-Dimethylformamide was used to remove the excess #ux, and washing with ether revealed small orange/red crystals as a pure phase. The presence of all four elements was detected in several of the crystals with semiquantitative EDS using a SEM. The powder di!raction pattern indicated a new phase. Preparation of Li0.5Pb1.75GeS4. In a nitrogen-"lled glovebox 0.093 g (0.45 mmol) of Pb, 0.022 g (0.3 mmol) of Ge, 0.007 g (0.15 mmol) of Li S, and 0.077 g (2.4 mmol) of S were loaded into a graphite tube. The graphite tube was inserted into a 13 mm Pyrex tube and #ame-sealed under vacuum (approximately 2;10\ mbar). The Pyrex tube was then placed into a programmable furnace and heated from 503C to 5003C in 24 h. The reaction was kept at this temperature for 96 h and then cooled at 2.53C per hour to 2503C followed by rapid cooling to 503C in 2 h. The product was isolated as described above as pure orange/red crystalline chunks. Analysis of the small orange/red chunks using semiquantitative EDS attached to a SEM indicated the presence of Pb, Ge, and S (lithium cannot be detected by EDS). Analysis preformed using inductively coupled plasma (ICP) con"rmed the presence of Li, Pb, and S in a 0.57:1.73:4 molar ratio (there was no standard made for germanium). The powder di!raction pattern indicated a new phase. In order to obtain crystals suitable for single-crystal X-ray di!raction, the same starting mixture was loaded into a graphite tube. The graphite tube was inserted into a 13 mm quartz tube and #ame-sealed under vacuum (approximately 2;10\ mbar). This tube was heated from 503C to 6503C in 12 h. The reaction was kept at this temperature for 72 h and then cooled at 2.753C per hour to 2503C followed by rapid cooling to 503C in 2 h. The product was isolated in the same manner as previous to reveal beautiful, red plate-like single crystals as a pure phase.

Physical Measurements

Powder X-ray diwraction. Analyses were performed using a calibrated Rigaku-Denki/RW400F2 (Rota#ex) rotating anode powder di!ractometer controlled by an IBM

AITKEN ET AL.

computer, operating at 45 kV/100 mA and with a 13/min scan rate, employing Ni-"ltered Cu radiation in Bragg} Brentano geometry. Powder patterns were calculated with the Cerius software package (33). Electron microscopy. Quantitative microprobe analysis of the compounds were performed with a JEOL JSM-6400V SEM equipped with a Noran Vantage EDS detector. Data were acquired with an accelerating voltage of 25 kV and a 40 s accumulation time. Inductively coupled plasma spectroscopy (ICP). Samples were submitted to the Animal Health Diagnostics Laboratory at Michigan State University for analysis. Experiments were carried out on a Thermo Jarrel Ash Polyscan 61E Simultaneous/Sequential inductively coupled plasmaatomic emission spectrometer (ICP-AES) with vacuum spectrometers and Ar-purged optical paths. The solid powders were weighed onto an analytical balance and then digested in a Te#on container in concentrated nitric acid overnight at 953C. The digest was transferred to a 25 ml volumetric #ask and diluted with water. Yttrium was used as an internal standard in 2% HNO. Multielemental analyses were done by nebulizing the liquid sample into an argon #ame (plasma) that was sustained by a surrounding high-frequency magnetic "eld. The photons emitted are collimated and directed by a di!raction grating onto a semicircular array of photomultiplier tubes, one for each element to be measured. A computer then converts the photomultiplier signals to concentration units. Diwerential thermal analysis (DTA). Di!erential thermal analysis (DTA) was performed with a computer-controlled Shimadzu DTA-50 thermal analyzer. Approximately 20 mg of the samples were sealed in a carbon-coated quartz ampoule under vacuum. A quartz ampoule containing alumina of equal mass was sealed and placed on the reference side of the detector. The sample was heated to 8003C at 103C/min, isothermed for 5 min, and then cooled at a rate of 103C/min to 1003C, followed by rapid cooling to room temperature. Residues of the DTA experiments were examined by powder X-ray di!raction. The stability and reproducibility of the samples were monitored by running multiple heating and cooling cycles. Single-crystal UV/vis spectroscopy. Optical transmission measurements were made at room temperature on single crystals using a Hitachi U-6000 microscopic FT spectrophotometer with an Olympus BH-2 metallurgical microscope over a range of 380}900 nm. Solid-State UV/vis/near IR spectroscopy. Optical di!use re#ectance measurements were performed at room temperature using a Shimadzu UV-3101PC double-beam, doublemonochromator spectrophotometer. The instrument is

equipped with an integrating sphere and controlled by a personal computer. BaSO was used as a 100% re#ec tance standard. The sample was prepared by grinding the crystals to a powder and spreading them on a compacted surface of the powdered standard material, preloaded into a sample holder. The re#ectance versus wavelength data generated were used to estimate the band gap of the material by converting re#ectance to absorption data (34). Infrared spectroscopy. FT-IR spectra were recorded as solids in a CsI matrix. The samples were ground with dry CsI into a "ne powder and pressed into translucent pellets. The spectra were recorded in the far-IR region (600}100 cm\, 4 cm\ resolution) with the use of a Nicolet 740 FT-IR spectrometer equipped with a TGS/PE detector and silicon beam splitter. Raman spectroscopy. Raman spectra were recorded on a Holoprobe Raman Spectrograph equipped with a 633 nm HeNe laser and a CCD camera detector. The instrument was coupled to an Olympus BX60 microscope. For each sample, crystals were simply placed onto a small glass slide and a 50; objective lens was used to choose the area of the crystal specimens to be measured. The spot size of the laser beam when using the 50; objective lens was 10 m.
Single-Crystal X-Ray Crystallography
Na1.5Pb0.75PSe4. A plate-like crystal with dimensions 0.16;0.09;0.03 mm was mounted on a glass "ber. A Bruker SMART Platform CCD di!ractometer, operating at 50 kV/40 mA and using graphite-monochromatized MoK radiation, was used for data collection. No initial cell is needed to collect data using this procedure. A full sphere of data was collected in three major swaths of frames with 0.303 steps in and an exposure time of 45 s per frame. Crystal stability was determined at the end of the data collection by recollecting the "rst 50 frames of the data and comparing them to the original "rst 50 frames. No crystal decay was detected. An initial cell was obtained by with the SMART (35) program to extract re#ections from the frames of the actual data collection. This orientation matrix was used to integrate the data with the SAINT (35) program. The "nal cell constants were determined from a set of 6673 strong re#ections obtained from data collection. An empirical absorption correction was done using SADABS (36), and all re"nements were done with the SHELXTL (37) package of crystallographic programs. The systematic absences pointed clearly to the space group I43d. One lead, one sodium, one phosphorus, and two selenium atoms were found in special positions except for one of the selenium atoms. However, the thermal displacement parameter, ;(eq), of Pb(1) was high (0.046 A), and the thermal s displacement parameter of the sodium atom, which we will call Na(2), was also high (0.048 A) compared to that of the s

other atoms (R1"10.5% wR2"29%). For the Pb(1) position, a disorder model was applied by introducing another atom, Na(1), so that the sum of the occupancies was set equal to full occupancy. Na(2) was found to generate a symmetry-equivalent position &0.8 A from itself. This position s was then constrained to be half-occupied. After leastsquares re"nement, the disorder site between Pb(1) and Na(1) re"ned to 49.4% Pb(1) and 50.6% Na(1), the thermal displacement parameter of the Pb(1)/Na(1) site dropped to 0.027 A, and the Na(2) position had a reasonable s thermal displacement parameter (0.024 A) (R1"4.3% s wR2"14.8%). The Pb(1)/Na(1) site was constrained to 50%/50%, which gave a charged-balanced formula and after re"nement, no change in the R values was noted. Next, all the atoms were re"ned anisotropically except for Na(2) (R1"2.26%, wR2"5.17%). Re"ning Na(2) anisotropically was not deemed statistically signi"cant. The maximum and minimum peaks on the "nal Fourier di!erence map corresponded to 1.134 and !0.781 e\/A. Crystallos graphic data for Na Pb PSe are given in Table 1, and fractional atomic coordinates and anisotropic temperature factors are in Tables 2 and 3. Na0.5Pb1.75GeS4. A block-like crystal having dimensions ca. 0.10;0.10;0.05 mm was glued at the tip of a Lindemann glass capillary. The data collection was carried out on a P4 Siemens di!ractometer. To avoid severe absorption e!ects and ease re"nements, a graphite-monochromatized Ag radiation ( "0.56087 A) was used. The orientation s matrix and the cubic cell constant were obtained from the centering of 28 high value re#ections. The intensities of all re#ections but the Friedel pairs were collected in an scan mode. The intensity decay observed during the room temperature measurements was less than 1%. The measured intensities were obtained by "tting of the re#ection pro"les. They were corrected for scale variations by means of three standard re#ections. Data reduction, absorption corrections, and all re"nements were carried out with the JANA98 program package (38). Prior to a Gaussian-type analytical absorption correction, an optimization of the crystal size and shape based upon -scan measurements was performed with the X-Shape program (39). The 9713 re#ection intensities were then merged according to the I43m point group, with an internal R(obs) value of ca. 10.0% and an I'2 (I) cuto! as a criterion for observed re#ections. Two lead, one germanium, and two sulfur atoms were found, all of which occupied special positions except for one of the sulfur atoms. After least-squares re"nement, the atomic displacement parameter, ;(eq) of Pb(2) was extremely high compared to that of the other atoms. When this position was assigned as sodium, the atomic displacement parameter was almost zero. This position was assigned as Pb(2) (R1"13.0%, wR2"31.8%), and it was found to

0.7500 0.7500 0.7500 0.7698(1) 0.9302(1) 0.8380(1) 0.7500 0.7500 0.7500 0.77614(5) 0.9347(1) 0.8469(2) 0.2500 0.2500 0.2500 0.2229(1) 0.0654(1) 0.1419(2)
28(1) 28(1) 18(2) 12(1) 20(1) 16(1) 29.3(1) 36.2(7) 36.2(7) 13.6(1) 13.7(3) 23.5(5) 23(1) 23(1) 23(1) 10(1) 9(1) 27(1)
0.5 0.5 0.5 1.0 1.0 1.0 1.0 0.167 0.333 1.0 1.0 1.0 1.0 0.167 0.333 1.0 1.0 1.0
Pb GeS 0.0000 0.0000 0.0000 0.77614(5) 0.9347(1) 0.6779(2)
Li Pb GeS 0.0000 0.0000 0.0000 0.2229(1) 0.0654(1) 0.3228(2)
*;(eq) is de"ned as one-third of the trace of the orthogonalized U tensor. GH ?@The letters a and b are used to help the reader distinguish between the two 24d positions. &&a'' represents &&cation site A.'' This position generates a symmetry-equivalent atom 3.807(2), 3.856(3), or 3.716(1) A from itself. &&b'' represents &&cation site B.'' This position generates a symmetry-equivalent atom s 0.81(1), 1.021(4), or 0.968(6) A from itself and is therefore half-occupied. s
Li0.5Pb1.75GeS4. A plate-like, 0.19;0.03;0.03 mm crystal was mounted on a glass "ber. The data were collected using the same Bruker SMART CCD that was used for the data collection of Na Pb PSe. The same data collection and processing techniques used for Na Pb PSe were utilized. The exposure time was 60 s per frame. No crystal decay was detected. The data were integrated using the same procedure as in Na Pb PSe. The "nal cell constants were determined from a set of 4427 strong re#ections obtained from data collection. I43d was the space group chosen on the basis of the systematic absences. Two lead, one germanium, and two sulfur atoms were found, all of which occupied special positions except for one of the sulfur atoms. After leastsquares re"nement, the thermal displacement parameter, ;(eq), of Pb(2) was extremely high (0.161 A) compared to s that of the other atoms (R1"12.2%, wR2"32.2%). This position was found to generate a symmetry-equivalent position about 1 A from itself. Pb(2) was then constrained to be s 50% occupied which reduced its thermal displacement parameter to 0.089 A as well as the R1 and wR2 values to s
12.2% and 36.2%, respectively. At this stage, lithium atom, Li(2), was found to be disordered with Pb(2). The sum of the occupancies was set equal to 50% and after least-squares re"nement the occupancy of Pb(2) dropped to 19% while the occupancy of Li(2) re"ned to 31%. The thermal displacement parameter of Pb(2)/Li(2) dropped dramatically to 0.027 A, giving R1"9.2% and wR2"30.5%. Since s 16.7% lead gives a charge-balanced formula, the occupancy of Pb(2) was constrained to 16.7% and the occupancy of Na(2) was constrained to 33.3% with no change in the R values observed. All atoms were subsequently re"ned anisotropically (R1"2.73%, wR2"6.37%). Re"ning the Pb(2)/Li(2) site anisotropically was acceptable for this structure because the data were collected to a higher resolution and the symmetry-equivalent position was about 1 A away s in this case as opposed to ca. 0.8 A in the case of s Na Pb PSe. The maximum and minimum peaks on the "nal Fourier di!erence map corresponded to 1.633 and !0.984 e\/A, respectively. Crystallographic data for s Li Pb GeS are given in Table 1, and fractional atomic coordinates and anisotropic temperature factors are in Tables 2 and 3.

TABLE 3 Anisotropic Displacement Parameters* (A2 103) for s Na1.5Pb0.75PSe4, Na0.5Pb1.75GeS4, and Li0.5Pb1.75GeS4
Atom ; 24 (1) 24(1) 12(1) 20(1) 17(1) 22.0(2) 61(1) 61(1) 13.6(5) 13.7(5) 25.0(9) 13(1) 48(2) 48(2) 10(1) 9(1) 32(1) ; ; ; ; ;
Pb(1)? Na(1)? P(1) Se(1) Se(2) Pb(1)? Pb(2)@ Na(2)@ Ge(1) S(1) S(2) Pb(1)? Pb(2)@ Li(2)@ Ge(1) S(1) S(2)
Na Pb PSe 36(1) 23(1) 2(1) 36(1) 23(1) 2(1) 12(1) 12(1) !1(1) 20(1) 20(1) !4(1) 16(1) 15(1) !1(1)
!1(1) !1(1) !4(1) !4(1) !3(1) 5(1)
Na Pb GeS 44.0(3) 22.0(2) 3.7(2) 0.0 0.0 22(1) 26(1) 5(1) 0.0 0.0 22(1) 26(1) 5(1) 0.0 0.0 13.6(5) 13.6(5) !0.5(2) !0.5(2) !0.5(2) 13.7(5) 13.7(5) 0.3(5) 0.3(5) 0.3(5) 19.4(8) 26.0(9) 6.1(7) !11.4(7) !2.8(7) Li Pb GeS 40(1) 15(1) 4(1) 9(1) 10(1) 3(1) 9(1) 10(1) 3(1) 10(1) 10(1) !1(1) 9(1) 9(1) 1(1) 13(1) 35(2) 1(1) !1(1) !1(1) 1(1) 1(1) 1(1) 1(1)
[PS ]\, which tends to be much more prevalent relative to that of [P S ]\. However, with the use of the selenophos phate #ux and our ability to tune its properties, we have been able to stabilize the [PSe ]\ unit a number of times (8, 11, 19, 40}42). Na Pb GeS was synthesized by reacting a mixture of Pb, Ge, Na S, and S in a 1:3:2:8 molar ratio at 5303C for 4 days. This compound could be synthesized relatively pure in the #ux but could not be prepared in pure form by direct combination of the elements. Di!erential thermal analysis of the compound shows that Na Pb GeS melts incon gruently at 7103C. After melting, some of the compound recrystallizes while some decomposes to PbS and residual #ux. Na Pb GeS is air- and water-stable for at least a few years. Li Pb GeS was synthesized by reacting a mixture of Pb, Ge, Li S, and S in a 3:2:1:16 molar ratio at 5003C for 4 days. The compound is similar in nature to its Na analogue. It melts incongruently at 6683C and slowly GeS is air- and waterprecipitates out PbS. Li Pb stable for a least a couple of weeks.

Structure

The cubic structure adopted by all three compounds is three-dimensional, noncentrosymmetric, and similar to that of Ba CdSn S and Ba CdAg Sn S (30). We will describe in detail the structure of Na Pb PSe and then com pare/contrast Na Pb GeS and Li Pb GeS , respectively. If one looks at the unit cell of Na Pb PSe shown in Fig. 1, the structure seems to be rather complicated; however, when it is broken down into pieces, one can see things more clearly. The structure of Na Pb PSe is made up of near-perfect [PSe ]\ tetrahedral building blocks, with Se}P}Se bond angles of 107.14(8)3 and 111.70(8)3. A view of only the tetrahedral building blocks, looking down the body diagonal, displayed in Fig. 2, shows that the structure is obviously noncentrosymmetric because all of the tetrahedra point in the same direction. These tetrahedral units coordinate to sodium or lead cations, which occupy two crystallographic sites in the structure. They are both special positions (24d) and will be referred to as cation site A and cation site B, corresponding to the numbering of the atoms which occupy these sites. Cation site A is disordered (50%/50%) between Na(1) and Pb(1). A symmetry-equivalent position is generated 3.807(2) A away, see Fig. 3a, and s can be described as a close contact. One can see in Fig. 3b the arrangement of the tetrahedral units surrounding this position. There are two Pb(1)/Na(1)}Se distances of 2.9124(3) A and two distances of 3.054(1) A. There are also s s four long Pb(1)/Na(1)}Se interactions, two of which are 3.4017(9) A; the other two are 3.4385(9) A, see Fig. 3c. Thus, s s when Pb(1) is in this position it can be considered to be 4-coordinate, with a distorted see-saw-like geometry, having

*The anisotropic displacement factor exponent takes the form !2 [ha*; #kb*; #lc*; #2hka*b*; #2hla*c*; # 2klb*c*; ]. ?@The letters a and b are used to help the reader distinguish between the two 24d positions. &&a'' represents &&cation site A''. This position generates a symmetry-equivalent atom 3.807(2), 3.856(3), or 3.716(1) A from itself. &&b'' s represents &&cation site B''. This position generates a symmetry-equivalent atom 0.81(1), 1.021(4), or 0.968(6) A from itself and is therefore half-occus pied.

RESULTS AND DISCUSSION

Na Pb PSe was synthesized by reacting a mixture of Pb, P Se , Na Se, and Se in a 1.5:1.0:1.5:1.5 molar ratio at 4953C for 4 days. We were never able to make this compound pure. Although the best ratio to synthesize Na Pb PSe was close to direct combination, it only formed in 60% yield. The other 40% of the isolated product was PbSe. Reactions that were richer in Na Se or Se gave more PbSe. Reactions at lower temperatures did not improve the synthesis, and reactions at higher temperatures only favored the formation of PbSe. Na Pb PSe is only stable in air and water for less than 1 day. This compound features the [PSe ]\ unit, whose occurrence in solid-state compounds is relatively uncommon. There are many more compounds containing the [P Se ]\ unit, which suggests that the tetrahedral P> species is unstable with respect to the dimeric P> species under selenide-rich conditions. This is in contrast to the sul"de analogue,
FIG. 1. Unit cell of Na Pb PSe. The [PSe ]U tetrahedral units are represented by gray polyhedra. Bonds between Pb and Se are omitted for clarity. Cation site A, the position that is disordered 50%/50% between Na and Pb, is shown in black, while the Na atoms, which sit on cation site B, are shown in white.
lithium atoms. As in Na Pb PSe there are two metal sites which are special positions (24d) and referred to as cation sites A and B; however, they are now occupied di!erently. For both of the thiogermanates, cation site A is occupied only by Pb(1). This position generates a symmetryequivalent atom 3.856(3) or 3.716(1) A away, which corress ponds to a weak Pb}Pb lone pair interaction (43). For Na Pb GeS , there are two Pb(1)}S bond distances of 2.846(2) A and two of 2.867(2) A, while there are four long s s interactions, two which are 3.443(2) A and two which are s 3.455(2) A. For Li Pb GeS , there are two Pb(1)}S s bond distances of 2.828(2) A and two of 2.8623(6) A, while s s there are four long interactions, two which are 3.403(2) A s and two which are 3.508(3) A. Cation site B, in the case of s both thiogermanates, is disordered between Na(2)/Li(2) and Pb(2) (33.3%/16.7%). This position generates a symmetryequivalent atom 1.021(4) A away in the case of s Na Pb GeS and 0.968(6) A away for Li Pb GeS s and therefore can have a maximum occupancy of only 50%. In Na Pb GeS , there are four shorter Na(2)/Pb(2)}S distances: two are 2.806(2) A, and two are 3.065(2) A. Furs s ther out there exist two distances of 3.315(3) A and two even s longer distances of 3.716(1) A. In the case of s Li Pb GeS , there are four shorter Li(2)/Pb(2)}S dis tances two of which are 2.601(3) A and two of which are s 2.860(4) A, while there are two longer distances of 3.496(4) A s s and two even longer distances of 4.308(4). This site can be considered as a very large 8-coordinate pocket when

some additional long interactions. However, when Na(1) is in this position it can be considered as 8-coordinate since the interaction between Na and Se is less covalent and more ionic. Cation site B is occupied only by Na(2). The coordination environment of Na(2) is an irregular, 8-coordinate pocket shown in Fig. 4. This position generates a symmetryequivalent atom 0.81(1) A from itself, and therefore its maxs imum occupancy is 50%. Six of the Na(2)}Se distances range from 2.936(1) to 3.381(6) A, while two are longer, s 4.052(6) A. See Tables 4 and 5 for additional distances and s angles. The sites occupied by Na and/or Pb in these compounds are slightly larger than the sizes of both these cations, and a certain degree of &&rattling'' motion of these cations may be expected. If multiple equivalent energy minima exist within these cavities and if the positions of the Na and/or Pb ions in the 24d sites (see Table 2) could be in#uenced by the application of electric "elds, then ferroelectric properties may be possible in these compounds. Na Pb GeS and Li Pb GeS are isostructural to Na Pb PSe. They are both made up of [GeS ]\ tetrahedral building blocks, with S}Ge}S bond angles of 105.40(8)3 and 113.21(8)3 for Na Pb GeS and 105.39(7)3 and 113.23(6)3 for Li Pb GeS. These [GeS ]\ tetrahedra are less perfect than the [PSe ]\ tetrahedra found in Na Pb PSe. The [GeS ]\ tetrahedral units are coordinated to lead and sodium or
FIG. 2. [PSe ]\ tetrahedral anions of Na Pb PSe viewed down the body diagonal of the unit cell. It is easy to see how the structure is noncentrosymmetric because all of the tetrahedra are pointing in the same direction. The P atoms are shown in white, while the Se atoms are shown in gray.
TABLE 4 Selected Distances (A) for Na1.5Pb0.75PSe4, Na0.5Pb1.75GeS4, s and Li0.5Pb1.75GeS4
Distances M(1)}Q(1) M(1)}Q(2) A(1)}Q(1) A(1)}Q(2) A(1)}Q(2) A(1)}Q(2) A(1)}A(1) A(1)}B(2) B(2)}Q(2) B(2)}Q(2) B(2)}Q(2) B(2)}Q(2) B(2)}B(2) Na Pb PSe 2.228(3) 2.212(1);3 2.9124(3);2 3.054(1);2 3.4017(9);2 3.4385(9);2 3.807(2) 3.847(3);2 2.936(1);2 3.139(3);2 3.381(6);2 4.052(6);2 0.81(1) Na Pb GeS 2.238(2) 2.214(2);3 2.867(2);2 2.846(2);2 3.443(2);2 3.455(2);2 3.856(3) 3.750(3);2 2.806(2);2 2.860(4);2 3.315(3);2 3.716(1);2 1.021(4) Li Pb GeS 2.244(4) 2.209(2);3 2.828(2);2 2.8623(6);2 3.403(2);2 3.508(3);2 3.716(1) 3.728(1);2 2.601(3);2 2.860(4);2 3.496(4);2 4.308(4);2 0.968(6)

TABLE 6 Comparison of A6B3M4Q16 Structures
A (Ba>) (Ba>) (Na>) (Pb>) (Pb>) (Pb>) B ? (Cd>) Ag (Cd>) (Na>) (Na>) (Pb>) (Li>) (Pb>) M (Sn>) (Sn>) (P>) (Ge>) (Ge>) Q Empirical formula (Z"4) Ba (Cd )Sn S Ba (Ag Cd)Sn S (Na Pb )(Na )P Se (Pb )(Na Pb)Ge S (Pb )(Li Pb)Ge S Ba Cd (SnS ) (30)@ Ba Ag Cd(SnS ) (30)A Na Pb (PSe ) Na Pb (GeS ) Li Pb (GeS )
(S\) (S\) (Se\) (S\) (S\)
? Site B for the "rst two compounds is a 12b position; however, site B for the last three compounds is a 24d position, which can only have a maximum occupancy of 50% owing to the presence of a short distance to a symmetry-equivalent position. @ The authors allowed the Ba and Cd sites to re"ne freely and obtained the formula of Ba Cd Sn S , where the 24d position is 94% occupied by Ba and the 12b site is 70% occupied by Cd. No constraint was added to the re"nement to make the formula charge balance. Therefore, Ba CdSn S or Ba Cd Sn S can be considered as an idealized formula. A For the compound Ba Ag CdSn S it was proposed that the 24d position is fully occupied by Ba and that the 12b site is occupied 66.67% by Ag and 33.33% by Cd. However, only a powder pattern was obtained, and no single-crystal structure determination was performed.
the sample (45) showed a second harmonic generation (SHG) signal which was 7 to 8 times greater than that observed for similarly prepared powder samples of LiNbO. The presence of SHG is consistent with the current cubic space group and crystal class (43m) (46). Further experi ments are needed to characterize the NLO behavior (47).

CONCLUDING REMARKS

All three compounds reported here are isostructural to one another and crystallize in the cubic, noncentrosymmetric space group I43d with a structure similar to that of Ba CdSn S. This structure seems to be quite stable and
FIG. 5. Single-crystal optical spectra of (A) Na Pb PSe and (B) Na Pb GeS. (C) Optical spectrum obtained from a polycrystalline powdered sample of Li Pb GeS.
vibrations of the [PSe ]\ and [GeS ]\ tetrahedra, re spectively. Since the average atomic number of Ge}S is greater than that of P}Se, the Ge}S stretches in Na Pb GeS and Li Pb GeS occur at a lower cm\ than those of the P}Se in Na Pb PSe. Table 7 lists the most important Raman and far-IR spectroscopic absorption peaks for all three compounds.
Nonlinear Optical (NLO) Behavior
Preliminary experiments on powder samples of Na Pb GeS using &150 J laser light at 3.5 m from

FIG. 6. Far-IR spectra of (A) Na Pb PSe , (B) Na Pb GeS , and (C) Li Pb GeS.
TABLE 7 Infrared and Raman Data (cm 1) for Na1.5Pb0.75PSe4, Na0.5Pb1.75GeS4, and Li0.5Pb1.75GeS4
Na IR Pb PSe Raman 442 w 430 m 408 vs 418 w 409 w 232 m 394 vs 361 m 346 w 252 m 232 vs 221 vs 160 m 124 w 216 vs 188 vw 161 vw Na Pb GeS IR Raman 408 m 392 w 363 s 350 s 261 w Li Pb GeS IR Raman 408 m 392 w 362 s 350 s 260 w
395 vs 363 m 349 w 253 m 232 vs

216 vs 188 vw 161 vw

Note. vs"very strong, s"strong, m"medium, w"weak, vw"very weak.
may form with this structure type. Synthetic investigations into the existence of some predictable formulas are worGeS shows interesting propthwhile (48) since Na Pb erties. Preliminary measurements of the second harmonic generation signal of polycrystalline powder samples of GeS indicate that this compound could be Na Pb a potential NLO material. Further investigations are necessary to explore the NLO behavior.

ACKNOWLEDGMENTS

Financial support from the National Science Foundation (Grant DMR9817287) is gratefully acknowledged. This work made use of the SEM facilities of the Center for Electron Optics at Michigan State University. The Bruker SMART platform CCD di!ractometer at Michigan State University was purchased with funds from the National Science Foundation (CHE-9634638). We acknowledge the use of the W. M. Keck Microfabrication Facility at Michigan State University, a NSF MRSEC facility. In addition, we acknowledge the Animal Health Diagnostic Laboratory in the College of Veterinary Medicine at Michigan State University for the ICP measurements. We gratefully acknowledge our collaborators at Rockwell International (M. Rosker and M. Eubank) for the NLO measurements.
FIG. 7. Raman spectra of (A) Na Pb PSe , (B) Na Pb GeS , and (C) Li Pb GeS.
#exible since it tolerates disorder and/or vacancies (see Table 6). It is also remarkable that the structure can be formed with [PSe ]\, [GeS ]\, or [SnS ]\ tetrahedra, since the charges of these building blocks are di!erent. This happens because of the ability of both cation site A and B to accommodate #1 or #2 metals alone or simultaneously by introducing disorder and/or vacancies. The amount of disorder and/or vacancies is adjusted based on the chargebalancing requirements of the particular tetrahedral unit ([PSe ]\, [GeS ]\, or [SnS ]\). Table 6 can be used to predict other quaternary and quinary compounds, which

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