New Polar Oxides: Synthesis, Characterization, Calculations, and

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Chem. Mater. 2009, 21, 1654–1662

New Polar Oxides: Synthesis, Characterization, Calculations, and Structure-Property Relationships in RbSe2V3O12 and TlSe2V3O12 Hong Young Chang,† Sang-Hwan Kim,† Kang Min Ok,‡ and P. Shiv Halasyamani*,† Department of Chemistry, UniVersity of Houston, 136 Fleming Building, Houston, Texas 77204-5003, and Department of Chemistry, Chung-Ang UniVersity, 221 Heukseok-dong, Dongjak-gu, Seoul 155-756, Republic of Korea ReceiVed January 28, 2009. ReVised Manuscript ReceiVed March 3, 2009

Two new polar noncentrosymmetric oxides, RbSe2V3O12 and TlSe2V3O12, have been synthesized and characterized. The oxides are isostructural, as Tl+ exhibits an inert rather than a stereoactive lone pair. The reported materials were structurally characterized by single-crystal X-ray diffraction. The materials exhibit a two-dimensional hexagonal tungsten oxide (HTO) type topology with layers of corner-shared VO6 octahedra. The layers are capped, above and below, by SeO3 polyhedra. The Rb+ and Tl+ cations are found between the layers. The V5+ and Se4+ cations are in asymmetric coordination environments attributable to second-order Jahn-Teller (SOJT) effects. In addition to structural characterization, secondharmonic generation (SHG), piezoelectric, and polarization measurements were performed. SHG measurements using 1064 nm radiation revealed doubling efficiencies ranging from ∼40-50 × R-SiO2. Piezoelectric experiments revealed d33 values of ∼6-12 pm V-1. Polarization measurements indicate the materials are not ferroelectric, i.e., the polarization is not “switchable”. The materials are pyroelectric, with a total pyroelectric coefficient, p, at 45 °C, of -4.4 and -2.6 µC m-2 K-1 for RbSe2V3O12 and TlSe2V3O12, respectively. Thermogravimetric measurements, UV-vis, and infrared spectroscopy were also performed, as were electronic structure calculations. Crystal data: RbSe2V3O12, hexagonal, space group P63 (No. 173), a ) b ) 7.1364(4) Å, c ) 11.4687(13) Å, V ) 505.83(7) Å3, and Z ) 2; TlSe2V3O12, hexagonal, space group P63 (No. 173), a ) b ) 7.1248(3) Å, c ) 11.4287(11) Å, V ) 502.43(6) Å3, and Z ) 2.

Introduction Polar noncentrosymmetric (NCS) materials, i.e., compounds exhibiting a dipole moment, are of significant interest because of their technologically important properties such as ferroelectricity and pyroelectricity.1,2 In molecular compounds such as H2O, CO, and NH3, the concept of polarity is well-understood. In solid-state materials, a compound is polar if it is found in one of 10 crystal classes (1, 2, 3, 4, 6, m, mm2, 3m, 4mm, or 6mm).3 With these crystal classes, specific polar directions have also been defined.3 Yet the question remains of how to synthesize a new polar material? We have focused on synthesizing oxides containing cations susceptible to second-order Jahn-Teller (SOJT) effects.4-11 These cations are octahedrally coordinated d0 transition * Corresponding author. E-mail: [email protected]. † University of Houston. ‡ Chung-Ang University.

(1) Auciello, O.; Scott, J. F.; Ramesh, R. Phys. Today 1998, 51, 22. (2) Lang, S. B. Phys. Today 2005, 58, 31. (3) Hahn, T. International Tables for Crystallography, Volume A, Space Group Symmetry; Kluwer Academic: Dordrecht, The Netherlands, 2006. (4) Opik, U.; Pryce, M. H. L. Proc. R. Soc. London, Ser. A 1957, 238, 425. (5) Bader, R. F. W. Mol. Phys. 1960, 3, 137. (6) Bader, R. F. W. Can. J. Chem. 1962, 40, 1164. (7) Pearson, R. G. J. Am. Chem. Soc. 1969, 91, 4947. (8) Pearson, R. G. J. Mol. Struct.:THEOCHEM 1983, 103, 25. (9) Wheeler, R. A.; Whangbo, M. H.; Hughbanks, T.; Hoffmann, R.; Burdett, J. K.; Albright, T. A. J. Am. Chem. Soc. 1986, 108, 2222. (10) Kunz, M.; Brown, I. D. J. Solid State Chem. 1995, 115, 395. (11) Goodenough, J. B. Annu. ReV. Mater. Sci. 1998, 28, 1.

metals (Ti4+, Nb5+, W6+, etc.) and lone-pair cations (Se4+, Te4+, Pb2+, etc.). In oxides, both families of cations are in inherently asymmetric polar coordination environments. For the d0 cations, an intraoctahedral distortion is observed usually toward a corner, edge, or face of the oxide octahedron.11 This distortion is thought to occur attributable to mixing between the empty metal d- and filled oxygen p-orbitals. For the lone-pair cations, the situation is slightly more complicated. Not only is metal s-p mixing involved,12 but it has been recently pointed out that oxygen 2p orbitals are also involved.13,14 Understanding the local polar environments is critically important because these environments are one of the dominant factors influencing the materials’ functional properties. As stated, we have focused our efforts on synthesizing new polar oxides containing cations susceptible to SOJT effects.4-11 In doing so, we have synthesized a host of new polar materials.15-23 In this paper we report on the synthesis (12) Orgel, L. E. J. Chem. Soc 1959, 3815. (13) Waghmare, U. V.; Spaldin, N. A.; Kandpal, H. C.; Seshadri, R. Phys. ReV. B 2003, 67, 12511–1. (14) Stoltzfus, M. W.; Woodward, P.; Seshadri, R.; Park, J.-H.; Bursten, B. Inorg. Chem. 2007, 46, 3839. (15) Goodey, J.; Broussard, J.; Halasyamani, P. S. Chem. Mater. 2002, 14, 3174. (16) Goodey, J.; Ok, K. M.; Broussard, J.; Hofmann, C.; Escobedo, F. V.; Halasyamani, P. S. J. Solid State Chem. 2003, 175, 3. (17) Ra, H. S.; Ok, K. M.; Halasyamani, P. S. J. Am. Chem. Soc. 2003, 125, 7764. (18) Ok, K. M.; Halasyamani, P. S. Angew. Chem., Int. Ed. 2004, 43, 5489.

10.1021/cm9002614 CCC: $40.75  2009 American Chemical Society Published on Web 03/24/2009

New Polar Oxides: RbSe2V3O12 and TlSe2V3O12

and characterization of two new polar oxides, RbSe2V3O12 and TlSe2V3O12. The materials are structurally similar to the previously reported KSe2V3O12 and CsSe2V3O12.24,25 We also resynthesized KSe2V3O12 and CsSe2V3O12. With all four compounds, second-harmonic generation, piezoelectricity, polarization, thermogravimetric, infrared, and UV-vis measurements were performed. In addition, electronic structure calculations were done. These calculations indicate Tl+ exhibits an inert rather than stereoactive lone pair. We also explore and discuss structure-property relationships. Experimental Section Reagents. K2CO3 (Alfa Aesar, 99%), Rb2CO3 (Alfa Aesar, 99%), Cs2CO3 (Alfa Aesar, 99%), Tl2CO3 (Alfa Aesar, 99%), SeO2 (Alfa Aesar, 99%), and V2O5 (Aldrich, 99%) were used as received. Synthesis. For RbSe2V3O12, 0.381 g (1.70 × 10-3 mol) of Rb2CO3, 1.098 g (9.90 × 10-3 mol) of SeO2, and 0.300 g (1.70 × 10-3 mol) of V2O5 were combined with 8 mL of H2O. For TlSe2V3O12, 0.400 g (8.50 × 10-3 mol) of Tl2CO3, 0.474 g (4.30 × 10-3 mol) of SeO2, and 0.464 g (2.60 × 10-3 mol) of V2O5 were combined with 10 mL of H2O. The respective solutions were placed in 23 mL Teflon-lined autoclaves that were subsequently sealed. The autoclaves were gradually heated to 230 °C, held for 3 days, and cooled slowly to room temperature at a rate 6 °C h-1. The mother liquor was decanted from the products, and the products were recovered by filtration and washed with distilled water. Pale green color crystals, the only product from each reaction, were obtained in approximately 90% yield based on V2O5 for RbSe2V3O12 and TlSe2V3O12. The previously reported oxides, KSe2V3O12 and CsSe2V3O12, were synthesized as described.24,25 Single-Crystal X-ray Diffraction. For RbSe2V3O12 and TlSe2V3O12, a green rod-shaped (0.05 × 0.1 × 0.1 mm3) and a green hexagonal rod-shaped (0.08 × 0.08 × 0.1 mm3) crystal were used for single-crystal data collection, respectively. Data were collected using a Siemens SMART APEX diffractometer equipped with a 1K CCD area detector using graphite-monochromated Mo KR radiation. A hemisphere of data was collected using a narrow-frame method with scan widths of 0.50° in ω, and an exposure time of 40 s per frame. The first 50 frames were remeasured at the end of the data collection to monitor instrument and crystal stability. The data were integrated using the Siemens SAINT program,26 with the intensities corrected for Lorentz, polarization, air absorption, and absorption attributable to the variation in the path length through the detector face plate. Psi-scans were used for the absorption correction on the data. The data were solved and refined using SHELXS-97 and SHELXL-97, respectively.27-29 All of the atoms (19) Ok, K. M.; Halasyamani, P. S. Inorg. Chem. 2005, 44, 9353. (20) Chi, E. O.; Ok, K. M.; Porter, Y.; Halasyamani, P. S. Chem. Mater. 2006, 18, 2070. (21) Sivakumar, T.; Chang, H. Y.; Baek, J.; Halasyamani, P. S. Chem. Mater. 2007, 19, 4710. (22) Kim, J.-H.; Baek, J.; Halasyamani, P. S. Chem. Mater. 2008, 20, 3542. (23) Chang, H. Y.; Kim, S.-H.; Halasyamani, P. S.; Ok, K. M. J. Am. Chem. Soc. 2009, 131, 2426. (24) Harrison, W. T. A. Acta Crystallogr., Sect. C 2000, 56, 422. (25) Harrison, W. T. A.; Dussack, L. L.; Jacobson, A. J. Acta Crystallogr., Sect. C 1995, 51, 2473. (26) SAINT Program for Area Detector Absorption Correction, 4.05; Siemens Analytical X-ray Systems; Madison, WI, 1995. (27) Sheldrick, G. M., SHELXS-97 - A program for automatic solution of crystal structures. University of Goettingen: Goettingen, Germany, 1997. (28) Sheldrick, G. M., SHELXL-97-A Program for Crystal Structure Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (29) Sheldrick, G. M. SHELXTL DOS/Windows/NT, 5.10; Bruker Analytical X-Ray Instruments, Inc.: Madison, WI, 1997.

Chem. Mater., Vol. 21, No. 8, 2009 1655 Table 1. Crystallographic Information for RbSe2V3O12 and TlSe2V3O12 formula fw T (K) λ (Å) cryst syst space group a (Å) c (Å) R (deg) γ (deg) V (Å3) Z Fcalcd (g/cm3) µ (mm-1) 2θmax (deg) R (int) Flack parameter GOF R (F)a Rw (Fo2)b a R (F) ) Σ||Fo| Fc2)2/Σw(Fo2)2]1/2.

RbSe2V3O12 588.21 296.0(2) 0.71073 hexagonal P63 (No. 173) 7.1364(4) 11.4687(13) 90 120 505.83(7) 2 3.862 14.752 58.08 0.0329 -0.012(10) 0.99 0.0189 0.0349 -

|Fc||/Σ|Fc|.

b

Rw

TlSe2V3O12 707.10 296.0(2) 0.71073 hexagonal P63 (No. 173) 7.1248(3) 11.4287(11) 90 120 502.43(6) 2 4.674 25.998 56.38 0.0870 -0.009(12) 1.09 0.0285 0.0716 (Fo2)

)

[Σw(Fo2

-

Table 2. Atomic Coordinates for RbSe2V3O12 and TlSe2V3O12 RbSe2V3O12 atom

x

y

z

Ueq (Å2)a

Rb(1) V(1) Se(1) Se(2) O(1) O(2) O(3) O(4)

0.6667 0.21288(9) 0.3333 0 0.1984(4) 0.1145(4) 0.4566(4) 0.2545(4)

0.3333 0.87974(9) 0.6667 0 0.7791(4) 0.8656(3) 0.9206(4) 0.1213(4)

0.02243(7) 0.20889(5) -0.01649(5) 0.43070(5) 0.0474(2) 0.3670(2) 0.2378(2) 0.1756(2)

0.0336(2) 0.01012(13) 0.01095(14) 0.01056(14) 0.0138(6) 0.0124(5) 0.0146(6) 0.0139(5)

Tl(1) V(1) Se(1) Se(2) O(1) O(2) O(3) O(4)

0.3333 0.8778(2) 0.6667 0 0.7821(8) 0.8657(8) 0.5377(8) 0.7446(8)

TlSe2V3O12 0.6667 0.92776(8) 0.2116(2) 0.10744(11) 0.3333 0.88063(12) 0 0.32960(12) 0.1999(8) 0.9447(5) 0.1168(8) 0.2658(5) 0.0794(9) 0.1363(5) -0.1311(8) 0.0729(6)

0.0536(3) 0.0150(3) 0.0138(3) 0.0121(3) 0.0161(11) 0.0139(10) 0.0178(11) 0.0161(10)

a Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.

were refined with anisotropic thermal parameters and the refinement converged for I > 2σ(I). All calculations were performed using the WinGX-98 crystallographic software package.30 A symmetry analysis on the structures, using the program PLATON,31 revealed considerable pseudosymmetry, consistent with small atomic shifts of the V, Se, and O atoms from possible positions in the NCS space group P63mc. However, the hh0l, l + 2n, condition required for P63mc is strongly violated in the data collected for the two structures. Crystallographic data, atomic coordinates, and selected bond distances for RbSe2V3O12 and TlSe2V3O12 are given in Tables 1-3, with additional details in the Supporting Information. Powder X-ray Diffraction. X-ray powder diffraction data were collected using a PANalytical X’Pert PRO diffractometer using Cu KR radiation. The 2θ range was 10° - 60° with a step size of 0.008° and a fixed time of 0.3s. The experimental powder XRD data are in good agreement with the calculated data based on the singlecrystal models. In addition, the powder XRD patterns for KSe2V3O12 and CsSe2V3O12, are good agreement with the reported data (see Supporting Information).24,25 (30) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837. (31) Spek, A. L. PLATON; Utrecht University: Utrecht, The Netherlands, 2001.

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Table 3. Selected Bond Distances (Å) for RbSe2V3O12 and TlSe2V3O12 RbSe2V3O12 Rb(1)-O(1) × 3 Rb(1)-O(2) × 3 Rb(1)-O(3) × 3 Rb(1)-O(4) × 3 V(1)-O(1) V(1)-O(2) V(1)-O(3) V(1)-O(3) V(1)-O(4) V(1)-O(4) Se(1)-O(1) × 3 Se(2)-O(2) × 3

TlSe2V3O12 3.540(2) 3.138(2) 3.551(3) 3.095(3) 1.970(2) 1.929(2) 1.647(2) 2.168(2) 1.641(2) 2.190(2) 1.697(2) 1.705(2)

Tl(1)-O(1) × 3 Tl(1)-O(2) × 3 Tl(1)-O(3) × 3 Tl(1)-O(4) × 3 V(1)-O(1) V(1)-O(2) V(1)-O(3) V(1)-O(3) V(1)-O(4) V(1)-O(4) Se(1)-O(1) × 3 Se(2)-O(2) × 3

3.542(5) 3.163(6) 3.488(6) 3.032(6) 1.968(5) 1.919(5) 1.660(5) 2.141(5) 1.670(5) 2.168(5) 1.702(5) 1.713(5)

Infrared Spectroscopy. Infrared spectra were recorded on a Matterson FTIR 5000 spectrometer in the 350-3000 cm-1 range. UV-vis Diffuse Reflectance Spectroscopy. UV-visible diffuse reflectance data for the reported compounds were collected on a Varian Cary 500 scan UV-vis-NIR spectrophotometer over the spectral range 200-1500 nm at room temperature. Poly(tetrafluoroethylene) was used as a reference material. Reflectance spectra were converted to the absorbance using the Kubelka-Munk function.32,33 Thermogravimetric Analysis. Thermogravimetric analyses were carried out on a TGA 951 thermogravimetric analyzer (TA instruments). The sample was placed in a platinum crucible and heated at a rate of 10 °C min-1 from room temperature to 800 °C under flowing nitrogen. Second Harmonic Generation. Powder SHG measurements were performed on a modified Kurtz-NLO system34 using a pulsed Nd:YAG laser with a wavelength of 1064 nm. A detailed description of the equipment and methodology has been published.35 SHG efficiency has been shown to depend strongly on particle size, thus polycrystalline samples were ground and sieved into distinct particle size ranges (20-45, 45-63, 63-75, 75-90, >90 µm). To make relevant comparisons with known SHG materials, crystalline R-SiO2 and LiNbO3 were also ground and sieved into the same particle size ranges. No index matching fluid was used in any of the experiments. Piezoelectric and Polarization Measurements. Piezoelectric measurements were performed using a Radiant Technologies RT66A piezoelectric test system with a TREK (model 609E-6) highvoltage amplifier, Precision materials analyzer, Precision highvoltage interface, and MTI 2000 fotonic sensor. A maximum voltage of 500 V was applied to the samples. The polarization was measured on a Radiant Technologies RT66A Ferroelectric Test System with a TREK high voltage amplifier between room temperature and 185 °C in Delta 9023 environmental test chamber. The unclamped pyroelectric coefficient, defined as dP/dT (change in the polarization with respect to the change in temperature) was determined by measuring the polarization as a function of temperature. A detailed description of the methodology used has been published elsewhere.35 The samples were pressed into pellets (∼12 mm diameter, ∼1 mm thick) and sintered, well below the decomposition temperatures, at 280 °C for 10 h for all four materials (KSe2V3O12, RbSe2V3O12, CsSe2V3O12, and TlSe2V3O12). Silver paste was applied to both sides as electrodes and cured at 200 °C for 3 h. To measure the ferroelectric behavior, we completed polarization measurements at room temperature under a static electric field of 30 kV cm-1 at (32) (33) (34) (35)

Tauc, J. Mater. Res. Bull. 1970, 5, 721. Kubelka, P.; Munk, F. Z. Tech. Phys. 1931, 12, 593. Kurtz, S. K.; Perry, T. T. J. Appl. Phys. 1968, 39, 3798. Ok, K. M.; Chi, E. O.; Halasyamani, P. S. Chem. Soc. ReV. 2006, 35, 710.

various frequencies (50, 100, and 200 Hz). For the pyroelectric measurements, the polarization was measured statically from room temperature to 185 °C in 20 °C increments, with an electric field of 20 kV cm-1. The temperature was allowed to stabilize before the polarization was measured. Electronic Structure Calculations. First-principle electronic band structures for the title compounds were calculated with the tight-binding linear muffin-tin orbital (TB-LMTO)36,37 and plane wave pseudopotential methods.38 The TB-LMTO calculations within atomic sphere approximation (ASA) were employed with the von Barth-Hedin local exchange-correlation potential39 used for the local density approximation (LDA). The radial scalar relativistic Dirac equation was solved for obtaining the partial waves. A total of 16 and 15 empty spheres were necessary to achieve space filling for RbSe2V3O12 and TlSe2V3O12, respectively. A total of 72 irreducible k-points from 10 × 10 × 4 grid was used for Brillouin zone integrations by tetrahedron method.40,41 The basis set consisted of Rb-5s/[5p]/[4d]/[4f], Tl-6s/6p/[6d], V-4s/4p/3d, Se-4s/4p/[4d] and O-2s/2p orbitals where the orbital in bracket was treated with the downfolding technique.42 Self-consistency was achieved within the total energy change smaller than 1 × 10-5 Rydberg. In the planewave pseudopotential calculations as implemented in Quantum ESPRESSO (4.0.2 version) package,43 norm-conserving MartinsTroullier (MT) pseudopotentials44 were utilized with a generalized gradient approximation (GGA) for the exchange-correlation corrections.45 The peudopotentials generated from FHI-98 code are converted for the calculations. A plane wave energy cutoff was set to 37 Rydberg. A k-point grid of 6 × 6 × 4 was used for Brillouin zone integrations. A total energy convergence threshold was set to 1 × 10-6 Rydberg.

Results Structures. RbSe2V3O12 and TlSe2V3O12 exhibit layered hexagonal tungsten oxide (HTO) type topologies consisting of corner shared VO6 octahedra that are capped, above and below, by SeO3 polyhedra. The layers are separated by Rb+ or Tl+ cations (see Figure 1). Each VO6 octahedron is linked to four additional VO6 octahedra and two SeO3 polyhedra through oxide bonds. In connectivity terms, the structure may be written as {3[VO6/2]- 2[SeO3/2]+}- with charge balance maintained by the Rb+ or Tl+ cation. The connectivity of the VO6 and SeO3 polyhedra result in puckered rings, in the ab-plane, with a Rb+ or Tl+ cation in the center of the ring (see Figure 2). The V-O and Se-O bond distances range from 1.641(2) to 2.190(2) Å and 1.697(2) to 1.713(5) Å for the V-O and Se-O bonds, respectively. The Rb+ and Tl+ cations are in 12-coordinate environments with bond distances ranging from 3.032(6) to 3.551(3)Å (see Figure 3 and (36) (37) (38) (39) (40) (41) (42)

Andersen, O. K. Phys. ReV. B 1975, 12, 3060. Andersen, O. K.; Jepsen, O. Phys. ReV. Lett. 1984, 53, 2571. Pickett, W. E. Comput. Phys. Rep. 1989, 9, 115. Von Barth, U.; Hedin, L. J. Phys. C: Solid State Phys. 1972, 5, 1629. Jepsen, O.; Andersen, O. K. Solid State Commum. 1971, 9, 1763. Jepsen, O.; Andersen, O. K. Phy. ReV. B 1984, 29, 5965. Lambrecht, W. R. L.; Andersen, O. K. Phys. ReV. B: Condens. Matter Mater. Phys. 1986, 34, 2439–2449. (43) Baroni, S.; Dal Corso, A.; de Gironcoli, S.; Giannozzi, P.; Cavazzoni, C.; Ballabio, G.; Scandolo, S.; Chiarotti, G.; Focher, P.; Pasquarello, A.; Laasonen, K.; Trave, A.; Car, R.; Marzari, N.; Kokalj, A.http:// www.pwscf.org/. (44) Troullier, N.; Martins, J. L. Phys. ReV. B 1991, 43, 1993. (45) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865.

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Figure 1. Ball-and-stick figures of RbSe2V3O12 (left) and TlSe2V3O12 (right) in the ac-plane. Note that the SeO3 polyhedra are pointed in opposite directions.

Table 3). Bond valence calculations46,47 resulted in values of 0.79, 0.83, 3.91-4.08, and 5.05-5.15 for Rb+, Tl+, Se4+, and V5+, respectively. Both the V5+ and Se4+ cations are in asymmetric coordination environments attributable to secondorder Jahn-Teller (SOJT) effects. With the V5+ cation, an out-of-center distortion toward an edge of the VO6 octahedron is observed, whereas for the Se4+ cations, a stereoactive lone pair is found, resulting in the asymmetric coordination environment (see Figure 4). The edge, or C2-type, distortion for V5+ results in two “short” (1.641(2)-1.670(5) Å), two “long” (2.141(5)-2.190(2) Å), and two “normal” (1.919(5)1.970(2) Å) V-O bonds. In addition, using continuous symmetry measures,48-50 we were able to calculate the magnitude of the out-of-center distortion for the V5+ cation.51 The magnitudes are 0.135 Å2 and 0.121 Å2 for RbSe2V3O12 and TlSe2V3O12, respectively. Infrared Spectroscopy. The infrared spectra for RbSe2V3O12 and TlSe2V3O12 revealed V-O and Se-O vibrations between 400 and 1000 cm-1. The stretches, in the ranges 900-700 and 500-400 cm-1, can be attributed to V-O and V-O-V vibrations, respectively, whereas the stretches in the 950-900 cm-1 range can be attributed to Se-O vibrations. The assignments are consistent with those previously reported.21,52 UV-Vis Diffuse Reflectance Spectroscopy. The UV-vis diffuse reflectance spectra indicate the absorption energy for (46) Brown, I. D.; Altermatt, D. Acta Crystallogr., Sect. B 1985, B41, 244. (47) Brese, N. E.; Okeeffe, M. Acta Crystallogr., Sect. B 1991, 47, 192. (48) Zabrodsky, H.; Peleg, S.; Avnir, D. J. Am. Chem. Soc. 1992, 114, 7843. (49) Alvarez, S.; Avnir, D.; Llunell, M.; Pinsky, M.; New, J. Chem. 2002, 26, 996. (50) Alvarez, S.; Alemany, P.; Casanova, D.; Cirera, J.; Llunell, M.; Avnir, D. Coord. Chem. ReV. 2005, 249, 1693. (51) Llunell, M.; Casanova, D.; Cirera, J.; Bofill, J. M.; Alemany, P.; Alvarez, S.; Pinsky, M.; Avnir, D. Shape Program, Version 1.1b, University of Barcelona: Barcelona, Spain, 2004. (52) Kwon, Y.-U.; Lee, K.-S.; Kim, Y.-H. Inorg. Chem. 1996, 35, 1161.

KSe2V3O12, RbSe2V3O12, CsSe2V3O12, and TlSe2V3O12 is approximately 2.4 eV. Absorption (K/S) data were calculated from the Kubelka-Munk function33 F(R) )

(1 - R)2 K ) 2R S

(1)

with R representing the reflectance, K the absorption, and S the scattering. In a K/S versus E (eV) plot, extrapolating the linear part of the rising curve to zero provides the onset of absorption at 2.43, 2.39, 2.40, and 2.44 for KSe2V3O12, RbSe2V3O12, CsSe2V3O12, and TlSe2V3O12, respectively (see the Supporting Information). Thermogravimetric Analysis. The thermal behavior of RbSe2V3O12 and TlSe2V3O12 was investigated using thermogravimetric analysis (TGA) (see the Supporting Information). The two oxides decompose near 300 °C. The TGA curve for RbSe2V3O12 reveals a single-step decomposition from 300 to 400 °C attributable to the sublimation of two moles of SeO2, exp. (calcd) 36.68% (37.73%), resulting in the formation of RbV3O8. For TlSe2V3O12, the TGA curve also shows single-step decomposition from 300 to 430 °C attributable to the sublimation of two moles of SeO2, exp. (calcd) 30.63% (31.38%), resulting in the formation of TlV3O8. Second Harmonic Generation. The new materials, RbSe2V3O12 and TlSe2V3O12, as well as KSe2V3O12 and CsSe2V3O12, crystallize in the noncentrosymmetric space group P63. Powder SHG measurements using 1064 nm radiation revealed SHG efficiencies ranging from 40-50 × R-SiO2. SHG measurements on sieved particles ranging from 20 to 120 µm indicated that all of the materials are nonphasematchable (see the Supporting Information). On the basis of the SHG efficiencies and phase-matching measurements, the four materials fall into the class C category of SHG materials, as defined by Kurtz and Perry.34 The phase-

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Figure 2. Ball-and-stick figures of RbSe2V3O12 (top) and TlSe2V3O12 (bottom) in the ab-plane. Note that the Rb+ and Tl+ cations reside in the center of the “ring” formed by the VO6 and SeO3 polyhedra.

matching measurements also allow us to estimate 〈deff〉, the average NLO susceptibility for the materials. For KSe2V3O12, RbSe2V3O12, CsSe2V3O12, and TlSe2V3O12, 〈deff〉 is approximately 1.3, 1.2, 1.2, and 1.5 pm V-1, respectively. Piezoelectric and Polarization Measurements. Piezoelectric measurements were performed on KSe2V3O12, RbSe2V3O12, CsSe2V3O12, and TlSe2V3O12. A maximum voltage of 500 V was applied to the samples. With each sample, 20 measurements were performed and an average was taken. Graphs of the piezoelectric data have been deposited as Supporting Information. We estimate d33 values of 12, 6, 6, and 10 pm V-1 for KSe2V3O12, RbSe2V3O12, CsSe2V3O12, and TlSe2V3O12, respectively. All of the materials discussed are not only noncentrosymmetric but also are polar, i.e., exhibiting a macroscopic dipole moment. Ferro-

electric measurements were performed to determine if the dipole moment was reversible, or switchable. Although “hysteresis loops” were measured, none of the materials are ferroelectric. In other words, the macroscopic polarization is not switchable. The reasons for the nonreversible polarization will be given in the discussion. The macroscopic polarity in the materials does indicate that pyroelectric behavior is possible. The total, or unclamped, pyroelectric coefficient was measured for all four materials. The pyroelectric coefficient, p ) dPs/dT, for KSe2V3O12, RbSe2V3O12, CsSe2V3O12, and TlSe2V3O12 at 45 °C is -5.5, -4.4, -6.9, and -2.6 µC m-2 K-1, respectively. The magnitudes of these coefficients are consistent with other known nonferroelectric pyroelectrics, e.g., ZnO (-9.4 µC m-2 K-1) and tourmaline (-4.0 µC m-2 K-1).2

New Polar Oxides: RbSe2V3O12 and TlSe2V3O12

Figure 3. ORTEP (50% probability level ellipsoids) diagrams for the RbO12 and TlO12 polyhedra are shown. Note that both cations have very similar coordination polyhedra, indicating the lone-pair on Tl+ is inert, rather than stereoactive.

Calculations. The electronic band structures of RbSe2V3O12 and TlSe2V3O12 were performed using pseudopotential calculations. Figure 5 shows the total density of states (TDOS) and projected density of states (PDOSs) of both compounds. An energy gap appears at the Fermi level (EF) indicative of a band gap. The gaps are approximately ∼ 2.3 and 2.2 eV for RbSe2V3O12 and TlSe2V3O12, respectively, consistent with the energy gap experimentally observed in the UV-vis diffuse reflectance spectra. Overall, both electronic structures are similar except for small differences in the Tl and Rb contributions. Two valence bands are shown below the Fermi level where narrow and broad valence bands are observed near -11 eV and from approximately -7 eV to EF, respectively. For a detailed examination of the orbital contributions through the PDOS analysis, we first considered chemical bonds, i.e., Se-O,

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V-O, and Rb-O (or Tl-O). As shown in Table 3, only the oxide ligands in the “normal” V-O bonds, O(1) and O(2), are also bonded to the Se4+ (hereafter called Oax), whereas the other oxide ligands in the “short” and “long” V-O bonds, O(3) and O(4), are loosely bonded to Rb (or Tl) (hereafter called Oeq). The narrow band is composed of the Se(4s) and Oax(2s) as further detailed in the PDOS analyses. More complexity is shown in the broadband where the lower part of the band (-7 to -5 eV) is mainly composed of Se(4p) and Oax(2p), whereas the upper part (-5 eV to EF) consists of larger contributions of V(3d) and Oeq(2sp) and relatively small contributions of Se(4sp) and Oax(2sp). In particular, the top part (approximately -1 eV to EF) is solely from Oeq(2sp), corresponding to its nonbonding character in the both electronic structures. The calculations indicate the covalency of the Tl-Oeq bond is comparable to Rb-Oeq. Consequently, noticeable differences are not observed between the two electronic band structures. For further elucidation of the electronic structures, we examined bonding characters of the bands through the crystal orbital Hamilton population (COHP) analysis.53 The COHP for the Se-O, V-O, and Tl-O bonds was carried out using TB-LMTO-ASA calculations that also provided qualitatively equivalent TDOSs and PDOSs in comparison to those calculated by the pseudopotential method. The COHP curves shown in Figure 6 indicate similar Se-O and V-O bonding patterns in both structures. Analyzing the PDOSs, we note that the V(3d)-O(2p) bonding interaction mostly appears near the top part of the valence band, and the Se(4p)-Oax(2p) antibonding interaction is dominant at the bottom part of the conduction band. Thus, the band gap energy is influenced by not only the V-O interaction but also the Se-O bond. The COHPs of the Tl-O and Se-O bonds reveal the antibonding characters near the Fermi level, corresponding to the antibonding interactions of the Se(4s)-O(2p) and Tl(6s)-O(2p) bonds, respectively. Such character is intimately related to the lone-pair distortion in the presence of mixing of the cation p orbitals.54-56 As shown in Figure 6, the Se(4p) contributes considerably below the Fermi level but the Tl(6p) contribution is negligible. Thus, stereoactive lone-pair formation is predicted only near Se4+ but not Tl+. The lone pair of Tl+ is highly symmetric attributable to the weak covalency of Tl-O and the absence of the Tl(6p) mixing into the Tl-O interaction. To examine the character of the lone pairs of the Se4+ and Tl+, we performed the electron localization function (ELF) calculations for RbSe2V3O12 and TlSe2V3O12 using the pseudopotential method. These calculations will be described in the discussion. Discussion Although two new compounds are reported, RbSe2V3O12 and TlSe2V3O12, it is relevant if we discuss these compounds in conjunction with the previously reported KSe2V3O12 and CsSe2V3O12. It is interesting to note that the four materials (53) (54) (55) (56)

Dronskowski, R.; Bloechl, P. E. J. Phys. Chem. 1993, 97, 8617. Watson, G. W.; Parker, S. C. J. Phys. Chem. B 1999, 103, 1258. Watson, G. W.; Parker, S. C.; Kresse, G. Phys. ReV. B 1999, 59, 8481. Lefebvre, I.; Szymanski, M. A.; Olivier-Fourcade, J.; Jumas, J. C. Phys. ReV. B. 1998, 58, 1896.

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Figure 4. ORTEP (50% probability level ellipsoids) diagrams for the VO6 and SeO3 polyhedra in RbSe2V3O12 (top) and TlSe2V3O12 (bottom) are shown along with V-O and Se-O bond lengths. The arrows give the approximate direction of the local dipole moment for each polyhedron.

are iso-structural. This may not be too surprising for the K, Rb, and Cs phases, but it was expected that Tl+ would exhibit a stereoactive lone pair, resulting in an asymmetric coordination environment. However, the coordination environment of Tl+ is similar to K+, Rb+, and Cs+ in the reported materials. The isostructural nature of all four compounds indicates that the lone-pair associated with Tl+ is more inert than stereoactive.14 This observation also is supported by our calculations. We performed the electron localization function (ELF) calculations57,58 for RbSe2V3O12 and TlSe2V3O12 using the pseudopotential method. For both, ELF visualization with (57) Becke, A. D.; Edgecombe, K. E. J. Chem. Phys. 1990, 92, 5397. (58) Savin, A.; Jepsen, O.; Flad, J.; Andersen, O. K.; Preuss, H.; Von Schnering, H. G. Angew. Chem. 1992, 104, 186–8. Angew. Chem., Int. Ed. 1992, 31, 187-188.

η ) 0.9 is shown in Figure 7 where a lobelike isosurface is clearly observed near the Se4+ in the both compounds. The isosurface may be considered as a stereoactive lone-pair. On the other hand, a spherelike isosurface exhibited at the Tl+ site can be considered as an inert pair, attributable to the absence of the Tl(6p) mixing into the weak Tl(6s)-O(2p) interaction below the Fermi level.14 Attributable to the highly symmetric inert pair, the polarization associated with Tl+ is negligible. Thus, the noncentrosymmetric functional properties result from the distortion and polarization associated with the V5+ and Se4+ cations. In Table 4, we report the bond valence sums, bond strain index (BSI),59 and global instability index (GII)60 for all four materials. As seen in Table 4, the bond valence sums for the cations are consistent with their reported oxidation state.

New Polar Oxides: RbSe2V3O12 and TlSe2V3O12

Figure 5. TDOS and PDOSs of the RbSe2V3O12 (top) and TlSe2V3O12 (bottom) using the norm-conserving pseudopotential calculations. The vertical line at 0 eV indicates the Fermi level. Black solid line: TDOS. Orange, Rb(5s) or Tl(6sp); blue, V(3d); green, Se(4sp); brown solid line, Oax(2sp); brown dotted line, Oeq(2sp), PDOS.

The BSI and GII values are greater than 0.05 vu (valence units) indicating structural strain in the materials. As seen in Table 4, the BSI and GII values for all four materials are substantially above zero. In fact for each material BSI > GII indicating the strains attributable to the electronic distortion, i.e., SOJT effects, are greater than the lattice induced strain.61 SOJT effects are observed in the V5+ and Se4+ cations. Attributable to SOJT effects the V5+ and Se4+ cations are in locally asymmetric coordination environments (see Figure 4). With the V5+ cations, the distortion is toward an edge, local C2 direction, of the VO6 octahedron and results in two “short”, two “long”, and two “normal” V-O bonds. We can quantify the magnitude of this distortion through continuous symmetry measurers48-50 utilizing the Shape program.51 With the four materials, the magnitude of the V5+ distortion ranges from 0.121-0.135 Å2 (see Table 4). The magnitudes of the distortions are smaller than the 0.148Å2 average reported earlier.62 For the Se4+ cation, a stereoactive lone pair is observed. Structurally the lone-pair pushes the oxide ligands (59) Preiser, C.; Losel, J.; Brown, I. D.; Kunz, M.; Skowron, A. Acta Crystallogr., Sect. B 1999, 55, 698. (60) Salinas-Sanchez, A.; Garcia-Munoz, J. L.; Rodriguez-Carvajal, J.; SaezPuche, R.; Martinez, J. L. J. Solid State Chem. 1992, 100, 210. (61) Brown, I. D. The Chemical Bond in Inorganic Chemistry: The Bond Valence Model, 1st ed.; Oxford University Press: Oxford, 2002. (62) Ok, K. M.; Halasyamani, P. S.; Casanova, D.; Llunell, M.; Alemany, P.; Alvarez, S. Chem. Mater. 2006, 18, 3176.

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Figure 6. COHP curves present Se-O (green solid line), V-O (blue), and Tl-O (red) interactions for RbSe2V3O12 (top) and TlSe2V3O12 (bottom) calculated using the TB-LMTO-ASA method. The vertical line indicates EF.

Figure 7. Visualization of the stereoactive lone-pair (purple) through electron localization function (ELF) for TlSe2V3O12 with η ) 0.9 from the pseudopotential calculations. The electron density around Tl+ is shown as a purple sphere, indicating the absence of a stereoactive lone pair.

bonded to Se4+ toward one side of the polyhedra resulting in a highly asymmetric coordination environment (see Figure 3). In addition to bond valence and distortion data, the results of our dipole moment calculations, second-harmonic generation, piezoelectric, and polarization measurements are also given in Table 4. We used a method described earlier to calculate the magnitude of the dipole moments for the SeO3

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Table 4. Bond Valence, Dipole Moment, SHG, Piezoelectric, and Polarization Data for KSe2V3O12, RbSe2V3O12, CsSe2V3O12, and TlSe2V3O12 bond valence sum V5+

A+a

bond strain index BSI

global instability index GII

5.14 5.15 5.05 5.05

0.59 0.79 1.08 0.83

0.195 0.197 0.189 0.180

0.167 0.111 0.086 0.092

Se4+

compd KSe2V3O12 RbSe2V3O12 CsSe2V3O12 TlSe2V3O12

4.08, 4.08, 3.95, 4.03,

3.89 4.00 3.96 3.91

compd

V5+ distortion Shape (Å2)

Se(1)

dipole moment Se(2)b

V

net momentc

SHGd〈deff〉 (pm V-1)

piezo. d33 (pm V-1)

pyro. const.e (µC/m2K)

max polarizationf (µC/cm2)

KSe2V3O12 RbSe2V3O12 CsSe2V3O12 TlSe2V3O12

0.134 0.135 0.130 0.121

7.30 7.44 7.06 7.18

6.64 6.99 6.54 6.56

9.27 9.35 9.04 8.87

8.43 8.22 7.99 8.83

1.3 1.2 1.2 1.5

12 6 6 10

-5.5 -4.4 -6.9 -2.6

0.32 0.22 0.27 0.14

a A+ ) K+, Rb+, Cs+, or Tl+. b Direction of moment opposite of Se(1). c Assumes magnitude of Se(2) is negative, and includes only z-component of V moment. d Using 1064 nm radiation. e Measured at 45 °C. f Measured at 25 °C and 50 Hz.

and VO6 polyhedra.63-65 With the former, the magnitude of the dipole moment ranges from 6.54 to 7.44 D, whereas for the latter, the range is from 8.87 to 9.35 D. Both are consistent with those reported earlier.21,66 It should be noted that the direction of the dipole moment associated with Se(2) is in the opposite direction of Se(1). This is shown clearly in Figures 1 and 7. Also, although the V5+ distorts locally toward an edge, only the z-component of its polarization constructively adds with the Se4+ polarization. Thus the net moment that is given in Table 4 assumes the magnitude of the dipole moment associated with Se(2) is negative, and includes only the z-component of the V5+ moment (see the Supporting Information). The net moments with the four materials are very similar, ranging from 7.99 to 8.83 D. Not surprisingly, the SHG, piezoelectric, and pyroelectric magnitudes for the four materials are also very similar (see Table 4). These magnitudes are not only similar but also rather small. The relatively weak SHG efficiency, 40-50 × R-SiO2, and piezoelectric response, ∼6-12 pm/V, may be attributable to the partial cancelation of the local polarizations. As seen in Figure 1, the polarization associated with the SeO3 polyhedra effectively cancel. In addition, only the z-component of the polarization associated with the VO6 octahedra constructively add. The lack of greater constructive addition of the local dipole moments profoundly impacts the functional properties, resulting in their relatively weak magnitudes. Although all four materials are polar, they are not ferroelectric. In other words, the macroscopic polarization is not switchable. Macroscopic polarization reversibility implies microscopic, or local, polarization reversibility. There are two types of polar polyhedra in the reported materials: the VO6 octahedra and the SeO3 polyhedra. As previously stated, the V5+ is displaced from the center of its oxide octahedron toward an edge (see Figure 3), resulting in a local dipole moment. With this octahedron, it is straightforward to visualize the local dipole moment being switched from one edge to the opposite without any substantial rearrangement or breaking of bonds. This is not the situation with the SeO3 polyhedra. For the dipole to be reversed, the entire (63) Maggard, P. A.; Nault, T. S.; Stern, C. L.; Poeppelmeier, K. R. J. Solid State Chem. 2003, 175, 27. (64) Izumi, H. K.; Kirsch, J. E.; Stern, C. L.; Poeppelmeier, K. R. Inorg. Chem. 2005, 44, 884. (65) Ok, K. M.; Halasyamani, P. S. Inorg. chem 2004, 43, 4248. (66) Ok, K. M.; Halasyamani, P. S. Inorg. Chem. 2005, 44, 3919.

Figure 8. (a) Hypothetical polarization reversal for a SeO3 polyhedron. (b) Hypothetical ball-and-stick structure of a Se4+-V5+-oxide fragment with the polarization on SeO3 reversed. Note that if SeO3 polarization reversal occurs, the lone pair is very close to V-O bonds.

SeO3 polyhedron must be inverted (see Figure 8a). If this did occur, the lone pair would be pointing directly toward the vanadium oxide layer. A hypothetical structural diagram of this situation is shown in Figure 8b. If the polarization associated with SeO3 were to be reversed, or switched, the lone pair would be situated very close to V-O bonds. Clearly, this situation is structurally and energetically unfavorable. Thus it is not surprising that ferroelectric behavior with fourth and fifth period cations, Se4+, Te4+, I5+, etc., where the polarization is “flipped” has never been observed. Acknowledgment. H.Y.C., S.-H.K., and P.S.H. thank the Welch Foundation, NSF (DMR-0652150), and ACS PRF (47345-AC10) for support. K.M.O. thanks KOSEF for financial support, Nuclear R&D 3N researcher program (2008-03981). Supporting Information Available: X-ray crystallographic file in CIF format; powder X-ray diffraction data, thermogravimetric analyses, infrared data, powder SHG results, and piezoelectric and polarization results (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM9002614