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Chem. Mater. 2001, 13, 1910-1915
Synthesis and Characterization of Te2SeO7: A Powder Second-Harmonic-Generating Study of TeO2, Te2SeO7, Te2O5, and TeSeO4 Yetta Porter, Kang Min Ok, N. S. P. Bhuvanesh, and P. Shiv Halasyamani* Department of Chemistry, University of Houston, 4800 Calhoun Boulevard, Houston, Texas 77204-5641
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Received December 21, 2000. Revised Manuscript Received March 20, 2001
The synthesis and characterization of a noncentrosymmetric tellurium selenate, Te2SeO7, is reported. In addition, the powder second-harmonic-generating (SHG) properties of TeO2, Te2SeO7, Te2O5, and TeSeO4 have been measured, using 1064 nm radiation. Through the powder SHG experiments, we are able to determine that TeO2 is not phase-matchable, whereas Te2SeO7, Te2O5, and TeSeO4 are phase-matchable. Also, TeO2, Te2SeO7, Te2O5, and TeSeO4 have SHG efficiencies of 5, 200, 400, and 400 times SiO2, respectively. The relative SHG efficiencies may be understood by examining the structure of each material. Through the powder SHG measurements, we estimate the average nonlinear optical bond susceptibility, 〈d2ωijk〉, for each material.
Introduction One of the continuing challenges in materials chemistry concerns the elucidation of structure-property relationships. This is especially true with second-order nonlinear optical (NLO), i.e., second-harmonic-generating (SHG), materials.1-5 Viable SHG materials must possess the following attributes: transparency in the relevant wavelengths, ability to withstand laser irradiation, and chemical stability. Most importantly, the material in question must be crystallographically noncentrosymmetric (NCS). Mathematically, it has been known for some time that only a NCS arrangement of atoms may produce a second-order NLO response.6 Thus, to understand SHG from a materials standpoint, it is important to understand the chemical and structural implications of NCS. We recently reviewed the known NCS oxides3 and determined that cations susceptible to a second-order Jahn-Teller (SOJT) distortion were found in nearly half, ∼45%, of these materials. A SOJT distortion7-13 is concerned with structural changes attributable to a nondegenerate ground state interacting with a low-lying excited state. The distortion occurs when the energy gap between the highest occupied (1) Keszler, D. A. Curr. Opin. Solid State Mater. Sci. 1999, 4, 155. (2) Becker, P. Adv. Mater. 1998, 10, 979. (3) Halasyamani, P. S.; Poeppelmeier, K. R. Chem. Mater. 1998, 10, 2753. (4) Marder, S. R.; Sohn, J. E.; Stucky, G. D. Materials for NonLinear Optics: Chemical Perspectives; American Chemical Society: Washington, DC, 1991. (5) Chen, C.; Liu, G. Annu. Rev. Mater. Sci. 1986, 16, 203. (6) Nye, J. F. Physical Properties of Crystals; Oxford University Press: Oxford, U.K., 1957. (7) Opik, U.; Pryce, M. H. L. Proc. R. Soc. London 1957, A238, 425. (8) Bader, R. F. W. Mol. Phys. 1960, 3, 137. (9) Bader, R. F. W. Can. J. Chem. 1962, 40, 1164. (10) Pearson, R. G. J. Am. Chem. Soc. 1969, 91, 4947. (11) Pearson, R. G. J. Mol. Struct. (THEOCHEM) 1983, 103, 25. (12) Wheeler, R. A.; Whangbo, M.-H.; Hughbanks, T.; Hoffmann, R.; Burdett, J. K.; Albright, T. A. J. Am. Chem. Soc. 1986, 108, 2222. (13) Kunz, M.; Brown, I. D. J. Solid State Chem. 1995, 115, 395.
(HOMO) and lowest unoccupied (LUMO) molecular orbitals is small and there is a symmetry-allowed distortion permitting the mixing of the HOMO and LUMO states. With oxides, two families of metals can undergo SOJT distortions: octahedrally coordinated d0 transition metals and cations with nonbonded electron pairs. With the former, the mixing of HOMO and LUMO is always symmetry-allowed, with the energy between the orbitals correlated with the size and charge of the cation.7-13 With the latter, the stereoactive lone pair is attributable to the mixing between the s and p orbitals of the metal and oxygen atoms, respectively. An example using Sb3+ is illustrative. Four-coordinate antimony might be expected to have tetrahedral symmetry (Td). However, in this geometry the s2 electron pair would occupy a strongly antibonding a1* orbital (HOMO). As such, a distortion occurs to square-pyramidal geometry (C4v) that lowers the energy of the HOMO s orbital by mixing it with the LUMO p orbital, i.e., s-p mixing. Thus, the HOMO is stabilized, and the lone pair becomes stereochemically active, resulting in the asymmetric coordination environment. We suggest that one manner in which to increase the incidence of NCS is to synthesize oxides that contain cations susceptible to SOJT distortions. We also suggest that the SOJT distortions observed in these materials will not only alter the symmetry from centrosymmetric to NCS but also will occur in a cooperative manner. This cooperative distortion will polarize the M-O bonds, resulting in a large SHG response. Specifically, we have been investigating the synthesis of oxides that contain cations with nonbonded electron pairs.14-16 Recently, we reported the synthesis and NLO behavior of TeSeO4 that (14) Halasyamani, P. S.; O’Hare, D. Chem. Mater. 1997, 10, 6646. (15) Halasyamani, P. S.; O’Hare, D. Inorg. Chem. 1997, 36, 6409. (16) Porter, Y.; Bhuvanesh, N. S. P.; Halasyamani, P. S. Inorg. Chem. 2001, 40, 1172.
10.1021/cm001414u CCC: $20.00 © 2001 American Chemical Society Published on Web 05/04/2001
Synthesis and Characterization of Te2SeO7
has a SHG efficiency of 400 times quartz.16 In this paper we report the synthesis, characterization, and SHG behavior of Te2SeO7. In addition, we compare the powder SHG behavior of TeO2, Te2SeO7, Te2O5, and TeSeO4 and discuss their structure-property relationships, as well as approximate their NLO susceptibilities. Experimental Section Synthesis. Caution! Use appropriate safety measures to avoid toxic SeO2 and TeO2 dust contamination. TeO2 (Aldrich, 99%) was used as received. Te2SeO7 was synthesized by combining TeO2 (0.500 g, 3.13 × 10-3 mol) with H2SeO4 (Aldrich; a 40 wt % solution, 2.20 mL, 8.54 × 10-3 mol) in a large test tube. The mixture was stirred and heated in an oil bath to 160 °C for 4 h. The resultant white powder was washed with water, to remove excess H2SeO4, and dried. Te2O5 was synthesized by combining TeO2 (Aldrich; 99%) and TeO3‚H2O. TeO3‚H2O was obtained by heating Te(OH)6 (Aldrich) at 200 °C overnight. The resultant yellow powder is amorphous to X-rays and is assumed to be TeO3‚H2O based on H2O loss during the dehydration. If Te(OH)6 is fully dehydrated to TeO3 and subsequently reacted with TeO2, a mixture of Te2O5 and Te4O9 is formed. TeO3‚H2O (0.576 g, 2.98 × 10-3 mol) was combined with TeO2 (0.476 g, 2.98 × 10-3 mol), pressed into a pellet, and placed in a quartz tube that was subsequently evacuated and sealed. The tube was held at 400 °C for 24 h and cooled at a rate of 2 °C min-1 to room temperature. The resultant pale yellow powder was found to be a mixture of Te2O5 and TeO2 by powder XRD. TeO2 was removed by washing the powder with 1 M HCl. The XRD pattern of the purified powder was shown to be in excellent agreement with that reported for Te2O5. TeSeO4 was synthesized as previously reported.16 Powder Diffraction. Powder XRD data for TeO2, Te2SeO7, Te2O5, and TeSeO4 were recorded on a SCINTAG XDS2000 automated diffractometer at room temperature (Cu KR radiation, θ-θ mode, flat-plate geometry). Infrared Measurements. Infrared spectra for Te2SeO7 were recorded on a Matteson FTIR 5000 spectrometer in the 400-4000 cm-1 range, with the sample pressed between two KBr pellets. IR (cm-1): ν(Se-O) 1162, 1131, 919, 876, 840, 800; ν(Te-O) 775, 649; ν(Te-O-Te) 510, 435. The assignments are in good agreement with those reported earlier.17 Second-Order NLO Measurements. Powder SHG measurements were performed on a modified Kurtz-NLO18 system using 1064 nm light. A Continuum Minilite II laser, operating at 15 Hz, was used for all measurements. Because the SHG efficiency of powders has been shown to depend strongly on the particle size,19 polycrystalline TeO2, Te2SeO7, Te2O5, and TeSeO4 were ground separately and sieved (Newark Wire Cloth Co.) into distinct particle size ranges,
