Synthesis, Crystal Growth And Characterization Of Novel Optical Material BaTeW₂O₉

It is an efficient way to expand the range of lasers'wavelengths by employing the non-linear effect of optical crystals to shift the frequency of the coherent light. Therefore, it is always of great interest to search for new optical crystal materials with better non-linear efficiency and better optical behavior, especially in the IR region. Recently, Halasyamani et al. have synthesized a large group of new compounds by introducing d⁰ transition metals together with VI group elements with lone pair electrons into crystal lattice, within which many exhibit large bandgap and strong SHG efficiency, thus indicates their potential application in non-linear optics. Among them, bulk single crystals of ?/?-BaTeMo₂O₉?BTM?, Cs₂TeMo₃O₁₂, MgTeMoO₆ ?MTM? and Cs₂TeMo₃O₁₂ ?CTW? have been successfully grown by TSSG flux method in our group since 2008. Characterization of these crystals implied that all of them fit the requirement of type-I phase matching, and at the same time their optical and thermal properties are attractive for second-order nonlinear optical applications. Raman laser experiments with large frequency shift and high efficiency have been successfully achieved by employing single crystals of BTM, indicating its possible application as Raman laser crystals.BaTeW₂O₉ ?BTW, which will be referred as Monoclinic-BTW in this paper to be indentified from its orthorhombic polymorph?, is another kind of newly synthesized compound with high powder SHG efficiency. Monoclinic-BTW exhibits very similar crystal structure to that of P-BTM, with Mo⁶⁺replaced by W⁶⁺. It is possible that monoclinic-BTW could have larger bandgap comparing with BTM, therefore exhibits better resistence to laser damage when employed in higher power optical applications. The topic of this paper will be focused on BTW material and could be described as following:?1? For the first time that another polymorph of BTW, orthorhombic-BTW, was obtained from flux. Crystal structure was determined for the first time by single crystal X-ray diffraction measurements. Orthorhombic-BTW has the same chemical composition with monoclinic-BTW, but belongs to a centrosymmetric crystal structure.?2? Polycrystalline powder of orthorhombic-BTW with high purity was synthesized by solid state reaction.?3? Compositions of products by spontaneous crystallization from different kinds of flux ?TeO₂:WO₃=x:y? were systemically explored:Crystalline of orthorhombic-BTW could be obtained from either pure TeO₂ or the mixture of TeO₂ and WO₃ with x:y=3:1 or 4:1.Monoclinic-BTW could only be obtained from mixture of TeO₂:WO₃=3:1 by decreasing temperature rapidly. Orthorhombic-BTW will be grown if temperature was decreased slower.?4? Crystal of orthorhombic-BTW was succesffuly grown by TSSG growth with TeO₂ as flux. A b- oriented seed was used in the growth and crystal with dimensions of 24 mm× 22 mm × 15 mm was obtained. The crystal exhibits regular shape and quality inside it is high.?5? Characterization of the optical and thermal properties of the orthorhombic-BTW single crystal was investigated:DSC/TGA measurements indicated that orthorhombic-BTW melt incongruently, thus single crystals could only be grown from flux. Thermal conductivity of orthorhombic-BTW is comparable to that of BTM, however, conductivity along b-aixs is much larger than along the other two axes, which is unforvable for high power application.Transmittance spectra indicated that single crystal of orthorhombic-BTW exhibits high transmittance from 363 nm to 4.5 ?m, corresponding with a bandgap of 3.42 eV. The curves of refractive indexes referred with wavelength were obtained by fitting measured refractive indexes at 13 different wavelengths. Refractive indexes polarized along b- axis are much bigger than the other two kinds of polarizations. Therefore orthorhombic-BTW could find its application as prism material.?6? Theoritical simulation was performed on orthorhombic-BTW using CASTEP. Geomistric optimization calculations were performed using crystal structure files of both monoclinic-BTW and orthorhombic-BTW. Calculated structures of both materials were close to original ones. Electronic properties were calculated using calculated crystal structure of both materials and and contributions of different atoms to the electronic distribution around Fermi levels were analized.Optical properties were calculated using optimized crystal structure of orthorhombic-BTW with higher k-points setting. Simulated results of transmittance spectra and refractive index exhibit similar behavior with experimental results. Therefore, it provides the possibility to predict the properties of a material from its crystal structure.