Bulk crystal growth and characterization of a new polar polymorph of BaTeMo2O9: α-BaTeMo2O9

Bulk single crystals of alpha-BaTeMo2O9 with dimensions up to 51 x 30 x 20 mm(3) were grown successfully by a top-seeded solution growth (TSSG) method using TeO2-MoO3 mixture as a flux. High-resolution X-ray diffraction measurement on the (400)-faced plate indicates that the full-width at half-maximum (FWHM) of the rocking curve is 16.55 ''. The as-grown crystal exhibits {010}, {002}, {110}, {111} and {201} facets, which are in good accordance with the predicted growth morphology based upon the Bravais-Friedel and Donnay-Harker (BFDH) method. In addition, thermal properties including thermal expansion, specific heat, thermal diffusivity and thermal conductivity were investigated as a function of temperature. The average linear thermal expansion coefficients along the a-, b-, and c-axis from 30 degrees C to 500 degrees C were measured to be alpha(a) = 9.10 x 10(-6) 6 K-1, alpha(b) = 19.58 x 10(-6) K-1 and alpha(c) = 11.94 x 10(-6) K-1, respectively. The specific heat was measured to be 0.433-0.566 J (g K)(-1) over the temperature range of 30-540 degrees C. The thermal conductivity along the alpha-axis is larger than those along other directions (k(a)> k(b)> k(c)). The transmission spectra and refractive indices were also measured. The alpha-BaTeMo2O9 crystal exhibits a wide transmission window ranging from 380 nm to 5530 nm. The large birefringence of alpha-BaTeMo2O9 (Delta n = 0.305 for light at 404.7 nm) indicates that it is not only Type I and II phase-matchable, but also a very promising candidate for optical devices.