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

Abstract

Bulk single crystals of α-BaTeMo₂O₉ with dimensions up to 51 × 30 × 20 mm³ 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 °C to 500 °C were measured to be α_a = 9.10 × 10⁻⁶ K⁻¹, α_b = 19.58 × 10⁻⁶ K⁻¹ and α_c = 11.94 × 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 °C. The thermal conductivity along the a-axis is larger than those along other directions (κ_a>κ_b> κ_c). The transmission spectra and refractive indices were also measured. The α-BaTeMo₂O₉ crystal exhibits a wide transmission window ranging from 380 nm to 5530 nm. The large birefringence of α-BaTeMo₂O₉ (Δn = 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.