Understanding the role of flux, pressure and temperature on polymorphism in ThBO.

A novel polymorph of ThBO, denoted as β-ThBO, was synthesised under high-temperature high-pressure (HT/HP) conditions. single crystal X-ray diffraction measurements, β-ThBO was found to form a three-dimensional (3D) framework structure where thorium atoms are ten-fold oxygen coordinated forming tetra-capped trigonal prisms. The only other known polymorph of ThBO, denoted α, synthesised herein using a known borax, BO-NaBO, high temperature solid method, was found to transform to the β polymorph when exposed to conditions of 4 GPa and ∼900 °C. Compared to the α polymorph, β-ThBO has smaller molar volume by approximately 12%. Exposing a mixture of the α and β polymorphs to HT/HP conditions further demonstrated the preferred higher-pressure phase being β, with no α phase material being observed Rietveld refinements against laboratory X-ray powder diffraction (PXRD) measurements. heating PXRD measurements on α-ThBO from RT to 1030 °C indicated that α-ThBO transforms to the β variant at approximately 900 °C a 1st order mechanism. β-ThBO was found to exist only over a narrow temperature range, decomposing above 1050 °C. calculations using density functional theory (DFT) with the Hubbard U parameter indicated, consistent with experimental observations, that β is both the preferred phase at higher temperatures and high pressures. Interestingly, it was found by switching from BO-NaBO to HBO-LiCO flux using consistent high temperature solid state conditions for the synthesis of the α variant, β-ThBO could be generated. Comparison of their single crystal measurements showed this was identical to that obtained from HT/HP conditions.