Understanding the role of flux, pressure and temperature on polymorphism in ThB2O5

A novel polymorph of ThB2O5, denoted as beta-ThB2O5, was synthesised under high-temperature high-pressure (HT/HP) conditions. Via single crystal X-ray diffraction measurements, beta-ThB2O5 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 ThB2O5, denoted alpha, synthesised herein using a known borax, B2O3-Na2B4O7, high temperature solid method, was found to transform to the beta polymorph when exposed to conditions of 4 GPa and similar to 900 degrees C. Compared to the alpha polymorph, beta-ThB2O5 has smaller molar volume by approximately 12%. Exposing a mixture of the alpha and beta polymorphs to HT/HP conditions ex situ further demonstrated the preferred higher-pressure phase being beta, with no alpha phase material being observed via Rietveld refinements against laboratory X-ray powder diffraction (PXRD) measurements. In situ heating PXRD measurements on alpha-ThB2O5 from RT to 1030 degrees C indicated that alpha-ThB2O5 transforms to the beta variant at approximately 900 degrees C via a 1st order mechanism. beta-ThB2O5 was found to exist only over a narrow temperature range, decomposing above 1050 degrees C. Ab initio calculations using density functional theory (DFT) with the Hubbard U parameter indicated, consistent with experimental observations, that beta is both the preferred phase at higher temperatures and high pressures. Interestingly, it was found by switching from B2O3-Na2B4O7 to H3BO3-Li2CO3 flux using consistent high temperature solid state conditions for the synthesis of the alpha variant, beta-ThB2O5 could be generated. Comparison of their single crystal measurements showed this was identical to that obtained from HT/HP conditions.