Understanding the role of flux, pressure and temperature on polymorphism in ThB 2 O 5

A novel polymorph of ThB 2 O 5 , denoted as β-ThB 2 O 5 , was synthesised under high-temperature high-pressure (HT/HP) conditions. Via single crystal X-ray diffraction measurements, β-ThB 2 O 5 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 ThB 2 O 5 , denoted α, synthesised herein using a known borax, B 2 O 3 –Na 2 B 4 O 7 , 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-ThB 2 O 5 has smaller molar volume by approximately 12%. Exposing a mixture of the α and β polymorphs to HT/HP conditions ex situ further demonstrated the preferred higher-pressure phase being β, with no α phase material being observed via Rietveld refinements against laboratory X-ray powder diffraction (PXRD) measurements. In situ heating PXRD measurements on α-ThB 2 O 5 from RT to 1030 °C indicated that α-ThB 2 O 5 transforms to the β variant at approximately 900 °C via a 1st order mechanism. β-ThB 2 O 5 was found to exist only over a narrow temperature range, decomposing above 1050 °C. Ab initio 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 B 2 O 3 –Na 2 B 4 O 7 to H 3 BO 3 –Li 2 CO 3 flux using consistent high temperature solid state conditions for the synthesis of the α variant, β-ThB 2 O 5 could be generated. Comparison of their single crystal measurements showed this was identical to that obtained from HT/HP conditions.