Synthesis, structure, and thermodynamic analysis of the polymorphs of thorutite, ThTi2O6

Polycrystalline α-ThTi₂O₆ was synthesised by a solid-state route for the first time and characterised by neutron diffraction, Raman spectroscopy, and drop solution calorimetry. Alongside the brannerite-structured β-ThTi₂O₆, the structure of α-ThTi₂O₆ was refined from high resolution time-of-flight neutron diffraction data, with both structures showing a small degree of Ti mixed occupancy on the 2a Th-site, 0.024 (2) f. u. and 0.018 (2) f. u. for the α and β polymorphs, respectively. A detailed description of the α-ThTi₂O₆ structure is presented, as well as discussion of the narrow compositional field of stability that it inhabits, relative to the brannerite, aeschynite, and euxenite structures seen more commonly for titanate AB₂O₆ materials. High temperature oxide melt drop solution calorimetric measurements show for the first time that, whilst β-ThTi₂O₆ is only entropically stabilised (ΔH_f,ox = 11.5 ± 5.1 kJ mol⁻¹) at elevated temperatures, α-ThTi₂O₆ is enthalpically stabilised (ΔH_f,ox = −22.3 ± 3.6 kJ mol⁻¹).