Formation enthalpy of ThSiO4 and enthalpy of the thorite → huttonite phase transition

Inelastic Neutron Scattering, Lattice Dynamics, Computer Simulation and Thermodynamic Properties

“…Our Gibbs free energy calculations show that the zircon phase transforms (Figure 3.9) to the scheelite and huttonite phases at high pressure for HfSiO 4 and ThSiO 4 , respectively, which is in good agreement with the experimental observations [60,[63][64][65]. It is likely that the phase transition pressure of ThSiO 4 in experiments is overestimated as these were performed with only increasing pressure and some hysteresis is expected [63][64][65]. Our calculated transition pressure agrees with that estimated from an analysis of the measured enthalpies [63][64][65].…”

“…This result in larger vibrational entropy in the huttonite phase, which favors [79] this phase at high temperatures. Our Gibbs free energy calculations show that the zircon phase transforms (Figure 3.9) to the scheelite and huttonite phases at high pressure for HfSiO 4 and ThSiO 4 , respectively, which is in good agreement with the experimental observations [60,[63][64][65]. It is likely that the phase transition pressure of ThSiO 4 in experiments is overestimated as these were performed with only increasing pressure and some hysteresis is expected [63][64][65].…”

“…It is likely that the phase transition pressure of ThSiO 4 in experiments is overestimated as these were performed with only increasing pressure and some hysteresis is expected [63][64][65]. Our calculated transition pressure agrees with that estimated from an analysis of the measured enthalpies [63][64][65]. For ThSiO 4 , at further high pressure, the scheelite phase is found (Figure 3.9) to be stable.…”

“…However, thorium silicate, ThSiO 4 has a zircon structure (I4 1 /amd) at low temperature [25], whereas the high temperature form of ThSiO 4 has huttonite structure (P2 1 /n). Zircon to huttonite transition is unusual [63][64][65] since a less dense phase usually occurs at high temperature. High-pressure studies have not been reported for USiO 4 .…”

The low-temperature heat capacity and standard entropy of synthetic huttonite ThSiO4

“…The thermodynamic properties of thorite and huttonite have been reported by Mazeina et al [3] recently. These authors focussed their studies on the determination of the enthalpies of formation of the two polymorphs.…”