Thermal expansion and phase transition behaviour of thorium orthosilicate

ThSiO4 adopts the common zircon structure type, space group I41/amd, occurring naturally as the mineral thorite. On heating to ~ 1225 C, however, thorite collapses to a denser monazite-structured phase, that also occurs naturally as the mineral huttonite. This behaviour is unusual amongst the zircon-structured actinide silicates, the only other example being PaSiO4. Similarly, on compression, thorite transforms to huttonite with a negative Clapeyron slope, dP/dT. There is considerable experimental and computational disagreement regarding the thermodynamics of the transition, both in P and T, with a factor of four difference in the published transition enthalpies and transition pressure. Our proposals seek to rectify this by carrying out the first in situ studies of the transition both at high T (at room pressure) on HRPD and as a function of P and T on PEARL.