Phase Decomposition of Rare Earth Apatite and Formation of Perovskite Rare Earth Silicate at High Pressure and High Temperature Conditions

The crystal structure of rare earth apatite was studied at high pressure and room temperature conditions. A reversible subtle phase transition was found at pressure. The high-pressure phase has also a hexagonal unit cell but with a lower symmetry reduced from \textit{P6}$_{3}/m$ to \textit{P6}$_{3}$. Due to the symmetry change, the high-pressure phase has an unusual lower bulk modulus as compared with the corresponding ambient structure. Laser heating of La-Si-O apatite at high pressure conditions revealed that apatite structure is not stable at temperatures higher than 1500 K and decomposes into two different phases. The decomposed phase is a perovskite-type structure. Alkaline earth silicate minerals can easily form the 6-coordianted high-pressure phase in the deep earth environment. However, a rare earth silicate with the perovskite structure has not been previously reported. The experimental results also suggest that the La$_{0.67}$SiO$_{3}$ perovskite structure is at least partially quenchable.