New High-Pressure Phases of Titanite-Type CaTiSiO₅ Predicted from First-Principles

Titanite-type CaTiSiO5 is one of the common accessory minerals in igneous, metamorphic, sedimentary, and ultra-high-pressure rocks. Determination of the high-pressure crystal-chemical behavior of silicates is crucial to comprehending the crystalline materials in deep-earth geospheres. The present study implements a density functional theory calculation assisted structure search method using evolutionary algorithms and reveals two new structural phase transitions: displacive-type monoclinic-II (C2/c) → triclinic (P1¯) and reconstructive-type triclinic (P1¯) → orthorhombic (Pnma) at 10 and 18 GPa, respectively. Ab initio molecular dynamics study confirms that the phase transformation sequence remains unaltered at 300 K. Moreover, the orthorhombic structure is quenchable at 0 GPa. The topology of the high-pressure orthorhombic structure can be identified as a combination of corner-shared distorted zigzag SiO6 octahedra along the [100] direction and an edge-shared TiO6 prism (with SiO6) along the [001] direction. This novel topological arrangement has not been observed in any of the high-pressure structural prototypes of silicates reported in the literature.