Spinel and post-spinel phase assemblages in Zn2TiO4: an experimental and theoretical study

Zn2TiO4 spinel (Zn2TiO4-Sp) was synthesized by a solid-state reaction method (1573 K, room P and 72 h) and quasi-hydrostatically compressed to similar to 24 GPa using a DAC coupled with a synchrotron X-ray radiation (ambient T). We found that the Zn2TiO4-Sp was stable up to similar to 21 GPa and transformed to another phase at higher P. With some theoretical simulations, we revealed that this high-P phase adopted the CaTi2O4-type structure (Zn2TiO4-CT). Additionally, the isothermal bulk modulus (K (T)) of the Zn2TiO4-Sp was experimentally obtained as 156.0(44) GPa and theoretically obtained as 159.1(4) GPa, with its first pressure derivative as 3.8(6) and 4.37(4), respectively. The volumetric and axial isothermal bulk moduli of the Zn2TiO4-CT were theoretically obtained as K (T) = 150(2) GPa ( = 5.4(2); for the volume), K (T-a) = 173(2) GPa ( = 3.9(1); for the a-axis), K (T-b) = 74(2) GPa ( = 7.0(2); for the b-axis), and K (T-c) = 365(8) GPa ( = 1.5(4); for the c-axis), indicating a strong elastic anisotropy. The Zn2TiO4-CT was found as similar to 10.0 % denser than the Zn2TiO4-Sp at ambient conditions. The spinel and post-spinel phase assemblages for the Zn2TiO4 composition at high T have been deduced as Zn2TiO4-Sp, ZnTiO3-ilmenite + ZnO-wurtzite, ZnTiO3-ilmenite + ZnO-rock salt, ZnTiO3-perovskite + ZnO-rock salt, and Zn2TiO4-CT as P increases, which presumably implies a potential stability field for a CT-type Mg2SiO4 at very high P.