Equations of state of Co2TiO4-Sp, Co2TiO4-CM, and Co2TiO4-CT, and their phase transitions: an experimental and theoretical study

Co2TiO4 spinel (Co2TiO4-Sp) was synthesized at 1573 K and room P by heating in an argon atmosphere for 72 h, and quasi-hydrostatically compressed to ~ 24 GPa using a diamond-anvil cell in conjunction with a synchrotron X-ray radiation (ambient T). We found that the Co2TiO4-Sp was stable up to ~ 21 GPa and transformed to a new phase at higher P. With some theoretical simulations, we revealed that this new phase adopted the CaMn2O4-type structure (Co2TiO4-CM), which might further transform to the CaTi2O4-type structure (Co2TiO4-CT) at ~ 35 GPa. The isothermal bulk modulus (KT) was experimentally obtained as 175.5(36) GPa for the Co2TiO4-Sp and 161(7) GPa for the Co2TiO4-CM, with its first pressure derivative \(K_{{\text{T}}}^{'}\) as 2.8(5) and 7.3(8), respectively. Furthermore, the KT was theoretically constrained (the GGA method) as 138(3) GPa for the Co2TiO4-CM and 196.8(14) GPa for the Co2TiO4-CT, with the \(K_{{\text{T}}}^{'}\) as 7.6(3) and 5.0(1), respectively. Consequently, the Co2TiO4-CM is ~ 12.3% denser than the Co2TiO4-Sp at ~ 21 GPa, whereas the Co2TiO4-CT is just ~ 0.8% denser than the Co2TiO4-CM at ~ 35 GPa. The spinel and post-spinel phase assemblages for the Co2TiO4 composition at some high T have been tentatively deduced as Co2TiO4-Sp, CoO-B1 (NaCl-type structure) + CoTiO3-Ilm (ilmenite-type structure), 2CoO-B1 + TiO2-α-PbO2 (α-PbO2-type structure), Co2TiO4-CM and Co2TiO4-CT, as P increases.