The phase transitions of AB2 oxide compound under high pressure: AB initio constant pressure study

In this study, we investigated the phase transition of the tetragonal anatase (I41/amd) structure of the TiO2 crystal from the AB2 type oxide structures under high pressure using the ab initio technique and the Generalized Gradient Approach (GGA). The siesta simulation program was used for the simulation in this study. During the simulation process, the structure was first allowed to relax under 0 GPa pressure, and then the pressure was increased in increments of 10 GPa. It was observed that the symmetry of the structure changed for the first time at pressures of 40 and 50 GPa. These structures, C2/c and C2/m, acted as intermediate phases. When a pressure of 60 GPa was applied, significant changes were observed in the lattice parameters and volume of the simulation cell, with the volume decreasing by around 13%. When the structure was analyzed using the KPLOT program, a first-order phase transition was observed, and the structure exhibited orthorhombic space group (Pnma) symmetry. The anatase unit cell parameters were a = 3.7806Å, b = 3.7806Å, c = 9.4997Å, and the Pnma unit cell parameters were a = 5.0028Å, b = 2.9265Å, c = 5.8755Å. The phase transition was calculated from enthalpy using the Birch-Murnaghan equation of state. When the anatase and Pnma pressure-enthalpy graphs were drawn, the intersection of these two graphs indicated the phase transition point. This value was determined to be around 13.5 GPa. The results were found to be in agreement with previous experimental and theoretical research.