Electron Hybridization and Anharmonic Thermal Vibration Effect on Structure Transition of Srtio3 at High-Pressure and Low-Temperature

We execute electron density analysis of SrTiO3 at low temperatures up 80 K and high pressures up to 11.88 GPa using X-ray single-crystal diffraction and ab initio quantum chemical molecular orbital (MO) calculation. By changing pressures, the cubic SrTiO3 with perovskite structure goes through a antiferroelastic distortion to tetragonal symmetry above the critical pressure P-c=7 GPa with c/a & $2lt;1 and decreasing with increasing pressure. On the other hand, by changing temperatures, the tetragonal distortion occurs below the critical temperature T-c=105 K with c/a & $ 2gt;1 and increasing with lowering temperature.Difference Fourier (D-F) synthesis experimentally proves the residual electron densities Delta rho(xyz) are associated with two different effects: electron hybridization bonding electron and anharmonic thermal vibration atoms. The d-p-pi hybridization between Ti(3d) and O(2p) orbitals is confirmed in the residual electron density, which is deformed from the ideal spherical density conducted by the atomic scattering factor fi using Hartree-Fock (HF) approximation. MO calculation also reveals the electron hybridization. Anharmonic thermal vibration of atoms yields a large effect to the structure transition. Mulliken charges analysis of MO calculation indicates much smaller charges than their formal ionic charges. Their ionicity increases from cubic to tetragonal above P-c and below T-c.