Theoretical study of quantum effect on ferroelectric transition in 18O-isotope exchanged SrTiO3

A theoretical picture for soft mode behavior and the phase transition in STO18-x is investigated throughout the concentration range x from 0 to 1. The approach was realized through extending the quasi-harmonic approximation to a quantum particle within a local potential of the double-Morse type, and combining the predominant mass effect with the cell volume effect and ferroelectric distortion. It is found that the quantum effects govern the low temperature regime and, the displacive-type phase transition is recognized with perfect softening at T_c for x ≥ x_c =0.318.