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Quasiharmonic model for ferroelectric transition in 18O-isotope exchanged SrTiO3

By extending the quasiharmonic approximation to a quantum particle within a local potential of the double Morse type, a model describing the ferroelectric phase transition in SrTiO3 induced by oxygen isotope replacement is developed. The theory uses the variational principle method at finite temperature with emphasis on the quantum effect manifested in zero-point vibration. The ferroelectric–paraelectric transition in SrTi(16O1−x18Ox)3 is analyzed and the x–Tc phase diagram is compared with the experimental one in order to confirm the qualitative validity of the model. The quantum mechanical effect and the evolution of the ferroelectricity in SrTi(16O1−x18Ox)3 are also demonstrated theoretically and the soft mode of SrTi(16O1−x18Ox)3 shows a perfect softening at the ferroelectric phase transition for x≥xc≈0.32. Finally, the study shows the importance of the mass of component ions and that isotope introduction has several effects even if the mass effect is predominant.