Theory of ferroelectric phase transition in SrTiO3 induced by isotope replacement

A theory to describe the dielectric anomalies and the ferroelectric phase transition induced by oxygen isotope replacement in SrTiO3 is developed. The proposed model gives consistent explanation between apparently contradictory experimental results on macroscopic dielectric measurements and microscopic lattice dynamical measurements by neutron-scattering studies. The essential feature is described by a three-state quantum order-disorder system characterizing the degenerated excited states in addition to the ground state of TiO6 cluster. The effect of isotope replacement is taken into account through the tunneling frequency between the excited states. The dielectric properties are analyzed by the mean-field approximation (MFA), which gives qualitative agreements with the experimental results throughout full range of the isotope concentration. The phase diagram in the temperature-tunneling frequency coordinate is studied by a quantum Monte Carlo method to confirm the qualitative validity of the MFA analysis.