Semi-microscopic Vibronic Theory of the Properties of Quantum Paraelectrics and Ferroelectrics of SrTiO3 Type

Abstract

In this work the semi-microscopic vibronic theory of quantum paraelectrics and ferroelectrics of oxide perovskites (SrTi16O3, CaTiO3, SrTi(18Ox16O1-x)3, KTaO3 etc.) has been developed further. It is shown that the electron-lattice couplings of the filled A1g and empty F1u electronic states of the BO6 cluster in ABO3 perovskite-oxides with the potential ferroelectric soft mode F1u (Γ15) and the zone-boundary phonon mode F2u (Γ25) lead to the antiferrodistortive phase transition at Ta = 105 K in SrTiO3. The vibronic interaction causes the softening of the ferroelectric F1u mode in the antiferrodistortive phase with the D4h symmetry. The electron-lattice coupling-hybridizations of the 1A1g and F1u electronic states by the F1u and R-corner F2u modes lead to the splitting of the F1u mode frequencies ωf (in the cubic phase) into the frequencies ωf,c and ωf,a, respectively, in the phase D4h. The Helmholtz free energies of such quantum paraelectrics and ferroelectrics are obtained.