Theory of isotope effect in displacive ferroelectrics

The isotope exchange effect on the stability of the paraelectric phase and on the Curie temperature in displacive ferroelectrics is considered. It is shown that ferroelectric (FE) lattice instability induced by the isotope exchange is the anharmonic and quantum effect and the associated FE phase transition is the result of the suppression of the zero-point atomic motion due to the enhancement of atomic masses caused by the substitution of a heavy isotope for a light one. A criterion for the low-temperature isotopically induced ferroelectricity is formulated. It is found that only in quantum paraelectrics with nearly complete compensation of the harmonic contribution into FE soft mode frequency by the zero-point motion contribution this isotope effect may occur. The isotope effect in SrTi(16O1−x18Ox)3 discovered recently is explained. The influence of 105 K antiferrodistortive phase transition in SrTiO3 on the FE isotope effect is discussed. It is shown that unlike SrTiO3 the stability of the paraelectric phase of KTaO3 is scarcely affected by oxygen isotope exchange. Thus, no ferroelectricity induced by such a mechanism must be observed in KTaO3.