Quantum Effects of Phase Transitions in Ferroelectric Crystals—Experiments and Models

From structural data obtained experimentally, some microscopic parameters can be estimated unambiguously. By using these parameters, a quantum Ising model can explain the isotope effect in SrTiO 3 and KH 2 PO 4 . The ferroelectric transition in 18 O isotope crystal of SrTiO 3 is successfully explained only by a mass effect in quantum state, which demonstrates importance of quantum effects in ferroelectrics.