Ferroelectric to Quantum Paraelectric Crossover in Mixed Crystals of Isotopic Strontium Titanate

Strontium Titanate (SrTiO3), the most studied and well known quantum paraelectric since sixty years ago, results in a typical ferroelectric when substituting 16O by 18O. An effective field approach including quantum considerations of zero point energy explains satisfactorily the behavior of different ferroelectric-quantum paraelectric mixed systems. The isotopic strontium titanate SrTi (16O1-x 18Ox)3 can then be modelled as a mixed system with a critical composition (xc = 0.33) at which a crossover between ferroelectric and quantum paraelectric behavior takes place. A discussion is made in terms of dipole interaction strength for both isotopes.