Effect of quantum fluctuations and isotopic substitution on the curie temperature of BaTiO3, PbTiO3, and KNbO3

The Hartree-Fock formalism and density functional theory are used in first-principles calculations of the Landau-Ginzburg free-energy expansion coefficients in the Devonshire-Barrett one-ion model for BaTiO3, PbTiO3, and KNbO3. The Curie temperature T0 and the Curie-Weiss constant are calculated. The effect of quantum fluctuations on T0 is considered in the approximation of the first quantum correction to the free energy. The quantum shift of the Curie temperature ΔT0quant for BaTiO3, PbTiO3, and KNbO3 and the isotopic shift ΔT0iso for barium titanate due to the substitution 48Ti → 46Ti or 48Ti → 50Ti are calculated.