Shinji Nambu

Lattice Energy Calculations for Ordered and Disordered Ba(Zn1/3Ta2/3)O3

Lattice energies were calculated for ordered and disordered Ba( Zn 1/3 Ta 2/3 ) O 3 . Lattice summations were carried out for both long-range electrostatic and short-range Born-Mayer type interaction energies by using the rigid ion model. Electronic polarizations were taken into account to calculate the energy contribution from deformable charges of oxygen and barium ions as a strong local field exists in the ordered structure. The interaction energy and the internal energy of dipole moments induced by local field were determined. It was shown that the lattice energy of the ordered structure is 2.15 eV lower than that of the disordered structure. This result is consistent with the reported experimental results which demonstrated the stability of the ordered structure for Ba( Zn 1/3 Ta 2/3 ) O 3 .

Theory of diffuse phase transition in complex perovskite relaxor ferroelectrics

Abstract A Landau type mean field theory of diffuse phase transition in complex perovskite relaxor ferroelectrics considering a gradual condensation of local clusters of polarizations was proposed. Numerical calculations for Pb(Mg1/3Nb2/3)O3 (PMN) and highly B-site ordered Pb(Sc1/2Ta1/2)O3 (PST) were conducted with appropriate phenomenological parameters. It was shown that, no macroscopic phase transition occurs in PMN in a short observation time scale, whereas in the case of highly ordered PST, a macroscopic first order transition occurs with the existence of the local clusters of frozen polarizations. These results confirm a picture of diffuse phase transition in relaxor ferroelectrics, the existence of local clusters and a gradual condensation of local polarizations in these clusters.

Microscopic calculation for thermal properties of ionic crystals

A microscopic calculation is carried out for thermal properties of ionic crystals on the basis of the thermodynamic variational principle. Self-consistent Einstein approximation is used to obtain effective force constants between constituent ions. Thermal expansion, temperature dependencies of the elastic constants and dielectric constants of ionic crystals are evaluated in terms of interatomic potentials. The results of numerical calculations for alkali halide crystals are in good agreement with experimental data.

Spinodal decomposition in tetragonal TiO2‐SnO2 system

A microscopic calculation was carried out for spinodal decomposition in the tetragonal TiO2‐SnO2 system by taking the contribution of the elastic free energy into account. Necessary elastic constants for the solid solutions and the elastic free energy of modulated structure were calculated in terms of interatomic potentials. Theoretical study and computer simulation were carried out for the dynamics of spinodal decomposition on the basis of a nonlinear diffusion equation derived from a coarse grained free energy. The time evolution of the microstructure and the effect of elastic strain on the decomposition process were investigated by Langer’s approximate method and the finite difference method. It was shown that the composition fluctuation along the [001] direction develops in the first stage, the formation of interface and the grain‐growth appear in the second stage, and the interface dislocations are introduced in the third stage. These results agree with the experimental observations in the tetragonal...

A MICROSCOPIC CALCULATION OF THERMAL PROPERTIES OF IONIC SOLID SOLUTIONS

A microscopic calculation is carried out for thermal properties of ionic solid solutions on the basis of the thermodynamic variational principle. The self-consistent Einstein approximation is used to obtain effective force constants between constituent ions. Thermal expansion and temperature dependence of elastic constants of solid solutions are evaluated in terms of interatomic potentials. Numerical calculations for alkali halide solid solutions are in good agreement with experimental data.