Diffusive phase transitions in mixed halide ferroelectrics

Abstract

Molecular dynamics simulations have been performed for the NaMg_1-x Ca _x F₃ materials where Ca ions were substituted for Mg in NaMgF₃ in varying concentrations of randomly distributed defects representing constitutional crossover. Small clusters of Ca defects in NaMgF₃ and clusters of Mg defects in NaCaF₃ were also studied. First-principles calculations were used to calculate the potentials between ion pairs in these samples. The change of defect concentration results in a gradual shift from nonfer-roelectric behavior of NaMgF₃ to ferroelectric behavior of NaCaF₃. Doping in these materials leads to a ‘‘smeared out'’ transition ‘‘range'’ instead of a sharp transition temperature—the so-called diffusive transition.

Keywords:

• diffusive phase transition
• perovskite