New, Lead Free, Perovskites With a Diffuse Phase Transition: NaNbO_{3} ~ Solid Solutions

Some of (1-x)NaNbO_3-(x)ABO_3 perovskite solid solutions exhibit a dramatic diffusion of the dielectric permittivity epsilon' maximum and relaxor-type behavior when the second component concentration exceeds a threshold value x_0. The concentration phase transition to the relaxor-like phase is abrupt (of the first order kind) that is seen from the step in the dependence of the epsilon'(T) maximum temperature, T_m, on x. The precursor of this transition is a giant (up to 100 K) temperature hysteresis of epsilon'(T). Some relaxor-like properties appear even at x < x_0 in the course of cooling while disappear in the course of heating. The experimental data obtained are qualitatively described within a Landau-type phenomenological approach, assuming the relaxor-type behavior to be local stress-induced.Comment: 5 figure

First Principles Studies of KNbO3_3, KTaO3_3 and LiTaO3_3 Solid Solutions

KTaO_3-based solid solutions exhibit a variety of interesting physical phenomena. To better understand these phenomena, we performed first-principles calculations on [K_{1-x}, Li_x]TaO_3 (KLT) and K[Ta_{1-x}, Nb_x]O_3 (KTN) supercells. Our results show Li displacements and potential barrier heights in KLT that are in excellent agreement with values obtained from experimental fits. Dramatic changes in B-site dynamical charges occur in KTN in response to changes in near neighbor (nn) coordination. These effects can be explained by heterogeneity in the local electronic dielectric permittivity.Comment: 4 figure

Dielectric Response in Microscopically Heterogeneous Dielectrics: Example of KTaO_3:Nb

New experimental data on solid solutions of quantum paraelectrics with KTaO_3:Nb as an example are considered within a framework of a quantum theory of ferroelectric phase transitions. In order to describe the effect of local heterogeneities a percolation type theory together with a random field approach were employed.Comment: 4 figure

Diffusive phase transitions in ferroelectrics and antiferroelectrics

In this paper, we present a microscopic model for heterogeneous ferroelectric and an order parameter for relaxor phase. We write a Landau theory based on this model and its application to ferroelectric PbFe$_{1/2}$Ta$_{1/2}$O$_3$ (PFT) and antiferroelectric NaNbO$_3$:Gd. We later discuss the coupling between soft mode and domain walls, soft mode and quasi-local vibration and resulting susceptibility function.Comment: 6 pages, Fundamental physics of ferroelectrics 200