R. A. Guenther

Ferroelectric phase transitions in NaCa-halide perovskites

Abstract We have used lattice statics and dynamics, as well as molecular dynamics to investigate the possibility of structural phase transitions in NaCaX3 compounds in which X is one of the halides; F, Cl, Br, I. These simulations were done using 320-ion clusters and ab initio potentials with no adjustable parameters. Our computations lead to the prediction that these compounds all have high temperature phases which are distorted perovskites and that each of them undergoes a phase transition to a polar state, although the structure of the ground state and the direction of the polar axis varies with the size of the halide ion.

A double well oscillator model for the ferroelectric phase transition in SBSi

Abstract The dielectric response spectrum for antimony sulfoiodide has been computed using a model in which the material is simulated by a damped double well oscillator. It is found that with the use of simple damping and relaxation terms and realistic well parameters the experimental dielectric response, including the central peak, can be reproduced quite closely. The choice of well depth, in particular, plays a major role in determining the frequency of the first peak above the soft mode.

The crossover of phase transitions from NaCaF3 to KCaF3

We have performed molecular dynamics simulations on a series of 320 ion samples of NaCaF3 with specific amounts of randomly distributed K substituted for Na. Our computations used ab initio potentials with no adjustable parameters to calculate the short range interactions between ion pairs in these samples. Our results indicate the gradual crossover of the phase transitions from NaCaF3 to KCaF3. The polar state seems to disappear at a K to Na ratio of three.

The effect of K defect clusters on the ferroelectric phase transitions in NaCaF3

We have performed a number of molecular dynamics computations on 320 ion samples of NaCaF3 containing K substitutional defects in various concentrations and configurations. Ab initio potentials with no adjustable parameters were used to obtain the short range interactions between ion pairs. Our results indicate that the presence of these defects, in most cases has a profound effect on the characteristics of the polar state, including a diminished sharpness of the transition temperature. We believe that these results may lend some insight into the mechanisms which produce “relaxor” ferroelectrics.

Spontaneous polarization of the perovskite NaCaF3 by simulated annealing

Abstract Electric polarization vs. applied electric field is determined by a simulated annealing Monte Carlo method for the cubic perovskite NaCaF3. A simulation system consisting of 8 40-ion subdomains with periodic boundary conditions is chosen as the model for first determining the average domain polarization at the solids saturation point. The maximum polarization of this system occurs along the polar [111] axis of NaCaF3 at 475K where a value of 64.8 °C/cm2 is calculated. The spontaneous polarization measured with an electric field then reduced reversibly along [111] is equal to 38.0 °C/cm2. Upon reduction of the field from its maximum value of 5.17 × 105 V/cm this spontaneous polarization is maintained and thus the ferroelectric behavior of this perovskite is demonstrated by a constant pressure Monte Carlo (MC) simulation.

Ferroelectric phase transitions in na-ca-halide perovskites

Abstract We have examined the possibility of structural phase transitions in the NaCaX3 compounds, where X is F, Cl, Br or I, using both lattice statics and molecular dynamics techniques. We find the structures of these materials to be distorted perovskites at room temperature. We predict that each of these compounds should become ferroelectric, with the fluoride having the highest polarization (21μC/cm2 at room temperature).

Phase Transitions in RbCaF3 and KCaF3

Calculations have been performed to determine the lowest static energy configurations associated with R-pint lattice instabilities in RbCaF3 and KCaF3. Minimum static energies and ionic displacements have been obtained for rotations of the Fluorine octahedra about the [1, 0, 0] [1, 1, 0] and [1, 1, 1] directions. Lattice configurations and energy minima will be discussed and compared with experimental values.