Molecular-dynamics study of successive phase transitions in potassium selenate

A unified description of the double perovskite family Sr2MWO6 within a rigid ion model

The sequence of phase transitions and structural instabilities of the Sr₂MWO₆ double perovskites are investigated using a rigid ion model. The parametrization of the short range empirical potential allows the control of the cation sizes by means of independent parameters, and in particular, the effective size of the M cation can be tuned to reproduce the behaviour of the whole family. The coupling of symmetry modes and its role in the stability of the phases are discussed, and molecular dynamics simulations are carried out to determine structural phase transitions as a function of temperature. A satisfactory agreement between experiments and ab initio calculations is obtained for the relevant range of ionic radii and temperatures, indicating that the range of stability of the structures is mainly governed by steric effects.

First-principle study of materials involved in incommensurate transitions

We discuss the applicability of the density functional theory to the study of incommensurate crystals. After a brief introduction to these aperiodic but ordered materials we present several types of ab initio methodologies that are adequate in the context of incommensurate transitions and phases. We also give a survey of the corresponding applications, while providing two case studies: Pb2MgTeO6 and K2SeO4.