Models for the phase diagrams of A₂BX₄ and similar compounds

Abstract

Incommensurate systems have been studied on a number of essentially onedimensional models. Examples are the ANNNI and the DIFFOUR models. These have one degree of freedom per site. The incommensurate phases of a large class of compounds with basic structure belonging to the space group Pcmn can qualitatively be understood with these models. Here two types of generalizations are discussed. First models with more degrees of freedom per site are studied. These give the possibility to restrict the range of the interactions. In the second place a coupling to elastic degrees of freedom is discussed. This may influence the type of phase transition. For strong enough coupling a second-order transition may turn into a first-order one.