Pseudospins and Their Collective Modes in Displacive Crystals

Abstract

Among crystalline systems undergoing phase transitions, there is a group of crystals where the structural change is signified by active ions or molecules involved in continuous displacements. At the threshold of such transitions, these active groups begin to exhibit linear or rotational displacements in some cases and distortion in others, as evidenced by X-ray diffraction [19] and magnetic resonance results [20], [21]. Arising from continuous displacements, these so-called displacive phase transitions are second order according to the Ehrenfest classification, and characterized by a classical displacement vector signifying symmetry change at the transition temperatures.