An Atomic Order Parameter in Disordered Crystals Undergoing Structural Phase Transitions and Its Determination by the Crystal Structure Analysis

Abstract

An atomic order parameter is defined by the difference of the probabilities with which the atom occupies two local equilibrium positions in the split-atom potential. An expression of this order parameter is given as a function of the temperature and the potential difference between two local equilibrium positions. It is shown that the thermal averages of the atomic position and the square of the atomic thermal amplitude, which can be determined from the crystal structure analysis, are expressed as a function of this atomic order parameter. The characteristic temperature variation of these values observed in the ferroelectric CsD₂PO₄ is successfully explained by the predicted temperature dependence of the atomic order parameter.