Symmetry And Order Parameters Of Displacive Phase Transition

Crystal structure phase transition is a kind of continuous phase transition of symmetry breaking, at present, Landau抯 theory is still used to analyse this phase transition. In research of it, symmetry and order parameter(OP) are two concepts having important physical significance, basic character of OP is which can explain the symmetry changes in front and behind phase transition, symmetry and OP are closely contacted by Curie抯 principle. Ferroelectric phase transition, antiferroelectric phase transition and 0 phase transition(or called antiferrodistortive phase transition) belong to displacive type of structure phase transition. In ferroelectric phase transition, spontaneous polarization vector F~ can reflect the symmetry changes pergectly, having the symmetry C,,~., and F~ has chosen the OP of ferroelectric phase transition. Except ferroelectric phase transition, many OP is not suitable. For example, suitable OP havn抰 found for antiferroelectric phase transition and 0 phase transition. We can deduce or guess the OP抯 symmetry of antiferroelectric and 0 phase transition, using Curie抯 principle, right OP can be selected. For antiferroelectric phase transition, an axial vector R is proposed, the definition -* x Zrjx is as follows: R = ? , and the symmetry is C~1 For 0 V V phase transition, we guess the OP is a three-rank complete symmetry polarization tensor.