Research On The First Order And Second Order Phase Transition Of Ferroelectrics

Landau theory is still used to analyze ferroelectric phase transition (PT), in which the free energy (FE) were expanded as power series of order parameter, and the conditions of the continuous PT and its stability conditions under the single variant situation were given. After that, Devonshire studied the ferroelectric PT of BaTiOs (belongs to Oh group at high temperature phase, and there aren't third-order terms in its FE expansion) , whose FE was expanded into the invariant expression of group in the case of the three variants, but the stability of PT wasn't discussed. Up to now, most of papers are only concerned with the single variant situationIn this paper, the stability conditions under the multivariate situation are given, and we take boracites as an example to discuss its PT and its stability in detail. We can assert that boracites' PT is the first order PT, which is consistent with the experimental result. Therefore, we can conclude that the PT of a crystal must be the first order PT when there are the third-order terms in the FE expansion, whether other thermodynamic variables are single variable or multivariable. At the same time, the basic function of the irreducible representations of thirty-two kinds of point group is extended to the third-order terms in this paper, by which we find that the FE expansion of a crystals presence the third-order terms when it belongs to one of 18 point groups that are C1, C2, C3, D2, C2v, C4, S4, C4v, D2d, C3, D3, C3v, C6, C3h, C6v, D3h. T, Td.