Ab initio calculation of the ferroelectric phase transition in disordered and ordered PbSc0.5Ta0.5O3 and PbSc0.5Nd0.5O3 solid solutions with the use of a model Hamiltonian

To describe the ferroelectric phase transition in ordered and disordered PbSc_0.5Ta_0.5O₃ (PST) and PbSc_0.5Nd_0.5O₃ (PSN) solid solutions, a model Hamiltonian is written in the local-mode approximation, in which long-range dipole-dipole and short-range interactions of local modes are taken into account. The Hamiltonian parameters are determined from the set of energies of a series of distorted structures, calculated within the nonempirical generalized Gordon-Kim model. The statistical mechanics of the systems with a model Hamiltonian is investigated by the Monte Carlo method. It is revealed that PSN solid solutions undergo a transition to the ferroelectric rhombohedral phase through an intermediate phase, existing in a narrow temperature range. PST solid solutions undergo a ferroelectric phase transition directly from the paraelectric cubic to the ferroelectric rhombohedral phase. Original Russian Text ? V.I. Zinenko, N.G. Zamkova, 2008, published in Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2008, Vol. 133, No. 3, pp. 622–631.