Abstract
The lattice vibration spectra, rf permittivity, and dynamic Born charges of disordered PbB′1/2B″1/2O3 (B′ = Ga, In, Lu; B″ = Nb, Ta) solid solutions are calculated in terms of the generalized Gordon-Kim model. All the compounds are found to have soft modes related to the center (Γ15) and boundary point R (R 15) of the Brillouin zone; the frequencies of these modes are close. The ferroelectric phase transition temperatures and the spontaneous polarization in the ferroelectric phase are calculated for the solid solutions using a model Hamiltonian in the local mode approximation and the Monte Carlo method. These temperatures for the tantalum compounds are found to be higher than for the niobium compounds, and temperature t s increases with the atomic number of ion B′. A model where the antiferroelectric state is related to the condensation of modes Γ15 and R 15 is proposed. It is found that, for all compounds except for PbSc1/2Nb1/2O3 and PbSc1/2Ta1/2O3, the ferroelectric phase, which is only related to the condensation of mode R 15, and the antiferroelectric phase have similar energies. In PbIn1/2Ta1/2O3, the antiferroelectric phase turns out to be energetically favorable.
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