Lattice dynamics, ferroelectric and antiferroeelctric instabilities, and the ferroelectric phase transition in disordered PbB1/2 ' B-1/2 '' O-3 (B ' = Ga, In, Lu; B '' = Nb, Ta) solid solutions

摘要

The lattice vibration spectra, rf permittivity, and dynamic Born charges of disordered PbB1/2'B-1/2 '' O-3 (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 (Gamma(15)) 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 Gamma(15) 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.