NONEMPIRICAL CALCULATION OF THE PB(SC0.5TA0.5)O-3-PBTIO3 QUASI-BINARY PHASE-DIAGRAM

The nonempirical potential induced breathing (PIB) method has been used to calculate structural parameters and total energies for PbTiO3 (PT), ten different ordered structures with stoichiometry Pb(Sc0.5Ta0.5)O-3 (PST), and 104 different structures with stoichiometries between PST and PT. The PIB results for intermediate stoichiometries predict two ordered ground-state phases at X(PbTiO3)=1/4 and X=1/2. The differences in total energies were used as a basis for fitting a set of effective cluster interactions, and the resulting Hamiltonian was used as input for a cluster-variation method (CVM) phase diagram calculation. The CVM calculation predicts a third ordered phase at X approximate to 1/4. This phase is not a ground state, but it is predicted to be stable at higher temperatures than either of the ground-state phases and, therefore, to be more likely to be found experimentally.