Lattice energies and structural distortions in Pb(ZrxTi1-x)O3 solid solutions

The changes in Madelung and non-electrostatic energies of Pb(ZrxTi1-x)O-3 (PZT) solid solutions in the high-symmetry (Pm3m) phase have been calculated using the heats of formation from the oxides. The non-electrostatic contribution (DeltaE(N)) decreases with decreasing x and becomes negative for compositions x less than or equal to 0.35, corresponding to perovskite tolerance factors t greater than or equal to 1. Correlation of the strong increase in tetragonal distortion in the ferroetectric phase with more exothermic values of DeltaE(N) suggests a softening of short-range repulsions for Ti-rich compositions. The influence of complex solid solution behaviour on the character of Pm3m <----> P4mm transition is investigated by high-temperature specific heat and unit-cell parameter measurements. The implications of the thermochemical results with respect to mean-field theoretical descriptions of lattice instabilities and phase boundaries in the PZT system are briefly discussed.