²⁰⁷Pb NMR of PbZrO₃ and PbZr_1-xTi_xO₃ solid solutions

PbZrO₃ undergoes a phase transition to a paraelectric phase at 230 °C. During this phase transition the unit cell changes from orthorhombic to cubic. Structural changes of PbZrO₃ have been monitored using solid state NMR by measuring the variation in the ²⁰⁷Pb chemical shielding tensor as a function of temperature. The two distinct lead sites show rather different behavior as a function of temperature. The less shielded lead maintains an almost constant asymmetry parameter η = 0.2 from 0°C to 200 °C while the more shielded lead resonance becomes more axially symmetric as the temperature is raised going from η = 0.24 at 0 °C to η = 0.08 at 200°C. Powder pattern singularities become less distinct near the phase transitions, hut the temperature dependence of the chemical shift tensor principal values remains continuous, and there is no evidence of an intervening higher-symmetry phase. At the phase transition, both resonances collapse into a single narrow line characteristic of a nucleus at a high-symmetry site. Results of a preliminary study of the PbZr_1-xTi_xO₃ solid solutions by ²⁰⁷Pb solid state NMR are also presented.