Variable temperature ²⁰⁷Pb NMR of PbTiO₃

A variable temperature ²⁰⁷Pb solid-state NMR study of lead titanate was conducted between-150 and 60 °C. The isotropic lead chemical shift is only weakly dependent on temperature, probably because of changes in the average Pb-O distance, which is known to be correlated with the chemical shift. In contrast, the chemical shielding anisotropy increases markedly with lower temperature, and an excellent correlation is observed between the magnitude of the shielding anisotropy and the square of the tetragonal distortion parameter c/a-1. The chemical shift powder pattern remains axially symmetric over the entire temperature range, ruling out any substantial lowering of symmetry; it is unlikely, however, that the very small orthorhombic distortions detected in recent powder X-ray work can be observed by NMR.