Lead-Oxygen Bond Length Distributions of the Relaxor Ferroelectric 0.67PbMg_1/3Nb_2/3O₃-0.33PbTiO₃ from ²⁰⁷Pb Nuclear Magnetic Resonance

We investigate changes in the local environment of ²⁰⁷Pb sites in the relaxor ferroelectric 0.67PbMg_1/3Nb_2/3O₃-0.33PbTiO₃ using variable temperature magic angle spinning nuclear magnetic resonance. We observe a Gaussian distribution of ²⁰⁷Pb chemical shifts with a mean chemical shift of -1469 ± 8 ppm and a standard deviation of 229 ± 8 ppm at 306 K and a mean chemical shift of -1410 ± 1 ppm and a standard deviation of 275 ± 1 ppm at 117 K. This corresponds to a decrease in the mean Pb-O bond length and a concurrent decrease in the effective coordination number from 6 to 5.8. An observed change in the asymmetry parameter from 0.4 to 0.8 for deshielded sites as well as a change in the shielding anisotropy of 300 ppm compared to 180 ppm for the more shielded resonances, indicating that the lead sites experience a more asymmetric environment at low temperature. Our observations support the unique direction model for Pb²⁺ ion displacements, in line with similar relaxor ferroelectric systems near room temperature. Multifield T₁ relaxation behavior observed between 9.4 and 21.1 T is indicative of a slowing down of dynamics around 200 K, and appears to be determined by spin-rotation fluctuations at high temperatures and a magnetic-field-dependent relaxation pathway at low temperatures.