Coupled anion and cation dynamics of silver orthophosphate in the picosecond range

Quasielastic neutron scattering spectra of Ag3PO4, taken at the time-of-flight spectrometer FOCUS, yield various details of the anion motion which turns out to be well described as a thermally activated isotropic rotational diffusion. The rotational diffusion constant varies between 0.36 ps− 1 (863 K) and 0.55 ps− 1 (1073 K), the activation energy being 0.17 eV. In addition, we find that anion rotation is not only performed by phosphate ions alone, but also by anions who carry a cation along. The dynamic coupling of cation and anion motion detected in our data explains why the phosphate centers are slightly shifted away from the origin of the FCC lattice in silver orthophosphate: the center of mass of the coupled AgPO4 units is no longer identical with the position of the phosphorus atom. The effect is stronger in silver orthophosphate than in sodium orthophosphate due to the higher mass of the silver atom.