The self trapped hole in alkaline earth fluorides: II. Hopping motion

For pt. I, see ibid., vol.6, 229 (1973). The activation energies for the hopping motion of a self trapped hole (Vk centre) have been calculated for CaF2, SrF2 and BaF2. The estimates use the well known small polaron model, together with the microscopic model used successfully for the static properties in the previous paper. The results are in reasonable agreement with observation. Unfortunately, the only data available refer to temperatures so low that simple thermal activation is not expected, and the most serious approximation appears to be the relation of the calculated and observed parameters. In CaF2 90 degrees jumps are not observed, in contrast to all simple theories. Possible explanations are discussed; chemical bonding between the Vk ions and the nearest axial anions is not adequate, but a modification of the corresponding repulsive interaction has the correct properties.