An investigation was made of the high-temperature super-ionic transition in an anion-excess fluorite, Ca0.94Y0.06F2.06; using both monochromatic and time-of-flight Laue single-crystal neutron diffraction techniques. The measured Bragg intensities indicated that the cubo-octahedral defect clusters which were found at ambient temperatures began to break up into smaller fragments even below the super-ionic transition temperature of about 1200K. Information concerning the local defect configurations at 1173K was obtained by modelling the measured distribution of coherent elastic diffuse scattering within the (1¯10) plane of reciprocal space (figure 3). The high-temperature defects were of so-called Willis type, and closely resembled the short-lived Frenkel clusters which were found in pure fluorites such as CaF2, at temperatures above the super-ionic transition. Both the Willis and Frenkel-type clusters contributed to ionic diffusion. The presence of extensive disorder was reflected by extremely high thermal vibration parameters. Analysis of the diffuse scattering, which indicated short-range correlations between disordered ions, provided independent confirmation of the defect geometry and permitted more reliable values of the position and thermal vibration parameters of the interstitial and relaxed ions to be obtained.

M.Hofmann, S.Hull, G.J.McIntyre, C.C.Wilson: Journal of Physics - Condensed Matter, 1997, 9[4], 845-57