Neutron diffraction study of the superionic transition in (Ca1-xYx)F-2+x with x=0.06

We have investigated the high-temperature superionic transition of the anion-excess fluorite (Ca1-xYx) F-2+x with x = 0.06 using both monochromatic and time-of-flight Laue single-crystal neutron diffraction. The measured Bragg intensities indicate that the cuboctahedral defect clusters found at ambient temperature start to break up into smaller fragments even below the superionic transition temperature, T-c similar to 1200 K. Information concerning the local defect configuration at T = 1173 K has been provided by modelling the measured distribution of the coherent elastic diffuse scattering within the (1(1) over bar0$) plane of reciprocal space. The high-temperature defects are of the 'Willis' type and strongly resemble the short-lived Frenkel clusters found in the pure fluorites such as CaF2 above T-c.