Defect structure of calcium fluoride containing excess anions I. Bragg scattering

Bragg neutron diffraction studies have been carried out on fluorite solid solutions of YF 3 in CaF 2 . The samples investigated were a single crystal containing 6 mol% YF 3 (studied at room temperature (RT)) and polycrystalline samples of compositions (Ca/Y)F 2.10 (500 degrees C), (Ca/YF)F 2.15 (RT and 900 degrees C), (Ca/Y)F 2.25 (RT) and (Ca/Y)F 2.32 (RT). The results in all cases indicate two types of interstitial site located along the (110) and (111) directions from the cubic, 1/2, 1/2, 1/2 positions (F, and F" respectively). Vacancies in the normal fluorine sites (F n ) are also detected. Variations in the relative numbers of F', F" and F n atoms with excess fluoride content are observed. The clustering of defects has been considered and possible models which would account for the observed occupation numbers have been proposed. A revised neutron scattering length for calcium of 0.477+or-0.004*10 -12 cm is reported.