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

Abstract

Bragg neutron diffraction studies have been carried out on fluorite solid solutions of YF3 in CaF2. The samples investigated were a single crystal containing 6 mol% YF3 (studied at room temperature (RT)) and polycrystalline samples of compositions (Ca/Y)F2.10 (500 degrees C), (Ca/YF)F2.15 (RT and 900 degrees C), (Ca/Y)F2.25 (RT) and (Ca/Y)F2.32 (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 (Fn) are also detected. Variations in the relative numbers of F', F" and Fn 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.