Neutron diffraction evidence for anion sublattice disordering in alkali earth fluorides at high temperatures

High temperature powder neutron diffraction studies of calcium, strontium and barium fluorides indicate that there are large thermal motions associated with the anions. This suggests that the anion sublattice in these compounds with the fluoride structure may disorder at temperatures below the melting points. In SrF2 this disordering appears at about 1370K and for BaF2 at about 1300K. This is in good agreement with transitions of 1440K and 1240K, predicted from thermodynamic data. (Auth.)