A neutron diffraction study of SrF2and CaF2

Accurate integrated intensities for the Bragg reflexion of neutrons by crystals of SrF2 and CaF2, both of which have the cubic fluorite structure, have been measured at room temperature. Measurements were made at three different wavelengths in each case, in order to investigate the effects of extinction, which were found to be quite large, and corrections were made for both extinction and thermal diffuse scattering. The experimental data show significant contributions from a third order anharmonic term in the one particle potential of the fluorine atoms in both cases. This term is consistent with the tetrahedral symmetry of the fluorine atom site and its magnitude was found to be similar to that observed previously for the isostructural BaF2. Thermal parameters for the three fluorides show that the relative mean-square vibration amplitude of the cations is essentially the same in all three but that for the fluorine ion it increases linearly with the atomic number of the cation neighbour. The ratio of the scattering amplitudes bSr/bF was refined to 1.229 (± 0.007), which gives a value for the nuclear scattering amplitude of strontium of bSr = 0.688 (± 0.013) × 10−12 cm, assuming bF = 0.560 (± 0.010) × 10−12 cm.