A neutron diffraction study of anharmonic temperature factors in SrF2

Accurate integrated intensities for the Bragg reflexion of neutrons from two single crystals of SrF2 have been measured at temperatures between 295 and 1043 °K. Corrections for extinction and thermal diffuse scattering were applied and the experimental data were analysed in terms of a model which includes both harmonic and anharmonic components in the thermal vibrations of the atoms. Significant contributions from the third-order anharmonic term, βF, in the one-particle potential of the fluorine atoms, consistent with their tetrahedral site symmetry, were observed at all temperatures. At 773 °K significant anisotropic fourth-order components, δSr and δF, became apparent in the thermal vibration of both the strontium and the fluorine atoms. At this temperature βF was refined to - 3.45 ( ± 0.11) × 10-12 erg Å-3, with δSr = 1.30 ( ± 0.75) × 10-12 erg Å-4 and δF = 0.84 (± 0.29) × 10-12 erg Å-4. The positive signs of the parameters δSr and δF result from the influence of Coulombic forces on the atomic vibrations.