Neutron inelastic scattering study of the lattice dynamics of strontium titanate: harmonic models

Extensive neutron inelastic scattering measurements of the frequencies of normal modes in strontium titanate propagating in the crystallographic directions (0, 0, zeta ), ( zeta , zeta , 0), ( zeta , zeta , zeta ), (1/2, 1/2, zeta ) and ( zeta , zeta , 1/2) are reported. The temperature dependence of the 'soft' modes at points Gamma and R have been investigated with particular emphasis on the mode of symmetry Delta 2, propagating in the direction (1/2, 1/2, zeta ); quantitative estimates of the isotropy of the dispersion surface at Gamma and R are given. Rigid ion, rigid shell, and deformable shell models of the crystal dynamics have been developed. Based on these, the frequency distribution, specific heat, one-phonon X ray scattering intensity. Debye-Waller factor, and two-phonon Raman spectrum have been calculated; where possible, these are compared with the available experimental measurements.