X-ray scattering by phonons in aluminium

Measurements have been obtained of the diffuse scattering of X-rays from an aluminium crystal over the temperature range 100-500 K. It is found that the scattering from transverse modes is as expected from the known phonon frequencies; neither anharmonicity nor atomic deformation with displacement from equilibrium position modifies this scattering. The scattering from longitudinal modes at medium values of the scattering vector is adequately represented by including the lowest order anharmonic process, but at high scattering vectors the next order anharmonic term must be included. At very low scattering vectors an additional contribution is found in the total intensity from longitudinal scattering vectors an additional contribution is found in the total intensity from longitudinal modes. The extra scattering is not associated with anharmonic behaviour although it is temperature dependent. It may be due to atomic deformation or electron-phonon interaction.