The inelastic nature of diffuse X‐Ray scattering near the α‐β transition in quartz

By inelastic scattering of neutrons, the origin of diffuse X‐ray scattering along [ζ00] directions has been investigated. Phonon frequencies are found to be anisotropic and especially low for the [ζ00] direction. Phonons with low frequencies cause enhanced X‐ray scattering in pronounced diffuse streaks. It is probable that this occurs because a soft optic phonon, which drives the α‐β transition and which has been investigated at q = 0 by other authors, becomes soft very predominantly for the [ζ00] direction and forces down an acoustic dispersion branch. The measured phonons are strongly damped at 560 °C (13° below the transition) so that there is a remarkable tail at zero energy transfer, but elastic scattering as expected from static disorder was not observed. The dispersion for deviations from the [ζ00] direction shows a steep valley, which explains the width of the diffuse scattering perpendicular to the [ζ00] direction.