The pulsed-neutron diffraction method of studying acoustic phonons in barium fluoride and calcium fluoride

Earlier papers [Willis (1986). Acta Cryst. A42, 514-525; Schofield & Willis (1987). Acta Cryst. A43, 803,-809] have discussed the nature of the thermal diffuse scattering (TDS) arising from the interaction of a 'white' beam of thermal neutrons with the acoustic modes of vibration in a single crystal. A simpler version of the scattering theory is now given which does not have recourse to the numerous equations employed in previous treatments. The theory is applied to the interpretation of TDS data from BaF₂ and CaF₂ to give the velocity of sound in these crystals as a function of the direction of propagation.