Theory of Inelastic Neutron Scattering from Orientationally Disordered Molecular Crystals, with Particular Application to ND4Br and ND4Cl

It is argued that the enhancement and narrowing of several single-phonon peaks observed in the Raman scattering on N${\mathrm{H}}_4$Br as the transition temperature is approached should also appear in the inelastic diffuse neutron scattering. Whereas the expression for the Raman scattering cross section contains several unknown parameters (i. e., the components of the Raman tensor), all of the major parameters in the neutron scattering cross section are known. Diffuse inelastic scattering from ammonium chloride is also discussed. The results of this paper should be at least qualitatively applicable to other orientationally disordered molecular crystals such as ice and NaN${\mathrm{O}}_2$ and to some magnetic systems.