Lattice Dynamics of Hexamine—A Rigid-Molecule Approximation

Lattice dynamics of molecular solids is analyzed using a force model which treats the molecule as an extended point mass which can undergo translations and librations. The model is similar to the rigid-molecule formulation of Cochran and Pawley but contains fewer force constant parameters because definite relations now exist among translation, libration, and cross coupling force constants. These relations depend on the relative positions of an interacting pair of molecules and introduce an effective size parameter for each type of neighbors. The model has been used to discuss the lattice modes in hexamine. Recent neutron dispersion relation measurements on deuterated hexamine are successfully analyzed in terms of first- and second-neighbor interactions. Frequency distribution functions, including separate translation and libration components, are computed, and the results are used to obtain satisfactory correlations with specific heat data and inelastic neutron scattering spectra of normal hexamine. The ability of the model to predict dispersion curves using only elastic constant values and the triply degenerate optical mode frequency is discussed.