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Intermolecular interactions in molecular crystals
Abstract
An improved method for light scattering from acoustic phonons in small, low- symmetry solids is described. The instrumentation developed greatly facilitates the determination of elastic constants in highly anisotropic materials. Elastic scattering can be greatly reduced so that weaker phonon modes can be observed. Errors associated with the determination of the polarization directions of an anisotropic material are minimized by considering crystal optics. All thirteen elastic constants of a vapor-grown, uncut, single crystal of anthracene were determined from acoustic phonon velocities obtained by Brillouin scattering. The phonon velocities are plotted for three crystallographic planes containing the crystal axis. The relationships between phonon velocities and lattice dynamics are discussed. A minimization procedure is introduced for converting phonon velocities of low-symmetry systems into elasticity coefficients. The procedure is shown to have several advantages over previous methods used. The results are compared with those obtained by previous studies of anthracene elastic constants. The molecular crystal 1,2,4,5-tetrabromobenzene (TBB) has been studied using Brillouin scattering and lattice dynamical calculations. A method for mapping acoustic mode anharmonicity is developed. The maps show a directional dependence that correlates well with the directions of the molecular movements associated with the phase transition. It is shown that insight can be gained into the displacive phase transition and the lattice dynamics of TBB by comparing the calculated sound velocities with the experimental values. The sound velocities are plotted for three crystallographic planes containing the crystal axes. The relationships between sound velocities and lattice dynamics are discussed. The phonon dispersion curves for three directions are also presented.
Subject Area
Chemistry
Recommended Citation
Dye, Robert Craig, "Intermolecular interactions in molecular crystals" (1989). ETD collection for University of Nebraska-Lincoln. AAI8925235.
https://digitalcommons.unl.edu/dissertations/AAI8925235