Lattice Dynamics of Molecular Crystals.
Abstract
Lattice dynamical calculations have been performed for two molecular crystal systems, 7,7,8,8tetracyanoquinodimethane (TCNQ) and the chargetransfer complex anthracene1,2,4,5 tetracyanobenzene (ATCNB). The calculations were carried out in the framework of the harmonic approximation and the rigid molecule model. Suitable potential energy functions for the crystal lattices of both systems were found in the atomatom potential method, using the "exp6" and "exp 61" functions. With these, lattice potentials were constructed for large arrays of molecules and the lattice energies were minimized with respect to the geometry of the lattice to ascertain the equilibrium condition necessary for the application of the harmonic approximation. The so calculated crystal structural constants were close to the experimental values, affirming that the potentials used were physically reasonable and appropriate. For both systems the phonon dispersion curves were calculated for several directions in the reciprocal lattice. The mode frequencies in the longwavelength limit agreed well with experimental observations. From symmetry analysis of the normal mode eigenvectors it was possible to assign the experimentally observed Raman modes according to symmetry species. For the TCNQ crystal this afforded the recognition of the demarcation in energy between the internal and external vibrations. It was also possible to calculate the complete elastic constants tensor of the crystal at T = 0 K. The linear compressibility was found to be highly anisotropic. Furthermore, the contribution of the lattice vibrations to the thermodynamic functions, including the specific heat, were evaluated from the phonon density of states function. The values were found to be similar to those of other molecular organic crystals. The sensitivity of the internal strains and vibrational frequencies to the application of external pressure was also examined and a comparison with experimental data is presented. For the anthracenetetracyanobenzene crystal system both the high temperature and low temperature phases were calculated. In the high temperature phase it was possible to identify soft phonon modes that are postulated to drive the orientational phase transition. Based on this, a mechanism of the phase transition is presented which is consistent with experimental results.
 Publication:

Ph.D. Thesis
 Pub Date:
 1988
 Bibcode:
 1988PhDT.......101B
 Keywords:

 Chemistry: Physical; Physics: Condensed Matter