Reply to ‘‘Comment on ‘Brillouin-scattering study of the elastic constants of phenothiazine through the phase transition’ ’’

Disagreement in the ordering of relative magnitudes of the elastic constants of phenothiazine with those of Ecolivet et al. is resolved. Dubious methodologies and assumptions employed by Ecolivet et al. in their measurements, approximations, and arguments [C. Ecolivet et al., Phys. Rev. B 44, 4185 (1991)] are noted. Samples employed by these investigators in both their ultrasonic and Brillouin measurements are shown to exhibit properties consistent with impure samples and inconsistent with those of Sartwell and Eckhardt

Semiempirical determination of molecular motions associated with phonon modes in molecular crystals

An approach using piezomodulated Raman scattering is presented which permits determination of the molecular motions associated with the eigenmodes of molecular crystals. When coupled with a lattice dynamical calculation for the eigenmodes, the experiment can be used to validate the calculation. The case of 1,2,4,5-tetrabromobenzene is treated and it is shown that the phase relations of the piezomodulated Raman signals confirm the calculated eigenmodes. The Journal of Chemical Physics is copyrighted by The American Institute of Physics

Translational-rotational modes in noncentrosymmetric lattices: A lattice-dynamical interpretation of the phenanthrene phase transition

A lattice-dynamical study has been used to elucidate the mechanism of the phase transition in phenanthrene. The calculation is consistent with the known lattice energy and crystal structure. Combination of experimental results with the calculated phonon dispersion curves and eigenmode symmetries yields a dynamic mechanism for the transition based on increasing density of higher-frequency modes upon approach to the transition temperature to the high-temperature phase. This motion, which may be associated with motion of hydrogen atoms in the molecule, confers a more three-dimensional character to the interactions and correlates well with the observed change in heat capacity

Anisotropic anharmonicity of lattice and molecular vibrations of 1,2,4,5 tetrabromobenzene determined by piezomodulated Raman spectroscopy

Strain-induced coupling constants for the anharmonicity of Raman-active lattice and certain molecular modes of vibration in 1,2,4,5-tetrabromobenzene (TBB) crystals have been determined using piezomodulated Raman spectroscopy. These constants, which are directly related to the first anharmonic term in the potential energy expansion for lattice dynamical calculations, are a quantitative measurement of the modal anharmonicities in the TBB molecular crystal. Application of uniaxial stress in the experiments permits the anisotropy of the anharmonicity to be determined as well as its magnitude. The TBB lattice modes are significantly coupled by the induced strains and the effects of coupling were observed to be dependent on the direction and symmetry of the strains. The molecular modes investigated were, by comparison, less coupled by the acoustic phonons and generally exhibited less anharmonic response with increasing frequency

Semiempirical determination of molecular motions associated with phonon modes in molecular crystals

An approach using piezomodulated Raman scattering is presented which permits determination of the molecular motions associated with the eigenmodes of molecular crystals. When coupled with a lattice dynamical calculation for the eigenmodes, the experiment can be used to validate the calculation. The case of 1,2,4,5-tetrabromobenzene is treated and it is shown that the phase relations of the piezomodulated Raman signals confirm the calculated eigenmodes. The Journal of Chemical Physics is copyrighted by The American Institute of Physics

Assignment of the lattice modes in TCNQ0 single crystals

A complete assignment of the six librational modes of the TCNQ0 crystal has been made employing both polarized Raman spectroscopic measurements and lattice dynamical calculations. Agreement between theory and experiment is good and modes at 40.5, 74.5, and 96 cm-1 are assigned as Ag active and those at 63, 75.5, and 104 cm-1 are Bg active. The lattice modes are found to be clearly separated from the molecular modes. The study emphasizes the importance of crystal optics and quality in the measurement of the polarized Raman spectra of biaxial crystals. The Journal of Chemical Physics is copyrighted by The American Institute of Physics