G.S. Pawley

The Determination of Rigid-Unit Modes as Potential Soft Modes for Displacive Phase Transitions in Framework Crystal Structures

This paper describes a computational method for the determination of all possible phonon modes in framework crystal structures that leave the fundamental structural units (tetrahedra and octahedra) undistorted. Such rigid-unit modes (RUMs) are prime candidates as soft modes for displacive phase transitions, such as in the perovskite structure, and this computational method can be used to rationalize the phase transitions in any framework structure. The method has been programmed for general use. The RUM approach is illustrated by consideration of the perovskite, quartz and cristobalite structures.

Molecular dynamics simulation of the plastic phase; a model for SF6

A model for simulating the condensed phase of SF6 is developed, using a potential function which is satisfactory in lattice statics and which gives, as required, an unstable performance in lattice dynamics. A sample of 54 molecules is used, in which the molecular motion is restricted to angular displacements only. Molecular orientation is described by the quaternion formulation, into which the molecular symmetry operations are worked. The quaternion parameters are altered whenever a molecule reorients on its site, and a record is kept of every such event. The extra energy associated with molecular reorientation is normally passed on to a nearest neighbour which may then undergo a rotation, more often about a perpendicular axis. This cooperative motion is described using a topological diagram. Future calculations on the DAP are suggested.

Orientational ordering and the low temperature structure of SF6

The crystal structure of sulphur hexafluoride in its low temperature phase has been solved from neutron powder diffraction measurements. At both 23 K and 85 K the structure is triclinic, space group P1, with Z = 3. It is in good agreement with the structure predicted previously by molecular dynamics simulations. No evidence was found for the existence of an hexagonal phase. The phase transition is interpreted in terms of two separate lattice distortions from the cubic, high temperature phase which couple to different stages of orientational ordering. The mechanism driving the transition is the resolution of orientational frustration as the temperature is reduced. The present results confirm the validity of the simple intermolecular force model employed in the simulations for SF6 and they have been used to improve the parameters of this model.

Collective excitations in an orientationally frustrated solid; neutron scattering and computer simulation studies of SF6

Collective excitations in the orientationally disordered phase of SF6 have been studied by inelastic neutron scattering and molecular dynamics simulation techniques. Experimental measurements to observe acoustic modes were made along the high symmetry directions at temperatures of 100 K and 200 K. The excitations observed showed little evidence of discrete peaks but were all broad and overdamped. They showed little temperature dependence. The dynamical structure factors S(Q, ω) calculated from the simulation are in qualitative agreement with the observed spectra but quantitatively show discrepancies. For smaller wave vectors than those studied experimentally the calculations show the existence of well-defined, long wavelength acoustic phonons. The wave vector at which the transition occurs between propagating and overdamped excitations was found to be temperature dependent. The results are interpreted in terms of the concept of orientational frustration. Some difficulties in the application of molecular d...

Monoclinic phase of SF6 and the orientational ordering transition

The structure of the low temperature phase of SF6 is described in terms of a monoclinic unit cell. The mechanism of the phase transition is discussed for this unit cell setting. It can be analysed in terms of three strain components, two molecular displacements and two molecular orientations.

CRYSTALLIZATION OF MOLECULAR LIQUIDS THROUGH SHEAR-INDUCED NUCLEATION

We report that a shear-induced transition to an ordered state occurs in simulations of condensed molecular systems and that positional ordering can persist after shearing has stopped. Moreover, we calculate that shear fluctuations sufficient to induce microscopic ordering can occur in equilibrium fluids. We conclude that microscopic ordering induced by shear fluctuations provides a mechanism for homogeneous nucleation of the phase transition from liquid to crystal in real molecular systems and may therefore be used as the basis of a computational technique for investigating the crystallization of molecular liquids.

Determination of phonon eigenvectors in naphthalene by fitting neutron scattering intensities

The coherent neutron scattering intensities from five phonon modes in naphthalene have been measured at twenty four points in reciprocal space. These intensities are determined by a set of eigenvectors, and a fit to these intensities has been obtained by altering the eigenvectors iteratively. Significantly the best result has been obtained starting from eigenvectors derived from a model calculation, which were found to be close to the final result. This supports our method of assigning neutron scattering results by a comparison with model calculations.

Raman scattering study of monoclinic and cubic tetracyanoethylene under high pressures

Raman scattering spectra of the cubic and monoclinic phases of tetracyanoethylene in the external-mode region have been investigated as a function of hydrostatic pressure in a diamond-anvil cell. There is evidence of a possible transition from the cubic phase to a new unknown phase at 13 kbar. The intensities of the Raman spectra are found to vanish at high pressures. Calculation of phonon frequencies as a function of pressure has been carried out in both the cubic and monoclinic phases using atom-atom potentials with a semi-rigid molecule model which takes account of both the external vibrations and those internal vibrations up to 300 cm-1. The calculated frequencies and mode-Gruneisen parameters are compared with the experiments, and Zallens vibrational scaling law has been successfully investigated.

The pressure dependence of the low-frequency Raman spectra of crystalline biphenyl and p-terphenyl

Abstract Raman spectra as a function of hydrostatic pressure are presented for crystalline biphenyl and p -terphenyl. The observed changes in the low-frequency Raman spectra of crystalline biphenyl indicate that there are probably some changes in the crystal or molecular structure with increasing pressure. The Raman spectra of p -terphenyl have no evident anomalies at pressures up to 33 kbar.

Interatomic Forces in Hexamethylenetetramine

A formalism expressing the intermolecular mode frequencies of rigid molecules explicitly in terms of phenomenological interatomic force constants is developed for a general uni-molecular crystal. The restraints on the force constants necessary for the dynamical matrix to be hermitian for all symmetries and general force fields are given. This ‘interatomic’ lattice dynamical model is used to analyse the intermolecular mode dispersion curves of deuterated hexamethylenetetramine (DHMT), assuming various interatomic force systems. It is found that an unambiguous choice of the best force system cannot be made from this analysis, but the shortest D. . . N bond is found to be the dominant bond. A ‘6-exponential’ form is then assumed for the interatomic pair potentials in DHMT, and the constants of these potentials are determined by fitting to the DHMT dispersion curves. Most of the constants are poorly determined by this analysis, only the long-range part of the D-D interaction showing a significant change from the values independently estimated from hydrocarbons. The ‘rigid molecule’ assumption is then removed, and using the fitted interatomic pair potentials and the known internal force field the effect of the internal vibrations on the intermolecular modes is calculated. Significant shifts, larger than the experimental error in some cases, are found in many external mode frequencies due to the molecular distortions. Small shifts are also found in the internal mode frequencies, some of them being unexpectedly negative. The rigid molecule approximation is indicated to be of doubtful validity for most molecules, but an approximate procedure is suggested for retaining it in analyses of external mode dispersion curves. Certain systematic discrepancies are noted in the fitting and it is suggested that neglect of the octopole moment of the DHMT molecule may be the cause.