Showing papers on "Molecular solid published in 1982"

Theory of elastic and phonon softening in ionic molecular solids. Application to alkali cyanides

Abstract: We have carried out a theoretical study of the effect of coupling between rotational and translational degrees of freedom first proposed by Michel and Naudts on the elastic constants and phonon frequencies of ionic molecular solids. We have applied our theory to the high-temperature plastic phase of alkali cyanides NaCN, KCN, and RbCN. We find that the competition between short-range repulsion and the interaction of the electric quadrupole moment of the C${\mathrm{N}}^{\ensuremath{-}}$ ion with the fluctuating electric field gradient strongly influences the elastic softening and ferroelastic instabilities in these systems. The effect of direct intermolecular interaction and anharmonicity is found to be significant in some cases. The ferroelastic transition temperatures for the above three compounds are found to be 337.5, 190, and 179 K which compare favorably with the experimental values 255.4, 156, and 130 K if we note the mean-field nature of our theory. Within our model we can understand the qualitative differences between the cyanides and the superoxides, a similar class of compounds showing drastically different ferroelastic behavior. Our calculations provide a microscopic justification for the use of certain phenomenological parameters by Strauch et al. in their calculation of phonon frequencies in NaCN and KCN at 300 K.

Elastic Softening and Ferroelastic Instabilities in Alkali Cyanides and Superoxides

Abstract: It is shown that the competition between short-range repulsion and the interaction between quadrupole moment and fluctuating field gradient strongly influences the elastic softening and ferroelastic instabilities of ionic molecular solids. This theory is applied to explain the ferroelastic properties of alkali cyanides and superoxides.

A molecular dynamics study of dephasing in solid N2 and O2

Abstract: We have carried out a theoretical study of the isotropic Raman vibrational band shape in high temperature molecular solids. We employ a new vibrational exciton formalism to determine the line shape in the weak coupling limit for solids with several inequivalent lattice sites. We demonstrate that resonant vibration–vibration coupling can lead to mixing of the contributions from various lattice sites, so that it is no longer possible to associate a particular band in the spectrum with a single lattice site; the effect of V–V coupling on relative band intensities can be particularly large. We have used a molecular dynamics simulation to study the vibrational line shape of solid β‐N2. The relative contributions of different parts of the potential are discussed. We have also simulated the isotropic Raman line shape of solid γ‐O2, which has a lattice structure with eight molecules per unit cell in two inequivalent lattice sites. The shifts, widths, and intensities of the two Raman bands that result are calculat...

NMR pulse studies of molecular solids: 15N2 and H2. II. Stimulated echoes and slow rotational motions

Abstract: For pt.I see ibid., vol.15, no.23, p.4881 (1982). Following a description of the formation of stimulated echoes in orientationally ordered crystals such as solid N2 and solid H2 the authors discuss the exploitation of these echoes for the detection of slow rotational motions in these systems. For solid N2 at low temperatures where the molecular orientations remain fixed the 'natural' decay of the echoes is very slow as a result of the quenching of the nuclear spin flip-flop transitions between different molecules. The 'natural' decay is also very slow in the orientationally ordered phases of solid H2 and the authors have used this property to search for residual rotational motion in the quadrupolar glass phase of solid H2. Such motion would lead to a strong additional damping of the stimulated echoes. The experimental results show that the molecular orientations in the glass phase of H2 are frozen for time scales up to 10-2 s. The temperature dependence of the echo amplitudes in the glass transition region clearly indicates a collective freezing of the molecular orientations. A new model of an inhomogeneous dynamical freezing is proposed to account for the experimental results.

Ultraviolet photoelectron spectroscopy of the valence bands of solid NH3, H2O, CO2, SO2 and N2O4

Abstract: A photoelectron study is presented for the outermost bands of the molecular solid phase of: NH3, H2O, CO2, SO2 and N2O4. Both gaseous and solid phase spectra were acquired with 40.81 eV ultraviolet photoelectron spectroscopy (UPS) techniques (except for gaseous N2O4). For the solids, charging effects were measured systematically and appropriate corrections made. Some valence band shifts were observed, as well as significant changes in valence band widths, between the gas and solid phase spectra, but the shifts were in general much less than those measured by previous workers. These results are interpreted in terms of molecular solid bonding and relaxation effects.

NMR pulse studies of molecular solids: 15N2 and H2. I. Solid echoes and orientational ordering

Abstract: The authors develop a theory of solid echoes following a two-pulse RF sequence in orientationally ordered crystals such as solid 15N2 and solid H2 which takes into account the dipolar interactions between nuclei of different molecules. These interactions result in a damping of the solid echoes which is found to depend on the angle and axis of the rotation of the nuclear spins by the second pulse. The theoretical predictions are in quantitative agreement with the experimental results obtained both in solid 15N2 and in solid H2. The authors discuss the use of solid echoes for the study of orientational ordering in molecular solids, and in particular how one can take advantage of the unique possibility offered by these systems of suppressing the damping of solid echoes.

Highly Anisotropic Molecular Solids

Abstract: The physicochemical properties of quasi-one-dimensional crystals formed by charge-transfer complexes, radical-ion salts, and coordination and organometallic compounds are examined. The characteristics of the electrophysical, optical, and mechanical properties of these compounds due to the crystal structure and the nature of the movement of the electrons and (or) holes in them are discussed. The bibliography includes 230 references

On the formation of clusters in secondary ion mass spectrometry of molecular solids

Abstract: Water strongly promotes the formation of ion clusters containing several undamaged methanol molecules during the sputtering of water–methanol layers condensed on titania at 120 °K.

Analysis of Electronic Structure from Electron Density Distributions of Transition Metal Complexes

Abstract: Accurate experimental determination of the electron density distribution in molecular solids has been realized in the past decade. For molecules containing only light atoms, careful experiments are found to yield charge densities in excellent agreement with the most sophisticated theoretical calculations.1,2 In the majority of cases, however, the observed electron distributions of light atom systems could have been predicted, at least qualitatively, from simple valence bond models. Experimentally determined electron distributions are therefore potentially more useful in understanding the electronic structures of transition metal complexes were the mechanisms of metal-metal and metal-ligand bonding are often far less obvious.