Simple Molecular Crystals Research Articles

Low-temperature thermal conductivity of solid carbon dioxide

Preliminary measurements of the thermal conductivity of pure carbon dioxide in the temperature range 1.5–35 K are reported. The first data below 25 K have been obtained. The thermal conductivity reaches very high values, about 700 W/(m⋅K), which is unusual for simple molecular crystals. A straightforward analysis of the data shows a coarse-grained sample.

Superconductivity under high pressure

Abstract In part 1, we review techniques developed in our laboratory for producing the complex extreme condition of very low temperature and ultra-high pressure and those for measuring electrical resistance and magnetization of the sample confined in the extremely small space of the used pressure cell. In part 2, we review our experimental results in search for pressure-induced superconductivity, which have been obtained by the use of developed techniques. Typical examples are shown in the case of simple inorganic and organic molecular crystals, ionic crystals, and magnetic metals.

Pressure-induced insulator-to-metal transition and superconductivity in iodanil, C 6I 4O 2

Electrical properties of iodanil (C 6I 4O 2) have been studied at low temperatures and at very high pressures. An insulator-to-metal transition in this compound is found at about 30 GPa. Further, iodanil shows superconductivity at about 2 K above 52 GPa. The magnetic field dependence of the transition temperature was measured at low temperatures under 78 GPa. The superconductivity of iodanil is suppressed by an applied magnetic field. This is the first observation of superconductivity in simple organic molecular crystals. The electronic spectra of a thin film of iodanil were measured up to 30 GPa at room temperature. The absorption intensity in the near infrared region increases markedly with increasing pressure above 16 GPa. This optical behavior is closely related to the insulator-metal transition at about 30 GPa. The insulator–metal transition and the superconductivity of iodanil at very high pressures are discussed.

Magnetic properties of oxygen clusters

Magnetic properties of oxygen pair clusters are investigated theoretically for different geometries of clusters which can be realized by doping molecular cryomatrices with oxygen. Anomalous temperature and pressure behavior of the magnetic susceptibility, magnetic heat capacity and magnetic entropy is predicted. It is shown that the low-temperature magnetic susceptibility of these solid solutions is very sensitive to the orientational structure of impurity oxygen clusters, which makes it possible to use the susceptibility data for studying structural and dynamics properties of the host lattice, including high-pressure phases of simple molecular crystals.

High-Temperature Thermoconductivity of Nitrogen-Type Crystals

We firstly report on the leading scattering mechanism responsible for the heat resistivity in simple molecular crystals. The analysis is given in the terms the phonon and libron three-particle-scattering processes in crystals formed by linear molecules (N2, CO, CO2, N2O, etc.) On the basis of a comparison of heat resistivity estimations with known experimental data the dominated microscopic scattering mechanism is established, namely, the process of absorption (and emission) of the one libron by the two acoustic phonons.

Pressure dependence of intramolecular mode frequencies in solidN2,O2, and CO2

A microscopic description of the pressure dependence of intramolecular vibrational modes in simple molecular crystals has been formulated using a classical perturbation theory. Quantitative agreement with experiment is demonstrated and it is shown that frequency changes at phase transitions are large enough to be observed optically.

Thermodynamic properties of simple anharmonic crystals

The equation of state earlier suggested for simple molecular crystals in the high-temperature region is applied to the calculation of thermodynamic functions of solid argon, krypton, xenon, and radon.

On the dielectric properties of molecular crystals: II. First-order spatial dispersion and optical activity

The general formal expression for the wavevector- and frequency-dependent dielectric susceptibility tensor of a simple molecular crystal, derived in paper I, is applied to the case of a Bravais lattice with gyrotropic molecules. The dielectric susceptibility tensor is calculated with first-order spatial dispersion and from this an expression for the gyration tensor is derived. As an example the optical rotary power is calculated for light propagating along the optical axis of a gyrotropic tetragonal crystal.

Self-consistent phonon calculation of the β phase and the α-β transition in solid nitrogen

The self-consistent phonon approximation of anharmonic lattice dynamics is applied to solid β nitrogen. Completely disordered molecular orientations and a Lennard-Jones 6–12 intermolecular potential are assumed. The quantities calculated are the phonon spectrum and the thermal expansion as a function of temperature at zero pressure. The α-β fcc-hcp transition temperature is estimated on the basis of two models for the molecular ordering in the β phase. Although the computations are intended primarily to explore the application of the self-consistent phonon method to a treatment of structural phase transitions in simple molecular crystals, comparison is made with existing experimental data.

Exciton surface states in molecular crystals

A new Hamiltonian partition method, used previously for cooperative excitations in molecular crystals, has been adopted for the treatment of surface exciton states in molecular crystals. The formation of surface excitons depends on the relative magnitude of the exciton transfer integrals, J terms, as compared to the environmental shift integrals, D terms. This was established for a sample calculation on a simple cubic molecular crystal. It was found that when the absolute value of the nearest neighbor D term exceeds the corresponding J term two localized states emerge for each value of a two dimensional wavevector. The two localized states are degenerate in the limit of an infinitely thick crystal. The localization of the surface states increases with an increase in the magnitude of the D term.

Trapping States Formed by Structural Defects in Molecular Crystals

Abstract Results of calculations of the energetic distributions of trapping states in simple molecular crystals are given in the paper. A crystal with primitive regular unit cells has been taken into account, and three kinds of simple isolated linear defects have been considered: an edge dislocation, a linear array of vacancies, and a defect formed by a linear array of molecules larger than the host ones. Equilibrium positions of molecules in the vicinity of the defects have been calculated describing the intermolecular interactions by the Lennard-Jones type potential function. Local values of the polarization energy and hence depths and distributions of traps formed by molecules neighbouring to the defects have been calculated in the ion-dipole approximation. It has been shown that the formation of trapping states by thermal vibrations can be neglected as being a small effect.

Cluster Expansion Method for the Density of Excited Electronic States of Substitutionally Disordered Crystals

In this paper we introduce a cluster expansion method which is suitable for numerical calculations of the density of electronically excited states of substitutionally disordered molecular crystals. This method is based on an expansion of the density of states of the guest subband in terms of the localized densities corresponding to various clusters and is valid for a large scattering potential and for concentrations of guest molecules below the percolation concentration. The general features of the cluster expansion method are illustrated by calculations for a simple one-dimensional molecular crystal. These numerical calculations for the one-dimensional model are compared with the results obtained by the coherent potential approximation. The role of the localized states, which are not accounted for by the coherent potential method can be elucidated.

Optical Constants of Simple Molecular Crystals. I. Results for CO and O2

The reflection and transmission spectra of unannealed films of α-CO and α-O2 at 20.4°K have been obtained, and values for the optical constants (n and k) and the real and imaginary part of the dielectric constant (ε1 and ε2) determined. The oscillator strength of the A 1Π←X 1Σ+ transition (fourth-positive group) of α-CO has been estimated to be in the range 0.035 and 0.068 (compared to the gas-phase value of 0.094). The oscillator strength for the B 3Σu−←X 3Σg− (Schumann—Runge) transition in α-O2 is estimated to be between 0.22 and 0.35 (compared to the gas-phase value of approximately 0.156). The broad absorption of α-O2 is thought to be due to the mixing of the B 3Σu− and A 3Σu+ states of O2 in the crystal.

Soft Modes in Molecular Crystals

The self-consistent phonon approximation is applied to study the temperature dependence of the libron and phonon modes for simple molecular crystals. Several linear chain models are studied to illustrate the soft mode behavior of these excitations near the orientational order—disorder transitions that occur in many molecular crystals. It is shown that both libron and phonon frequencies are strongly temperature dependent in the neighborhood of the transition in qualitative agreement with experimental observations. Both the cases of strong and weak orientational coupling are considered. The effects of the cubic anharmonic terms are also discussed.

Optical Constants of Simple Molecular Crystals. II. Results for Kr and Xe

The vacuum uv reflection and transmission spectra of unannealed films of Kr and Xe at 20.4°K have been obtained, and values of the optical constants (n and k) and the real and imaginary part of the dielectric constant (ε1 and ε2) determined. The total oscillator strength for the transition to the n=1 Γ (3/2), Γ(1/2) exciton states has been found to be less than than that of the analogous P13, P11←S01 transition in the gas phase. It has also been found that the ratio of the absorption strength due to the Γ(3/2) exciton to that of the Γ(1/2) exciton is larger than the comparable gas-phase value. These data are taken to show that the exciton excited orbital is more diffuse than the gas-phase atomic orbital.

Adiabatic compressibility of solid gases

Adiabatic compressibilities of solidCO (20–67 K) and NH3 and ND3 (90–190 K) have been determined with an ultrasonic method. The temperature dependence of adiabatic compressibility is discussed for two groups of molecular crystals: solid inert gases (Ar, Kr, Xe, Ne) and a number of simple molecular crystals with nonspherical particles (CO, CO2, NH3, ND3). The effect of orientational disordering processes on molecular crystal compressibilities has been considered.

Low-temperature photoconductivity of matrix-isolated molecules

It is proposed that photoionization of molecules may be studied by observing the photo-conductivity induced by them in matrices of simple molecular crystals with wide band-gaps. Photoconductivity has been observed in the near ultra-violet in dilute solid solutions of iodine in carbon dioxide at 77 K and in a series of substitued anilines in xenon at 40 K. Both the wave-length and light intensity dependence of the photoconductivity differ considerably from those of either the pure solutes or the pure matrices.

Central force field approximation for the calculation of displacement lattice vibrations of molecular crystals

Displacement lattice vibration frequencies are calculated for a number of simple molecular crystals using a central force field approximation. The results are compared with experimental results. In the case of ammonia, where more experimental results are available, the calculation is extended to include electrostatic effects.

Some Cases of Apparent High Conductivity in Simple Molecular Crystals

WHEN one approaches the examination of dark conductivity of molecular crystals, a good first approximation is that ‘molecular crystals are electrical nonconductors’1. Most measurements of dark conductivity on molecular crystals such as anthracene must be regarded as involving carrier injection from electrode material, ambient gas, impurities or surface imperfections. Simple d.c. measurements by two-probe techniques which neglect polarization or space-charge effects are relatively meaningless. These measurements are, however, commonly performed to screen materials in an attempt to find high conductivity crystals.

Energy Exchange Processes

It is interesting to review the types of energy transfer process shown by different systems. By energy transfer, I mean the coupling mechanism between two parts of the system. I shall not go into the question of energy migration, which involves a knowledge of the relaxation around the excited unit of the crystal. The rare-gas crystal is a very simple molecular crystal, although, as Dr. Knox has shown, it may not be so simple electronically as some organic crystals like anthracene. Inorganic ionic crystals have a range of binding types much wider than organic molecular crystals do, and at the same time there is a much wider variety of energy transfer phenomena. It will be interesting here to present several examples from these different categories and to compare them. A classification scheme which is useful in making these comparisons is to divide the examples into those having overlapping and those having nonoverlapping charge distributions (1). In the second class, we may have spherically connected or not spherically connected charge distributions. By this is meant that a sphere of minimum size can be drawn about each charge distribution, and these spheres may or may not intersect. The case where the charge distributions do not overlap and are not spherically connected is the easiest one to treat mathematically. In this case, the interaction energy reduces to that of a set of classical point multipoles, as long as exchange can be neglected. This state of affairs seems to be a relatively good description of the organic molecular crystal. Davydov splitting can be described in terms of dipoledipole (2) or octupole-octupole coupling (3), and intensity anomalies due to the crystalling state can be similarly accounted for-with the addition of the dipoleoctupole interaction (4) in some cases. Some molecules are also able to be treated as a collection of nearly nonoverlap-