Thermal conductivity of solid cyclohexane in orientationally ordered and disordered phases

Thermal conductivity Λ _P of solid cyclohexane is measured at a pressure P = 0.1 MPa in the temperature range from 80 K to the melting point, which covers the ranges of low-temperature orientationally ordered phase II and high-temperature orientationally disordered phase I. Thermal conductivity Λ _V is measured at a constant volume in orientationally disordered phase I. The thermal conductivity measured at atmospheric pressure decreases with increasing temperature as Λ _P ∝ T ^−1.15 in phase II, whereas Λ _P ∝ T ^−0.3 in phase I. As temperature increases, isochoric thermal conductivity Λ _V in phase I increases gradually. The experimental data are described in terms of a modified Debye model of thermal conductivity with allowance for heat transfer by both phonons and “diffuse” modes.     

The rotational stateJ=1 in orientationally disordered and ordered solid hydrogen

The rotational transitionJ=1J=0 in solid H₂ was measured by incoherent neutron scattering. Results are presented for the mean energy of this transition as a function of the ortho concentration at temperatures 4.2 and 1.2 K. The data at 4.2 K are compared with theoretical calculations resolving a discrepancy between these calculations and previous experiments. At 1.2 K and high ortho concentrations, our data cannot be interpeted in terms of an orientationally orderedfcc phase as would be expected. This observation is discussed in terms of an orientationally orderedhcp phase originating perhaps from the effects of internal strains.     

Structural relationships between the orientationally disordered (plastic) and ordered phases of tetrahedral molecules

A systematic study of the crystal structures of both the plastic and ordered phases of a number of pseudo-spherical tetrahedral molecules has revealed some generalized relationships which apply to all the molecular crystals studied, including those which exhibit significant hydrogen bonding. The following rule is proposed: equal-sphere closest-packing principles control the crystal structures of both orientationally-disordered and ordered phases of tetrahedral, pseudo-spherical molecules.     

Theory of phonon linewidth in orientationally disordered crystals

The dynamics of coupled translational (T) and rotational (R) modes is studied with special reference to the experimental situation in the alkali cyanides. The dynamic equations are extended and, in addition to the bilinear T-R coupling, anharmonic interactions of the form T-T-R are considered. These interactions lead to additional friction coefficients for translational and rotational motion. Transport coefficients are evaluated with mode-mode coupling theory. The validity of orientational relaxation models coupled to lattice vibration is confirmed as a particular case of the present theory at low frequencies. An extension to the high frequency regime is proposed.Dedicated to Prof. Dr. H.E. Müser ob the occasion of his 60th birthday     

Lattice dynamics of KCN and NaCN in the disordered cubic phase

The rotational degrees of freedom of the CN^- dumbbell have been included in a shell model treatment of the lattice dynamics of KCN and NaCN. Recent experiments are discussed and compared with the model.     

Localization in disordered,nonlinear dynamical systems

We study localization and wave trapping in disordered, nonlinear dynamical systems. For some models of classical, disordered anharmonic crystal lattices, we prove that, with large probability, there are quasiperiodic lattice vibrations of finite total energy which lie on some infinite-dimensional, compact invariant tori in phase space. Such vibrations remain localized, for all times, and there is no transport of energy through the lattice. Our general concepts and techniques extend to other systems, such as disordered, nonlinear Schrödinger equations, or randomly coupled rotors.     

Modelling of dynamical processes in a molecular crystal by NMR

In this contribution we report on the plastic crystal 1-chloroadamantane dynamics via conventional frequency dependent (¹H and ¹³C) and field cycling NMR measurements. A suitable microscopic dynamical model, worked out from from X-ray analysis is developed and the molecular motions are interpreted in terms of: self diffusion and dipolar molecular axis combined with uniaxial rotation. In the rotator phase the molecules execute a bimodal reorientation process whereas the uniaxial rotation solely persists in the low temperature phase. In both phases, the residence times exhibit an Arrhenius temperature dependence. The results confirm the existence of a dynamic crossover transition predicted by molecular dynamics simulation.     

Phonon damping by translation-rotation coupling in orientationally disordered molecular crystals

A simple model for the translation-rotation coupling in orientationally disordered molecular crystals (ODIC) is used to investigate the effect of the coupling to the damping of acoustic phonons in an fcc-ODIC composed of tetrahedral molecules. The influence of the coupling is calculated as a function of molecular and lattice parameters in the framework of linear response theory. The results are applied to the plastic phase I of solid CD₄, revealing a strong damping of short-wavelength phonons. This is consistent with experimental and molecular dynamics observations.     

Brillouin scattering study of the orientationally disordered phase of cyclohexanol

Brillouin scattering was used to measure the velocity and attenuation of longitudinal hypersonic waves in the orientationally disordered (OD) phase of a cyclohexanol single crystal and at the OD-liquid transition. Our results show that the coupling with a high-frequency relaxation mechanism, which dominates the behaviour of the hypersonic waves in the liquid, is still efficient in the OD phase.     

First-principles investigations on thermodynamic properties of the ordered and disordered Si0.5Ge0.5 alloys

The structure, formation energy, and thermodynamic properties of Si_0.5Ge_0.5 alloys are investigated through first-principles method. The ordered and disordered structures of Si_0.5Ge_0.5 compounds are considered. Our results show that thermodynamic instabilities of Si_0.5Ge_0.5 alloys at 0 K can be judged from the calculated formation energy. However, the alloy might be prepared at specified environment owing to the entropy effects considered. Moreover, the temperature dependence of the heat capacity, Debye temperature and thermal expansion coefficient of ordered and disordered structures are discussed.     

The thermoelastic behaviour of the orientationally disordered ionic crystals in two dimensions

The anomalous temperature dependence of the elastic constantsc ₁₁ andc ₆₆ and the elastic instability temperatureT _c are obtained in the two-dimensional microscopic model of the alkali cyanides crystals. It has been found, by means of the Michel and Naudts' theory of the translation rotation coupling, that the orientational disorder in the high-temperature phase leads to the critical softening of the shear elastic constantc ₆₆.     

On the dynamics of the orientationally disordered ionic crytals in two dimensions

The translational and rotational generalized susceptibilities have been calculated in a two-dimensional model crystal with orientational disorder. The Michel and Naudts' method has been employed to account for the rotation-translation coupling. The elastic and displacive character of the phase transition in the system has been shown to be independent of the mass and of the size of the disordered molecular ions. The plausibility of the third order continuous fraction expansion of the dynamic susceptibilities has been evidenced. The influence of the rotation-translation coupling on the dispersion relations are shown to be of the character observed experimentally.     

Lattice vibrations in ordered and disordered thiourea

Raman scattering and infrared reflection spectra of single crystals of thiourea were recorded at several temperatures between 2 K and 240 K in the spectral region (03500) cm^–1. The intention was to search for changes in frequency, linewidth, and intensity of optical phonon modes near the various phase transitions which are known to exist in this material. All observed phonon modes have been tabulated according to a group-theoretical enumeration, and they are attributed to internal motions of the molecular groups and external lattice vibrations. A survey of the temperature dependence of all observed lines is given.Some modes are in fact influenced by the phase transitions in a characteristic manner. The observed temperature dependences can be interpreted by a simple pseudospinphonon coupling model (following contribution II).     

Lattice vibrations in ordered and disordered thiourea

The critical dynamics of the Syozi model for dilute ferromagnetism is considered by the use of master equations. The dynamics is soluble as it is assumed that the time scale of motion on the sublattice on which the impurities move is so much faster than on the other sublattice that fast relaxing variables may be adiabatically eliminated, leaving a new soluble master equation. It is found that the linear and non-linear relaxation of magnetization exponents ^(l) and ^(nl) increase on dilution to ^(l)/(1–) and ^(nl)/(1–) respectively ( is the specific heat exponent for the pure system, which itself changes on dilution to –/(1–)). Thus if the exponents for the pure system obey the scaling law of Rácz and Fisher ^(nl)= ^(l)– ( is the magnetization exponent which changes on dilution to /(1–)) then so do the exponents for the diluted system. Similarly the exponent for spin diffusion changes on dilution to /(1–).