Table of contents

A universal functional form is proposed for the direct correlation function of the one-component plasma. This form depends on only one scaling parameter which is determined from a novel hypernetted-chain compressibility equation through a single numerical integration. The results compare well with the numerical solutions of the hypernetted-chain integral equation as well as with recent Monte Carlo computations.

The authors suggest that the disordered anions in superionic fluorites do not occupy the cube-centre interstitial positions, which they occupy below the superionic transition temperature. The authors base the suggestion on the idea that interstitials in such positions are destabilised by nearby vacancies and support this idea by lattice-statics calculations. They then combine the suggestion with other recent results to give a tentative picture of the superionic state in fluorites.

Based on the results of ENDOR measurements it was proposed that the self-trapped exciton in KCl is displaced from the centrosymmetric position in the lattice along the (110) direction. Using the same model presented earlier for defect formation in alkali halides, it is shown that axial shift of the self-trapped exciton is very likely to occur. The magnitude of the axial displacement is estimated from the experimental values of the quadrupole interaction parameters.

Liquid-gas transitions in an asymmetrical spin-1 Ising model are studied using the Blume-Emery-Griffiths-Mukamel Hamiltonian. Within the Bragg-Williams approximation it is shown that the liquid-gas transition in the symmetrical model may become first order or second order provided the crossed dipole-quadrupole term vanishes, and that no tricritical point can exist. But an asymmetrical model always permits both first-order and second-order liquid-gas transitions, and a tricritical point can exist for some critical value of the crossed dipole-quadrupole coupling.

The individual s electron densities at the nuclear sites of Fe³⁺, Ru³⁺, and Os³⁺ ions under the influence of a spherically symmetric electrostatic potential in ionic solids have been calculated and compared with the free-ion values using the non-relativistic Hartree-Fock-Slater wavefunctions. It is found that the inner s shell densities increase and the outermost s shell densities decrease relative to the free-ion values in all the three cases considered.

The band structures for nickel and copper phthalocyanides are compared and the difference attributed to the change in lattice parameter.

A microscopic model is introduced to study the time-dependent spatial diffusion of optical excitations and concomitant spectral transport in inhomogeneously broadened random systems. The optical excitations migrate among a low concentration of optically active impurity ions distributed at random with a site-symmetric transfer rate assumed to be independent of energy mismatch. An explicit analytic calculation is given for dipolar interaction. The result agrees reasonably well with that of a recent Monte Carlo calculation.

Surface states in (111) covalent faces are obtained as a function of the alkali coverage of the surface. The theoretical results are in good agreement with the available experimental evidence.

Neutron quasi-elastic and elastic scattering experiments from NH₄I are reported in the temperature range 213K-300K. It is shown that both while cooling and heating, one obtains an NaCl phase coexisting with low-temperature phases. This calls for great care in interpreting any data on NH₄I in this temperature range. The reorientation times for NH₄⁺ ions in the CsCl phase are determined. In the NaCl phase it is shown that a simple rotational diffusion model does not describe NH₄⁺ reorientations properly.

For pt.I see ibid., vol.12, p.209 (1979). Neutron scattering from NH₄I at room temperature and (NH₄)_0.16K_0.84I and (NH₄)_0.16K_0.84Br from 98K to room temperature are reported. Measurements using a fixed elastic window technique show that reorientation rates of NH₄⁺ ions in all the salts are fast ( tau approximately 5 ps) and similar at room temperature. This is a feature typical of the octahedral environments in these salts. Quasi-elastic scattering data has been utilised to derive reorientation rates, the NH₄⁺ ion form factor and information regarding the reorientational barrier. A number of reorientational models have been considered and it is shown that none of the models proposed in the literature can explain neutron diffraction, quasi-elastic and inelastic scattering data simultaneously. A new model, which has the qualitative features required to explain all these data, is proposed.

Colour centres in single crystals are stable. The crystals can be stored in the dark for several months without any appreciable loss of colouration. The colouration of the microcrystalline powders, on the other hand, was found to decay very fast. This fast decay has been attributed to the defects inherent in the microcrystallites. The microcrystalline powders of the mixed crystals of KBr:KI could, surprisingly, retain their colouration for several days.

Some simple domain walls which arise as stationary points of the free energy in the A₁ phase in a strong magnetic field are discussed. Their stability is investigated using orbital dynamics.

The Hartree-Fock (HF) energies of a new set of HF orbitals with long-range order are calculated for two different models which involve a system of many fermions interacting via repulsive delta potentials as a preliminary step facing such problems as the electron gas system, ³He systems, etc., where, however, the calculation of the relevant matrix elements is considerably more difficult. The models considered illustrate paramagnetic and antiferromagnetic behaviour. The authors find a whole new set of HF orbitals which are more stable than the (trivial) plane wave orbitals as well as the classic Overhauser ones.

The band structures of several A15 compounds have been established by calculating XPS and SXE one-electron spectra and comparing them with experimental data. The method of calculating is general for muffin-tin potentials where l-decomposed density-of-states functions are available.

Electron band structure calculations reveal that ZrSe₃ is a semiconductor with E_g approximately 1.5 eV whereas NbSe₃ and TaSe₃ are both highly anisotropic metals. The Peierls transition in NbSe₃ is related to the presence of four quasi-one-dimensional free-electron-like bands crossing the Fermi energy. This feature does not exist in TaSe₃, and the differences in properties are related to structural differences.

The authors have calculated the self-energy potential of valence and conduction electrons in zinc sulphide. When this potential is included in a calculation of the band structure appreciable differences from the local pseudopotential calculation appear. Although these differences can be explained by simple physical arguments, they act to increase discrepancies with experiment, showing that non-locality from other sources is important.

The compound (Ni(C₅H₅NO)₆)(NO₃)₂ crystallises in the space group R3 with the metal atom at the centre of the unit cell, which imposes a centre of inversion, and a three-fold rotation axis at the metal atom. The NO₃^- groups are positionally disordered. Susceptibility measurements at 1.60K on oriented single crystals of (Ni(C₅H₅NO)₆)(NO₃)₂ in applied fields clearly shows a singlet-doublet crossing at H_1c=46.2+or-1.5 kOe. Susceptibilities at zero field are also presented. The data analyses yield D/k_B approximately=5.69K for the singlet-doublet separation and zJ/k_B approximately=-1.45K for the (subcritical) antiferromagnetic interaction.

Measurements of the thermal conductivity of MgO doped with manganese in the temperature range 0.4 to 300K indicate the existence of resonant scattering by Mn³⁺ ions at about 160 and 630 GHz. This cannot be accounted for by simple crystal-field theory but is consistent with the dynamic Jahn-Teller theory of Fletcher and Stevens (1969) with the parameters delta (tunnelling frequency)=480 GHz and D (spin-orbit splitting)=120 GHz. These frequencies and parameters are the same within experimental error as those determined for the isoelectronic Cr²⁺ in MgO using the same technique by Challis et al. (1978).

The linear Jahn-Teller system E(X) epsilon , Gamma ₈(X) tau ₂ and Gamma ₈(X)( epsilon (+) tau ₂) (in equal coupling) can all be reduced to similar sets of differential equations. Wavefunctions, energies and Ham factors for low-lying levels of such systems in strong coupling are found. The agreement between the results of analytical and numerical calculations is good.

Published drift mobility measurements on amorphous PVK, TNF, TNF:PVK mixtures and dispersions of TNF in polyester strongly suggest that transport is by small-polaron hopping controlled by charged traps. The existence of a characteristic temperature at which the extrapolated mobility loses its field dependence is ascribed to the temperature dependence of the permittivity. With the aid of a first-order model of Coulomb-trapping in hopping systems it is argued that the transport activation energy at room temperature is only 0.4-0.45 eV, not 0.65 eV as suggested by the experiments, and that the trap-free hopping mobility is of the order 10^-3 cm² V^-1 s^-1. The experiments suggest that the pre-factor mu ₀ in the mobility of pure PVK or TNF is of the order 1 cm² V^-1 s¹, and this is shown to be a plausible value for aromatic hopping systems.

Neutron diffraction experiments have been carried out at room temperature and 4.2K on polycrystalline samples of Fe_zO with z=0.943, 0.938 and 0.929. At room temperature, the ratio of octahedral iron vacancies to tetrahedral iron interstitials is close to 3 and is therefore in agreement with the defect cluster proposed by Catlow and Fender (1975). Examination of the 4.2K data has led to a model for magnetic ordering around the clusters. The interstitial cations and surrounding iron atoms are coupled by an antiferromagnetic exchange interaction. The spins in this region lie in the (111) plane, not along (111) as in the defect-free regions. The number of spins lying in the (111) plane is consistent with Catlow and Fender's model.

A position space renormalisation group technique (decimation) is used to treat site- and bond-diluted Ising ferromagnets on square and triangular lattices. In every case the renormalisation equations lead to two fixed points, one corresponding to the pure Ising system and one to percolation. Linearising around the fixed points, eigenvalues and hence the critical exponents nu _T, nu _P (correlation length), alpha (specific heat) and phi (crossover) are obtained. Numerical iteration of the recursion relations provides the critical curves. Values of critical temperatures, critical concentrations, exponents, and of limiting slopes of critical curves are in very satisfactory agreement with all known exact or series results. Points on the critical curve flow into the Ising fixed point under the transformation. The sign of the pure Ising exponent alpha is consistent with this flow for the site-diluted cases but not for the bond-diluted cases.

Donor-acceptor recombination has been investigated in ZnS by optically detected magnetic resonance (ODMR). Resonances due to both donors and A-centre acceptors have been observed in hexagonal and cubic ZnS containing group IIIB and group VIIB dopants. Spectral dependence measurements show that both resonances are associated with the blue self-activated emission, which lies at 2.70 eV in ZnS:Cl and 2.60 eV in ZnS:Al,I. The resonances are observed as increases of order 0.4% in the total emission intensity and are accounted for in terms of a donor-acceptor recombination process. It is found that the signals are predominantly from donor-acceptor pairs that are sufficiently close for recombination to occur before thermalisation is achieved but which are sufficiently well separated for the exchange interactions and associated line broadening to be small. The investigation confirms that the self-activated emission is due to radiative recombination of an electron from a shallow donor with a hole on an A-centre.

A phenomenological explanation with a pseudo-spin-lattice coupled-mode model is given for the ferroelectric phase transition from space group C₂c/ to C_c in hydrogen-bonded NH₄H(ClCH₂COO)₂ at 120K. From the fitting of the Curie constant and the transition temperature, the Blinc-de Gennes model parameters have been calculated.

The soft mode Raman spectra of ferroelectric Pb₅Ge₃O₁₁ have been carefully measured as a function of temperature. These new results were analyzed to obtain the parameters of various models for the soft mode spectral response for frequencies larger than 4 cm^-1. The modes employed were: (i) a Debye relaxational function for the overdamped spectra; (ii) a classically damped harmonic oscillator at all temperatures and (iii) a model which allows for coupling between the soft mode and phonon density fluctuations. It is found that the low-frequency Raman spectra are well described by the damped harmonic oscillator model for 30K<or approximately=T_c-T<or approximately=80K. Further, the resonance frequency and damping parameters obtained do not show the expected temperature dependence but diverge for T_c-T<30K. It is shown that the self-energy of this simple model does not have the appropriate frequency dependence and that a more suitable form can be obtained when coupling of the soft mode to phonon density fluctuations is considered.

Iodine in AgBr behaves as an isoelectronic trap with the hole being bound first. The bound-exciton zero-phonon photoluminescence line is split by exchange into an A-line and a B-line separated by 0.1 meV. The Zeeman splitting at H=80 kG is isotropic in agreement with iodine being a substitutional point-like impurity in AgBr. In the low-temperature limit optic and acoustic phonons contribute to the phonon side wing with nearly the same coupling strength. At temperatures below 10K, up to n=11 phonons are observed in the phonon side band. The authors present experimental evidence that these are due to the emission of n optic phonons which, for n>3, are statistically independent. A multiphonon model which neglects the k dependence of transition rates is introduced; however, the model is useful only qualitatively.