Table of contents

The authors show that the recent numerical results in this model can be understood in terms of the global phase diagram for coupled XY models.

A recently proposed phenomenological chemical scale chi is shown to provide excellent structural separation of all binary compounds with a given stoichiometry A_mB_n within a single-two-dimensional structure map ( chi _A, chi _B). The following structure maps are plotted and discussed: AB, AB₂, AB₃, AB₄, AB₅, AB₆, AB₁₁, AB₁₂, AB₁₃, A₂B₃, A₂B₅, A₂B₁₇, A₃B₄, A₃B₅, A₃B₇, A₄B₅, and A₆B₂₃. These maps are important for both pedagogical and predictive purposes. A structural energy difference theorem is proved which allows the microscopic origin of the structural trends to be explored in terms of physically intuitive concepts such as atomic size, electronegativity difference, and valence. The remaining papers of this series examine theoretically the pd-, dd- and sp-sp-bonded AB compounds.

For pt.I see ibid., vol.19, p.285 (1986). The relative stability of the seven most common crystal structures of the pd-bonded AB compounds is examined within a tight-binding model which allows the influence of atomic size, p and d energy level separation, and band filling to be studied directly. The simple theory, which neglects the valence s electrons, accounts qualitatively for the observed domains of NaCl, CsCl, NiAs, MnP and boride stability, only the narrow FeSi domain remaining unaccounted for.

Positron lifetime measurements are used to study various doped and undoped GaAs samples in the as-grown state. In undoped GaAs the authors detect vacancy defects, the concentration of which varies across the sample and is of the order of 10¹⁸ cm^-3. During isochronal annealing the defects disappear at 500 degrees C. The nature of the vacancy defects is discussed and their behaviour is compared with those of non-stoichiometric defects and radiation defects. It is concluded that the defects observed to trap positrons in undoped GaAs are neutral As vacancies. Systematic trends of the doping effects on positron lifetime are observed. The lifetime values in n-type GaAs are 10-20 ps higher than in p-type GaAs. This effect is attributed to positron trapping by acceptor-type Ga-vacancy-dopant complexes.

The phenomenon of acoustical activity, occurring in all noncentrosymmetric crystals, is analysed in terms of a new fourth-rank pseudotensor which is completely equivalent to the higher-rank (fifth-rank) acoustic gyrotropic tensor used so far in the literature. The nonvanishing independent components of the fourth-rank tensor are tabulated for all the crystal classes. The term 'pure ferro-acoustogyrotropy' is introduced to describe the effect in which the acoustic rotatory power of a crystal can be switched in sign for the transverse acoustic waves travelling along crystallographic axes of three-fold or higher rotational symmetry (acoustic axes). This effect can occur in ferroic crystals only. The possible ferroic species allowing it are determined and tabulated. A number of crystals are identified as candidates for demonstrating this potentially useful property, the optical analogue of which is already familiar.

An electron diffraction and imaging study of charge-density-wave phases in 1T-VSe₂ is presented. Two phase transitions are found. At 110K a 3q-state is formed which at 80K transforms into an irregular pattern of 2q-domains. A Landau theory of these transitions is presented. In order to explain the 3q-2q transition sixth-order terms must be included in the free-energy expansion.

The electronic structure of the ideal, i.e. neutral, unrelaxed vanadyl-oxygen vacancy in V₂O₅ is studied by means of a tight-binding Green function method, based on a single-layer model of the band structure. The perturbation, including certain charge transfer effects, is described by an infinite potential step on the oxygen atom and a shift of the 3d level of the neighbouring vanadium. Four bound states are obtained at the bottom of the conduction band. Their nature and origin is discussed with the help of small-cluster calculations. The changes in total and local densities of states within the conduction band are obtained and their relation to the corresponding perfect crystal quantities is clarified. The optical properties are discussed on the basis of local densities of states and the earlier interpretation of the transitions as being from central vanadium defect states towards neighbouring vanadium conduction band states is confirmed. The electron-electron and spin-orbit interaction effects on the two-electron neutral vacancy ground state are studied within a strong-field scheme. The spin-orbit interaction is found to split the S=1 ground state, thus requiring an effective S=¹/₂ spin Hamiltonian in EPR. The experimental EPR spectra are discussed from this point of view.

Pr³⁺-Cl^- intrinsic crystal field parameters A_k have been calculated for three internuclear separations, between the equilibrium distance (R₀=5.5812 au) in PrCl₃ and 4.0812 au. The results show that pi -bonding and charge penetration contributions both tend to increase the A₄/A₆ ratio for interionic separations much smaller than the equilibrium spacing. It follows that mechanisms similar to those which have been successfully employed for crystal field calculations in ionic solids may also be invoked to explain the observed crystal fields in covalently bonded solids, where this ratio is much larger, and metals, where it is negative.

The authors have studied the temperature and the magnetic field dependence of the susceptibility of the quasi-two-dimensional ferromagnet Rb₂CrCl₄ by means of a mutual inductance method. The zero-field susceptibility measured along the easy intraplanar axis diverges with a critical exponent gamma , which varies from approximately 1.9 to 1.35 on approaching T_c⁺=52.1K. This indicates crossover from two-dimensional to three-dimensional XY-model behaviour. Below T_c the field-dependent susceptibility along the hard intraplanar axis drops discontinuously to zero at a critical field B_c, which varies between B_c approximately=0.2 T at T=0 and B_c=0 at T=T_c. The step-like behaviour of chi corresponds to a second-order canted-to-collinear ferromagnetic phase transition. It is discussed within the framework of a mean-field theory in the limits T to 0 and T to T_c^-.

Diffusion of interstitial carbon atoms in iron creates the phenomenon of the 'diffusion after-effect'. After a review of the main features of this effect, this paper reports on an experimental study of this phenomenon in a SiFe crystal. The various waveshapes generated by the motion of a single Bloch wall are analysed by an autoregressive signal processing technique which allows the study of the 'instantaneous' power spectrum. A simple model is proposed in order to account for the experimental results.

Dynamical electric, magnetic and magneto-electric susceptibilities of axial ferromagnets exhibiting linear magneto-electric effects are studied in the case of spin waves coupled resonantly with optical phonons. Both ionic and electronic contributions to the susceptibilities are calculated. Employing classical Maxwell equations these susceptibilities have been used to find the dispersion relations of mixed magnon-phonon polaritons. The coupling between spin waves and phonons is shown to imply the irreversible character of wave propagation.

Valence holes can react to various elementary excitations, such as plasmons and phonons, causing them to relax coherently in the final states of electron spectroscopies. This can produce interesting effects on the lineshapes. Here the photoemission spectra of a chemisorbate or an impurity are discussed in the framework of a model that explicitly deals with the dynamics of two different boson modes. The exact solution is given and leads to the conclusion that coherence effects are important when the two excitations give comparable contributions to the relaxation energy. The dynamical effects due to the plasmons end to enhance the vibrational structures.

The Cu L₃M_4,5M_4,5 Auger spectra have been measured for Cu and the alloys Al₂Cu, Mg₂Cu and Ni₉₀Cu₁₀. The Cu 3d and Cu 2p binding energies have also been measured for Cu, Al₂Cu and Mg₂Cu using X-ray photoelectron spectroscopy. The energy difference between the ³F and ¹G components of the Auger spectrum and the relative intensity of the ³F compared with the ¹G component decrease on going from Mg₂Cu to Al₂Cu to Cu and quantities are reduced compared with a calculated L₃M_4,5M_4,5 spectrum for atomic Cu. It is shown that these changes are in quantitative agreement with the Cini model in its simplest form.