Table of contents

By using LEED and workfunction measurements it has been found that a previously reported c(2*2) structure for Cs on W(100) was due to a small amount of H accumulated on the surface at the beginning of Cs deposition. These H atoms, in the presence of Cs, form patches of c(2*2).

Some comments are made on the problem of localization, in the Anderson sense, of electrons moving on a Bethe lattice. It is shown that in one dimension a simple treatment yields both the previously derived critical condition and an accurate solution to the self- consistent problem. Interesting exact limits are presented for the probability distributions of the real and imaginary parts of the self-energy.

The contributions of the large and the small components of the radial wavefunctions to the relativistic crystal field are calculated from Wybourne's (1965) expression. It is shown that both mechanisms for the ground-state splittings of S-state ions, proposed by Lulek (1969) and Chatterjee et al (1973), are obtained from the small and the large components of the radial wavefunctions respectively.

By means of time-differential-perturbed angular correlation of gamma -rays the electric quadrupole interaction for the Hf site in PbHfO₃ is measured as a function of temperature. It is thereby shown that there are two different Hf sites in the high- temperature antiferroelectric phase. The transition temperature is 431(1)K.

It is shown that the contribution to the Helmholtz free energy of the cubic and quartic terms of the Taylor expansion of the potential energy can be factorized in terms of correlation functions for a two-body Mie-Lennard-Jones potential. An investigation into the rate of convergence of the terms with respect to the number of summed wavevectors is performed. Instead of the usual summation over the set of wavevectors determined by the Born-von Karman periodic boundary conditions, several numerical integration methods are employed which use selected wavevectors are integration points.

Expressions are derived for the long-wavelength limits of the various partial structure factors occurring in aqueous solutions. The limits are given in terms of known or measurable thermodynamic quantities, namely the density, compressibility and osmotic pressure.

The adiabatic compressibility of liquid Mg-Sn alloys has been determined to an accuracy of 0.5% through measurements of the sound velocity (up to 920 degrees C) and knowledge of the density. Comparison is made with the compressibility of liquid Mg-Bi. Whereas the compressibility of liquid Mg-Bi displays a distinct maximum at the composition Mg₃Bi₂, only a slight anomaly is observed in the Mg-Sn system, at the Mg₂Sn composition. These anomalies are found to correlate with a minimum in the concentration fluctuations which have been evaluated from available thermodynamic data. In addition, the zero wavevector partial structure factors have been evaluated for the liquid Mg-Sn system.

The thermal transport properties of solid mixtures of the neon isotopes ²⁰Ne and ²²Ne have been studied by means of heat-pulse and steady-state thermal conductivity measurements. The results have been interpreted numerically using the Callaway formalism to establish the relaxation rates for phonon scattering. The rate for three-phonon normal processes which is of particular interest can be represented by tau _N^-1=(4.5+or-0.5)*10⁵x²T⁴s^-1, where x=h(cross) omega /kT and W is the phonon frequency. The phonon scattering by isotopic impurity atoms was found to be 40% stronger than that predicted on the basis of the mass difference alone, but accords well with calculations that include the effects of strain field scattering. The intrinsic process relaxation rates have been used to assess the possibility of second-sound propagation in perfect neon crystals.

The convergence properties of both APW wavefunctions and various matrix elements calculated from these wavefunctions have been studied using Cu and Gd metals as examples. The authors find that s-p-like states converge much faster than d-like states in local properties (e.g. the discontinuity in slope at the muffin-tin sphere radius); surprisingly however, even the d-like states converge rapidly for integrated properties (e.g. the charge density and optical matrix elements). The authors conclude that for the materials and matrix elements studied, there is no need to utilize methods involving the matching of basis function derivatives at the APW sphere boundary; and that matrix elements can converge faster than the energy eigenvalue using the wavefunctions from the standard APW method.

The function of this work is twofold. One is to present exact values for the first ten moments of the density of states for various disordered alloy models (including off-diagonal randomness and short-range order). These may serve as a standard for the comparison of various approximate theories. In particular, it is shown that the truncation in the range of the T matrix or the mass operator often assumed in multiple scattering formalisms can introduce serious errors when there is off-diagonal randomness. Second, the moments are used to generate approximate expressions for the density of states. In particular it is shown how the technique yields a sensitive test for the effective band edges. This is a new use of a rather old technique. The strengths and weaknesses of the approach are discussed.

The KKR band structure method is developed to obtain suitable expressions for band gaps in both ordered and disordered semiconductors. The theory is applied to three cases: firstly to the bottom of an s-band to allow comparison to be made with the theory of Wigner and Seitz, secondly to the crystalline semiconductors diamond and germanium, and thirdly the cancellation of the free particle density of states is explicitly demonstrated for free bands in amorphous materials.

Transient photoconductivity techniques have been used to investigate carrier transport in SnI₄, a molecular solid. The hole drift mobility is equal to 2.2 cm² V^-1 S^-1 at 295 K and varies approximately as T^-4 over the temperature range 240 K to 370 K. This unusually rapid diminution with temperature is interpreted in terms of a nonlinear electron-phonon scattering process. It is estimated that the upper limit of the electron mobility is of order 10^-3 cm² V^-1 s^-1.

A coherent potential approximation for insulating antiferromagnetic alloys is presented, in which the off-diagonal (transfer) parts of the exchange interaction are treated on the same basis as the diagonal parts. The theory uses three parameters which are adjusted to ensure no net scattering on average from a nearest-neighbour pair of atoms when immersed in the coherent crystal. This type of treatment is especially suitable for Heisenberg exchange where the diagonal and off-diagonal terms have the same magnitudes. Results are presented for the one-magnon densities of states at various concentrations for the systems K(Mn, Ni)F₃, K(Ni, Zn)F₃ and K₂(Mn, Ni)F₄.

For Pt. I, see abstr. A34131 of 1973. A suggestion made briefly by Elliott and Thorpe for determining selection rules for one-magnon inelastic scattering of thermal neutrons is examined in some detail and modified. The method is applied to the determination of these selection rules for simple ferromagnetic FCC, BCC, and HCP metals. For FCC and BCC metals it is found that there is no restriction on the wavevector of a magnon which can participate in the scattering, but for HCP metals it is found that wavevectors which terminate on certain planes cannot participate.

Electron spin resonance and electron nuclear double resonance measurements on free radicals of structure CD₃C(COOH)₂ in gamma -irradiated single crystals of partially deuterated methyl malonic acid are reported. The results show that the CD₃ groups undergo rapid tunnelling rotation at 4.2 K indicating a lower hindering barrier than in the analogous radical in the undeuterated crystal. Small anomalous shifts of the ENDOR frequencies and broadening of some components of the electron spin resonance hyperfine structure are attributed to a spin rotation interaction arising from the effect of the deuteron quadrupole moment on the hindering barrier, making the probability for tunnelling rotation of the methyl group through 2 pi /3 depend on the spin state of that deuteron which traverses the hindering barrier during the rotation.

From a study of the EPR of Mn²⁺ in K₂SO₄ as a function of temperature, Chowdari and Venkateswarlu reported a possible phase transition at 143 K. To test whether a phase transition really occurs, the spectra due to Mn²⁺ in K₂SO₄ have been re-examined in our laboratory as a function of temperature. It was found that the Mn²⁺ ions occupy two sites, which are quite different from those proposed by Chowdari and Venkateswarlu, and these have been identified and analysed in detail at 4.2, 77 and 300 K at both X- and Q-band frequencies. The variation of the spin hamiltonian parameters B₂⁰ and B₂² for the site with the stronger spectrum has been determined throughout the temperature range 4.2-300 K. No evidence for any phase transition was found.

A study of the Mossbauer effect as a function of temperature has been carried out for the two crystallographic phases of FeSi₂. The spectra show a quadrupole splitting for all Fe positions which slightly decreases with increasing temperature. The isomer-shift values are characteristic for covalent Fe compounds. For each structure a band model is proposed, explaining the differences in electrical and magnetic properties.

PrAlO₃ has the cubic perovskite structure at high temperatures but is trigonally distorted at room temperature. At about 200 K it undergoes a first-order phase transition to an orthogonal phase, and there is a further phase transition to a triclinic phase at about 150 K. The authors have studied this system by Raman scattering, and optical fluorescence spectroscopy, and by observing the EPR of Gd³⁺ impurities. A further transition to a tetragonal phase at 99+or-5 K has been established. From the observed behaviour of the electronic energy levels of the Pr³⁺ ion, it is found that the main features of the phase transitions can be explained by consideration of the coupling of the Pr³⁺ ion to the soft Gamma ₂₅ R-point phonon of the high-temperature cubic phase. A molecular-field model for this coupling which qualitatively accounts for the existence of the observed phases has been developed.