Table of contents

The crystal structure of irradiation/field treated triglycine sulphate has been determined by single crystal X-ray methods. The configuration of glycine molecules, which are relevant to a ferroelectric switching mechanism, show differences compared to the accepted structure of untreated material.

Neutron diffraction measurements on polycrystalline MnF₂ are reported. The experiments were carried out at room temperature and 4.2 K, using the Badger diffractometer and neutrons of wavelength 1.11 AA. The results are in good agreement with those of Erickson (1953).

A hydrogen-containing trapped hole centre (V_OH) has been identified from its ESR spectra in X-irradiated strontium oxide crystals. ENDOR studies confirm the nature and geometry of the centre as similar to that in MgO and in CaO. The g components g/sub ///=2.0010(3) and g_{perpendicular to} =2.0750(3) are similar to those for CaO.

It is shown that deflections of many degrees can occur in the trajectories of sputtered ions due to electrostatic interaction with the incident ion beam.

The connection between the diagrammatic expansion obtained by Sheard and Tombs (see abstr. A37362 of 1971) for the free energy of a coupled spin-phonon system and the coupled harmonic oscillator result obtained by Muller and Tucker is established.

Good results are obtained for the band gaps in aluminium by setting up a pseudopotential in terms of phase shifts for a neutral atomic sphere and deriving a 'minimum perturbation' model.

The single site coherent potential approximations for the cases of cellular and structural disorder are contrasted. In particular Ziman's medium propagator approximation is analysed in relation to the exact coherent potential recently put forward by Thornton and it is shown how Ziman's procedure (see abstr. A44879 of 1969) may be modified to yield qualitatively valid results.

The electroreflectance spectrum of the layer compound gallium selenide shows that the peaks at 3.63 eV and 4.9 eV of the absolute reflectivity are due to threedimensional saddle points. The first transition is resolved in a doublet arising from a spin-orbit split valence state associated with the p orbitals of Se; this contrasts with some theoretical models and favours the view that the p_xp_y states of selenium are important in forming the upper valence band.

The Penn dielectric function is often used in the theory of semiconductors. It is shown that although the Penn model is reasonable as regards the electronic spectrum it considerably overestimates the interband matrix element. A correction factor is determined by group theoretical considerations and is shown to give agreement with an appropriate effective mass.

It is shown that the electric quadrupole-quadrupole interaction between the molecules in the alpha phase of solid nitrogen energetically favours the Pa3 structure over the P2₁3 structure.

An explanation is given for the ferromagnetic resonance lineshapes observed from a series of thick nickel discs when magnetized in the perpendicular orientation and also for variation with disc thickness of the resonance field of the main perpendicular absorption signal. See abstr. A56661 of 1970.

The effect of recoil on the mobilities of impurities in liquid ⁴He is investigated by using the Van Hove scattering function. The recoil of ions from ion-roton scattering gives a weak dependence of the mobility on the effective mass of the ion. The diffusion coefficient of ³He is also calculated for dilute solutions of ³He in liquid ⁴He.

Lattice sums on ionic crystals have been evaluated by a modified Ewald method by splitting them into sums on cubic Bravais lattices. The values of the sums for the CsCl and the NaCl structures are reported in a dimensionless form, from which the values of the sums of the fluorite and the zinc blende structures can also be obtained as suitable linear combinations.

The authors find inverse (D₀-ZI+S)^-1 where the matrix S has only a small number of elements different from zero. It is shown that the usual formulae for the inversion can be simplified if the matrix S is singular.

The dependence of the saturation level of the first stage F colouring of gamma irradiated sodium chloride on the gamma radiation intensity (in the range 250-3*10⁴ R min^-1) has been quantitatively explained. The proposed model takes into account, in addition to the radiation induced mechanisms, a thermally activated process of free anion vacancy trapping whose main characteristics have been previously reported.

The point symmetry of the anion site in alkaline earth fluorides is T_3d and the F centre (an electron in an anion vacancy) in these crystals lacks inversion symmetry. Point ion calculations have been performed for the F centre in alkaline earth fluorides with trial functions of sufficient flexibility to incorporate the anisotropy of the potential. The predicted strong admixture of s and f orbitals in the ground Gamma ₁ state yields calculated fluorine hyperfine interactions in substantial agreement with results of Endor measurements. An even stronger admixture of p and d orbitals in the first excited state leads to a linear Stark effect and the magnitude of this effect has been calculated for the F band. Experimental investigation of the Stark effect of the F band in SrF₂ provides an upper bound to the linear Stark splitting which is consistent with the predicted value. The quadratic Stark effect is discussed.

The dynamical matrix is split into ionic and electronic parts. Ionic elements of the matrix have been obtained considering purely central forces up to the fourth neighbour and the electronic elements have been derived following Bhatia's (1955) work on cubic metals. Phonon dispersion relations and the lattice specific heats have been computed for gadolinium and dysprosium. Lattice specific heats have been compared with the experimental data.

A general method is formulated for the calculation of the phonon spectrum of anharmonic metals by applying the pseudopotential method. On the basis of the recently developed theory of anharmonic crystals, the frequencies of selfconsistent phonons and their damping in the adiabatic approximation are obtained, taking into account the effective cubic anharmonic interactions. The importance of indirect many body forces in anharmonic metals is stressed. The three body interactions are explicitly taken into consideration.

The temperature dependence of thermal and magnetic properties, namely Schottky specific heat magnetic susceptibility, torque in unit magnetic field and mu _eff, has been calculated for Pr³⁺:LaBr₃, Er³⁺:GdCl₃ and Er³⁺:LaBr₃ systems over a temperature range 10-400 K. The spectroscopic splitting factors and the extra entropy associated with the Schottky anomaly are also derived. Comparison of the calculations with experiments (for isosymmetric systems) leads to the conclusion that the crystal electric field parameters employed are sufficiently accurate. Furthermore, the paramagnetic Curie temperature turn out to be 10.6 K for Pr³⁺:LaBr₃, 6.3 K for Er³⁺:GdCl₃ and 2.8 K for Er³:LaBr₃.

A method is described for analysing the explicit anharmonic contribution to the entropy which is obtained from experimental C_p data. It is known that there exists a temperature interval in which the experimental entropy at fixed volume, S^exp(V₀, T), fulfils Salter's expansion; it is called the apparent quasiharmonic region. According to Barron's frequency shift, the anharmonic contribution to the entropy is found to be Delta S^anh(V₀, T)=(A(V₀)/3NK)E_vC_v where A(V₀) is a constant characteristic of the crystal, E_vC_v is fitted in the apparent quasiharmonic region to an expansion, Sigma _n=0^infinity a_nT^-2n, up to a temperature T". For T>T", the difference between the experimental values and the extrapolated values, obtained with the Salter expansion, is called Delta S_exp^anh(V₀, T) and therefore is equal to the difference between the anharmonic contribution to the experimental entropy and the anharmonic contribution to the extrapolated entropy. The method is applied to KF, and A=(-2.1+or-0.5)10^-5K^-1 is found. In the potassium halides the absolute value of A theta (2) is found to decrease when the radius of halide ion increases.

The thermal diffusivity of BN has been measured from 100 K-800 K using a 'flash' method; published specific heat data has been used to evaluate the thermal conductivity. Over a limited temperature range the results have been confirmed by steady state measurements. The thermal conductivity parallel to the deposition plane has been found to have a maximum value of about 2.5 W cm^-1 K^-1 at 235 K, while perpendicular to it the thermal conductivity shows no maximum but decreases from about 2.5*10^-2 W cm^-1 K^-1 at 100 K to 1.7*10^-2 W cm^-1 K^-1 at 800 K. The conductivity parallel to the deposition plane has been analysed using a two dimensional mode in which the vibrations in and out of the plane are independent and the latter depend only upon the resistance of the layer to bending. Characteristic temperatures for the two modes of 2290 K and 1200 K respectively have been estimated. Above 200 K the conductivity can be explained in term of Umklapp processes and boundary scattering from crystallites estimated to have average diameters in the deposition plane of 580 AA and 290 AA for the two samples investigated.

A linear approximation to the Fermi-Dirac distribution function is proposed which might be of interest in practical applications. Three different ways of choosing its parameters are suggested. In order to assess the goodness of the different choices, the specific heat curves for a closed parabolic band are calculated within this approximation and compared with the numerically computed, exact specific heat.

NaCl single crystals have been subjected to fast neutron irradiations during which the neutron beam was interrupted for a specific time interval. The variation of crystal decrement with strain amplitude (for strain amplitudes between 5*10^-8 and 5*10^-6) was measured at a series of times during the interrupted irradiations. Similar experiments were carried out at a number of temperatures between -16 and 22 degrees C. The results were analysed in terms of the theory of strain amplitude dependent internal friction developed by Granato and Lucke (1956). This theory yielded the variation with time in the number of pinning points added to each original dislocation loop. When the irradiation was interrupted, a decrease occurred in the pinning point number. This decrease reached completion within 1 minute at temperatures below 17 degrees C but required about 4 minutes to reach completion at higher temperatures. An explanation for this effect is offered which involves recombination of the pinning point parts.

It is pointed out that the current theory of the quantum size effects (QSE) assumes effectively free conduction electrons, and that this assumption is not obviously valid in this QSE regime. Exact band theoretical results are given for a simple model of the QSE. The results are similar, but not identical, to those obtained by assuming effectively free conduction electrons.

The author describes an investigation into the two methods of band structure calculation, APW and KKRZ, in order to determine their relative advantages and disadvantages. The main conclusion is that the APW method represents the better method generally but that there are circumstances when the use of the KKRZ method is to be preferred.

In the Anderson tight binding model the author attempts to find a 'frozen' configuration that gives some localized states and a density of states that is similar to that suggested by Mott and Cohen, Fritsche and Ovshinsky (M-CFO). When there is no overlap between orbitals on different lattice sites, the configuration must obey certain rules to ensure that the states at a particular energy are localized or extended. When there is no overlap, it is shown that the convergence of a particular form of perturbation series is a necessary and sufficient condition for a state to be localized. Some arguments are provided to support the conjecture that all states in which the zeroth order part is the orbital on one lattice site. Such localized states are thus highly localized. It is shown that to zeroth order the localized states and extended states occupy different orbitals; thus the band of extended states can be considered separately from the localized states and has the typical band edges expected from a band of extended states.

A brief general discussion is given of the calculation of the ac conductivity of an electron moving in a finite group of potential wells. The calculation extends one carried out by Pollak and Geballe (1961) for the two potential well case except that the question of configuration averages is not considered. The possibility of a region in which sigma ( omega ) alpha omega ² is discussed and the relevance of this work to experiments on polymeric CS₂ is suggested.

A technique involving eddy current heating has been used to study the Sondheimer oscillations in the resistivity of single crystal discs of indium. Oscillations were observed for magnetic fields in all directions in the (001), (100), and (110) crystallographic planes. With a few exceptions the oscillation periods agree to better than 5% with those arising from orbits in the second zone of a Fermi surface model which has been proposed on the basis of de Haas-van Alphen data. The field dependence of the oscillation amplitudes have been measured, and large discrepancies with the simple theory were found.

The electrical conductivity of NaCl and NaCl+SrCl₂ was measured over a wide temperature range and fitted by a least squares method to the conventional Schottky defect model which allows for specific nearest neighbour defect interactions and Coulomb interactions treated in the Debye-Huckel approximation, at greater separations. Doped crystals showed small systematic deviations between theory and experiment and gave abnormally high enthalpies and entropies of association between cation vacancies and impurities. Defect formation and migration parameters are quoted. They predict single vacancy diffusion coefficients in moderate agreement with experiment although the predicted anion activated energy appears to be definitely too large.

A precision study has been made of the temperature dependence of the Hall effect and resistivity in five carefully selected natural semiconducting diamonds. The donor and acceptor concentrations in each sample have been calculated from the Hall effect data, and the acceptor concentrations are shown to correlate excellently with the intensities of the acceptor infrared absorption spectrum in each diamond. The behaviour of the resistivity is characteristic of a single acceptor centre, and the much lower activation energy observed at low temperatures is attributed to the tunnelling of electrons from ionized to neutral acceptor centres. The aluminium concentration in each crystal has been measured by slow neutron activation analysis and found to be substantially lower than the acceptor concentration in every case. Aluminium therefore cannot form the acceptor centre as was previously thought. Boron is the only group III element that neutron activation analysis has not eliminated as a possible acceptor, and the results of other workers point to boron as a likely candidate. The revised interpretation, that the acceptor centre in semiconducting diamond is definitely not aluminium and probably boron, is discussed, and its significance to previous publications is critically considered.

Hot electron transport properties in germanium have been investigated using a Monte Carlo method with the principal objective of elucidating the origin of the negative differential mobility observed at low temperatures and high electric fields. The model takes full account of electron transfer from the (111) to the (100) valleys and incorporates the ellipsoidal constant energy surfaces associated with both these sets of minima. The scattering rates within the (111) valleys are determined from low field data, while constraints are placed on the strength of the scattering to and within the (100) valleys by recent data on the variation of low field resistivity with pressure. It is demonstrated that this model can give a consistent explanation of the magnitude of the negative differential mobility and its threshold field as functions of temperature orientation, uniaxial stress and hydrostatic pressure. The relative importance of f((100) from or to (010)) and g((100) from or to (100)) scattering in the (100) valleys is discussed in relation to uniaxial stress experiments and it is concluded that g scattering must be an important process.

Hall effect measurements on n type Ge to 65 kbar are described. The Hall mobility of electrons through the transfer from (111) to (100) states has been measured and an assessment of the scattering parameters for the different intervalley and intravalley processes has been made following the analysis of Nathan et al. (1961). The Hall mobility of electrons in the (100) valleys is determined as 1020+or-170 cm²V^-1s^-1, the (100) effective mass is estimated to be approximately 50% greater than for n type Si and the (100)-(111) energy sub-band gap is 0.186+or-0.010 eV. Nonequivalent intervalley scattering between the (111) and (100) valleys is shown to reduce the mobility by a factor of two near band cross-over. Results are directly relevant in determining coupling constants between valleys in high electric field calculations, involving (100) and (111) minima, as in InP.

Using the antisymmetrized version of a two-body interaction in a many electron system the author demonstrates its general exchange character and derives the s-d exchange interaction of Kasuya (1956).

A method is described for obtaining initial-slope values for magnetization curves of superconductors which is appreciably more accurate thab that of relying on the first few low field points.

The consequence of incorporating crystal field interactions into the molecular field theory of magnetism are investigated, with special reference to noncubic symmetry. Spin-orbit coupling is assumed to dominate both the exchange and the crystal field interactions, but no approximations are made concerning the relative magnitudes of exchange and crystal field. Topics discussed include differential susceptibilities in the paramagnetic and ferromagnetic phases, the effect of the crystal field on the Curie temperature and on the saturation magnetization, the temperature dependence of the direction of spontaneous magnetization, and crystal field quenching of the cooperative phase. The theory is illustrated by numerical calculations for ions with an incomplete f shell in a crystal field of hexagonal symmetry.

The transverse susceptibility is derived by means of Green function techniques in the itinerant model for a ferromagnet. Correlations within the many band system were taken into account through the t matrix approximation. The expression for the susceptibility was found to decompose naturally into two parts from which the collective excitations and, in particular, the spin wave modes can be determined. Explicit expressions were obtained for both the acoustic and the optical modes for some simple cases.

A crystal of KZnF₃ containing added Mn²⁺ and Cu²⁺ has been shown to have three absorption regions associated with nearest neighbour pairs of Cu²⁺ and Mn²⁺. Two correspond to electronic excitation energy on the Mn²⁺ and the third is an electron transfer process. All three occur only for the pair in which the tetragonal axis of the Jahn-Teller distortion around the Cu²⁺ is directed towards the Mn²⁺. Temperature dependence measurements of the absorption intensity can be accounted for by using an exchange energy of 130+or-10 cm^-1 for this pair in the ground state. The two absorption regions corresponding to Mn²⁺ excitation comprise mainly the Delta S=0 transitions (⁴A_1g at 25052 cm^-1 and ⁴E_g^b at 29980 cm^-1) although three other lines are assigned to Delta S=-1 transitions (⁴A_1g at 25244.7 cm^-1 and 25254.3 cm^-1 (split by spin-orbit interaction) and ⁴E_g^b at 30460 cm^-1). These latter assignments demand exchange energies of -100+or-5 cm^-1 and -240+or-30 cm^-1 respectively, for the two excited states and are of opposite sign to the ground state value.

A theory of transition dipole-electromagnetic radiation interaction in a dielectric medium, developed by Dissado (1970), is applied to the calculation of the absorption intensities of solids in terms of the refractive index. Two cases are considered, that of the mixed crystal (solid solution of tetracene in anthracene) and of the pure crystal (anthracene-1st singlet state), for which the polarization ratios are determined and compared with experiment. Additionally, a value of the Davydov splitting is obtained for the case of the pure crystal which is compared to earlier experimental and theoretical values obtained by other workers.

The selection and polarization rules for the optical absorption lines of Pr in LaEtS are discussed and compared with experimental spectra obtained at liquid nitrogen temperatures. It is found that the rules obeyed are those of C_3h symmetry rather than those of the D_3h symmetry usually assumed in crystal field treatments.

Trigonal HgS is a wide bandgap semiconductor possessing a twofold coordinated low symmetry structure highly unusual among II-VI compounds. The authors have investigated the photoelectronic properties of two types of high purity natural HgS crystals. Measurements of temperature dependent electrical conductivity, thermal and infrared quenching of photoconductivity, and luminescence have been used to locate the energies of several localized levels in the forbidden gap. A similar distribution of energies is deduced for the two varieties of natural crystals, although luminescence and the optical quenching of photoconductivity are weaker in one type containing a small amount of Fe impurity. Despite its distinctive crystal structure, the results presented for trigonal HgS are quite similar to those documented for zinc blende and wurtzite structure II-VI semiconductors.

NQR studies are presented for HgCl₂. The asymmetry parameters of the electric field gradient tensor at the sites of chlorine nuclei in mercuric chloride were determined by Zeeman effect studies in both single crystals and polycrystalline forms of the substance. These investigations gave an average value of 0.08+or-0.03 for the asymmetry parameter. The results of the work are discussed and compared with the results of previous work on this substances.

The lineshape and spin-lattice relaxation times have been measured in solid 1,3,5 cycloheptatriene between 50 K and the melting point. The linewidth change which occurs below the heat capacity transition point of 154 K indicates considerable molecular motion. The very low value for the second moment of the lineshape (0.8 G²) cannot be explained by any simple motion, and various possibilities are discussed. This motion is governed by an activation energy of 6.1+or-0.5 kcal mol^-1. Above the transition point, where the lattice spacing is known, free quasi-isotropic reorientation is occurring, coupled with molecular diffusion at an increasing rate.