The reasons and conditions for applicability of the Cauchy distribution law for crystal structure factor components are discussed. It is shown that the standardized structure factors of centrosymmetric crystals are quite prone to be Cauchy distributed for crystals having planar molecules in their unit cells.

The impurity induced charge density is calculated in jellium by solving the Schrödinger equation self-consistently following the procedure of Manninen and Nieminen and using Kohn-Sham density functional formalism. The host-ion contribution is included through the spherical solid model potential (SSMP). The calculated activation energy 0.27 eV is found in good agreement with experimental value 0.28±0.02 eV. The estimated residual resistivity 1.02 µΩ cm/at% for Lu-H system using the resulting phase shifts agrees reasonably well with the observed value 1.75±0.10 µΩ cm/at%. The calculated configurational energy shows that hydrogen prefers tetrahedral(T)-sites over octahedral(O)-sites in Lu matrix. This has been confirmed by Bonnet experimentally. A very shallow value ofs-type bound state of energy −0.00316 Ryd predicts that there is no formation of lutetium hydride solution and H⁺ exists as a free ion in Lu matrix.

The isotropic Compton profile of iridium, measured using 59.54 keVγ-rays, is reported in this paper. The results are compared with the theoretical Compton profiles from APW method with and without incorporating electron correlation effects. It is seen that correlation effects improve the agreement between the experiment and theory. Comparison with the renormalized-free-atom (RFA) model calculations has also been made. Behaviour ofd-band electrons in Ir and some other 5d transition metals is discussed in terms of broadening in their Compton profiles.

Neutron diffraction and Mössbauer measurements have been carried out on the cubic Laves phase intermetallic TbMnFe. The magnetic moment on the transition metal atom is found to be low, 0.2µ_B, at room temperature. This moment is temperature independent down to 10 K. Magnetic moment on the rare earth atom varies from 2.5µ_B at 296 K to 7.27µ_B at 10 K. Mössbauer spectra recorded at 298 K and 78 K have magnetic character but there is a large distribution of hyperfine field values. Both these features arise due to magnetic frustration created in the sample due to the competing ferro and antiferromagnetic interactions between the transition metal atoms.

We report the optical spectra and single crystal magnetic susceptibility of the one-dimensional antiferromagnet KFeS₂. Measurements have been carried out to ascertain the spin state of Fe³⁺ and the nature of the magnetic interactions in this compound. The optical spectra and magnetic susceptibility could be consistently interpreted using aS=1/2 spin ground state for the Fe³⁺ ion. The features in the optical spectra have been assigned to transitions within thed-electron manifold of the Fe³⁺ ion, and analysed in the strong field limit of the ligand field theory. The high temperature isotropic magnetic susceptibility is typical of a low-dimensional system and exhibits a broad maximum at ∼565K. The susceptibility shows a well defined transition to a three dimensionally ordered antiferromagnetic state atT_N=250 K. The intra and interchain exchange constants,J andJ′, have been evaluated from the experimental susceptibilities using the relationship between these quantities, andχ_max,T_max, andT_N for a spin 1/2 one-dimensional chain. The values areJ=−440.71 K, andJ′=53.94 K. Using these values ofJ andJ′, the susceptibility of a spin 1/2 Heisenberg chain was calculated. A non-interacting spin wave model was used belowT_N. The susceptibility in the paramagnetic region was calculated from the theoretical curves for an infiniteS=1/2 chain. The calculated susceptibility compares well with the experimental data of KFeS₂. Further support for a one-dimensional spin 1/2 model comes from the fact that the calculated perpendicular susceptibility at 0K (2.75×10⁻⁴ emu/mol) evaluated considering the zero point reduction in magnetization from spin wave theory is close to the projected value (2.7×10⁻⁴ emu/mol) obtained from the experimental data.

A new field emission theory of dislocation-sensitized photo-stimulated exo-electron emission (DSPEE) is proposed, which shows that the increase in the intensity of photo emission fromF-centres during plastic deformation is caused by the appearance of an electric field which draws excited electrons out of the deeper layer and, therefore, increases the number of electrons which reach the surface. The theory of DSPEE shows that the variation of DSPEE flux intensity should obey the following relation$$\frac{{\Delta J_e \left( \varepsilon \right)}}{{J_e \left( o \right)}} = \left[ {\frac{{Y_s }}{{d_F }}\exp \left( {\frac{\chi }{{kT}}} \right) - 1} \right]$$. The theory of DSPEE is able to explain several experimental observations like linear increase of DSPEE intensityJ_e with the strain at low deformation, occurrence of the saturation inJ_e at higher deformation, temperature dependence ofJ_e, linear dependence ofJ_e on the electric field strength, the order of the critical strain at which saturation occurs inJ_e, and the ratio of the PEE intensity of deformed and undeformed crystals. At lower values of the strain, some of the excited electrons are captured by surface traps, where the deformation generated electric field is not able to cause the exo-emission. At larger deformation (in between 2% and 3%) of the crystal, the deformation-generated electric field becomes sufficient to cause an additional exo-electron emission of the electrons trapped in surface traps, and therefore,t here appears a hump in theJ_e versusε curves of the crystals.

The nature of the temperature dependence of luminescence intensity from Er⁺ ions in GaInAsP, Si, InP, GaAs, AlGaAs, ZnTe, as observed by Favennecet al [1] has been examined in terms of a double exponential model. The smaller activation energy is found to be 58–100 meV, characteristic of a localized energy barrier at the Er⁺ centre while the higher activation energy is approximately 0.8E_g attributed to an Auger non-radiative process of carrier excitation into bands. This model has been found to describe the observed temperature dependences with reasonably good agreement.

Spectroscopic properties of Er³⁺:CBS (CdSO₄+B₂O₃ and R₂SO₄+CdSO₄+B₂O₃, R₂SO₄=Li₂SO₄.H₂O, Na₂SO₄, K₂SO₄ and Gd₂(SO₄)₃.8H₂O) glasses are reported. The assigned energy level data of Er³⁺(4f¹¹) in these glasses are analysed in terms of a parametrized model Hamiltonian. The standard deviations of the data fits are between 39 and 47 cm⁻¹ so that the energy level schemes of the Er³⁺(4f¹¹) ions in borosulphate (CBS) glasses are reasonably well reproduced. Radiative properties for the fluorescent levels of Er³⁺:CBS glasses are determined by using the Judd-Ofelt theory. The potential laser transitions are identified with the help of predicted radiative properties which are compared and discussed with similar results.

Collision strengths have been calculated for electron impact excitation of sodium-like TiXII for all 21 transitions amongst its lowest seven states. Configuration interaction wave functions have been used to represent the target states. The standard and no-exchangeR-matrix codes have been used to calculate the contribution of partial waves withL⩽8 andL>8 respectively. Collision strengths are tabulated at selected energies in the range 26 to 50 Ryd. Effective collision strengths are tabulated for electron temperatures in the range logT_e=4.0 to logT_e=6.0, withT_e in K. This is the first detailed calculation on this ion in which the effects of exchange, channel couplings and short-range correlation are taken into account.

We report for the first time the Compton profiles of electron momentum distribution inβ-gallium calculated along the crystallographic directions (100), (110) and (111). The conduction electron states for this purpose are determined in the energy band calculations using model potential. The core states, on the other hand, are represented each by a single tight-binding function. The results show that the Compton profiles are nearly isotropic with very little directional dependence, which is suggestive of a free-electron-like distribution of the conduction electrons in this system. The latter conclusion is in close confirmity with similar conclusions drawn in augmented plane wave (APW) calculation of energy bands and the derived Knight-shift results inβ-Ga.