The electronic band structure, metallization, structural phase transition and superconductivity of cubic zinc blende-type GaP and GaN are investigated. The equilibrium lattice constant, bulk modulus and pressure at which the compounds undergo a structural phase transition from ZnS structure to NaCl structure are predicted from the total energy calculations. The density of states at the Fermi level (N(E F )) is enhanced after metallization, which leads to the superconductivity in GaP and GaN. The superconducting transition temperatures (T c ) of GaP and GaN are obtained as a function of pressure for both the ZnS and NaCl structures and GaP and GaN come under the category of pressure-induced superconductors. When pressure is increased T c increases in both the normal and high-pressure structures. The dependence of T c on the electron-phonon mass enhancement factor λ shows that GaP and GaN are electron-phononmediated superconductors. Also, it is found that GaP and GaN retained in their normal structure under high pressure give appreciably high T c .

Using first principle Pseudo Impurity Theory (PIT) and variational method, the binding energies of group V shallow donors in silicon are estimated incorporating the effect of core squeezing of the substitutional impurity atoms, with an empirical squeezing scheme of the core orbitals. Scaling factors are defined for various core states of the impurity atom to take explicit account of the varying degree of rigidity on squeezing. From the extent of squeezing required to reproduce the experimental binding energies of the donors, the local relaxation due to the size difference between the host and donor atoms is deduced and compared with experimental results and theoretical predictions available in literature. It is demonstrated that the squeezing scheme of the present work combined with the first principle pseudo impurity theory could deduce information about a purely local phenomenon such as lattice distortion reasonably well from the binding energy of the donor impurity.

The electronic structure of substitutional group V (P, As and Sb) and group VI (S, Se and Te, both neutral and singly ionized) impurities in silicon has been investigated using self-consistent Multiple Scattering Xa (MSXa) method within the framework of the standard muffin-tin potential approximation, incorporating the effect of lattice relaxation surrounding the impurity. A systematic study of the effect of local lattice relaxation on the electronic structure of group V (shallow) and group VI (deep) donors in Si is attempted. It is interesting to note that the size difference between the impurity atom and that of the host Si atom seems to be shared almost equally by the impurity atom (either expansion or squeezing) and the nearest neighbours (inward or outward movement). The effect of size difference on charge transfer and the bonding characteristics between the impurity atom and Si atoms are also analyzed.

A fully self-consistent calculation using the multiple scattering X, method is used to study the electronic structure, the relation between charge transfer and electronegativity, the bonding characteristics, and the alloying of Mg with Al. The present investigation indicates that the charge transfer from the host A1 atoms to the central impurity atom increases with the increase of electronegativity of impurity atoms. It is found that the bonding is metallic for AlLMg and for the othcr impurities considered it gradually becomes ionic. The alloying of Mg with A1 is more favourable than with other impurities. The results are compared with the available experimental and theoretical data.

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