CALCULATION OF THE ELECTRONIC-STRUCTURE OF SP ELEMENTS IN BETA-SI3N4 WITH CORRELATION TO SOLUBILITY AND SOLUTION EFFECTS

Local electronic structures around the sp-element solutes in beta-Si3N4 have been examined from first principles by the discrete-variational (DV) Xalpha method for the first time. A solute atom, M, was put substitutionally into the Si sites of a (Si3N10)18- cluster, and Hartree-Fock-Slater equations were solved self-consistently. The relationship between the calculated overlap population of electrons between the s solute and nitrogen and the experimental solubility and the magnitude of the solution effects was examined, and a satisfactory correlation was found. Based on the present calculations, the sp-element solutes are classified into the following three categories: (1) Elements such as K, Ca, Na, and Mg are almost insoluble and localize at the intergranular glassy phase or precipitates. (2) Remarkable solution effects to lose the local covalency are expected for Li, Be, Ga, and Al. They may enhance the volume diffusivity and also decrease the elastic constants considerably. (3) For B, C, and Ge, no significant reduction in local covalency is expected when they dissolve. The correlations between the calculated overlap population and empirical ionic radii of solute atoms and Pauling's electronegativity were found to be weak, indicating that these empirical parameters are not sufficient to represent the electronic structure change due to the substitution and hence solubility and solution effects, although they are useful in describing general trends.