ELECTRONIC-STRUCTURE OF 3D TRANSITION-ELEMENTS IN BETA-SI3N4

Impurity levels due to the presence of 3d transition elements (M) in beta=Si3N4, the net impurity charge, and the chemical bonds around the impurity atom are all examined by the use of first-principles molecular-orbital calculations for atomic clusters consisting of 13 atoms. The structure of the valence band obtained from calculations on a (Si3N10)(18-) cluster is in good agreement with the experimental X-ray photoelectron emission spectra as well as the Si L X-ray emission spectra of bulk Si3N4. M-3d levels are located in a gap between occupied and unoccupied bands of Si3N4. The energy of M-3d levels decreases and the occupation number increases with rising atomic number. The net charge is between + 2.1 and + 1.6, depending on the atomic number of M when the formal charge is M(4+). nh, net charge increment from M(2+) to M(4+) is about 0.5 for V, Cr and Mn, and it is smaller in the other elements. When formal charges higher than M(4+) are assumed, viz, for V5+, Cr6+ and Mn7+, the net impurity charge turns out to be not significantly different from that obtained for M(4+). This means that N, rather than M, is oxidized when the oxidation number of M is higher than (4+). The overlap population between M and N is far below zero for Ca2+, and it increases with rising atomic number. It is between 0.0 and 0.2 for elements between Ti and Ni when the formal charge is M(4+). The overlap population is smaller for M(2+). The value is negative except for Zn2+ The variation of the overlap population is determined by the balance of two terms, i.e. (i) anti-bonding between M core electrons and N-2s/2p valence electrons, and (ii) bonding and anti-bonding between M-3d/4s, 4p valence electrons and N-2s/2p valence electrons. The overlap population may be correlated to the solid solubility and the magnitude of solution effects for 3d transition elements in beta-Si3N4. The results may be useful as guiding principles for additive selection and alloying design of new Si3N4 transition-element systems.