Crossover from semiconductor to magnetic metal in semi-Heusler phases as a function of valence electron concentration

Experimental and theoretical investigations of intermetallic semi-Heusler compounds (CoTiSn, FeTiSb, CoTiSb, NiTiSn, CoNbSn, CoVSb, NiTiSb) and their solid solutions (CoTiSn1-xSbx, CoTi1-xNbxSn) are presented. The physical properties of these systems are found to be mostly determined by the number of valence electrons. Resistivity experiments show that compounds with 18 valence electrons are either semiconductors (CoTiSb, NiTiSn) or semi-metals (CoNbSn). The electronic structure calculations performed on 18-valence-electron systems by the KKR method show nine valence bands below the Fermi level and a gap of order 0.4-0.9 eV. A decrease or increase of the number of valence electrons in CoTiSb, NiTiSn or CoNbSn leads in either case to a metallic state and either ferromagnetic (CoTiSn, CoVSb) or paramagnetic (FeTiSb, NiTiSb) properties. The KKR results concerning 17- and 19-valence-electron systems correspond well with experimental characteristics, except in the case of CoVSb which KKR calculations predict to be a half-metallic ferromagnet, which conflicts with experimental data. Magnetization and resistivity measurements indicate that semiconductor-metal crossovers occur together with the appearance of ferromagnetism in the CoTiSn1-xSbx and Co1-xNixTiSn series, for x near 0.4. This behaviour is discussed in the context of the KKR-CPA results.