Electronic phase diagram of the XTZ (X=Fe, Co, Ni; T=Ti, V, Zr, Nb, Mn; Z=Sn, Sb) semi-Heusler compounds

Some general aspects of electronic structure of the XTZ semi-Heusler systems are discussed using the Korringa-Kohn-Rostoker computations within the LDA framework. An energy gap appears for a total number of 18 valence electrons, which corresponds to semiconductors. This gap is still maintained in the density of states in other XTZ compounds with different electron concentration (EC). This peculiar electronic structure gives rise to a variety of different physical properties, starting from EC=16 (FeTiSn) and EC=17 (FeTiSb, CoTiSn) (paramagnetic or ferromagnetic) metals, through semiconductors (NiTiSn, NiZrSn, NiYSb, FeVSb, CoTiSb, etc.) and EC=19 (CoVSb, CoNbSb, NiTiSb) metallic phases. The reason for the instability of the semi-Heusler phase of NiVSb (EC=20) is discussed using the total energy KKR analysis. The characteristic gap in the density of states is also maintained for the minority spin projection in the case of the half-metallic ferromagnetism in CoMnSb (EC=21) and NiMnSb (EC=22). Some of the semiconducting compounds (NiTiSn, FeVSb, NiYSb) seem to possess promising thermoelectrical properties. (C) 2000 Elsevier Science S.A. All rights reserved.