Band gap and stability in the ternary intermetallic compounds NiSnM (M=Ti,Zr,Hf): A first-principles study

The structural stability and electronic properties of the ternary intermetallic compounds NiSnM (M=Ti,Zr,Hf) and the closely related Heusler compounds Ni2SnM are discussed using the results of ab initio pseudopotential total-energy and band-structure calculations performed with a plane-wave basis set using the conjugate gradients algorithm. The results characterize the lowest-energy phase of NiSnM compounds, with a SnM rocksalt structure sublattice, as narrow-gap semiconductors with indirect gaps near 0.5 eV, while the Ni2SnM compounds are described as normal metals. Two other atomic arrangements for NiSnM in the MgAgAs structure type result in energetically unfavorable compounds that are metallic. The gap formation in the lowest-energy structure of NiSnZr and relative stability of the three atomic arrangements are investigated within a tight-binding framework and by considering the decomposition of each ternary compound into a binary substructure plus a third element sublattice. The stabilization of the lowest-energy phase of NiSnZr is found to be mainly due to the relative stability of the SnZr rocksalt substructure, while the opening of the gap induced by the addition of the symmetry-breaking Ni sublattice makes a relatively minor contribution. The results from the theoretical calculations for the NiSnM compounds are compared with the existing experimental data. From analysis of structural and chemical trends in the NiSnM compounds, CoVSn is predicted to be a semiconducting intermetallic compound in the MgAgAs structure type. Preliminary first-principles calculations suggest an indirect gap of 0.8 eV.