Crystal Structures,Bonding and Electronic Structures of MM′As,a Series of New Ternary Arsenides (M = Zr,Hf; M′ = Fe,Co, Ni)

All six new arsenides were prepared by arc-melting of preheated mixtures of the monoarsenides MAs with the 3d metals Fe, Co, and Ni, respectively. The isostructural title compounds all form the Co₂Si structure type, in contrast to the corresponding phosphides with ZrNiP occurring in the Ni₂In type. The anomalous expansions of the unit cells from ZrCoAs to ZrNiAs (V = 178.5(3) ų versus V = 182.5(1) ų) and from HfCoAs to HfNiAs (V = 175.03(5) ų versus V = 177.0(1) ų) can be understood based on Extended Hückel calculations of the electronic structure of HfCoAs.