TiAs2-type Phases

Abstract

THE structure of TiAs2, which is of a new type, has recently been determined by Wenglowski et al.1. The orthorhombic unit cell contains eight formula units which comprise two kinds of titanium and four kinds of arsenic atoms. The co-ordination of TiI and TiII is very similar. Both have six As neighbours forming a trigonal prism and three more neighbours lying at the corners of a triangle perpendicular to the prism axis. The corners of this triangle are occupied by 3As in the case of TiI and by 2As + 1TiII in the case of TiII. AsI is located at the centre of a tetrahedron formed by 1TiII + 2TiII + 1AsI. AsII and AsIII lie at the centre of a distorted tetrahedron of 3TiI + 1TiII. Finally, AsIV is placed at the centre of a trigonal prism of 2TiI + 4TiII, a co-ordination which is also found in α-TiAs (TiP type) and β-TiAs (NiAs type). This coordination immediately reveals that TiAs2 must be metallic though its composition would suggest the possibility of a semi-conductor of ionic formula Ti4+(As2)4−. However, only one of the four different arsenic atoms forms pairs, but half the cations are connected as well. Assuming single bonds between TiII–TiII and AsI–AsI, we are left with 44 anion valence electrons per unit cell for the cationanion bonds which cannot be balanced by the 28 available cation electrons and thus induce metallic conductivity. My resistivity measurements in fact confirm these conclusions.