The crystal structures of Hf 3±δNb 4±δAs 3 and Hf 7.2Nb 3.8As 4: Members of a homologous series combining W-type, Mg-type and AlB 2-type building blocks

Abstract

Two new partially ordered compounds were synthesized in the system Hf–Nb–As by arc melting and subsequent annealing at 1400 °C and they were structurally characterized by means of single crystal X-ray diffraction. The compound Hf 3± δ Nb 4± δ As 3 is isostructural to Zr 3Pd 4P 3 and Hf 3Pd 4P 3 (Space group: Pnma, Pearson symbol oP40) and shows a considerable homogeneity range between the limiting compositions Hf 2.4Nb 4.6As 3 and Hf 4.1Nb 2.9As 3. Mixed occupation of Hf and Nb were found at all seven independent metal sites, with varying Hf/Nb ratios, so the compound is stabilized by differential fractional site occupation (DFSO). The compound Hf 7.2Nb 3.8As 4 is isostructural with Zr 6.45Nb 4.55P 4 ( Immm, oI30) and was found to be metastable at the temperature of annealing (1400 °C). Similar to the type structure, Hf 7.2Nb 3.8As 4 is stabilized by (DFSO). Structural relations of the two compounds with Hf 1.5+ δ Nb 1.5− δ As ( Pnma, oP16) are discussed. All three compounds belong to a homologous series M k +2 l + mX 2 m combining W-type, Mg-type and AlB 2-type building blocks.