The crystal structures of Hf3±dNb4±dAs3 and Hf7.2Nb3.8As4: Members of a homologous series combining W-type, Mg-type and AlB2-type building blocks

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 Hf3±dNb4±dAs3 is isostructural to Zr3Pd4P3 and Hf3Pd4P3 (Space group: Pnma, Pearson symbol oP40) and shows a considerable homogeneity range between the limiting compositions Hf2.4Nb4.6As3 and Hf4.1Nb2.9As3. 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 Hf7.2Nb3.8As4 is isostructural with Zr6.45Nb4.55P4 (Immm, oI30) and was found to be metastable at the temperature of annealing (1400 °C). Similar to the type structure, Hf7.2Nb3.8As4 is stabilized by (DFSO). Structural relations of the two compounds with Hf1.5+dNb1.5-dAs (Pnma, oP16) are discussed. All three compounds belong to a homologous series Mk+2l+mX2m combining W-type, Mg-type and AlB2-type building blocks.