The crystal structure of Hf1.5+δNb1.5−δAs and structure–composition relations in the section Hf3As–Nb3As

The title compound Hf1.5+δNb1.5−δAs was characterized by means of single crystal X-ray diffraction. It represents a new structure type of intermetallic compounds (space group Pnma; lattice parameters a=7.142(2) Å, b=3.583(2) Å, c=11.640(2) Å) and shows a small homogeneity range corresponding to (0.1<δ<0.25) at 1400 °C. The crystal structure may be visualized by a combination of As-centred trigonal prisms of the metal atoms and bcc-like fragments formed by metal atoms. Structural relations with various binary arsenides are discussed. The structure of Hf1.5+δNb1.5−δAs shows significant preferred site occupation of Hf and Nb at the three independent metal positions (differential fractional site occupancy). Structure–composition relations in the section Hf3As–Nb3As which also contains the new phase Hf2+δNb1−δAs with Ti3P-type structure (space group P42/n) are discussed. Ground state energies of various ordered compounds with Hf1.5+δNb1.5−δAs-, Ti3P- and Ta3As-type structures were calculated from ab initio density functional theory. These energies were used for thermodynamic calculations employing the compound energy formalism (CEF) with the aim to model the experimentally observed site fraction data for both ternary compounds as well as Gibbs energies at the temperature of equilibration (1400 °C).