Novel quaternary metal-rich phosphides: Stabilization by differential fractional site occupancies and polar intermetallic bonding

The new phosphide Hf5Nb5Ni3P5 has been prepared by are-melting of a pressed stoichiometric mixture of HfP, Nb, and Ni. Single crystals suitable for the structure determination were obtained after annealing in an induction furnace at 1350 degrees C. Hf5Nb5Ni3P5 crystallizes in the hexagonal space group <P(6)over bar 2m> (No. 189). The early transition metal atoms Hf and Nb are found on three crystallographically different sites, with refined occupancies of 100% Hf for the M1, 59.5(4)% Hf and 40.5(4)% Nb for the M2, and 13.1(6)% Hf and 86.9(6)% Nb for the M3 site. Small variations of the Hf to Nb ratio are possible, occurring with significantly different lattice parameters. The early transition elements form a three-dimensional framework with numerous M-M (M = Hf, Nb) interactions, including Ni and P in its trigonal prismatic voids. The differential fractional site occupancies can be understood on the basis of the different preferences of Hf and Nb to form M-M, M-Ni, and M-P bonds. The metallic character is confirmed by the Pauli paramagnetism experimentally obtained.