THE CO P EXCHANGE IN HF5CO1+XP3-X AND ITS CONSEQUENCES FOR THE ELECTRONIC-STRUCTURE AND MAGNETIC-PROPERTIES/

The Co/P exchange observed in Hf5Co1+xP3-x is the first example for a statistical disordered Co/P occupancy on one position. In its crystal structure both different kinds of atoms, the late-transition metal ato m Co and the nonmetal P, are located in trigonal prismatic voids of th e Hf sublattice. The purely Co site has two P neighbours located in ne ighbouring trigonal prisms, each of which shares a rectangular face wi th the central Co prism, forming an infinite -Co-P- zigzag chain. The two other P sites have no contact to a Co atom. As a consequence of th is, a partial substitution of P by Co on the sites which have shorter distances to the surrounding Hf atoms can take place. This P position can be occupied by up to 50% Co, leading to a significant phase range (0 less than or equal to x less than or equal to 0.5 in Hf5Co1+xP3-x). However, increasing the Co to P ratio brings a higher density of stat es at the Fermi level, corresponding to higher magnetic susceptibiliti es as determined experimentally. The consequences of the Co/P exchange for the electronic structure are discussed.