Über Polyphosphide von Chrom, Molybdän und Wolfram

Abstract

MoP₂, α-WP₂ (OsGe₂ type structure) and β-WP₂ (MoP₂ type) were prepared for the first time in well crystallized form by reaction of the elemental components in a tin flux. Their crystal structures were refined from single-crystal X-ray data to conventional residuals ofR=0.047,R=0.048 andR=0.030 respectively. The proper space group for OsGe₂ (NbAs₂) type compounds is discussed. The previously reported high-pressure compound MoP₄ was prepared at ambient pressure in a tin flux. Solid solution studies show that MoP₂ and α-WP₂ can incorporate chromium up to the compositions Mo_0.94Cr_0.06P₂ and W_0.75Cr_0.25P₂, but CrP₂ cannot be prepared under normal pressure. MoP₂ and both modifications of WP₂ are diamagnetic. Electrical resistivity measurements of MoP₂ and α-WP₂ indicate semimetallic behaviour. In agreement with these properties the structures of MoP₂ and β-WP₂ can be rationalized completely if one d state of the P atoms of one site is assumed to participate fully in classical two-electron bonds. Although the structure of α-WP₂ is dominated by directional two-electron bonds, it has a tendency for higher coordination numbers and thus forms the transition to intermetallic compounds.