Ternary silicides M2Cr4Si5 (M=Ti, Zr, Hf):: filled variants of the Ta4SiTe4 structure type

The isostructural ternary silicides M2Cr4Si5 (M=Ti, Zr, Hf) were prepared by arc-melting of the elemental components. The single-crystal structure of Zr2Cr4Si5 was determined by X-ray diffraction (Pearson symbol oI44, orthorhombic, space group Main, Z = 4, a = 7.6354(12) Angstrom, b = 16.125(3) Angstrom, c = 5.0008(8) Angstrom). Zr2Cr4Si5 adopts the Nb2Cr4Si5-type structure, an ordered variant of the V6Si5-type structure. It consists of square antiprisms that have Zr and Cr atoms at the corners and Si atoms at the centers; they share opposite faces to form one-dimensional chains [Zr4/2Cr4/2Si] surrounded by additional Si atoms and extending along the c direction. In a new interpretation of the structure, additional Cr atoms occupy interstitial octahedral sites between these chains, clarifying the relation between this structure and that of Ta4SiTe4. The formation of short Si-Si bonds in Zr2Cr4Si5 is contrasted with the absence of Te-Te bonds in Ta4SiTe4. The compounds M2Cr4Si5 (M = Ti, Zr, Hf) exhibit metallic behavior and essentially temperature-independent paramagnetism. Bonding interactions were analyzed by band structure calculations, which confirm the importance of Si-Si bonding in these metal-rich compounds. (C) 2004 Elsevier Inc. All rights reserved.