La(3)X(3)Z-compounds with condensed La(6)Z octahedra helically connected in three dimensions

The subhalides La(3)X(3)Z (X = Br, I. Z = Si, P, As, Sb, C-2) were synthesi zed from stoichiometric mixtures of La, LaX3 and Z under Ar atmosphere in s ealed Ta ampoules at 950 degreesC to 1200 degreesC for 3-30 days. La(3)X(3) Z (X = Br, I, Z = Si, P, As, Sb) is isostructural to Gd3Cl3C (Z = 8, space group I4(1)32, No. 214) which can be described as a defect NaCl type. This structure is characterized by the main group element Z centered octahedra p ropagating helically in three dimensions. The lattice constants a are 12.16 3(3), 12.4267(5), 12.533(1) and 12.780(1) Angstrom for La3Br3Si, La3I3P, La 3I3As and La3I3Sb, respectively. The excess electrons in the La d(xy) condu ction band lead to a metallic behavior for these compounds. La3Br3Si underg oes a metal-insulator transition at 36 K which is attributed to a structura l change. La3Br3C2 crystallizes in a different space group C222(1) (No. 20) , Z = 16, a = 11.5330(6) Angstrom, b = 17.0698(6) Angstrom, c = 17.0540(8) A. The C-2 units center highly distorted La octahedra. This structure, howe ver, is related to the above I4(1)32 structure in that the edge-sharing La- 6 octahedra fill space in a similar way. This compound is a semiconductor ( electrical gap E-g = 0.02 eV) and its conducting property can be understood from the closed-shell electron configuration of (La3+)(3)(Br-)(3)(C-2)(6-) .