La15NbxGe9: a superstructure of the Mn5Si3 structure type with interstitial Nb atoms

The crystal structure and elementary properties of La15NbxGe9 are reported. Single-crystal X-ray diffraction, from a crystallite with only 0.12 Nb/formula unit, reveals that this compound, although transition metal deficient, crystallizes in a hexagonal “15-1-9”-like structure type, space group P63mc (no. 186) with lattice parameters a = b = 15.5017(2) A, c = 6.9173(2) A. The physical properties were examined by specific heat and resistivity measurements. La15Nb0.4Ge9 shows metallic behavior, and no superconductivity is observed above 0.4 K. The Sommerfeld coefficient (γ = 62.41(8) mJ mol−1 K−2) and the Debye temperature ( θ D = 267(1) K) were estimated from a fit to the low temperature heat capacity data. La15Nb0.4Ge9 is the first reported compound in the ternary La-Nb-Ge system.