Synthesis, structure, and electrical properties of Eu_0.59Nb₅Se₈

The new compound Eu_0.59Nb₅Se₈ has been prepared from a combination of the elemental powders at 1275 K. The material crystallizes with four formula units in space group C³_2h- C2 m of the monoclinic system in a cell (at 113 K) of dimensions a = 17.95(2), b = 3.388(4), c = 19.30(2)Å, β = 107.90(2)°. The structure has been solved and refined from single-crystal X-ray data. The asymmetric unit contains two independent Eu atoms, five independent Nb atoms, and eight independent Se atoms. Each Eu site is approximately 59% occupied; the Eu atoms are in an unusual distorted cube of Se atoms with EuSe interactions ranging from 3.108(6) to 3.454(6) Å. Three of the independent Nb atoms are in trigonal prisms of Se atoms with NbSe interactions ranging from 2.548(6) to 2.617(6) Å while the other two independent Nb atoms are in unusual distorted octahedral environments with NbSe interactions ranging from 2.526(9) to 2.718(7) Å. These latter two Nb atoms show short NbNb interactions of about 2.95 Å. The material exhibits excellent metallic conductivity along the needle (b) direction with σ_{298 K} of 35 × 10³ ohm^-1 cm^-1 and σ_{12 K} ≈ 1.4 × 10⁶ohm^-1 cm^-1.