The A_0.6Nb₅Se₈ (A = Eu, Pb, Sn, and Sr) structure: A trigonal prismatic variation of the TIV₅S₈ channel structure

Three new ternary niobium selenides A_0.6Nb₅Se₈, A = Sr, Pb, and Sn, have been synthesized. The new compounds have the previously reported Eu_0.6Nb₅Se₈ structure, as judged by the similarity in intensities and positions of lines on X-ray powder patterns. Lattice constants at 298 K are:. a = 17.925(13), b = 3.425(2), c = 19.464(12) A ̊, β = 107.89(6)^o(Eu_0.6Nb₅Se₈). a = 17.970(6), b = 3.420(1), c = 19.525(6) A ̊, β = 107.88(3)^o (Pb_0.6Nb₅Se₈). a = 17.941(7), b = 3.414(1), c = 19.510(6) A ̊, β = 107.85(3)^o(Sn_0.6Nb₅Se₈). a = 18.007(10), b = 3.425(2), c = 19.462(8) A ̊, β = 107.93(5)^o(Sr_0.6Nb₅Se₈). The A_0.6Nb₅Se₈ structure comprises a ³_∞[Nb₅S₈] framework built from NbSe₂-type trigonal prismatic layers parallel to the (001) plane that are interconnected by face sharing to double chains of staggered edge-shared [NbSe₆] octahedra along the b direction. This ³_∞[Nb₅S₈] framework leaves channels parallel to b in which the A atoms lie. The A_0.6Nb₅Se₈ structure is distinct from the TIV₅S₈ structure, in which all the transition-metal atoms are octahedrally coordinated.