Synthesis and Structure of a New Ternary Chalcogenide Nb₃Pd₀.72Se₇; Interrelationships in the Packing of Prisms and Planes

The ternary chalcogenide Nb₃Pd_0.72Se₇has been synthesized and its structure has been established through single-crystal X-ray measurements. The compound crystallizes in space group C³_2h—C2/m of the monoclinic system with four formula units in a cell of dimensions a = 12.803 (2) A, b = 3.413 (1) Å, c = 21.058 (2) A, and β = 95.47 (1)°. The structure is a new laminar type that consists of layers of formula ²_∞[Nb₆PdSe₁₄]. Between these layers additional Pd atoms statistically occupy (43%) a rhombic site. Occurrence of the layers ²_∞[Nb₆PdSe₁₄] and the known layers ²_∞[Nb₄PdSe₁₀] and ²_∞[Nb₂PdSe₆] allows formulation of the set of layers ²_∞[Nb_2nPdSe_4n+2], where n = 1, 2, and 3 have thus far been observed. This formulation includes the compositional behavior as well as the structural pattern that evolves with a serial change in the index n. This structural pattern is extended with comparisons between this set and the structural types NbSe₂, NbSe₂, Nb₂Pd₃Se₈, and Co₂Ta₄PdSe₁₂. From these comparisons, structural details, valence descriptions, and physical properties a model for the electronic structure of the ternary phases that contain Nb, Pd, and Se atoms is proposed.