A niobium phosphate with a tunnel structure: Ca0.5+xCs2Nb6P3O24

A new niobium phosphate, Ca_0.5+xCs₂Nb₆P₃O₂₄ has been isolated. It crystallizes in the R32 space group, with the following parameters of the hexagonal cell: a_H = 13.379 Å, c_H = 10.371 Å. The determination of the structure by a single crystal X-ray diffraction study shows that its host lattice [Nb₆P₃O₂₄]_∞ can be described as the assemblage of mixed chains [Nb₂PO₁₃]_∞ running along c_H in which one PO₄ tetrahedron alternates with two NbO₆ octahedra. This framework delimits huge tunnels where the cesium cations are located and cages formed by [Nb₆P₃O₃₆] units occupied by calcium. The most striking feature of this framework deals with its similarity with the hexagonal tungsten bronze of Magnéli (HTB). The latter is discussed here by considering the stacking along c of [Nb₂PO₈]_∞ layers whose geometry is closely related to that of the HTBs. The possibility of nonstoichiometry leading to a mixed valency of niobium is considered.