Na 2+ xNb 6P 4O 26: A novel niobium phosphate bronze isotypic of the m = 3 member of the MPTB p family
A new niobium phosphate bronze Na 2+ xNb 6P 4O 26 (0 ≤ x ≤ 0.75) has been isolated. Its structure was solved by single crystal X-ray diffraction. It crystallizes in the orthorhombic system with space group P2 12 12. The cell parameters are a = 19.8050(10), b = 14.3859(7), and c = 5.3960(4) Å. The framework is built up from ReO 3-type slabs of NbO 6 octahedra, perpendicular to a and linked by isolated PO 4 tetrahedra delimiting pentagonal tunnels running along c. The structure of this bronze corresponds to that expected for the hypothetical member m = 3 of the series of the monophosphate tungsten bronzes (MPTB p) with pentagonal tunnels (WO 3) 2 m(PO 2) 4. Relationships with other niobium phosphate bronzes are also discussed.