The crystal structure ofβ-NbPO5

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Abstract
The crystal structure of β -NbPO 5 has been solved by threedimensional single crystal X-ray analysis. The refinement in the cell of symmetry P 2 1 / c with a = 13.0969(16) Å, b = 5.2799(6) Å, c = 13.2281(16) Å, β = 120.334(8)°, Z = 8, has led to R = 0.035 and R w = 0.042 for 2888 reflections with σ ( I )/ I ≤ 0.333. The structure belongs to the structural family P 4 O 8 (WO 3 ) 2 m called monophosphate tungsten bronzes with pentagonal tunnels (MPTB P ) and characterized by ReO 3 -type slabs of various widths connected through single PO 4 tetrahedra. β -NbPO 5 and the isostructural compound β -TaPO 5 can thus be considered as the members m = 2 of the series and their framework described as built up of strands of two MO 6 (M = Nb, Ta) octahedra. Adjustment stresses induced by PO 4 tetrahedra lead to different octahedral deformations which explain the numerous supercells observed by electron diffraction.