Structural Redetermination and Photoluminescence Properties of the Niobium Oxyphosphate (NbO)₂P₄O₁₃

The structure of (NbO)₂P₄O₁₃ was solved and refined based on new single-crystal diffraction data revealing considerably more complexity than previously described. (NbO)₂P₄O₁₃ crystallizes in the triclinic space group P1̅ with Z = 6. The lattice parameters determined at room temperature are a = 1066.42(4) pm, b = 1083.09(4) pm, c = 1560.46(5) pm, α = 98.55(1)°, β = 95.57(1)°, γ = 102.92(1)°, and V = 1.7213(2) nm³. The superstructure contains 64 unique atoms including two disordered semioccupied oxygen positions. An unusual 180° bond angle between two [P₄O₁₃]^6- groups was refined to form half-occupied, split positions in agreement with previous reports. The IR and Raman spectra reflect the appearance of overlapping bands assignable to specific group vibrations as well as P-O-P linkages present in the [P₄O₁₃]^6- entities. Investigation of the powdered product concerning its photoluminescence properties revealed an excitability in the UV at 270 nm assigned to O2p-Nb4d charge transfer transitions. A resulting broad-band emission with the maximum in the visible region at 455 nm was determined.