Towards a unified description of the AMOB₂O₅ (a = K, Rb, Cs, TI; M = Nb, Ta) family of compounds

Single-crystal X-ray diffraction data (Mo Kα radiation) are used to re-refine the structure of RbNbOB₂O₅, rubidium niobium oxo pyroborate. The structure is refined as an incommensurate modulated structure with superspace group symmetry Pmn2₁(0,0.4,0)s and lattice parameters a = 7.406 (2), b = 3.939 (2) and c = 9.475 (2) A. Refinement on 3242 unique reflections converged to R = 0.031, while a previous conventional superstructure refinement led to R = 0.090. This lowering of the R factor goes hand-in-hand with a substantial reduction in the number of refined parameters. The refinement strongly suggests that the structure is effectively incommensurately modulated, despite an apparently rational magnitude of the primary modulation wave-vector and overlap of satellite reflections.