Towards a unified description of the AMOB₂O₅ (A = K, Rb, Cs, Tl; 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) Å. 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 wavevector and overlap of satellite reflections.