Two isostructural manganese(III) diphosphates, acid α-MnHP2O7 and double MnLiP2O7:: crystal structure determination from X-ray powder diffraction data

Ab initio crystal structure determination of the a form of MnHP2O7 and Rietveld refinement of the MnLiP2O7 structure were performed using powder X-ray diffraction data. The final structures have been obtained using soft restraints on interatomic bond lengths and bond angles for the diphosphate group. The two compounds were found to be to a great extent isostructural. They crystallize in the monoclinic space group P2(1), Z = 2, a 6.7547(6), b = 8.3160(7), c = 4.8769(5) angstrom, beta = 113.1791(18)degrees, R-p = 0.0283, R-wp = 0.0390, R-Bragg 0.0930, R-F = 0.0869 for alpha-MnHP2O7, and a = 7.0018(5), b = 8.1616(6), c = 4.7267(4)angstrom, beta = 109.404(3)degrees, R-p = 0.0307, R-wp = 0.0423, R-Bragg = 0.0960, R-F = 0.0861 for MnLiP2O7. In both compounds, diphosphate groups adopt nearly eclipsed configuration. Their structures present three-dimensional networks, which have tunnels including the lithium cations or hydrogen atoms. The behaviour of the crystalline forms of MnHP2O7 in H/Li exchange reactions is discussed, vs. their structural aspects.