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

Abstract

Summary: Ab initio crystal structure determination of the α 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 P2₁, Z = 2, a = 6.7547(6), b = 8.3160(7), c = 4.8769(5) Å, β = 113.1791(18)°, R _p = 0.0283, R _wp = 0.0390, R _Bragg = 0.0930, R _F = 0.0869 for α-MnHP 2₂O₇, and a = 7.0018(5), b = 8.1616(6), c = 4.7267(4)Å, β = 109.404(3)°, 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.