Temperature-Dependent Structural Changes of Na2Mn3(P2O7)(2) Phase in NaLiMnP2O7 Mixed Phase Compound

XRD measurements revealed that the investigated NaLiMnP2O7 compound consists of two phases: Na2Mn3(P2O7)(2) and LiMnPO4. We found by XRD measurements that at 313 K Na2Mn3(P2O7)(2) shows the triclinic symmetry of the compound (space group P-1), while LiMnPO4 phase showed the orthorhombic crystal symmetry (space group Pnma). The phase change between 773 K and 813 K was proved by thermal XRD, impedance spectroscopy and DTA analysis. No evident changes of peak intensities in thermal XRD were observed for LiMnPO4 phase. The impedance of NaLiMnP2O7 ceramics was measured at different temperatures and the total ionic conductivity of the compound was found to be of 5.3 x 10(-9) S/m at 400 K on heating stage and only slightly higher on cooling stage. However, the conductivity value at 800 K was much higher - about 1.45 S/m on both stages. The step-like phase transition resulted in the conductivity increase of about 3 orders of magnitude in a narrow range of temperature. The ionic transference number was found to be 0.91 at room temperature. The XRD spectra and conductivity analysis of synthesised two separate phases showed that the phase transition takes place only in Na2Mn3(P2O7)(2) ceramics.