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 × 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.