Synthesis, Structural Characterization, Magnetic Properties, and Ionic Conductivity of Na4MII3(PO4)2(P2O7) (MII = Mn, Co, Ni)

The new phases, Na4M3(PO4)2(P2O7) (M = Mn, Co, Ni), have been synthesized by solid-state reactions. Single crystals of Na4M3(PO4)2(P2O7) (M = Mn, Ni) have been isolated and their structure has been determined by X-ray diffraction techniques using as the starting model the structure of the isostructural compound Na4Co3(PO4)2(P2O7). These compounds crystallize in the orthorhombic noncentrosymmetric space group Pn21a with a = 17.991(3) Å, b = 6.6483(1), and c = 10.765(2) Å for the manganese compound and a = 17.999(2), b = 6.4986(6) Å, and c = 10.4200(9) Å for the nickel compound, with Z = 4. Magnetic measurements reveal the existence of antiferromagnetic interactions in the nickel compound. The manganese and cobalt compounds show canting antiferromagnetic behavior at low temperatures. Magnetic correlation is also studied from the analysis of possible superexchange pathways in the structure. The ionic conductivity, due to Na+ ions, is measured for the three compounds. The activation energy is nearly the same (0.81−0.86 eV), but the conductivity at 300 °C changes, increasing from 2.1 × 10-7 S cm-1 for the Ni compound to 1.3 × 10-6 S cm-1 for the Co one, and then from this value to 2.7 × 10-5 S cm-1 for the Mn compound.