Synthesis, characterization and vibrational spectroscopic study of Co, Mg co-doped LiMnPO₄

The isostructural olivine-like LiM(II)PO4 compounds [M(II)=Mn, Mn0.9Co0.1, Mn0.8Co0.1Mg0.1] were successfully generated through the solid state reaction from the synthesized NH4M(II)PO4∙H2O precursors. The TG/DTG/DTA, AAS/AES, FTIR and XRD methods were employed to confirm both NH4M(II)PO4∙H2O and LiM(II)PO4 compounds. Their morphologies were studied by SEM method. The shift of two theta angle of XRD to higher values was observed in metal doping compounds, which indicate the formation of the single phase of isodivalent doping of Co(2+) and Mg(2+) ions according to the change in the lattice parameters and cell volumes. Their infrared spectra are reported and discussed with respect to the normal vibrations of NH4(+), PO4(3-), P2O7(4-) and H2O molecules using factor group analysis. The correlation field splitting analysis of PO4(3-) in NH4M(II)PO4∙H2O (orthorhombic system, Pmn2(1), C(2v)(7) and Z=2, [(3×5)-6]×2=18 internal modes) symbolized as T(d)-C(s)-C(2v)(7) suggested the number of vibrational modes to be: Γ(Vib)=A1(6)+A2(3)+B1(6)+B2(3) and A1(6)+A2(3)+B1(3)+B2(6) for zx and yz plane respectively. While, LiM(II)PO4 crystallizes in the orthorhombic system the space group Pnma (D(2h)(16)), Z=4 and the site symmetry of PO4(3-) is C(s). The correlation field splitting of type T(d)-C(s)-D(2h)(16) were reported in relation to [(3×5)-6]×4=36 internal modes for PO4(3-) unit in the structure.