Raman spectroscopy of superionic Ti-doped Li3Fe2(PO4)3 and LiNiPO4 structures

The Raman spectra of two systems, namely titanium-doped lithium iron and lithium nickel phosphates have been investigated. At room temperature, Li3Fe2(PO4)3 is reported having a monoclinic structure with P21/n space group, and transforms to a rhombohedral structure (γ-phase) with Pcan space group above 513 K. An orthorhombic structure with Pnma space group is reported for LiNiPO4 and Ti-doped LiNiPO4 polycrystalline samples. The Raman spectra of the first system, specifically Li3Fe2(PO4)3 and Ti-doped Li3Fe2(PO4)3, are assigned with respect to band positions reported in the literature for Li3Fe2(PO4)3 and Fe2(SO4)3 and, Nasicon-type compounds Li3In2(PO4)3 and Na3Fe2(PO4)3. Although spectra show a marked change in the broadness and number of bands observed, the overall profile is similar to α-Li3Fe2(PO4)3 which indicates up to 20 mol% solubility of titanium in the Li3Fe2(PO4)3 phase. The second LiNiPO4 system shows mixed phase behaviour and is composed of orthorhombic LiNiPO4 and a secondary phase, likely a titanium phosphate. With increasing Ti-dopant concentration up to 20 mol%, there is an increase in the relative proportion of the secondary phase, characterised by a strong band at 784 cm−1.