Formation of Metastable Na 2 CrO 4 ‐Type LiNiPO 4 from a Phosphate–Formate Precursor

High‐pressure modification of LiNiPO 4 with a Na 2 CrO 4 ‐type structure was obtained at ambient pressure and low temperature from a mixed LiNi–phosphate–formate precursor, LiNiPO 4 H x (HCOO) x · y H 2 O (where x ≈ 1.2 and y ≈ 2.5). The structural and thermal characterization of the precursor and the LiNiPO 4 compositions were carried out by powder XRD analysis, IR spectroscopy, and DSC analysis. Thermal treatment of LiNiPO 4 H x (HCOO) x · y H 2 O precursors between 450 and 650 °C yields a mixture of the two structural modifications of LiNiPO 4 : the Na 2 CrO 4 type and the olivine type. It was established that the obtained Na 2 CrO 4 ‐type LiNiPO 4 is a metastable phase, which completely transforms at 700 °C into the olivine‐type phase. The enthalpy of the phase transition is Δ H = –43.40 kJ mol –1 . The mechanism of formation of the two forms of LiNiPO 4 from the LiNi–phosphate–formate precursor is discussed.