Effect of High Pressure on the Crystal Structure and Vibrational Properties of Olivine-Type LiNiPO4.

In this work, we present an experimental and theoretical study of the effects of high pressure and high temperature on the structural properties of olivine-type LiNiPO4. This compound is part of an interesting class of materials primarily studied for their potential use as electrodes in lithium-ion batteries. We found that the original olivine structure (α-phase) is stable up to ∼40 GPa. Above this pressure, the onset of a new phase is observed, as put in evidence by the X-ray diffraction (XRD) experiments. The structural refinement shows that the new phase (known as β-phase) belongs to space group Cmcm. At room temperature, the two phases coexist at least up to 50 GPa. A complete conversion to the β-phase was only obtained at high-pressure and high-temperature conditions (973 K, 6.5 GPa), as confirmed by both XRD and Raman spectroscopy. Ab initio calculations support the same structural sequence. The need of high-temperature conditions to obtain the complete transformation of the α-phase into the β-phase is indicative of the existence of a kinetic barrier for the phase transition. Here, we report the evolution of crystallographic parameters as a function of pressure for both phases, comparing them with the theoretical predictions. We also discuss the influence of pressure on the polyhedral units and report room-temperature equations of state. The dependence of the Raman phonons of both phases on pressure is also studied, assigning to each phonon its respective symmetry by comparison with the results of the ab initio simulations.