Evidence for a pressure-induced spin transition in olivine-type LiFePO4 triphylite

We present a combination of first-principles and experimental results regarding the structural and magnetic properties of olivine-type ${\mathrm{LiFePO}}_4$ under pressure. Our investigations indicate that the starting $Pbnm$ phase of ${\mathrm{LiFePO}}_4$ persists up to 70 GPa. Further compression leads to an isostructural transition in the pressure range of 70–75 GPa, inconsistent with a former theoretical study. Considering our first-principles prediction for a high-spin to low-spin transition of ${\mathrm{Fe}}^{2+}$ close to 72 GPa, we attribute the experimentally observed isostructural transition to a change in the spin state of ${\mathrm{Fe}}^{2+}$ in ${\mathrm{LiFePO}}_4$. Compared to relevant Fe-bearing minerals, ${\mathrm{LiFePO}}_4$ exhibits the largest onset pressure for a pressure-induced spin state transition.