Evidence for a pressure-induced spin transition in olivine-type LiFePO$_{4}$ triphylite

TL;DR

This study combines theoretical and experimental approaches to reveal a pressure-induced spin transition in LiFePO4 at around 72 GPa, marking the highest known onset pressure among Fe-bearing minerals.

Contribution

It provides the first combined experimental and first-principles evidence for a high-spin to low-spin transition in LiFePO4 under high pressure, extending understanding of spin transitions in minerals.

Findings

Persistence of Pbnm phase up to 70 GPa

Isostructural transition observed at 70-75 GPa

High-spin to low-spin transition predicted near 72 GPa

Abstract

We present a combination of first-principles and experimental results regarding the structural and magnetic properties of olivine-type LiFePO$_4$ under pressure. Our investigations indicate that the starting $Pbnm$ phase of 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 Fe$^{2+}$ close to 72 GPa, we attribute the experimentally observed isostructural transition to a change on the spin state of Fe$^{2+}$ in LiFePO$_4$. Compared to relevant Fe-bearing minerals, LiFePO$_4$ exhibits the largest onset pressure for a pressure-induced spin state transition.