Spectroscopic and ab initio studies of the pressure-induced $Fe^{2+}$ high-spin-to-low-spin electronic transition in natural triphylite–lithiophilite

Using optical absorption and Raman spectroscopic measurements, in conjunction with the first-principles calculations, a pressure-induced high-spin (HS)-to-low-spin (LS) state electronic transition of $Fe^{2+}$ (M2-octahedral site) was resolved around 76–80 GPa in a natural triphylite–lithiophilite sample with chemical composition $^{M1}Li^{M2}Fe^{2+} \ _{0.708}Mn_{0.292}PO_4$ (theoretical composition $^{M1}Li^{M2}Fe^{2+} \ _{0.5}Mn_{0.5}PO_4$). The optical absorption spectra at ambient conditions consist of a broad doublet band with two constituents $ν_1 (\sim 9330 cm^{−1})$ and $ν_2 (\sim7110 cm^{−1})$, resulting from the electronic spin-allowed transition $^5T_{2g} → \ ^5E_g$ of octahedral $HS^{M2}Fe^{2+}$. Both $ν_1$ and $ν_2$ bands shift non-linearly with pressure to higher energies up to ~ 55 GPa. In the optical absorption spectrum measured at ~ 81 GPa, the aforementioned HS-related bands disappear, whereas a new broadband with an intensity maximum close to $16,360 cm^{−1}$ appears, superimposed on the tail of the high-energy ligand-to-metal $O^{2−} → Fe^{2+}$ charge-transfer absorption edge. We assign this new band to the electronic spin-allowed dd-transition $^1A_{1g} →\ ^1T_{1g}$ of LS $Fe^{2+}$ in octahedral coordination. The high-pressure Raman spectra evidence the $Fe^{2+}$ HS-to-LS transition mainly from the abrupt shift of the P–O symmetric stretching modes to lower frequencies at ~ 76 GPa, the highest pressure achieved in the Raman spectroscopic experiments. Calculations indicated that the presence of $^{M2}Mn^{2+}$ simply shifts the isostructural HS-to-LS transition to higher pressures compared to the triphylite $^{M2}Fe^{2+}$ end-member, in qualitative agreement with our experimental observations