Spectroscopic and ab initio studies of the pressure-induced Fe2+ high-spin-to-low-spin electronic transition in natural triphylite–lithiophilite

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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 Fe2+ (M2-octahedral site) was resolved around 76–80 GPa in a natural triphylite–lithiophilite sample with chemical composition M1LiM2Fe2+0.708Mn0.292PO4 (theoretical composition M1LiM2Fe2+0.5Mn0.5PO4). The optical absorption spectra at ambient conditions consist of a broad doublet band with two constituents ν1 (~ 9330 cm−1) and ν2 (~ 7110 cm−1), resulting from the electronic spin-allowed transition 5T2g → 5Eg of octahedral HSM2Fe2+. Both ν1 and ν2 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 O2− → Fe2+ charge-transfer absorption edge. We assign this new band to the electronic spin-allowed dd-transition 1A1g → 1T1g of LS Fe2+ in octahedral coordination. The high-pressure Raman spectra evidence the Fe2+ 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 M2Mn2+ simply shifts the isostructural HS-to-LS transition to higher pressures compared to the triphylite M2Fe2+ end-member, in qualitative agreement with our experimental observations.