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

Abstract

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 ^M1Li^M2Fe²⁺_0.708Mn_0.292PO₄ (theoretical composition ^M1Li^M2Fe²⁺_0.5Mn_0.5PO₄). 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 ⁵T_2g → ⁵E_g of octahedral HS^M2Fe²⁺. 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 ¹A_1g → ¹T_1g 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.