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

• Published in: Physics and chemistry of minerals 2019-03, Vol.46 (3), p.245-258
• Main Authors: Taran, M. N., Núñez Valdez, M., Efthimiopoulos, I., Müller, J., Reichmann, H. J., Wilke, M., Koch-Müller, M.
• Format: Article
• Language: English
• Subjects: Absorption; Absorption spectra; Broadband; Charge transfer; Chemical composition; Crystallography and Scattering Methods; Earth and Environmental Science; Earth Sciences; Electron spin; First principles; Geochemistry; Mathematical analysis; Mineral Resources; Mineralogy; Organic chemistry; Original Paper; Qualitative analysis; Raman spectra
• ISSN: 0342-1791; 1432-2021
• DOI: 10.1007/s00269-018-1001-y

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 (~ 9330 cm −1 ) and ν 2 (~ 7110 cm −1 ), resulting from the electronic spin-allowed transition 5 T 2g  →  5 E 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 1 A 1g  →  1 T 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.