TY  - JOUR
A1  - Taran, Michail N.
A1  - Nunez Valdez, Maribel
A1  - Efthimiopoulos, Ilias
A1  - Müller, J.
A1  - Reichmann, Hans-Josef
A1  - Wilke, Max
A1  - Koch-Müller, Monika
T1  - Spectroscopic and ab initio studies of the pressure-induced Fe2+ high-spin-to-low-spin electronic transition in natural triphylite-lithiophilite
JF  - Physics and Chemistry of Minerals
N2  - 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 76-80GPa in a natural triphylite-lithiophilite sample with chemical composition (LiFe0.7082+Mn0.292PO4)-Li-M1-Fe-M2 (theoretical composition (LiFe0.52+Mn0.5PO4)-Li-M1-Fe-M2). The optical absorption spectra at ambient conditions consist of a broad doublet band with two constituents (1) (similar to 9330cm(-1)) and (2) (similar to 7110cm(-1)), resulting from the electronic spin-allowed transition (T2gEg)-T-5-E-5 of octahedral (HSFe2+)-Fe-M2. Both (1) and (2) bands shift non-linearly with pressure to higher energies up to similar to 55GPa. In the optical absorption spectrum measured at similar to 81GPa, the aforementioned HS-related bands disappear, whereas a new broadband with an intensity maximum close to 16,360cm(-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 (1)A(1g)(1)T(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 similar to 76GPa, the highest pressure achieved in the Raman spectroscopic experiments. Calculations indicated that the presence of Mn-M2(2+) simply shifts the isostructural HS-to-LS transition to higher pressures compared to the triphylite Fe-M2(2+) end-member, in qualitative agreement with our experimental observations.
KW  - Phosphates
KW  - Triphylite
KW  - Raman
KW  - Infrared
KW  - Optical absorption spectroscopy
KW  - High pressure
KW  - Spin transition
KW  - DFT
Y1  - 2019
U6  - https://doi.org/10.1007/s00269-018-1001-y
SN  - 0342-1791
SN  - 1432-2021
VL  - 46
IS  - 3
SP  - 245
EP  - 258
PB  - Springer
CY  - New York
ER  -