@article{NunezValdezEfthimiopoulosTaranetal.2018,
  author    = {Nunez Valdez, Maribel and Efthimiopoulos, Ilias and Taran, Michail and Mueller, Jan and Bykova, Elena and McCammon, Catherine and Koch-M{\"u}ller, Monika and Wilke, Max},
  title     = {Evidence for a pressure-induced spin transition in olivine-type LiFePO4 triphylite},
  series = {Physical review : B, Condensed matter and materials physics},
  volume    = {97},
  journal   = {Physical review : B, Condensed matter and materials physics},
  number    = {18},
  publisher = {American Physical Society},
  address   = {College Park},
  issn      = {2469-9950},
  doi       = {10.1103/PhysRevB.97.184405},
  pages     = {9},
  year      = {2018},
  abstract  = {We present a combination of first-principles and experimental results regarding the structural and magnetic properties of olivine-type LiFePO4 under pressure. Our investigations indicate that the starting Pbnm phase of LiFePO4 persists up to 70 GPa. Further compression leads to an isostructural transition in the pressure range of 70-75 GPa, inconsistent with a former theoretical study. Considering our first-principles prediction for a high-spin to low-spin transition of Fe2+ close to 72 GPa, we attribute the experimentally observed isostructural transition to a change in the spin state of Fe2+ in LiFePO4. Compared to relevant Fe-bearing minerals, LiFePO4 exhibits the largest onset pressure for a pressure-induced spin state transition.},
  language  = {en}
}
@article{TaranNunezValdezEfthimiopoulosetal.2019,
  author    = {Taran, Michail N. and Nunez Valdez, Maribel and Efthimiopoulos, Ilias and M{\"u}ller, J. and Reichmann, Hans-Josef and Wilke, Max and Koch-M{\"u}ller, Monika},
  title     = {Spectroscopic and ab initio studies of the pressure-induced Fe2+ high-spin-to-low-spin electronic transition in natural triphylite-lithiophilite},
  series = {Physics and Chemistry of Minerals},
  volume    = {46},
  journal   = {Physics and Chemistry of Minerals},
  number    = {3},
  publisher = {Springer},
  address   = {New York},
  issn      = {0342-1791},
  doi       = {10.1007/s00269-018-1001-y},
  pages     = {245 -- 258},
  year      = {2019},
  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 76-80GPa 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.},
  language  = {en}
}