Optical and X-ray absorption spectroscopy of the pressure-induced spin transition of Fe2+ in natural phosphates of the triphylite-lithiophilite series, Li (Fex2+Mn1-x2+)[PO4]

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Natural minerals of the triphylite-lithiophilite series, M1[6]LiM2[6](Fex2+Mn1-x2+)[PO4], will be studied by means of high-pressure optical absorption (UV/VIS/NIR/MIR) and high-pressure X-ray absorption spectroscopy. The aim is to trace in detail the evolution of the spectra up to pressures above the high spin to low spin (HS-LS) transition in octahedral Fe2+ and to understand the mechanism and driving forces of pressure-induced spin transitions in transition-metal bearing minerals. We believe that the proposed minerals are highly appropriate for investigating the pressure-induced HS-to-LS transition phenomenon of Fe2+ by optical and X-ray absorption spectroscopy. The spectra in the UV/VIS/NIR range consist of a split spin-allowed dd-band originated from 5T2g to 5Eg electronic transition of octahedral Fe2+(M2) with maximum at around 9100-9400 cm-1. With an appropriate sample preparation and orientation in the diamond anvil cell, this band allows for very reliable detection and careful tracing of the pressure-induced behavior. It is highly advantageous that the material is perfectly transparent in a wide spectral range from NIR to UV (ca. 12000 - 28000 cm-1). Particularly, the spectra are not affected by the Fe2+ high-energy absorption edge, which usually increases with pressure and significantly hampers spectrum acquisition, in contrast to other natural and synthetic minerals, studied so far (siderite, magnesiowüstite, ferropericlase, silicate perovskite). The data obtained by optical spectroscopy will be correlated with XANES data at the Fe K-edge and XANES spectra measured by X-ray Raman scattering at the Fe M-edge, Li K-edge and P L-edge. The X-ray spectra will provide unique insights to both electronic and local structure of all cations in the mineral phase. The combination of optical absorption, FTIR and X-ray absorption spectroscopy will provide unique insight for a better understanding of the relation of electronic and structural transitions in phases containing 3d-transition elements. Specifically, understanding the correlation between structural and electronic changes at pressure-induced transitions will be a central aim of the project.