@article{CerantolaWilkeKantoretal.2019, author = {Cerantola, Valerio and Wilke, Max and Kantor, Innokenty and Ismailova, Leyla and Kupenko, Ilya and McCammon, Catherine and Pascarelli, Sakura and Dubrovinsky, Leonid S.}, title = {Experimental investigation of FeCO3 (siderite) stability in Earth's lower mantle using XANES spectroscopy}, series = {American mineralogist : an international journal of earth and planetary materials}, volume = {104}, journal = {American mineralogist : an international journal of earth and planetary materials}, number = {8}, publisher = {Mineralogical Society of America}, address = {Chantilly}, issn = {0003-004X}, doi = {10.2138/am-2019-6428}, pages = {1083 -- 1091}, year = {2019}, abstract = {We studied FeCO3 using Fe K-edge X-ray absorption near-edge structure (XANES) spectroscopy at pressures up to 54 GPa and temperatures above 2000 K. First-principles calculations of Fe at the K-edge in FeCO3 were performed to support the interpretation of the XANES spectra. The variation of iron absorption edge features with pressure and temperature in FeCO3 matches well with recently reported observations on FeCO3 at extreme conditions, and provides new insight into the stability of Fe-carbonates in Earth's mantle. Here we show that at conditions of the mid-lower mantle, ~50 GPa and ~2200 K, FeCO3 melts and partially decomposes to high-pressure Fe3O4. Carbon (diamond) and oxygen are also inferred products of the reaction. We constrained the thermodynamic phase boundary between crystalline FeCO3 and melt to be at 51(1) GPa and ~1850 K. We observe that at 54(1) GPa, temperature-induced spin crossover of Fe2+ takes place from low to high spin such that at 1735(100) K, all iron in FeCO3 is in the high-spin state. A comparison between experiment and theory provides a more detailed understanding of FeCO3 decomposition observed in X-ray absorption spectra and helps to explain spectral changes due to pressure-induced spin crossover in FeCO3 at ambient temperature.}, language = {en} } @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} }