0 10 20 30 40 50 60 70 80 90 100 32 34 36 38 40 42 44 46 48 K 0 =199(6) GPa K 0 =221(3) GPa Low spin Fe 0.5 Mg 0.5 Al 0.5 Si 0.5 O 3 Volume per formula unit (Å 3 ) Pressure (GPa) Fe 0.96 Mg 0.5 Si 0.54 O 3 High spin 0.02 0.03 0.04 0.05 0.06 0.07 0.08 165 170 175 180 185 190 195 200 205 210 Normalized Pressure (GPa) Strain 40 GPa Structure, crystal chemistry, and compressibility of iron-rich silicate perovskite at pressures up to 95 GPa. I. Koemets 1 , Z. Liu 2 , E. Koemets 1 , B. Wang 1 , C. McCammon 1 , T. Katsura 1 , M. Hanfland 3 , A. Chumakov 3 , L. Dubrovinsky 1 10 20 30 40 50 60 7 8 9 10 15 16 17 18 oB (Al 0.5 Si 0.5 ) pA (Fe 0.61 Mg 0.39 ) oB (Si) oB (Fe 0.7 Mg 0.22 Si 0.08 ) Polyhedral volume (Å 3 ) Pressure (GPa) pA (Fe 0.5 Mg 0.5 ) 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 This study: Al-free sample Fe3+ on pA-site Fe3+ on oB-site Al-rich sample: Fe3+ on pA-site Fe2+ on pA-site Previous studies: Sinmyo et al. 2017 Liu et al. 2018 Potapkin et al. 2013 High spin oB-site Fe 3+ pA-site Fe 2+ Centre shift (mm/s) Quadrupole splitting (mm/s) pA-site Fe 3+ Low spin 1. Introduction Bridgmanite elasticity is influenced by chemical composition, oxygen vacancies, and pressure-induced high-spin to low-spin transition in iron. The effect of pressure-driven spin transition(s) in iron-bearing bridgmanite on bulk modulus is complex, because iron could occupy different crystallographic sites and can occur as both ferrous and ferric. In addition, high-pressure experiments with silicate perovskite end-members could provide us with a self-consistent elasticity database and clarify mechanism(s) of cations substitution. 2. Methods Samples synthesis in Large Volume Press, diamond anvil cells for pressure generation, synchrotron radiation based in situ single-crystal X-ray diffraction, and Mössbauer spectroscopy. 4. Results: Compressibility Cold compression behaviour of Fe 0.5 Mg 0.5 Si 0.5 Al 0.5 O 3 and Fe 0.96 Mg 0.5 Si 0.54 O 3 Black lines represent fits of P-V data with second order Birch-Murnaghan EoS. Blue lines are data from (Lin et al., 2000).At pressures above ~10 GPa samples with both compositions transform to (pseudo) orthorhombic perovskite-type structured phases. Inset: Above 40 GPa softening of the phase is obvious and related to spin transition of Fe 3+ located in the B’-site of double-perovskite. 3. Results: Structures and their relations Crystal structures of Fe 0.5 Mg 0.5 Al 0.5 Si 0.5 O 3 (a, b) and Fe 0.96 Mg 0.5 Si 0.54 O 3 (c, d) silicate phases and their structural relations (a,c) Phases stable at P<10 GPa, (b,d) Phases stable at P>10 GPa. Distinguishable crystallographic sites are different in color. pA refers to prismatic A-site, oB and oB’ refers to octahedral B- and B’-site 5. Results: Crystal chemistry Compressibility of individual polyhedra in Al-rich silicate perovskite (black) and Al-poor silicate double perovskite (red). 6. Results: Mössbauer spectroscopy A summary of quadrupole splitting and centre shift of silicate perovskite samples at high pressure. For comparison, we plotted data from previous studies involving data collection on Fe-bearing bridgmanites. Liu et al. 2018 (stars) used nuclear forward scattering; Sinmyo et al. 2017 (crosses) and Potapkin et al. 2013 (triangles) used SMS. Key points: First silicate double-perovskite New corundum-structure derivative Fe,Mg-bearing octahedral site in dPv 1 University of Bayreuth, Bayreuth, Germany 2 Jilin University, Changchun, China 3 ESRF, Grenoble, France