LaMnO(3) was studied by synchrotron x-ray diffraction, optical spectroscopies, and transport measurements under pressures up to 40 GPa. The cooperative Jahn-Teller (JT) distortion is continuously reduced with increasing pressure. There is strong indication that the JT effect and the concomitant orbital order are completely suppressed above 18 GPa. The system, however, retains its insulating state to approximately 32 GPa, where it undergoes a bandwidth-driven insulator-metal transition. Delocalization of electron states, which suppresses the JT effect but is insufficient to make the system metallic, appears to be a key feature of LaMnO(3) at 20-30 GPa.
High-pressure Mö ssbauer spectroscopy on several compositions across the (Mg,Fe)O magnesiowü stite solid solution confirms that ferrous iron (Fe 2؉ ) undergoes a high-spin to low-spin transition at pressures and for compositions relevant to the bulk of the Earth's mantle. High-resolution x-ray diffraction measurements document a volume change of 4 -5% across the pressure-induced spin transition, which is thus expected to cause seismological anomalies in the lower mantle. The spin transition can lead to dissociation of Fe-bearing phases such as magnesiowü stite, and it reveals an unexpected richness in mineral properties and phase equilibria for the Earth's deep interior.lower mantle ͉ Mö ssbauer ͉ magnesiowü stite W . S. Fyfe proposed 45 years ago that the effect of high pressure deep inside the Earth's mantle may be to collapse the atomic orbitals of iron from the high-spin to the low-spin state (1). This transition would represent a major change in chemical-bonding character for one of the Earth's most important elements (Fig. 1), with predictions suggesting as much as a 45% collapse in the ionic volume of ferrous iron in silicates and oxides (2). Elastic moduli, thermal conductivity, electrical transport, and other physical and chemical properties of Fe-bearing minerals could thus be dramatically altered at depth due to the spin transition. Consequently, there has been much interest in the high-to low-spin transition (3), and high-pressure studies of the past decade have demonstrated that it can indeed take place in oxides similar to those thought to be present in the deep mantle (4-10).In the present study, we investigate the high-to low-spin transition across the (Mg,Fe)O magnesiowüstite solid solution (see Supporting Text I, which is published as supporting information on the PNAS web site). This oxide is believed to comprise up to Ϸ30 molar percent of the lower mantle, and is thus the second-most abundant mineral phase of the Earth's rocky interior after (Mg,Fe)SiO 3 perovskite (11, 12). . The Fe 3ϩ content was in all cases below the detection limit of Mössbauer spectroscopy, hence below 1% of the total Fe. For each Mössbauer experiment, the sample was loaded together with several ruby chips (for pressure determination) in a 100-m diameter sample chamber drilled in a Re foil indented to 25-m thickness. The sample assemblages were compressed between 200-m diamond culets by using a modified piston-cylinder diamond-anvil cell. Mössbauer spectra were collected by using a 10-mCi 57 Co(Rh) point source (1 Ci ϭ 37 GBq). Materials and MethodsPowder samples of (Mg 0.8 Fe 0.2 )O, from the same batch as used for Mössbauer spectroscopy, were mixed with (Mg 0.1 Fe 0.9 )O (from the material studied in ref. 13) in a 1:3 volume ratio for our x-ray diffraction experiments. Two different series of experiments were performed, using Ar or a methanol:ethanol:water mixture (16:3:1 volume ratio) as a pressure-transmitting medium. Ruby chips were loaded together with the sample powder, and pressure for each run was determined by...
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