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...

Abstract:The effect of applied pressure on the magnetic properties of the Prussian blue analogue K 0.4Fe4[Cr(CN)6]2.8 · 16H2O (1) has been analyzed by dc and ac magnetic susceptibility measurements. Under ambient conditions, 1 orders ferromagnetically at a critical temperature (T C) of 18.5 K. Under application of pressure in the 0-1200 MPa range, the magnetization of the material decreases and its critical temperature shifts to lower temperatures, reaching T C ) 7.5 K at 1200 MPa. Pressure-dependent Raman and Mö ssbauer spectroscopy measurements show that this striking behavior is due to the isomerization of some Cr III -CtN-Fe II linkages to the Cr III -NtC-Fe II form. As a result, the ligand field around the iron(II) centers increases, and the diamagnetic low-spin state is populated. As the number of diamagnetic centers in the cubic lattice increases, the net magnetization and critical temperature of the material decrease considerably. The phenomenon is reversible: releasing the pressure restores the magnetic properties of the original material. However, we have found that under more severe pressure conditions, a metastable sample containing 22% Cr III -NtC-Fe II linkages can be obtained. X-ray absorption spectroscopy and magnetic circular dichroism of this metastable sample confirm the linkage isomerization process.