The magnetic properties of iron in cementite (Fe3C) have been measured by x-ray emission spectroscopy in a diamond cell up to 45 GPa. The Fe-K-beta fluorescence peaks reveal that Fe3C undergoes a magnetic collapse at approximately 25 GPa, consistent with theoretical predictions. This transition is likely to be a second-order phase transition without a major structural change. The magnetic collapse transition is expected to affect the elastic and thermodynamic properties of Fe3C; the nonmagnetic phase predicted theoretically has a higher incompressibility and density than the magnetic state. Our results support recent theoretical and thermodynamic calculations indicating that Fe3C is unlikely to be the major component in the Earth's inner core.

The magnetic properties of iron in cementite (Fe3C) have been measured by x-ray emission spectroscopy in a diamond cell up to 45 GPa. The Fe-K-beta fluorescence peaks reveal that Fe3C undergoes a magnetic collapse at approximately 25 GPa, consistent with theoretical predictions. This transition is likely to be a second-order phase transition without a major structural change. The magnetic collapse transition is expected to affect the elastic and thermodynamic properties of Fe3C; the nonmagnetic phase predicted theoretically has a higher incompressibility and density than the magnetic state. Our results support recent theoretical and thermodynamic calculations indicating that Fe3C is unlikely to be the major component in the Earth's inner core.

The electronic environment of the Fe nuclei in two silicate perovskite samples, Fe0.05Mg0.95SiO3 (Pv05) and Fe0.1Mg0.9SiO3 (Pv10), have been measured to 120 GPa and 75 GPa, respectively, at room temperature using diamond anvil cells and synchrotron Mossbauer spectroscopy (SMS). Such investigations of extremely small and dilute Fe-57-bearing samples have become possible through the development of SMS. Our results are explained in the framework of the "three-doublet" model, which assumes two Fe2+-like sites and one Fe3+-Iike site that are well distinguishable by the hyperfine fields at the location of the Fe nuclei. At low pressures, Fe3+/SigmaFe is about 0.40 for both samples. Our results show that at pressures extending into the lowermost mantle the fraction of Fell remains essentially unchanged, indicating that pressure alone does not alter the valence states of iron in (Mg,Fe)SiO3 perovskite. The quadrupole splittings of all Fe sites first increase with increasing pressure, which suggests an increasingly distorted (noncubic) local iron environment. Above pressures of 40 GPa for Pv10 and 80 GPa for Pv05, the quadrupole splittings are relatively constant, suggesting an increasing resistance of the lattice against further distortion. Around 70 GPa, a change in the volume dependence of the isomer shift could be indicative of the endpoint of a continuous transition of Fe3+ from a high-spin to a low-spin state.

The electronic environment of the Fe nuclei in two silicate perovskite samples, Fe0.05Mg0.95SiO3 (Pv05) and Fe0.1Mg0.9SiO3 (Pv10), have been measured to 120 GPa and 75 GPa, respectively, at room temperature using diamond anvil cells and synchrotron Mossbauer spectroscopy (SMS). Such investigations of extremely small and dilute Fe-57-bearing samples have become possible through the development of SMS. Our results are explained in the framework of the "three-doublet" model, which assumes two Fe2+-like sites and one Fe3+-Iike site that are well distinguishable by the hyperfine fields at the location of the Fe nuclei. At low pressures, Fe3+/SigmaFe is about 0.40 for both samples. Our results show that at pressures extending into the lowermost mantle the fraction of Fell remains essentially unchanged, indicating that pressure alone does not alter the valence states of iron in (Mg,Fe)SiO3 perovskite. The quadrupole splittings of all Fe sites first increase with increasing pressure, which suggests an increasingly distorted (noncubic) local iron environment. Above pressures of 40 GPa for Pv10 and 80 GPa for Pv05, the quadrupole splittings are relatively constant, suggesting an increasing resistance of the lattice against further distortion. Around 70 GPa, a change in the volume dependence of the isomer shift could be indicative of the endpoint of a continuous transition of Fe3+ from a high-spin to a low-spin state.

The magnetic behavior of a (BiFeO3)-Fe-57 powdered sample was studied at high pressures by the method of nuclear forward scattering (NFS) of synchrotron radiation. The NFS spectra from Fe-57 nuclei were recorded at room temperature under high pressures up to 61.4 GPa, which were created in a diamond anvil cell. In the pressure interval 0 < P < 47 GPa, the magnetic hyperfine field H-Fe at the Fe-57 nuclei increased reaching a value of similar to 52.5 T at 30 GPa, and then it slightly decreased to similar to 49.6 T at P = 47 GPa. As the pressure was increased further, the field H-Fe abruptly dropped to zero testifying a transition from the antiferromagnetic to a nonmagnetic state (magnetic collapse). In the pressure interval 47 < P < 61.4 GPa, the value of H-Fe remained zero. The field H-Fe recovered to the low-pressure values during decompression. (C) 2005 Pleiades Publishing, Inc.

The magnetic behavior of a (BiFeO3)-Fe-57 powdered sample was studied at high pressures by the method of nuclear forward scattering (NFS) of synchrotron radiation. The NFS spectra from Fe-57 nuclei were recorded at room temperature under high pressures up to 61.4 GPa, which were created in a diamond anvil cell. In the pressure interval 0 < P < 47 GPa, the magnetic hyperfine field H-Fe at the Fe-57 nuclei increased reaching a value of similar to 52.5 T at 30 GPa, and then it slightly decreased to similar to 49.6 T at P = 47 GPa. As the pressure was increased further, the field H-Fe abruptly dropped to zero testifying a transition from the antiferromagnetic to a nonmagnetic state (magnetic collapse). In the pressure interval 47 < P < 61.4 GPa, the value of H-Fe remained zero. The field H-Fe recovered to the low-pressure values during decompression. (C) 2005 Pleiades Publishing, Inc.

Iron is the most abundant transition-metal element in the mantle and therefore plays an important role in the geochemistry and geodynamics of the Earth's interior(1-11). Pressure-induced electronic spin transitions of iron occur in magnesiowustite, silicate perovskite and post-perovskite(1-4,8,10,11). Here we have studied the spin states of iron in magnesiowustite and the isolated effects of the electronic transitions on the elasticity of magnesiowustite with in situ X-ray emission spectroscopy and X-ray diffraction to pressures of the lowermost mantle. An observed high-spin to low-spin transition of iron in magnesiowustite results in an abnormal compressional behaviour between the high-spin and the low-spin states. The high-pressure, low-spin state exhibits a much higher bulk modulus and bulk sound velocity than the low-pressure, high-spin state; the bulk modulus jumps by similar to 35 per cent and bulk sound velocity increases by similar to 15 per cent across the transition in (Mg-0.83, Fe-0.17) O. Although no significant density change is observed across the electronic transition, the jump in the sound velocities and the bulk modulus across the transition provides an additional explanation for the seismic wave heterogeneity in the lowermost mantle(12-21). The transition also affects current interpretations of the geophysical and geochemical models using extrapolated or calculated thermal equation-of-state data without considering the effects of the electronic transition(5,6,22,23).

Iron is the most abundant transition-metal element in the mantle and therefore plays an important role in the geochemistry and geodynamics of the Earth's interior(1-11). Pressure-induced electronic spin transitions of iron occur in magnesiowustite, silicate perovskite and post-perovskite(1-4,8,10,11). Here we have studied the spin states of iron in magnesiowustite and the isolated effects of the electronic transitions on the elasticity of magnesiowustite with in situ X-ray emission spectroscopy and X-ray diffraction to pressures of the lowermost mantle. An observed high-spin to low-spin transition of iron in magnesiowustite results in an abnormal compressional behaviour between the high-spin and the low-spin states. The high-pressure, low-spin state exhibits a much higher bulk modulus and bulk sound velocity than the low-pressure, high-spin state; the bulk modulus jumps by similar to 35 per cent and bulk sound velocity increases by similar to 15 per cent across the transition in (Mg-0.83, Fe-0.17) O. Although no significant density change is observed across the electronic transition, the jump in the sound velocities and the bulk modulus across the transition provides an additional explanation for the seismic wave heterogeneity in the lowermost mantle(12-21). The transition also affects current interpretations of the geophysical and geochemical models using extrapolated or calculated thermal equation-of-state data without considering the effects of the electronic transition(5,6,22,23).

Full understanding of atomic arrangement in amorphous oxides both at ambient and high pressure is an ongoing fundamental puzzle. Whereas the structures of archetypal oxide glasses such as v-B2O3 at high pressure are essential to elucidate origins of anomalous macroscopic properties of more complex melts, knowledge of the high-pressure structure and pressure-induced coordination changes of these glasses has remained elusive due to lack of suitable in situ experimental probes. Here, we report synchrotron inelastic X-ray scattering results for v-B2O3 at pressures up to 22.5 GPa, revealing the nature of pressure-induced bonding changes and the structure. Direct in situ measurements show a continuous transformation from tri-coordinated to tetra-coordinated boron beginning at 4-7 GPa with most of the boron tetra-coordinated above 20 GPa, forming dense tetrahedral v-B2O3. After decompression from high pressure the bonding reverts back to tri-coordinated boron but with the data suggesting a permanent densification.

An electronic transition of iron in magnesiowustite has been studied with synchrotron Mossbauer and x-ray emission spectroscopies under high pressures. Synchrotron Mossbauer studies show that the quadrupole splitting disappears and the isomer shift drops significantly across the spin-paring transition of iron in (Mg-0.75,Fe-0.25)O between 52 and 70 GPa. Based upon current results and percolation theory, we reexamine the high-pressure phase diagram of (Mg,Fe)O and find that iron-iron exchange interaction plays an important role in stabilizing the high-spin state of iron in FeO-rich (Mg,Fe)O.