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.

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.

We report measurements of the valence band width in compressed Ge determined from x-ray emission spectra below the Ge K edge. The width of the valence band does not show any pressure dependence in the semiconducting diamond-type structure of Ge below 10 GPa. On the other hand, in the metallic beta-Sn phase above 10 GPa the valence band width increases under compression. Density-functional calculations show an increasing valence band width under compression both in the semiconducting phase (contrary to experiment) and in the metallic beta-Sn phase of Ge (in agreement with observed pressure-induced broadening). The pressure-independent valence band width in the semiconducting phase of Ge appears to require theoretical advances beyond the density-functional theory or the GW approximation.

We report measurements of the valence band width in compressed Ge determined from x-ray emission spectra below the Ge K edge. The width of the valence band does not show any pressure dependence in the semiconducting diamond-type structure of Ge below 10 GPa. On the other hand, in the metallic beta-Sn phase above 10 GPa the valence band width increases under compression. Density-functional calculations show an increasing valence band width under compression both in the semiconducting phase (contrary to experiment) and in the metallic beta-Sn phase of Ge (in agreement with observed pressure-induced broadening). The pressure-independent valence band width in the semiconducting phase of Ge appears to require theoretical advances beyond the density-functional theory or the GW approximation.

Synchrotron Mossbauer spectroscopy (SMS) was performed on an hcp-phase alloy of composition Fe92Ni8 at a pressure of 21 GPa and a temperature of 11 K. Density functional theoretical calculations predict antiferromagnetism in both hcp Fe and hcp Fe-Ni. For hcp Fe, these calculations predict no hyperfine magnetic field, consistent with previous experiments. For hcp Fe-Ni, however, substantial hyperfine magnetic fields are predicted, but these were not observed in the SMS spectra. Two possible explanations are suggested. First, small but significant errors in the generalized gradient approximation density functional may lead to an erroneous prediction of magnetic order or of erroneous hyperfine magnetic fields in antiferromagnetic hcp Fe-Ni. Alternately, quantum fluctuations with periods much shorter than the lifetime of the nuclear excited state would prohibit the detection of moments by SMS.

Synchrotron Mossbauer spectroscopy (SMS) was performed on an hcp-phase alloy of composition Fe92Ni8 at a pressure of 21 GPa and a temperature of 11 K. Density functional theoretical calculations predict antiferromagnetism in both hcp Fe and hcp Fe-Ni. For hcp Fe, these calculations predict no hyperfine magnetic field, consistent with previous experiments. For hcp Fe-Ni, however, substantial hyperfine magnetic fields are predicted, but these were not observed in the SMS spectra. Two possible explanations are suggested. First, small but significant errors in the generalized gradient approximation density functional may lead to an erroneous prediction of magnetic order or of erroneous hyperfine magnetic fields in antiferromagnetic hcp Fe-Ni. Alternately, quantum fluctuations with periods much shorter than the lifetime of the nuclear excited state would prohibit the detection of moments by SMS.