[1] Understanding the alloying effects of nickel and light element(s) on the physical properties of iron under core conditions is crucial for interpreting and constraining geophysical and geochemical models. We have studied two alloys, Fe0.92Ni0.08 and Fe0.85Si0.15, with nuclear resonant inelastic x-ray scattering up to 106 GPa and 70 GPa, respectively. The sound velocities of the alloys are obtained from the measured partial phonon density of states for Fe-57 incorporated in the alloys. Addition of Ni slightly decreases the compression wave velocity and shear wave velocity of Fe under high pressures. Silicon alloyed with Fe increases the compressional wave velocity and shear wave velocity under high pressures, which provides a better match to seismological data of the Earth's core.

The elastic properties of compressed Fe and FeO are examined using density of states measured by nuclear resonant inelastic X-ray scattering. We analyze the data both from non-hydrostatic and nearly hydrostatic experiments. The effects of preferred orientation in non-hydrostatic experiments could be substantial. We present also evidence in favor of strong softening of the Debye sound velocity due to the magnetoelastic coupling in iron oxide near the Neel transition. The geophysical implications resulting from the elastic and magnetoelastic properties of these and related materials under compression are discussed.

The elastic properties of compressed Fe and FeO are examined using density of states measured by nuclear resonant inelastic X-ray scattering. We analyze the data both from non-hydrostatic and nearly hydrostatic experiments. The effects of preferred orientation in non-hydrostatic experiments could be substantial. We present also evidence in favor of strong softening of the Debye sound velocity due to the magnetoelastic coupling in iron oxide near the Neel transition. The geophysical implications resulting from the elastic and magnetoelastic properties of these and related materials under compression are discussed.

Attributed to their specific atomic bonding, the soft, graphite-like, hexagonal boron nitride (h-BN) and its superhard, diamond-like, cubic polymorph (c-BN) are important technological materials with a wide range of applications(1). At high pressure and temperature, h-BN can directly transform to a hexagonal close-packed polymorph (w-BN)(2) that can be partially quenched after releasing pressure. Previous theoretical calculations(3-5) and experimental measurements (primarily on quenched samples)(6-9) provided substantial information on the transition, but left unsettled questions due to the lack of in situ characterization at high pressures. Using inelastic X-ray scattering to probe the boron and nitrogen near K-edge spectroscopy, here we report the first observation of the conversion process of boron and nitrogen sp(2)- and p-bonding to sp(3) and the directional nature of the sp(3) bonding. In combination with in situ X-ray diffraction probe, we have further clarified the structure transformation mechanism. The present archetypal example opens two enormous, element-specific, research areas on high-pressure bonding evolutions of boron and nitrogen; each of the two elements and their respective compounds have displayed a wealth of intriguing pressure-induced phenomena(10) that result from bonding changes, including metallization(11,12), superconductivity(13,14), semiconductivity(15), polymerization(16) and superhardness(2,17,18).

Attributed to their specific atomic bonding, the soft, graphite-like, hexagonal boron nitride (h-BN) and its superhard, diamond-like, cubic polymorph (c-BN) are important technological materials with a wide range of applications(1). At high pressure and temperature, h-BN can directly transform to a hexagonal close-packed polymorph (w-BN)(2) that can be partially quenched after releasing pressure. Previous theoretical calculations(3-5) and experimental measurements (primarily on quenched samples)(6-9) provided substantial information on the transition, but left unsettled questions due to the lack of in situ characterization at high pressures. Using inelastic X-ray scattering to probe the boron and nitrogen near K-edge spectroscopy, here we report the first observation of the conversion process of boron and nitrogen sp(2)- and p-bonding to sp(3) and the directional nature of the sp(3) bonding. In combination with in situ X-ray diffraction probe, we have further clarified the structure transformation mechanism. The present archetypal example opens two enormous, element-specific, research areas on high-pressure bonding evolutions of boron and nitrogen; each of the two elements and their respective compounds have displayed a wealth of intriguing pressure-induced phenomena(10) that result from bonding changes, including metallization(11,12), superconductivity(13,14), semiconductivity(15), polymerization(16) and superhardness(2,17,18).

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.