A. Kantor

BX90: A new diamond anvil cell design for X-ray diffraction and optical measurements

We present a new design of a universal diamond anvil cell, suitable for different kinds of experimental studies under high pressures. Main features of the cell are an ultimate 90-degrees symmetrical axial opening and high stability, making the presented cell design suitable for a whole range of techniques from optical absorption to single-crystal X-ray diffraction studies, also in combination with external resistive or double-side laser heating. Three examples of the cell applications are provided: a Brillouin scattering of neon, single-crystal X-ray diffraction of α-Cr(2)O(3), and resistivity measurements on the (Mg(0.60)Fe(0.40))(Si(0.63)Al(0.37))O(3) silicate perovskite.

Effect of iron oxidation state on the electrical conductivity of the Earth’s lower mantle

Iron can adopt different spin states in the lower mantle. Previous studies indicate that the dominant lower-mantle phase, magnesium silicate perovskite (which contains at least half of its iron as Fe(3+)), undergoes a Fe(3+) high-spin to low-spin transition that has been suggested to cause seismic velocity anomalies and a drop in laboratory-measured electrical conductivity. Here we apply a new synchrotron-based method of Mössbauer spectroscopy and show that Fe(3+) remains in the high-spin state in lower-mantle perovskite at conditions throughout the lower mantle. Electrical conductivity measurements show no conductivity drop in samples with high Fe(3+), suggesting that the conductivity drop observed previously on samples with high Fe(2+) is due to a transition of Fe(2+) to the intermediate-spin state. Correlation of transport and elastic properties of lower-mantle perovskite with electromagnetic and seismic data may provide a new probe of heterogeneity in the lower mantle.

Portable double-sided laser-heating system for Mössbauer spectroscopy and X-ray diffraction experiments at synchrotron facilities with diamond anvil cells

The diamond anvil cell (DAC) technique coupled with laser heating is a major method for studying materials statically at multimegabar pressures and at high temperatures. Recent progress in experimental techniques, especially in high-pressure single crystal X-ray diffraction, requires portable laser heating systems which are able to heat and move the DAC during data collection. We have developed a double-sided laser heating system for DACs which can be mounted within a rather small (~0.1 m(2)) area and has a weight of ~12 kg. The system is easily transferable between different in-house or synchrotron facilities and can be assembled and set up within a few hours. The system was successfully tested at the High Pressure Station of White Beam (ID09a) and Nuclear Resonance (ID18) beamlines of the European Synchrotron Radiation Facility. We demonstrate examples of application of the system to a single crystal X-ray diffraction investigation of (Mg(0.87),Fe(3+) (0.09),Fe(2+) (0.04))(Si(0.89),Al(0.11))O(3) perovskite (ID09a) and a Synchrotron Mössbauer Source (SMS) study of (Mg(0.8)Fe(0.2))O ferropericlase (ID18).

Iron spin state in silicate perovskite at conditions of the Earth's deep interior

We present a review of our recent work concerning the spin state of Fe2+ and Fe3+ in iron magnesium aluminium silicate perovskite, the most abundant phase in the Earths interior. Experimental results obtained using Mössbauer spectroscopy (with a radioactive source and a Synchrotron Mössbauer Source) and nuclear forward scattering for a range of different sample compositions in both externally heated and laser-heated diamond anvil cells show clear trends in the variation of hyperfine parameters with pressure and temperature. These trends combined with reported total spin state measurements using X-ray emission spectroscopy on samples of similar composition support the conclusion that Fe2+ undergoes a high-spin to intermediate-spin transition near the top of the lower mantle and an intermediate-spin to low-spin transition near the bottom of the lower mantle. No spin transition is observed to occur in Fe3+ for samples with compositions relevant for the lower mantle.

High-pressure structural studies of eskolaite by means of single-crystal X-ray diffraction

Abstract The structural behavior of Cr2O3 was investigated up to ~70 GPa using single-crystal X-ray diffraction under a quasi-hydrostatic pressure (neon pressure medium) at room temperature. The crystal structure remains rhombohedral with the space group R3̄c (No. 167) and upon compression the oxygen atoms approach an ideal hexagonal close-packing arrangement. An isothermal bulk modulus of Cr2O3 and its pressure derivative were found to be 245(4) GPa and 3.6(2), respectively, based on a third-order Birch-Murnaghan equation of state and V0 = 288.73 Å3. An analysis of the crystal strains suggest that the non-hydrostatic stresses can be considered as negligible even at the highest pressure reached.

High-pressure synthesis and physical properties of an orthorhombic phase of chromium dioxide

High-pressure synthesis and physical properties of an orthorhombic phase of chromium dioxide

A gigahertz ultrasonic interferometer for the diamond anvil cell and high-pressure elasticity of some iron-oxide minerals

A second-generation high-frequency acoustic interferometer has been developed for high-pressure and high-temperature elasticity measurements in the diamond anvil cell. The instrument measures single-crystal compressional and shear-wave travel times, which are converted to sound velocities and elastic moduli for direct application to problems in geophysics. The second-order elastic constants (cij) of several iron-bearing oxide minerals has been measured under hydrostatic pressures to ~10 GPa. Pressure-induced c44 mode softening is observed in magnetite (Fe3O4), wustite (Fe0.95O) and in iron-rich magnesiowustite-(Mg, Fe)O, indicating that strong magnetoelastic coupling is common among these iron-rich oxides well ahead of known structural phase transitions. In (Mg, Fe)O, the pressure derivative of c44 is highly sensitive to composition and switches sign between 1.2 ± 0.2 at 25 mol% FeO to −0.96 ± 0.3 at 75 mol% FeO, and is about zero for (Mg, Fe)O containing 50 mol% FeO. In wustite, a discontinuity in the pressure derivatives c11 and C12 at ~5 GPa is interpreted to result from the onset of magnetic ordering, implying that a partially ordered but still cubic phase of FeO exists between ~5 GPa and where the rhombohedral distortion occurs at ~17 GPa.

FeO and MnO high-pressure phase diagrams: relations between structural and magnetic properties

Magnetic and structural phase relations in FeO and MnO were studied in situ using Mössbauer spectroscopy, X-ray and neutron powder diffraction at high pressures and over a broad temperature range. Two principal transition boundaries were mapped: antiferromagnetic ordering (the Néel transition) and the structural distortion from the cubic B1 to a rhombohedral rB1 lattice. These two transitions are not necessarily coupled and the relations between the structural and magnetic properties of transition metal monoxides are discussed.

Trigonal distortion of ferropericlase (Mg0.8Fe0.2)O at high pressures

Transition-metal monoxides (MnO, FeO, CoO, NiO) are of great interest for solid-state physics in virtue of their magnetic, electron, and structural characteristics, which underlie the wide technological applications of this group of materials. These oxides are typical antiferromagnets with a cubic crystal structure of the NaCl type (structural type B1) above the Neel temperature ( T N ). Below T N , these oxides undergo a structural transition of the distortion type (corresponding to the lowering of symmetry to a trigonal or tetragonal one). The structural phase transition occurs within approximately the same temperature range in which the magnetic ordering arises. Therefore, until recently, it was commonly believed that the loss of symmetry in these oxides stems from the ordering of magnetic moments in the crystal structure [1]. The growth of pressure at room temperature leads to similar distortion-type phase transformations in MnO, FeO, CoO, and NiO. Since the Neel temperature has a positive baric coefficient (increases with pressure), it has been suggested [2] that the structural transitions in these materials under high pressures have the same nature as those occurring at low temperatures. Recent studies of the magnetic and elastic characteristics of wustite (FeO) at high pressures [3, 4] have demonstrated that the onset of magnetic ordering in wustite takes place at about 5 GPa. This pressure is significantly lower than that corresponding to the structural phase transition (about 17 GPa). In light of these results, it becomes clear that the relation between magnetic ordering and structural distortion needs a revision—for FeO at the very least.

Peculiarities of FeSi phonon spectrum induced by a change of atomic volume

We analyze in detail the results of experimental investigations of the evolution of the thermal vibration spectra for iron atoms in iron monosilicide FeSi depending on two external parameters, viz., temperature T (in the range 46–297 K at pressure P = 0.1 MPa) and pressure P (in the range 0.1 MPa–43 GPa at temperature T = 297 K), obtained by nuclear inelastic scattering of synchrotron radiation. The decrease of the atomic volume is accompanied by a rearrangement of the phonon spectrum, which is manifested, in particular, in the splitting of the low-energy peak in the spectrum and in an increase of the energy for all phonons. The changes of the average energy of the iron atom vibrational spectrum and of the Debye energy with decreasing atomic volume are analyzed. Different versions of FeSi electron spectrum variation, which can be used to explain the observed phonon anomalies, are considered.