A. G. Gavriliuk

Structural transformation of molecular nitrogen to a single-bonded atomic state at high pressures.

The transformation of molecular nitrogen to a single-bonded atomic nitrogen is of significant interest from a fundamental stand point and because it is the most energetic non-nuclear material predicted. We performed an x-ray diffraction of nitrogen at pressures up to 170 GPa. At 60 GPa, we found a transition from the rhombohedral (R3c) epsilon-N(2) phase to the zeta-N(2) phase, which we identified as orthorhombic with space group P222(1) and with four molecules per unit cell. This transition is accompanied by increasing intramolecular and decreasing intermolecular distances. The major transformation of this diatomic phase into the single-bonded (polymeric) phase, recently determined to have the cubic gauche structure (cg-N), proceeds as a first-order transition with a volume change of 22%.

Single-crystalline polymeric nitrogen

The authors synthesized polymeric nitrogen at pressures above 110GPa with laser heating above 2000K. To prove its cubic gauche (cg) structure, the authors have grown single crystals which produced strong reflections at large diffraction angles. The authors found nine peaks in the 2Θ=35° range with a special diamond anvil cell. All of the peaks agree with the cg-N structure. The authors checked various combinations of gasket materials and heaters to study possible contamination of the polymeric nitrogen by products of the reaction of the hot nitrogen with these materials or diamond anvils, and found that this is not the case.

Observation of superconductivity in hydrogen sulfide from nuclear resonant scattering

Peeking into a diamond pressure cell A defining characteristic of a superconductor is that it expels an external magnetic field. Demonstrating this effect can be tricky when the sample is under enormous pressures in a diamond anvil cell. Troyan et al. placed a tinfoil sensor inside a sample of H2S under pressure. They then bombarded it with synchrotron radiation and watched how the scattering of photons of tin nuclei changed over time. When H2S was in the normal state, an external magnetic field reached the sensor through the sample, causing the nuclear levels of tin to split. In the superconducting state, however, no splitting was observed because H2S expelled the field before it could reach the sensor. Science, this issue p. 1303 A tin foil sensor inside a pressurized superconducting sample of hydrogen sulfide is used to demonstrate the expulsion of magnetic field. [Also see Perspective by Struzhkin] High-temperature superconductivity remains a focus of experimental and theoretical research. Hydrogen sulfide (H2S) has been reported to be superconducting at high pressures and with a high transition temperature. We report on the direct observation of the expulsion of the magnetic field in H2S compressed to 153 gigapascals. A thin 119Sn film placed inside the H2S sample was used as a sensor of the magnetic field. The magnetic field on the 119Sn sensor was monitored by nuclear resonance scattering of synchrotron radiation. Our results demonstrate that an external static magnetic field of about 0.7 tesla is expelled from the volume of 119Sn foil as a result of the shielding by the H2S sample at temperatures between 4.7 K and approximately 140 K, revealing a superconducting state of H2S.

Spin reorientation effects in GdFe3(BO3)4 induced by applied field and temperature

Magnetic properties of GdFe3(BO3)4 single crystals were investigated by 57Fe-Mössbauer spectroscopy and static magnetic measurements. In the ground state, the GdFe3(BO3)4 crystal is an easy-axis compensated antiferromagnet, but the easy axis of iron moments does not coincide with the crystal C3 axis, deviating from it by about 20°. The spontaneous and field-induced spin reorientation effects were observed and studied in detail. The specific directions of iron magnetic moments were determined for different temperatures and applied fields. Large values of the angle between the Fe3+ magnetic moments and the C3 axis in the easy-axis phase and between Fe3+ moments and the a2 axis in the easy-plane phase reveal the tilted antiferromagnetic structure.

Structural and electronic transitions in gadolinium iron borate GdFe3(BO3)4 at high pressures

The optical properties and structure of gadolinium iron borate GdFe3(BO3)4 crystals are studied at high pressures produced in diamond-anvil cells. X-ray diffraction data obtained at a pressure of 25.6 GPa reveal a firstorder phase transition retaining the trigonal symmetry and increasing the unit cell volume by 8%. The equation of state is obtained and the compressibility of the crystal is estimated before and after the phase transition. The optical spectra reveal two electronic transitions at pressures ∼26 GPa and ∼43 GPa. Upon the first transition, the optical gap decreases jumpwise from 3.1 to ∼2.25 eV. Upon the second transition at P=43 GPa, the optical gap deceases down to ∼0.7 eV, demonstrating a dielectric-semiconductor transition. By using the theoretical model developed for a FeBO3 crystal and taking into account some structural analogs of these materials, the anomalies of the high-pressure optical spectra are explained.

Optimization of the conditions of synchrotron Mössbauer experiment for studying electronic transitions at high pressures by the example of (Mg, Fe)O magnesiowustite

The effect of the experimental conditions on the shape of the nuclear resonant forward scattering (NFS) from (Mg0.75Fe0.25)O magnesiowustite has been studied at high pressures up to 100 GPa in diamond anvil cells by the method of the NFS of synchrotron radiation from the Fe-57 nuclei at room temperature. The behavior of the system in the electronic transition of the Fe2+ ion from the high-spin to low-spin state (spin crossover) near 62 GPa is analyzed as a function of the sample thickness, degree of nonhydrostaticity, and focusing and collimation conditions of a synchrotron beam. It is found that the inclusion of dynamical beats associated with the sample thickness is very important in the approximation of the experimental NFS spectra. It is shown that the electronic transition occurs in a much narrower pressure range (±6 GPa) rather than in a broad range as erroneously follows from experiments with thick samples under strongly nonhydrostatic conditions.

Electronic and structural transitions in NdFeO3 orthoferrite under high pressures

The effect of high pressure up to 65 GPa on the crystal structure and optical absorption spectra of NdFeO3 orthoferrite single crystals is studied in diamond anvil cells. At P∼37.5 GPa, an electronic transition at which the optical absorption edge jumps from ∼2.2 to ∼0.75 eV is observed. The equation of state V(P) is studied on the basis of the X-ray diffraction data obtained under pressure. This study reveals a first-order structural phase transition at P∼37 GPa with a jump of ∼4% in the unit cell volume. It is shown that the phase transition observed in rare-earth orthoferrites at 30–40 GPa is a transition of the insulator-to-semiconductor type.

High-spin-low-spin transition and the sequence of the phase transformations in the BiFeO3 crystal at high pressures

The transition of Fe3+ ions from the high-spin (HS) state (S = 5/2) to the low-spin (LS) state (S = 1/2) has been observed in the BiFeO3 multiferroic crystal at high pressures in the range 45–55 GPa. This effect has been studied in high-pressure diamond-anvil cells by means of two experimental methods using synchrotron radiation: nuclear resonant forward scattering (NFS or synchrotron Mössbauer spectroscopy) and FeKβ high-resolution X-ray emission spectroscopy (XES). The HS-LS transition correlates with anomalies in the magnetic, optical, transport, and structural properties of the crystal. At room temperature, the transition is not stepwise, but is extended in a pressure range of about 10 GPa due to thermal fluctuations between the high-spin and low-spin states. It has been found that the transition of the BiFeO3 insulator to the metal occurs only in the low-spin phase and the cause of all phase transitions is the HS-LS crossover.

High-spin-low-spin transition in magnesiowüstite (Mg0.75,Fe0.25)O at high pressures under hydrostatic conditions

The spin states of Fe2+ ions in (Mg0.75,Fe0.25)O magnesiowüstite crystals at hydrostatic pressures up to 90 GPa created in a diamond-anvil cell with helium as a pressure-transmitting medium have been investi-gated by transmission and synchrotron Mössbauer spectroscopy at room temperature. An electron transition from the high-spin (HS) state to the low-spin (LS) state (HS-LS crossover) has been observed in the pressure range of 55–70 GPa. The true HS-LS transition occurs in a narrow pressure range and the extension of the electron transition to ∼15 GPa is attributed to the effect of the nearest environment and to thermal fluctuations between the high-spin and low-spin states at finite temperatures. It has been found that the lowest pressure at which the electron HS-LS transition can occur in the Mg1 − xFex system is 50–55 GPa.

Synchrotron Mössbauer spectroscopic study of ferropericlase at high pressures and temperatures

Abstract The electronic spin state of Fe2+ in ferropericlase, (Mg0.75Fe0.25)O, transitions from a high-spin (spin unpaired) to low-spin (spin paired) state within the Earth’s mid-lower mantle region. To better understand the local electronic environment of high-spin Fe2+ ions in ferropericlase near the transition, we obtained synchrotron Mössbauer spectra (SMS) of (Mg0.75,Fe0.25)O in externally heated and laser-heated diamond anvil cells at relevant high pressures and temperatures. Results show that the quadrupole splitting (QS) of the dominant high-spin Fe2+ site decreases with increasing temperature at static high pressure. The QS values at constant pressure are fitted to a temperature-dependent Boltzmann distribution model, which permits estimation of the crystal-field splitting energy (Δ3) between the dxy and dxz or dzy orbitals of the t2g states in a distorted octahedral Fe2+ site. The derived Δ3 increases from approximately 36 meV at 1 GPa to 95 meV at 40 GPa, revealing that both high pressure and high temperature have significant effects on the 3d electronic shells of Fe2+ in ferropericlase. The SMS spectra collected from the laser-heated diamond cells within the time window of 146 ns also indicate that QS significantly decreases at very high temperatures. A larger splitting of the energy levels at high temperatures and pressures should broaden the spin crossover in ferropericlase because the degeneracy of energy levels is partially lifted. Our results provide information on the hyperfine parameters and crystal-field splitting energy of high-spin Fe2+ in ferropericlase at high pressures and temperatures, relevant to the electronic structure of iron in oxides in the deep lower mantle.