V.E. Antonov

Phase transformations, crystal and magnetic structures of high-pressure hydrides of d-metals

This paper will briefly discuss the high-pressure-hydrogen techniques used at the Institute of Solid State Physics, Russian Academy of Sciences, the T–P diagrams of the studied binary metal–hydrogen systems and the crystal and magnetic structures of high-pressure hydrides formed in those systems. A compilation of the available experimental data and a list of relevant publications are provided for reference purposes.

Magnetic ordering in hydrofullerite C60H24

Abstract Hydrofullerites C 60 H x synthesised at hydrogen pressures of 0.6 and 3 GPa were found to possess ferromagnetic properties at room temperature. The magnitude of magnetisation varied from sample to sample and reached 0.001–0.16 Bohr magnetons per C 60 molecule at H =10 kOe. The coercivity of all the samples was about 100 Oe. The hydrofullerites had either an fcc or bcc lattice formed of C 60 H x units. The maximum values of magnetisation were observed for the fcc hydrofullerites with x ≈24.

Neutron diffraction investigation of the dhcp and hcp iron hydrides and deuterides

Abstract Iron hydride and deuteride with the dhcp metal lattice and iron deuteride with the hcp lattice were prepared under high hydrogen or deuterium pressures and studied by neutron diffraction in a metastable state at ambient pressure and 90 K. The crystal structures of the obtained phases were refined using the Rietveld method. The composition of the dhcp phase is close to FeH 1.0 or to FeD 1.0 and its metal lattice contains many stacking faults in agreement with the previous data. Hydrogen or deuterium atoms in this phase occupy all available octahedral interstitial sites in the metal lattice. The composition of the hcp phase is FeD 0.42±0.04 , deuterium atoms randomly occupying approximately 0.42 octahedral site of the one available per formula unit (L′3 type structure). The hcp phase is identified with the non-magnetic phase observed earlier in the Mossbauer spectra of Fe–H and Fe–D samples at 4.2 K.

Coherent control of the waveforms of recoilless γ-ray photons

The concepts and ideas of coherent, nonlinear and quantum optics have been extended to photon energies in the range of 10–100 kiloelectronvolts, corresponding to soft γ-ray radiation (the term used when the radiation is produced in nuclear transitions) or, equivalently, hard X-ray radiation (the term used when the radiation is produced by electron motion). The recent experimental achievements in this energy range include the demonstration of parametric down-conversion in the Langevin regime, electromagnetically induced transparency in a cavity, the collective Lamb shift, vacuum-assisted generation of atomic coherences and single-photon revival in nuclear absorbing multilayer structures. Also, realization of single-photon coherent storage and stimulated Raman adiabatic passage were recently proposed in this regime. More related work is discussed in a recent review. However, the number of tools for the coherent manipulation of interactions between γ-ray photons and nuclear ensembles remains limited. Here we suggest and implement an efficient method to control the waveforms of γ-ray photons coherently. In particular, we demonstrate the conversion of individual recoilless γ-ray photons into a coherent, ultrashort pulse train and into a double pulse. Our method is based on the resonant interaction of γ-ray photons with an ensemble of nuclei with a resonant transition frequency that is periodically modulated in time. The frequency modulation, which is achieved by a uniform vibration of the resonant absorber, owing to the Doppler effect, renders resonant absorption and dispersion both time dependent, allowing us to shape the waveforms of the incident γ-ray photons. We expect that this technique will lead to advances in the emerging fields of coherent and quantum γ-ray photon optics, providing a basis for the realization of γ-ray-photon/nuclear-ensemble interfaces and quantum interference effects at nuclear γ-ray transitions.

Neutron spectroscopy of fullerite hydrogenated under high pressure; evidence for interstitial molecular hydrogen

Inelastic neutron scattering spectra of a hydrofullerite quenched after synthesis at 620 K under a hydrogen pressure of 0.6 GPa, and of the same sample after annealing at 300 K for 35 h, which reduced the hydrogen content by molecules per unit, were measured at 85 K. The quenched sample is shown to consist of molecules with and of interstitial molecular hydrogen. The interstitial molecular hydrogen left the sample during annealing at room temperature, whereas the molecules were stable at this temperature. The intramolecular and intermolecular vibrations of and in the fullerite are discussed in view of the measured spectra.

On the isomorphous phase transformation in the solid f.c.c. solutions CoH at high pressures

Abstract Samples of Co containing 0.2 at.% Fe and consisting of a mixture of approximately equal amounts of the h.c.p. (stable) and f.c.c. (metastable) phases were loaded with hydrogen by a 24 h exposure to a hydrogen atmosphere at 325°C and pressures up to 9 GPa. X-ray diffraction at ambient pressure and 100 K has revealed a steep increase in the lattice parameter of the f.c.c. phase in the samples hydrogenated at 4–6 GPa, corresponding to an increase in the hydrogen content from few atomic per cent to an H/Co atomic ratio of approximately 0.95. This behaviour is interpreted as a supercritical anomaly of an isomorphous phase transformation in the f.c.c. solid solutions. The topology of the T-P phase diagram of the CoH system is discussed in view of these observations.

Neutron spectroscopy of MnH0.86, NiH1.05, PdH0.99 and harmonic behaviour of their optical phonons

Abstract Inelastic neutron scattering spectra from manganese, nickel and palladium hydrides synthesized under a high pressure of gaseous hydrogen have been measured in the energy region of 0–500 meV. The positions and intensities of the peaks in the higher energy parts of the spectra are well described by a contribution from the multiphonon neutron scattering in the harmonic approximation.

Thermally stable hydrogen compounds obtained under high pressure on the basis of carbon nanotubes and nanofibers

Compounds containing 6.3–6.5 wt % H and thermally stable in vacuum up to 500°C were obtained by annealing graphite nanofibers and single-walled carbon nanotubes in hydrogen atmosphere under a pressure of 9 GPa at temperatures up to 45°C. A change in the X-ray diffraction patterns indicates that the crystal lattice of graphite nanofibers swells upon hydrogenation and that the structure is recovered after the removal of hydrogen. It was established by IR spectroscopy that hydrogenation enhances light transmission by nanomaterials in the energy range studied (400–5000 cm−1) and results in the appearance of absorption bands at 2860–2920 cm−1 that are characteristic of the C–H stretching vibrations. The removal of about 40% of hydrogen absorbed under pressure fully suppresses the C–H vibrational peaks. The experimental results are evidence of two hydrogen states in the materials at room temperature; a noticeable portion of hydrogen forms C–H bonds, but the most of the hydrogen is situated between the graphene layers or inside the nanotubes.

Atomic ordering in the hcp cobalt hydrides and deuterides

Abstract Cobalt hydrides and deuterides with the hcp metal lattice and H(D)-to-metal atomic ratios 0.18≤ x ≤0.5 were prepared under high pressures of hydrogen or deuterium, respectively, and studied by neutron diffraction in a metastable state at 120 K and ambient pressure. A profile analysis of the spectra showed that in all samples the hydrogen and deuterium atoms occupy octahedral interstitial sites. In the samples with x ≤0.26, the hydrogen and deuterium atoms are randomly distributed over these sites. In the samples with x ≥0.34, they form layered superstructures, occupying every third octahedral base layer at x =0.34 and every second layer at x =0.38 and 0.5.

Hydrogen Caused Ordering in PdAg Alloy

Hydrides of PdAg alloy synthesized at high hydrogen pressures have been studied by neutron diffraction. At temperatures of about 470K saturation with hydrogen causes ordering of the metal atoms. At higher temperatures of about 640K no ordering occurs. The results have been explained by the theory taking into account an influence of interstitial atoms on the ordering temperature of binary alloys. It is expected that hydrogénation and subsequent dehydrogenation could be a perspective technique for manufacturing ordered alloys in the cases when the ordering cannot be made by the usual means.