M. Baier

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.

Mössbauer study of hydrides and deuterides of iron and cobalt

Abstract Hydrides and deuterides of iron and hydrides of cobalt prepared at pressures between 4 and 9 GPa and temperatures near 350 °C were studied by 57Fe Mossbauer spectroscopy. The d.h.c.p. e-FeH≈1.00 phase was found to have a deuteride analogue with nearly the same Mossbauer parameters. In addition to e-FeH≈1.00 and e-FeD≈1.00, three novel hydride (deuteride) components were observed in the Mossbauer spectra, one of them non-magnetic at 4.2 K. In h.c.p. e-CoHx the magnetic hyperfine field decreases by about 6% between x = 0 and x = 0.5. In f.c.c. γ-CoHx it is about 3% higher than in the e phase. The electron density at the 57Fe nuclei decreases with increasing hydrogen content without a discontinuity between the e and the γ phase.

Mössbauer study of the hydrides of Nb3Me with A15 structure

Abstract Hydrides of intermetallic Nb3Me (Me ≡ Au, Ir, Sn) with A15 structure were prepared at hydrogen pressures up to 7 GPa and studied by Mossbauer spectroscopy with the resonances on 197Au (77.3 keV), 193Ir (73.0 keV) and 119Sn (23.9 keV). In all these systems, hydrogenation was found to lead to a reduction in the electron density at the Mossbauer nuclei, but the decrease for 119Sn was much smaller than for 197Au and 193Ir. This difference suggests that in Nb3Au and Nb3Ir the hydrogen occupies interstitial sites which are closer to the metal atoms than those occupied in Nb3Sn.

Crystal structure and superconductivity of high pressure hydrides and deuterides of HfRu and ZrRu compounds

Abstract The solubility of hydrogen and deuterium in HfRu and ZrRu was measured at 600 K and pressures up to 7 GPa. HfRuH(D) ≈3.6 and ZrRuH(D) 1.6–2.2 were synthesized. At ambient pressure these were shown to be superconductors with T c ∼ 4.5 and 3 K respectively. No difference between the T c values of the hydrides and deuterides of the same compound was detected. X-ray diffraction at 100 K has shown the metal sublattices of HfRuH x (D x ) and ZrRuH x (D x ) to be base-centred orthorhombic with very similar unit cell parameters. The crystal structures of HfRuH 3.6 and HfRuD 3.6 determined by neutron diffraction at 120 K were of the ZrNiH 3 type. The observed superconducting and structural properties of HfRuH x (D x ) and ZrRuH x (D x ) agree with the predictions of the rigid d-band model. The applicability of this model to other high pressure hydrides is discussed.

197Au Mössbauer study of the hydrides of NiAu alloys

Abstract Hydrides of Ni0.70Cu0.30 alloys were prepared electrolytically or under high pressures of molecular hydrogen and studied by Mossbauer spectroscopy using dilute substitutional 57Fe as a probe. At a hydrogen pressure of 7 GPa, a hydrogen-to-metal ratio near x = 0.9 could be reached, while at 1.1 GPa or by cathodic charging hydrogen contents near x = 0.6 were obtained, which are already sufficient to suppress ferromagnetism completely. The hydrogen was found to be repelled by the iron probes, but less effectively than in pure nickel. The temperature dependence of the isomer shift showed that the samples began to lose hydrogen near 200 K. The reappearing magnetic phase had hyperfine parameters practically the same as those of the alloy before hydrogenation.

57Fe Mössbauer study of hydrides and deuterides of manganese

Hydrides and deuterides of manganese with hydrogen-to-metal ratiosx between 0.65 and 0.93 were studiéd by Mössbauer spectroscopy on dilute substitutional57Fe probes. No traces of magnetic ordering were found. Experiments on57Fe in MnD0.77 in external magnetic fields show that the iron has no measurable magnetic moment. The shapes of the Mössbauer patterns are compatible with superstructure ordering of the interstitials of the anti-Cdl2 type nearx=0.5, while the dependence of the isomer shifts onx shows that there are no substantial iron-hydrogen interactions.

Mössbauer and time-differential perturbed angular correlation studies of the hydrides and deuterides of vanadium☆

Abstract Hydrides and deuterides of vanadium were studied by source and absorber Mossbauer experiments with the resonances of 99Ru, 193Ir and 197Au and by perturbed angular correlation experiments with the 353 — 90 keV γ-γ cascade in 99Ru. The observed isomer shifts and electric quadrupole interactions are discussed in terms of the local environment of the probes in the different hydride and deuteride phases and compared with similar results for the Nb-H system.

Investigation of the hydrides of Ni0.70Cu0.30 by57Fe Mössbauer spectroscopy and magnetization measurements

Hydrides of Ni0.70Cu0.30 prepared both electrolytically and under high pressures of hydrogen gas were studied by Mössbauer spectroscopy on dilute57Fe probes and by magnetization measurements. At hydrogen-to-metal ratios abovex≈0.3 no ferromagnetism is observed down to 4.2 K. The dependence of the mean change of the isomer shift on the hydrogen content of the samples reveals a repulsive interaction between the hydrogen interstitials and the iron probes. The effect of this interaction is, however, less pronounced than for57Fe in the hydrides of pure nickel. This difference can be attributed to a competition of the repulsive Fe−H and Cu−H interactions.

A57Fe Mössbauer study of the high pressure hydride phases of Fe and Co

Hydrides of iron and cobalt prepared at pressures between 4.0 and 9.5 GPa were studied by57Fe Mössbauer spectroscopy at 4.2 K. Iron hydride was found to be nearly stoichiometric FeH. The two iron sites in its dhep lattice have hyperfine fields of 33.8 and 28.8 T. Practically the same results were found for the deuteride. In hep ε-CoHx, the hyperfine fields decrease with hydrogen content by about 6% betweenx=0 andx=0.5. In all studied hydrides the electron densities at the57Fe nuclei are smaller than in the pure metals.