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Hyperfine Interactions 54 (1990) 891-894

891

A 57Fe MOSSBAUER STUDY OF THE HIGH PRESSURE HYDRIDE PHASES OF Fe AND Co R. WORDEL, M. BAIER, G. SCHNEIDER and F.E. WAGNER
Physics Department, Technical Unioersity of Munich, D-8046 Garching, FRG

V.E. ANTONOV and E.G. PONYATOVSKY
Institute of Solid State Physics, Academy of Sciences of the USSR, Moscow, USSR

Yu. KOPILOVSKII and E.F. MAKAROV
Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow, USSR

Hydrides of iron and cobalt prepared at pressures between 4.0 and 9.5 GPa were studied by 57Fe MiSssbauer spectroscopy at 4.2 K. Iron hydride was found to be nearly stoichiometric Fell. The two iron sites in its dhcp lattice have hyperfine fields of 33.8 and 28.8 T. Practically the same results were found for the deuteride. In hcp c-Coil x, the hyperfine fields decrease with hydrogen content by about 6% between x = 0 and x = 0.5. In all studied hydrides the electron densities at the 57Fe nuclei are smaller than in the pure metals.

1. Introduction

Both iron and cobalt form hydrides at temperatures around 300~ and hydrogen pressures exceeding about 5 GPa for Fe and 2 GPa for Co [1-4]. Iron hydride was initially thought to have a hcp structure with the hydrogen in the octahedral interstices, but later shown [5] to rather have a dhcp stacking of closepacked planes, as had been suggested by MiSssbauer experiments [6] with samples that were only partially hydrogenated. We now present MiSssbauer data for virtually pure e-Fell x as well as first results for the corresponding deuteride. In hcp cobalt, hydrogen forms a continuous series of c-CoH~ solid solutions up to a hydrogen-to-metal atomic ratio of about x = 0.6 [1,4]. At higher hydrogen contents, the fcc y-phase with x = 1.0 begins to form. Like the hydride of iron, both hydride phases of cobalt are ferromagnetic [1,4], but the spontaneous magnetization of Coil x decreases with the hydrogen content. The present 57Fe M/Sssbauer results confirm these findings and show that between the iron probes and the hydrogen in CoH~, there is no strongly repulsive interaction like, for instance, in /3-NiH x at x = 1.0 [7].
9 J.C. Baltzer A.G. Scientific Publishing Company


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R. Wordel et al. / High pressure hydrides of Fe and Co

2. Experimental details At normal pressure and ambient temperature, the hydrides of iron, and to a lesser extent also those of cobalt, decay quickly into the metals and hydrogen. All M/Sssbauer experiments were therefore performed at 4.2 K. The hydrides were prepared at pressures between 4 and 9.5 GPa as described elsewhere [1] and cooled with liquid N 2 while still in the high-pressure cell. Between the preparation and the M/Sssbauer experiments, and during the transfer into the MiSssbauer cryostat, the samples were kept at liquid N= temperature. The hydrogen contents were determined by outgassing after the MiSssbauer measurements. Iron hydride and deuteride were prepared from 25 p.m thick iron foils, the hydrides of cobalt from 25/,m thick foils containing 0.5% of enriched 57Fe.

3. Results and discussion M/Sssbauer spectra of the hydride and deuteride of iron are shown in fig. 1. The hydride was prepared at a pressure of 8.5 GPa and a temperature of 350 ~ C. It exhibits two magnetic hyperfine patterns with hyperfine fields of Bhr = 33.8(1) and 28.8(1) T. Both have very small effective quadrupole interactions, 88 cos=0- 1)= -0.053(5) and +0.030(5) mm/s, respectively. This is not surprising since the c/a ratio of e-Fell,, is very close to the ideal value [1,2,5,8]. The isomer shifts of both patterns are also practically the same, IS = + 0.364(5) and + 0.370(5) mm/s with respect to the source of 57Co in Rh at 4.2 K, or about +0.49 mm/s with respect to a-iron. The hydrogen content of the sample was found to be x = 1.01(3). According to the MiSssbauer spectrum, the sample still contains 3.6(5)% of a-iron. For the ~-FeH~ phase one therefore obtains x = 1.05(4). This is compatible with a virtually stoichiometric Fell composition of the hydride, which explains the narrow M/Sssbauer lines of the hydride phase. The FED,. sample was prepared at 7.0 GPa and 350 ~ and still contained about 70% of a-iron. The hyperfine fields. Bhf = 33.5(2) and 29.0(2) T, as well as the isomer
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Fig. 1. M~ssbauer spectra of iron hydride (left) and iron deuteride (right) measured with a source of 57Co in Rh with the source and absorber at 4.2 K. Both samples still contain ~-iron, which is the weakest component in the hydride but dominant in the deuteride.


R. Wordel et al. / High pressure hydrides of Fe and Co
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Fig. 2. M~3ssbauer results for the hydrides of cobalt. On the left, spectra of 57Fe in hcp Co and COH0.56 are shown. The diagram on the right shows the dependence of the hyperfine field (dots), the isomer shift with respect to the 57Co: Rh source (triangles) and the projected electric quadrupole interaction, 88 cos ~- 0 - 1), (squares) on the hydrogen-to-metal ratio x.

shifts and quadrupole interactions of the deuteride are nearly the same as those of the hydride. The hydrogen content of the FeD~ phase could not be determined because of the high a-iron content, but the MiSssbauer data indicate that it is also virtually stoichiometric FeD. The two magnetic hyperfine patterns in both the hydride and the deuteride can be attributed to the cubic and hexagonal layers of the dhcp structure, but it is impossible to tell which of the two sites has the larger hyperfine field. With Bhr=32.4(2) T, ~eQV._,.(3 cos 2 0-1)=-0.055(3) mm/s and IS= -0.101(3) mm/s, the spectrum of the rolled 57Fe: Co foil (fig. 2) used to prepare the hydrides is typical for the hcp low temperature phase of Co [9,10]. Samples of e-Coil x prepared at 350 ~ and 4, 5 and 6 GPa reveal a decrease of the hyperfine field and an increase of the isomer shift with increasing hydrogen content (fig. 2), while the quadrupole splitting hardly changes. Although the hydrides are nonstoichiometric, their MiSssbauer lines are but slightly wider than those of the pure Co sample. The Co sample loaded at 8.2 GPa (x -= 0.84(4)) should be a mixture of hcp c-CoH~, with x = 0.65 and of fcc 7-CoH=I. 0 [1,4]. The M/Sssbauer spectrum, however, does not show any contribution of the e-phase and rather indicates that the sample is pure but understoichiometric fcc y-Coil,.. The line broadening is larger than in the E-phase hydrides. The shape of the spectrum indicates that this is due to a distribution of isomer shifts rather than hyperfine fields. The decrease of the hyperfine field with hydrogen content by about 6% between 5VFe in pure Co and in c-Coil0. 5 is somewhat smaller than the 13% decrease of the magnetic moment per Co atom in the same composition range [4]. On formation of the fcc y-Coil,. at higher hydrogen concentrations, the hyperfine field increases again, revealing structural influences in addition to those of the


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R. Wordel et al. / High pressure hydrides of Fe and Co

hydrogen. The isomer shift increases nearly linearly with x and extrapolates to about +0.40 mm/s at x = 1.0 with respect to 57Fe : Co. The approximately linear dependence on x as well as the absolute value of the shift as compared to that of c-Fell1. 0 indicate that iron probes in CoH~ may experience a slightly repulsive interaction with the hydrogen, but certainly not a strong repulsion Iike that found, e.g., for 57Fe in NiH x [7] or PdH~ [11].

Acknowledgement
This work has been funded in part by the Deutsche Forschungsgemeinschaft.

References
[1] E.G. Ponyatovsky, V.E. Antonov and I.T. Belash, in: Problems in Solid-State Physics, eds. A.M. Prokhorov and A.S. Prokhorov (Mir, Moscow, 1984) p. 109. [2] V.E. Antonov, I.T. Belash, V.F. Degtyareva, E.G. Ponyatovskii and V.I. Shiryaev, Dokl. Akad. Nauk SSSR 252 (1980) 1384; Sov. Phys. Doklady 25 (1980) 490. [3] V.E. Antonov, I.T. Belash and E.G. Ponyatovsky, Scripta Met. 16 (1982) 203. [4] I.T. Belash, W.Yu. Malyshev, B.K. Ponomarev, E.G. Ponyatovskii and A.Yu. Sokolov, Fiz. Tverd. Tela 28 (1986) 1317; Soy. Phys. Solid State 28 (1986) 741. [5] V.E. Antonov, I.T. Belash, V.F. Degtyareva, D.N. Mogilyansky, B.K. Ponomarev and V.Sh. Shekhtman, Int. J. Hydrogen Energy 14 (1989) 371. [6] R. Wordel, F.E. Wagner, V.E. Antonov, E.G. Ponyatovskii, A. Permogorov, A. Plachinda and E.F. Makarov, Hyp. Int. 28 (1986) 1005; Z. Phys. Chem. NF 145 (1985) 121. [7] M.A. Amer, F.E. Wagner and H.J. Bauer, Hyp. Int. 41 (1988) 539. [8] T. Butz, Physica Scripta 17 (1978) 87. [9] G.J. Perlow, C.E. Johnson and W. Marshall, Phys. Rev. 140 (1965) A875. [10] W. Karner, L. H~ggstr~Sm and R. W~ippling, Hyp. Int. 10 (1981) 867. [11] F. PriSbst and F.E. Wagner, J. Phys. F: Met. Phys. 17 (1987) 2459.