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Hypcrfine Interactions 69 (1991) 471-474

471

INVESTIGATION OF THE HYDRIDES OF Ni,.~,Cu,.3, BY STFe MOSSBAUER SPECTROSCOPY AND MAGNETIZATION MEASUREMENTS B. ZHANG, I. DUGAND~',I(~, J.BAUER
Sektion Physik, Ludwig-Muximilians-UniversitOt, D-8000 Mtichen, Germapo,

M. BAIER, F.E. WAGNER
Pto,sics Department, Technical Universi O, of Munich, D-8046 Garehing, Germat O'

V.E. ANTONOV, T.E. ANTONOVA
Institute of Solid State PIo,~ics, Academy of Sciences of the USSR, Chernogolovka District 142432, USSR

and S.M. FILIPEK
Institute of Pto,sical Chemistry Polish /leaden O' of Sciences, ll'arsa~; Pohmd

Ilydrides of Ni0.70Cu0.a0 prepared both electrolytically and under high pressures of hydrogen gas were studied by M6ssbauer spectroscopy on dilute STFe probes and by magnetization measurements. At hydrogen-to-metal ratios above 9 .-~ 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 for STFe in the hydrides of pure nickel. This difference can be attributed to a competition of the repulsive Fe-It and Cu-lt interactions.

1. Introduction
Nickel and copper form a continuous series of solid solutions, whose saturation magnetization and Curie temperature decrease with increasing copper content. Alloys containing more than about 60 at.% Cu do not become ferromagnetic down to 4.2 Ir [1]. Ni-Cu alloys with copper contents up to about 50 at.% can be loaded with hydrogen electrolytically or under high pressures of 112 gas [2-11]. The hydrides retain the fcc structure of the Ni-Cu alloys. Like in pure Ni [12], the ferromagnetism is suppressed by hydrogenation [2,4,5,8-11]. and a hydrogen-rich fl phase t is observed [4,10]. ReCently, first results of a M6ssbauer study of the hydrides of Ni0.ToCu0.a0 have been reported [13]. "].'he present paper describes a more detailed ii1vestigation of the dependence of the isomer shift on the hydrogen content and of the properties of the magnetic a ph~e containing a few percent of hydrogen only. t Alternatively [10], these two phases have been designated as 3'1 and 3'2At COl)per concentrations below about 40 at.%, a separation into an a phase containing very little hydrogen

%) J.C. Baltzer A.G., Sciclltilic Publishing Company


472

B. Zhang et al. / Investigation of the hydrides of NioToCUo.~o

2. Experimental Details
As in the previous MSssbauer experiments [13], the absorbers were made from an arcmelted Ni0.wCu0.30 alloy containing 0.15 at.% of isotopicaily enriched STFe. Rolled foils of about 15 Fm thickness were annealed at 850 ~ in H2 for 2 hours and then loaded with hydrogen either under high pressures of molecular hydrogen or electrolytically at ambient temperature in 0.5 n It2SO4 with 0.2 g/l of thiourea as a promoting agent [14] and a current density of about 20 mA/cm 2. tlydrogen pressures of up to 3 GPa were reached at room temperature in pistontype pressure cells [9,15-17], while the hydrogenation at 7 GPa was performed at 300 ~ in a high pressure cell using AIII3 as a hydrogen donor [18]. After the hydrogenation, the pressure cells were cooled with liquid nitrogen before release of the pressure and removal of the samples. After the hydrogenation, the specimens were stored in liquid nitrogen and transferred in.to the M6ssbauer cryostat without warnfing. The hydrogen content was determined by outgassing either the whole samples after the MSssbauer measurements or parts removed from the specimens beforehand.

3. Results
The MSssbaucr spectra of the hydrides of Ni0.v0Cu0.301t~ with hydrogcn-to-mctal ratios z~0.3 are broadened'patterns that do not show a magnetic hyperfine splitting [13]. The broadening call be mainly be attributed to the distribution of hydrogens on the six interstitial sites next to the substitutional iron probes. Moreover, the distribution of Ni and Cu atoms ill the vicinity of the iron causes some line broadening already at ~ = 0 [13]. In analogy to the model used previously for fitting the spectra of NiIt~ [19,20] and PdH~ [21], the spectra were fitted with a superposition of seven equidistant lines corresponding to 57Fe with 0 to 6 nearest hydrogen neighbours. The influence of the distribution of Cu and Ni neighbours was included in the width of these lines. For the isomer shift per hydrogen neighbour a value of AS =0.10 mm/s yielded good fits of the spectra and is in agreement with previous results [13,19-21]. Fig. 1 shows the dependence of the centre shift of the non-magnetic peak on the hydrogen content. Samples with an average hydrogen content below z ~ 0.3 usually are mixtures of the non magnetic /3 phase and tile ferromagnetic ~ phase containing a few percent of hydrogen. The hydrogen contents of the/3 phase in such samples were derived using a: = 0.03 for the the ~ phase (see below) and taking the relative amounts of the two phases from the areas in the MSssbauer spectra. Samples loaded at a hydrogen pressure of 0.33 GPa at ambient temperature contained no /3 phase and had hydrogen-to-metal ratios near z = 0.03. These small hydrogen contents have no visible influence on the isomer shift or the magnetic hyperfine field of (26.6 i 0.2) T at 4.2 K. Magnetization measurements in alternating magnetic fields performed as described in Ref. [22], however, show that at 9 ~ 0.03 the Curie temperature is reduced by about 20 I( and tile saturation magnetization by about 10 % (fig. 2).

4. Discussion
The hydrogen-induced centre shifts for S~Fe in Ni0.70Cu0.~011= show a strongly non-linear dependcncc on the hydrogen contcnt but are generally higher than those for STFc in hydrides of pure nickel (fig. 1), where below room temperature the/3 phase cxists only with hydrogen


13. Zhang et al. / Im,estigation of the Io,drides of NiaToCUoj o
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o 0.15 at.% SZFc Ni0 zCu0.sH~ /
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t~ 0.15 at.% SZFe:NiH= o 1.0 at.% STFc:NiIt/o

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0

0.00.0
,.ci (_)

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0.4

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Fig. i: Isomer shifts of sTI;e in Ni0.-t0Cu0.a0tlz hydrides as a function of the hydrogen content. Open circles represent data from single-phase, filled circles data from two-phase specimens. The shifts were measured at 100 K and are given relative to hydrogen-free Ni0.7oCu0.a0 at the same temperature. Shifts for dilute STFe in NilI. hydrides [19,20] with respect to unloaded nickel are shown for comparison (triangles and squares).l

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0.6 0.4 Ni0.TCUo.alt0.03 0.2

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0.0100 150 200 250 300 350

Temperature (K)
Fig. 2: Temperature dependence of the saturation magnetization of Ni0.ToCu0.a0110.0a and of the same alloy after complete outgassmg. contcnts x ~ 0.9 [7]. Thc shifts for/3-Nill~ show no correlation with the hydrogen content (fig. 1), presumably bccausc the accuracy of the dctcrmination of the hydrogen contcnt is ]nsuIficicnt to rcsol'vc thc steep incrcasc of the shift with increasing hydrogen content for z >,, 0.9. Such a steep increase is cxpccted if there is a strongly rcpulsivc interaction between thc hydrogen and the iron probes ft9-21], which largely prcvcnts hydrogen from occupying sites next to iron at except at hydrogcn contents close to x = 1. For the Ni0.70Cu0.a0 alloys the incrcase of thc isomer shift with the hydrogcn concentration


474

B.

Zhang et

al. / Investigation of the hydrides of Nio.7oCUo.~o

is sn]oother, showing that hydrogen penetrates into the vicinity of the iron probes already at lower hydrogen concentrations. This can be explained by a distribution of interstitial site energies in the disordered Ni-Cu alloys, where interstitial sites with many Cu neighbours will be less favourable for hydrogen occupation than sites with fewer or none, in a similar manner as has been proposed for hydrides of amorphous alloys [23,2@ Hydrogen sites with an iron neighbour will be distributed within the range of site energies and therefore become filled gradually together with other sites with comparable energy. Model calculations based on the model of a site energy distribution [23,24], whose description would exceed the scope of this paper, can indeed explain the dependence of the centre slfift on the hydrogen content and on temperature [13] with reasonable values of the model parameters. The lack of sensitivity of the S~Fe for probing the inlluence of the dissolved hydrogen in the c~ phase can also be understood in this way, since for small z the interstitial sites next to the iron will not be occupied even in Ni-Cu alloys. The magnetic properties, i. e. the Curie temperature and the saturation magnetization, however, are strongly affected, as issuggested by the rigid band model, since the incorporation of hydrogen fills the empty d states in Ni or Ni-Cu alloys.

Acknowledgenlcnt
We thank Prof. R. Sizmann and Prof. B. Baranowski for their support and for their interest in this work. Funding by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.

References
[1] 121 [3[ [4] [51 I6] [7] [8] [!1] [10] [1.1] [12] [13] [14] [15] [16] [17] [18] [19] [20] [2l] [22] [23] [24] R. M. Bozorth, Ferromagnetism, (Van Nustrand, I'rinceton, 19511 p. 308. 1I. J. Bauer, Z. Natnrforsch. 22a (1967) 1468. B. lh~ranowski, and S. Majchrzak, Roczniki Chemii 42 (1968) 1137. 11. J. Bauer, (;. Berningcr and G. Zinmwrmaun, Z. Naturforsch. 23a (19681 2023. II. J. Bauer, Z. a,,t;cw. ]'hys. ~6 (1969) 87. B. Baranowski, ttydrogcn in Metals II, Topics in Applied Physics 29, eds. G. Alefeld and J. V~lkl (Springer, Berlin 11t781 p. 157. B. Baranowski, Z. Phys. Chem. N. F. i1,1 (19791 59. B. Baranowski and M. Tkacz, Europhys. Conf. Abstr. la (19751 108; Pol. J. Chem. 54 (1980) 819. 11. J. Bauer, J. Mag. Magnet. Mat. 15-18 (1980) 1267. V. E. Antonov, 1. T. Bclash, V. F. Dcgtyareva, E. G. Ponyatovsky and V. G. Thiessen, Fiz. Metallov i Metallovcdenie 53 (1982) 677. W. 15. Martin, 1|. J. Bauer, S. M. Filipek and 13. Bara,mwski, J. Mag. Magnet. Mat. 54 - 57 (t9861 1505. II. J. Bauer and E. Schmidbauer, Naturwissenschaften 11 (1961) 425; Z. Physik 164 (1961) 367. B. Zhang, 11. J. Bauer, M. Baier, F. E. Wag,mr, V. E. A,Ltonov and T. E. Antonova, J. Less(Jonl|llon Metals, in print. B. Baranowski and M. Smialowski, J. Phys. Chem. Solids, 12 (19591 2(16. 13. Baranowskiand R. Wi~niewski, Bull. Akad. Polon. Sci. 14 (1966) 273. R. Wi~niewski, Rev. Set. Instr. 41 (1970) 464. B. 13armmwski and W. Bujnowski, RoczEfiki Chemii ,14 (1970) 2271. E. G. Ponyatovsky, V. E. Antonov and t. T. Belash, in Problems in Sohd 5talc Physics, eds. A. M. Prokhorov and A. S. Prokhorov (Mir, Moscow 1984). M. Amer, F. E. Wagner and 1t. J. Bauer, Ityp. lnt. 41 (1988) 539. M. Amer, M. Baler, II. J. Bauer and F. E. Wagner, Z. Phys. Chem. NF 164 (1989) 773. F. PrBbst and F. E. Wagner, J. Phys. F: Met. Phys. 17 (1987) 2459. W. E. Martin and J. Wieser, J. Phys. E: Sci. Instr. 18 (1985) 342. R. Kirchheim, Acta Metall. 30 (1982) 1069. R. Kirchheim, F. Sommer and G. Schluckbier, ActaMetall. 30 (1982) 1059.