Short Notes

phys. stat. sol. (b) 158, K105 (1990) Subject classification: 61.80 and 76.80; S1.3

KlOD

Institute of Physics and Power Engineering, Obninskl) Investimtion of Proton-Irradiated Platinum by Miissbauer Spectroscopy

BY V.V. BOGDANOV, V.V. ZAKURKIN, D.V. PETROV, and YU.P. PENKOV

Introduction The interaction of hydrogen atoms with point defects and substitutional impurity atoms in metals has profound consequences on the mobility of both defects and hydrogen, and essentially determines the kinetics of microstructure formation and its character in irradiated materials 111.

Until recently the information about the influence of dissolved hydrogen on the parameters of Mossbauer spectra was gained from experiments with hydride phases

containing impurity probe atoms 1'21. It was shown that the direction of the isomer shift is consistent with the decrease of the s-electron density (p,) on the resonant nucleus 57Fe in metallic matrices as a result of hydrogenation. However, whereas in the case of N b , Ti, and N i the decrease of p, was partly determined by the macroscopic increase of material volume, in the PdHx systems (x < 0.55) this decrease was insufficient in comparison with the expected volume change. In the latter case a component of the isomer shift which is not connected with the lattice expansion, corresponds to the increase of p, on the 57Fe nucleus and essentially compensates for a contribution due to macroscopic volume change. The increase of ps may be explained both by the influence of dissolved hydrogen in Pd on the electronic structure of 57Fe impurity atoms and by a volume decrease in its localization site surrounded by hydrogen neighbours. These mechanisms of hydrogen atom influence on p, at the resonant nucleus apparently take place also in the case of low average concentration of hydrogen which may be trapped at Mossbauer atoms. The character of the specific local environment will be manifested in the resonance spectra without additional effects caused by macroscopic lattice expansion.

As it has been shown in earlier investigations 131, the saturation of the Cu matrix with hydrogen in the case of radiation defect generation essentially alters the character of their interaction with 57C0 probe atoms, leading to the trapping of defects at the latter. In contrast, Mossbauer experiments with Cu Co sources performed following low temperature neutron and electron irradiations and subsequent annealings did not reveal any evidence of defect trapping at 57C0

impurity atoms /4/. A similar result was also obtained for electron irradiated Pt with the addition of 57C0. No trapping was detected in the case of Pt source

57

57 Co

) SU-249020 Obninsk, USSR.

K106

annealing after low temperature deformation 151. On the other hand, defect trapping at the 57C0 probe impurity led to the appearance of "defect!' lines in the spectrum of the proton irradiated Pt Co source 161. Therefore, one would expect Pt along with Cu to be a metallic matrix which demonstrates a high sensitivity of defect trapping at 57C0 probe atoms to the defect-hydrogen interactions. In this note we report on M6ssbauer investigations of the defect annealing in Pt in the presence of dissolved hydrogen.

The Mossbauer source P t Co (15 mCi) was prepared by thermal diffusion of electrolytically deposited 57C0 into the polycrystalline platinum matrix (99.986% purity). The final step of diffusion treatment was undertaken at 1420 K. A sample in the form of a plate (12~12~0.3 mm ) was mounted in a sample holder of a special cryostat and was irradiated at 90 K with 1 MeV protons. The total dose was 5 x d 7 cm-2 at a beam current density of 3 l.iAcm-2 ( 1 . 9 ~ 1 0 ~ ~ cm-'s-'). We have developed an .experimental facility that makes it possible to perform Mossbauer experiments with sources ion irradiated in the 2-MeV Van de Graaff accelerator at liquid nitrogen temperature, without warming up the satnples during transportation to the measuring set-up. Isochronal (10 min annealings of the irradiated source were carried out in a cryostat for the interval of 100 to 320 K and in evacuated quartz tubes for higher annealing temperatures. Mossbauer spectra of the source just after irradiation and after each step of annealing sequence were taken with an AME-50 type spectrometer with a programmable analyzer IN-96. Measurements were performed with a source in a cryostat at 77 K and a resonance detector mounted on a constant-acceleration drive.

To determine the parameters of the individual spectral components computer fitting of the spectra was performed. Special programmes for the analysis of the complex spectra with poorly resolved lines were utilized. In the first step of fitting the positions of the "defect" components relative to the substitutional line and the values of quadrupole splittings were determined. This procedure was performed by computer realization of the method of Mdssbauer spectra processing based on the formalism of "image restoration and quality improvement'' 171. In the final fitting the Mossbauer spectra were represented as a superposition of lines with the same width and fixed values for the central shifts and the quadrupole splittings. The only variable parameters were line intensities. The results of the computer fits for the fractional areas of the spectral components as a function of the annealing temperature give the basis of the analysis of the annealing process.

Results and discussion The single line Lorentzian shape spectrum of the Pt Co source before irradiation had a measured linewidth of 0.31 mmls . Characteristic Mossbauer spectra of a proton irradiated source after successive isochronal annealings up to 150 and 500 K , respectively, are shown in Fig. la and b. The fractional areas of the "defect" spectral components as a function of the

physica status solidi (b) 158

57

57 Experimental

3

57

Short Notes K107

I I I

l6 t

\ * . . . . . :$.. .. . . . . . . . .. . .. .. . . .. .. .' .

c- Y ( rnrnls)

Fig. 1. Typical Mdssbauer spectra of Pt Co source obtained oq isochronal annealing at a) 150 K and b) 500 K after 1 MeV proton irradiation at 90 K. The arrows indicate the positions of substitutional line (0), and the "defect" lines 1 to 4

57

annealing temperature are presented in Fig. 2. The best description of the spectrum of the ,as-irradiated source is provided by three additional doublets 1, 2, and 3, the parameters of these components are given in Table 1. In the case of annealing treatment, the following details of "defect" line behaviour can be recognized. The slow decrease of intensities of components 1 and 2 was accompanied by a remarkable increase of the area fraction of line 3, reaching a maximum

rt around 200 K. After 300 K annealing the "defect" lines 1 and 3 have disappeared, while the intensity of the component 2 was found to be approximately constant in the range of 200 to 300 K, and began to decrease at higher temperature. During annealing at 300 K a new "defect" doublet appeared in the Mjssbauer spectra. This line (component 4) reaches the maximum intensity at around 450 K , disappearing completely above 600 K.

T a b l e 1

Central shifts AS relative to the substitutional line and quadrupole splittings AE of the "defect" lines in Pt 57 Co source

The positive sign of AS means an increase of the electron density at the nucleus of the probe atom. The widths of "defect" lines are set equal to that of the substitutional line.

K108 physica status solidi (b) 158

Fig. 2. The fractional areas of "defecttf spectral components as a function of annealing temperature for proton irradiated Pt

A salient feature of the 57C0 atom location corresponding to site 1 is an increased value of ps at the probe nucleus. It was suggested that such an effect is a consequence of hydrogen atom trapping at the 57C0 probe 131. The transfer of d-electron charge from the Co atom to hydrogen can decrease its shielding effect on the 3s electrons, thus causing the increase of ps at the 57C0 nucleus

Little is known about th'e interactions of hydrogen with self-interstitial atoms (SIA) and their clusters. In accordance with the theoretical results 191 and ion- channeling data I101 hydrogen atoms may be trapped at SIAs with the formation of stable complexes. One might expect that the interaction of the SIA-H complex with the Co atom is likely to involve a more complex defect-impurity cluster. We suggest I1defect1' line 2 to be due to Co probe atoms with trapped SIA-H complexes and the disappearance of this line at temperatures from 300 to 450 K to the dissociation of such clusters (presumably small interstitial loops) with the detrapping of SIAs and their migration to sinks. The activation of vacancy migration and the annihilation of vacancies with trapped interstitials can also decrease the intensity of line 2.

1 8 1 .

During irradiation at 90 K SIAs interact with each other, forming di-interstitials (di-SiA). It is known that the migration enthalpies for di-SIAs in f.c.c. metals are markedly higher than that for single ones. We propose to explain the steep increase of the line 3 intensity by the formation of Co sites with trapped di-SIA- H complexes at annealing temperatures in the range of 100 to 200 K. Being

relatively unstable these clusters decay in a narrow temperature above

200 K. interval

S h o r t Notes K109

The trapping of h y d r o g e n atoms at Vacancies is a well established f a c t / I / . It is believed that the formation of V-H complexes can influence the vacancy mobility and, possibly, the interactions of vacancies with substitutional impuri t ies . The t empera tu re range of component 4 evolution correlates with the location of the peak population of the p r o b e atom-vacancy complexes in perturbed angular correlation (PAC) experiment with '"In as impur i ty atoms in Pt matrix /11 / . The identification of "defect" line 4 with V-H Complexes trapped at 57C0 probe atoms has been proved in an addi t ional experiment . Taking into account that the migration of interstitials is always accompanied by the annihi la t ion of vacancies, the P t Co s o u r c e irradiated and annealed at 450 K w a s subsequently pos t - irradiated at 90 K. A s a result of this t r ea tmen t it was revealed that the "defectt t line 4 was obviously suppressed in the spec t rum. Thus, the application of defect- an t ide fec t reactions in the present investigation s t r o n g l y supports the ass ignmen t of '!defect" line 4 to the trapping of vacancy type d e f e c t s at the 57C0 probe atoms.

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Refe rences / I / A.A. PISAREV, Atomnaya Energ iya 62, 109 (1987). I21 F. WAGNER and G. WORTMANN, in: H y d r o g e n in Metals, Vol. I ,

Ed. G. ALEFELD and J. VOLKL, Spr inge r -Ver l ag , 1978 ( p . 132).

in: Electronic Structure and P r o p e r t i e s of H y d r o g e n in Metals, Ed. P. JENA and C.B. SATTERTHWAITE, Plenum Press, 1983 (p. 567).

/ 3 / P. BOOLCHAND, C .T . MA, M. MARCUSO, and P. JENA,

/ 4 / G . VOGL, W. MANSEL, W. PETRY, and V. GR6GER,

/ 5 / W . PETRY, M. BRfjSSLER, V. GR6GER, H.G. MfjLLER, and G. VOGL, Hyper f ine Interactions 4, 681 (1978).

Hyper f ine In t e rac t ions 15/16, 371 (1983). IS/ V.V. BOGDANOV, V.V. ZAKURKIN, and YU.P. PENKOV,

Preprint FEI-1866, Obn insk 1987. 171 D.V. PETROV and V.V. ZAKURKIN, Preprint FEI-1892, Obn insk 1988. / 8 / P. JENA, F.Y. FRADIN, and D.E. ELLIS, Phys. Rev. B 20, 3543 (1979). / 9 / J .K. NORSKOV, F. BESENBACHER, J. BOTTIGER, B . BECH NIELSEN,

and A.A. PISAREV, Phys. Rev . Letters 49, 1420 (1982). / l o / F . BESENBACHER, H. BOGH, A.A. PISAREV, M . J . PUSKA,

S. HOLLOWAY, and J . K . NORSKOV, Nuclear I n s t r u m . and Methods B 4, 232, 374 (1984).

/11/ L. NIESEN, Hyper f ine In t e rac t ions lJ, 619 (1981).

(Received November 8, 1989)