THE STUDY OF STATICAL AND DYNAMICAL PROPERTIES OF OXYGEN AND HYDROGEN IMPURITIES IN TANTALUM METAL, USING THE 6.2 keV MÖSSBAUER TRANSITION OF 181Ta

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THE STUDY OF STATICAL AND DYNAMICAL PROPERTIES OF OXYGEN AND HYDROGEN IMPURITIES IN TANTALUM METAL, USING THE

6.2 keV MÖSSBAUER TRANSITION OF 181Ta

P. West, D. Salomon

To cite this version:

P. West, D. Salomon. THE STUDY OF STATICAL AND DYNAMICAL PROPERTIES OF OXY- GEN AND HYDROGEN IMPURITIES IN TANTALUM METAL, USING THE 6.2 keV MÖSS- BAUER TRANSITION OF 181Ta. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-616-C2-618.

�10.1051/jphyscol:19792214�. �jpa-00218594�

JOURNAL DE PHYSIQUE Colloque C2, supplcfment au n ^O 3, Tome 40, mars 1979, page C2-616

THE STUDY OF STATICAL AND DYNAMICAL PROPERTIES OF OXYGEN AND HYDROGEN IMPURITIES IN TANTALUM METAL, USING THE

6 , 2 k e v

MoSSBAUER TRANSITION OF

' ~ a

P. J. West and D. ~a?omon+

Fachbereich Physik, Freie Vniversitat Berlin, 0-1000 Berlin 33, Germany

Rhsum6.- L'influence dfimpuret6s d'oxygsne dans le tantale mhtallique sur la transition Mgssbauer B 6,2 keV de "'~a a ht6 6tudiEe pour des concentrations dfoxyg6ne de 500 1 50 pprn et pour des tempgratures situhes entre 300 et 2400 K. Cette influence est cornparse B celle de concentrations d'hydrogsne allant de 20 000 5 600 ppm, entre 30 et 400 K. Les tempsratures critiques sont observ6es pour llhydrogSne B 200 K et pour 110xyg6ne B 1400 K. Au-dessus de ces temp6rature.s critiques la mobilith des impuretCs interstitielles rsduit la probabilit6 d'excitation du voisin le plus proche.

Des 6nergies d'activation de 0,14 (1) et de 1,O (I) eV sont obtenues pour l'hydrogsne et pour l'oxygsne respectivement au-dessus des tempsratures critiques.

Abstract.- The influence of oxygen impurities in tantalum metal on the 6.2 keV MGssbauer transition of '"~a has been studied for oxygen concentrations of 500 pprn to 50 pprn and for temperatures ranging from 300 to 2400 K. This is compared with the influence of hydrogen concentrations from 20000 pprn down to 600 pprn over the temperature range of 3 0 to 400 K. Critical temperatures are observed for hydrogen at 200 K and for oxygen at 1400 K. Above these temperatures the interstitial impurity's mobility reduces the probability for the next neighbour excitation. Activation energies of 0.14 (1) and 1.0 (I) eV are obtained for hydrogen and oxygen respectively for temperatures greater than the critical temperature.

I. Introduction.- The diffusion of hydrogen in tan- talum metal at room temperature (R.T.) is characte- rized by a mean-stay-time of

lo-''

seconds /I/. Thus even at low temperature a fast diffusion is expected to persist for low H-concentrations in the a-phase.

On the other hand, above 500 K, the hydrogen readily leaves the metal 121. However, oxygen diffusion into the metal from the surface layers takes place as the temperature is further increased /3/. For oxygen at low concentrations

(%lo3

ppm) comparable diffusion rates to those of hydrogen at R.T. are obtained abo- ve 1400 K /4/.

In earlier '"~a-temperature studies of hydro- gen loaded tantalum foil /5,6/ the motional-narrowing of the resonance line was demonstrated. Both line- width and isomer shift varied linearly as a function of the hydrogen concentration at R.T. In this paper we confirm these results, but with proton implanted tantalum foils. These results are then compared with oxygen loading at very low concentrations and high temperatures.

2. Experimental.- In a previous study 171; the beha- viour of the 6.2 keV-resonance of '"~a at very high temperatures (up to 2400 K) was demonstrated. In the present work, using the same set-up, tantalum foils

+ present address : Ruhr Universitst Bochum, Exp.

Physik IV, D 4630 Bochum, Germany.

measured at different temperatures and partial pres- sures

%lo-'

to torr were monitored to contain approximately the same residual gas concentrations.

The concentration was determined by a R.T. measure- ment of the resonance after a fast shut down process

(<ms) of the heating current / 9 / . It may then be assumed that the concentration of oxygen at high temperatures is frozen in /7,8/.

The Ta-metal foils for proton implantation, first out-gassed at

lo-''

torr, were loaded with 80 keV protons in a Faraday cup setup at a pressure of torr. T1,e low-temperature measurements of the foils were then carried out in a Leybold cryos- tat (VMK -3-700) in a transmission geometry, where the source was kept outside at R.T.

In figure I, the resultant spectra for oxygen loaded foils are displayed, whereas in figure 2 a graphical summary of the isomer shift of oxygen and hydrogen loaded foils is shown.

3. Results and discussion.- The diffusion of the interstitial gas atom can be described as a modula- tion of the resonance energy with the parameters

A

and T~ /10,11/.

A

is the modulation amplitude, here of a fluctuating monopole interaction /5,10/, and T is the temperature dependent correlation time.

We will distinguish here three different ranges of temperature :

a) A temperature range for which the fast modulation

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792214

RT

r =a168 17) mmls

I

Fig. I : ~assbauer spectra for two different concen- trations of oxygen interstitials at temperatures above Tk. The R.T. spectra shown was obtained after a fast shut down for each measurement (see text) and was used for concentration determination as well.

A.T

<C h /10,13/ is valid. In this case the increa

se in linewidth and isomer shift is given /12/ by 6 W = ~ A and ~ S T= 2Ac1I2, ~ where c is the impurity concentration. Third order correlation effects on

S /I31 were neglected, because our measurements are not sufficiently ~recise to observe these.

b) A temperature range for which the modulation be- comes an intermediate state between the fast rela-

0 5

-.

-

V)

.

_{E \}

-

^\

xing and the stationary states of tantalum atoms with and those without interstitial neighbours

/lo/.

Since the mean lifetime of the 6.2 keV resonance cannot be a criterion for fast or slow modulation, we define for the intermediate case o f modulation a critical correlation time T k. T~ is associated with the impurity induced maximum isomer shift S '

m at c=l by T = 4 h / Sm. At the correlation time

k

T~ = T ~ , the maximum of line broadening as well as the maximum of the first derivative of the isomer shift with respect to temperature should then occur 16,131.

C) This temperature range establishes the condition

T~ >> T ~ , for which the modulation and the diffusion

process becomes stationary. Since the concentration of the interstitial in question is small, the non- perturbed pure Ta-resonance dominates. The weak per- turbed line is shifted by S/4c 151 to higher veloci- ty and most likely split by electric quadrupole in-

=

^{0 2}

L ly

t

teractions caused by nearest-neighbour interstitial atoms.

For a given interstitial (i.e. A) and concen- tration c T is proportional to exp(U/kT) where U is the activation energy of the interstitial in

\

-

question. Thus by varying the temperature one can then pass from the stationary to the fast modulation state of the resonance. This is indeed what is shown in figure 2 from which we conclude a critical tempe-

0 5wppm \

.

temperature (K)

- 0 1

- ^{0 4}

Fig. 2 : Summary of the isomer shift for Ta-0 and Ta-H-interstitial system as function of temperature. The second-order-Doppler shift (SOD) is indicated at the upper part in the left figure. The dashed and solid lines at the right figure are of graphical nature only.

-

^\

0 150 ppm \ \ 0

* M P P ~ ^\

\

-

⁰ concentratton ^\ \

~n To-metal \

\

\

\

200 700 1200 1700 2200

lernperoture I K I

C2-6 1 8 JOURNAL DE PHYSIQUE rature _{T k} of 200 K for H and 1400 K for the 0 inter-

stitial. The correlation time 7 associated with T k can directly be derived from T ~ . Correlations times of 7.2 x

lo-''

s/rad and 8.0 x

lo-'

s/rad are ob- tained for 0 and H interstitials respectively, con- sistent with diffusion time values calculated from macroscopic diffusion data /1,4/ at _{T k .} Above _T

k' the case of fast modulation, activation energies of 0.14(1) eV for H and 1.0(1) eV for 0 are obtained from the dependence of the experimental linewidth with temperature. These are also in good agreement with previously measured values /1,4/. Table I sum- marizes the results for 0 and H interstitial diffu- sion.

Therefore we assume that the change in the i s w mer shift and linewidth at T is only a time depen-

k

dent relaxation mechanism, and because of the rever- sibility of these effects with temperature no preci- pitation in the case of 0 or in the case of H occurs.

It should be noted that both H and 0 impurities show similar dynamic effects which seem to be independent of their ionic character.

4. Sumary.- It has been demonstrated that the 6.2 keV MGssbauer spectroscopy with '"~a offers a pro- mising microscopic tool for studying the dynamical properties of the diffusion gas impurities in the pure system of tantalum metal. At very high tempe- ratures ( > 1400 K), the '"~a XEssbauer effect, be- sides the positron annihilation method 1141, is uni- que in the investigation of the diffusion of 0 inTa.

Table I

Summary of the change of isomer shifts and linewidth, and activations energies for oxygen and hydrogen interstitials in tantalum metal at various temperatures and interstitials concentrations.

i Concentration

? Tk asl la^) I (asfae),

i

(aUlac)T (Activation ₁

i Energy i

j

(impurity/Ta-Atom)

j

^(K)

i

10-"m/s/deg) mm/s/at %

i

^m/s/at

% i

^{( e ~ )}

i

J

$ange of T !

i

T<Tk

i

T>Tk

;(

T<T

i

^T>Tk

i

T>Tk

i

T>Tk

i

k :

$xygen 150 ppm 1400

i -

^{8.0 (5)}

-

^{8.0 (5)}

!

^{3.3 (1) 6.8 ( 5 ) ; 4.5 (7)}

i

^{1.0 (I)}

i

i

500 ppm

:

. .

i

^0.06 X

i

I80

i -

^{7.8 (8)}

i -

^{9.0 (6)}

hydrogen i

^2.00 %

_i

₂₀₀

i -

^{8.0 (5)}

j -

^{8.2 (6)}

ⁱ

^: < 8 x

i

^{0.63 (2) 0.078(2)}

f

^{0. 14 (1)}

f

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