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Migration of vacancies in tantalum and stage III recovery in refractory bcc metals observed by PAC
Volume 117, number 2
PHYSICS LETTERS A
28 July 1986
MIGRATION OF VACANCIES IN TANTALUM AND STAGE III RECOVERY IN REFRACTORY bcc METALS O B S E R V E D BY PAC
R. SIELEMANN 1, H. METZNER, E. H U N G E R and S. K L A U M O N Z E R Hahn-Meitner-lnstitut fftr Kernforschung Berlin GmbH, Bereich Kern- and Strahlenpt~vsik, and Freie Universiti~t Berlin, Fachbereich Physik, D-IO00 Berlin, Germany' Received 2 April 1986, accepted for publication 19 May 1986
Annealing of radiation-induced defects in high-purity tantalum was studied by perturbed angular correlation (PAC) between 77 and 380 K. Vacancy migration around 270 K is deduced. From comparison with PAC studies on niobium, molybdenum, and tungsten a highly systematic picture of stage III defects in l llln traps in the refractory bcc metals evolves.
One of the still strongly disputed problems concerning point defects in metals is the temperature (or activation enthalpy) of vacancy migration in several refractory bcc metals, in particular for the group-V metals, V, Nb, Ta, and for Fe. The strong tendency of these metals to dissolve impurities (H, C, N, O) and the "blindness" of many experimental methods to distinguish between selfinterstitials and vacancies are major reasons for this dispute. Ta forms an outstanding example for a metal on which a large variety of experimental methods has been applied but a synopsis has not been reached [1,2]. Agreement has only developed on the position of the intrinsic annealing stage III, namely between 250 and 320 K (though shifts to 380 K have occurred under certain experimental conditions [3]). No agreement exists, however, whether this stage is due to vacancy migration. A recent quenching experiment by Tietze and coworkers [4] is compatible with vacancy migration below 300 K. However, a deduction of the vacancy migration enthalpy HI~ involving the formation enthalpy and self-diffusion data leads Maier et al. to conclude, that vacancies cannot migrate below 380 K [5,1]. In this situation experimental methods should favorably be applied which can supply information on the nature and microscopic structure of i Presently at Technische Hochschule Darmstadt, FRG.
the defects involved. In a recent study on Nb we used the PAC technique (perturbed angular correlation of 7 rays) in a large variety of experimental conditions that enabled us to identify defect migration around 250 K as vacancy migration [6,7]. To solve the intricate problem of doping the high-purity Nb material with radioactive tracers (the PAC probes) without spoiling the purity of the sample, ~11In probes were directly produced inside Nb via heavy-ion nuclear reactions [6]. Since l~lIn cannot be produced in Ta in that way, we used a recoil implantation technique which gives us complete freedom in the choice of the host material. A 2 rtm Nb foil was positioned in front of the high-purity polycrystalline Ta sample (90 ~tm thick, residual resistivity ratio 3000-5000) and ~ I n was produced by the reaction 93Nb(ZZNe,
4n)
111
75 s
35 min
Sb ---, 111Sn ~
1~1In,
employing a 22Ne beam of 90 MeV from the VICKSI heavy-ion accelerator. The nuclear reaction imparts energies up to 18 MeV to the resulting 111Sb ions which thereby leave the Nb foil and recoil into Ta (where they decay to ~lIn with the half-lives indicated). By that technique the PAC probes become distributed within a Ta layer of about 4 ~tm at an extremely low concentration ( < 1 at. ppm). The primary beam (22Ne) is stopped far (about 15 p,m) from the implanted activity.
0375-9601/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
87
Volume 117, number 2
PHYSICS LETTERS A
During irradiation the sample is cooled by a stream of liquid N 2. Simultaneously with the doping procedure defects are produced by the 22Ne ions (and also by the recoiling probe atoms). With a typical irradiation dose of 1 × 1017 Ne ions/cm 2 we estimate a defect density in Ta close to saturation. Following the irradiation an isochronal annealing program was performed. After each annealing step (10 min) a PAC spectrum employing the decay lllln ~ m C d ( T 1 / 2 - - - 2 . 8 d) was measured at 77 K. Fig. 1 shows two PAC spectra thus obtained. For details of the experimental technique and data evaluation see refs. [7,8], for details of the PAC method see ref. [8]. From the annealing sequence we obtain the following results: (i) At the irradiation temperature (around 77 K) the PAC time spectrum shows a damping of the amplitude, indicative of a distribution of electric field gradients due to an overall lattice damage. However, no distinct frequency can be detected. (ii) Starting at an annealing temperature of 200 K, a well-pronounced quadrupole precession pattern develops, showing that a fraction of the PAC probes has trapped migrating defects. The maximum of trapping occurs around 270 K. Fourier analysis very clearly reveals three distinct configurations and least squares fits to the PAC time spectra yield the following hyperfine parameters ~1. ~,Q~ = 91(1) MHz, ~/= 0, uo2 = 120(2) MHz,
, / = 0.63(3),
UQ2= 201(2) MHz,
~/= 0.18(3).
is the quadrupole interaction frequency and v/ the asymmetry parameter [7,8]. Fig. 2 shows the corresponding fractions f, (i = 1-3) of the r a i n probe atoms which have trapped defect i as a function of annealing temperature. Though the fractions ~ all appear at roughly the same temperature, closer inspection of fig. 2 shows, that defect 3 shows up at higher temperature than defects 1 and 2. On the high-temperature side, defect 1 disappears first as can be seen from the data at 348 K. VQ = e Z Q q / h
~1 First results of the measurements in Ta were communicated to the International Conference on Hyperfine interactions, Groningen, The Netherlands, 1983.
88
28 July 1986
R(t)
1111n in Tantalum
--0.05
no ~i
Traces = 77 K t ~
-o.10
-o. 4
.t
_o, --0.14~ . . . . . . . . . . . 0 1O0 200 time [ns]
,_,
.dJi f'
I"'
..500
Fig. 1. PAC spectra of 1111n/lllCd in Ta after heavy-ion irradiation at 77 K and after annealing at 292 K. Measuring temperature of the spectra is 77 K.
It is highly instructive to compare the results with those we recently obtained from an extensive PAC study on Nb. In that case electron and heavy-ion irradiations were performed on polycrystalline and single crystal Nb of high purity. Two different PAC probes, l°°Pd and rain, were employed and the following conclusions could be drawn [6,7]: (i) Trapping of thermally activated defects in three different configurations occurred at the Hi In probes around 250 K. All configurations 1-3 can be assigned to trapped vacancies, based on the trapping behavior at Pd and In and on defect geometries obtained from single crystal measurements. (ii) Configuration 1 is due to trapping of monovacancies. The argument is based on the dominant occurrence of this defect after e - irradiation and its (111) symmetry axis. From similar arguments configuration 2 can be ascribed to a trapped divacancy. In this case the single crystal measurements agree with a configuration where both vacancies are nearest neighbors to the probe atom but next nearest neighbors to each other. The asymmetry parameter ~ = 0.65 indicates a strong relaxation of the In-probe toward the
Volume 117, number 2
PHYSICS LETTERS A
10
fi [% ] 5
0
-G]-
10'
+ f2
0 i -9
.
.
.
.
.
10 fl 5
o
-q~-/-80
r~- r 200
300
t~O0
TAtK] Fig. 2. Fraction ~ of H q n / U l C d probe atoms which have
trapped defects of type pQi as a function of annealing temperature TA. Different symbols indicate 3 different samples.
vacancies. For configuration 3 a small vacancy cluster is suggested. (iii) 250 K is the temperature of free migration for at least the monovacancy proven by e - irradiation results. A remarkable similarity to the present Ta experiment is obvious. Defect trapping in both metals occurs in stage III which is almost identical in temperature for Ta and Nb. Furthermore, the same number of p r o b e - d e f e c t complexes is formed. The analogy becomes even more striking when the defect induced hyperfine parameters of these complexes are compared (see table 1), Each component in Ta has a counterpart in N b with respect to the quadrupole frequency UQ and asym-
28 July 1986
metry parameter 7. This correspondence clearly prompts an explanation that one observes analogous defects in Ta and Nb. Therefore we propose for Ta the same defect configuration as we have already described for Nb. Defect site 1: This is the trapped monovacancy in Ta. A single crystal experiment, yielding (111) symmetry for this defect gives further confirmation to this assignment ~2 Defect site 2: We propose a divacancy configuration as it is shortly discussed above for the respective defect in Nb. A detailed discussion is given in ref. [7]. The occurrence of an asymmetry parameter smaller than one (~/= 0.63) again as in the case of Nb points to a relaxation of the ~11In probe towards the vacancies. Defect site 3: This is very likely a small vacancy cluster as in Nb. The high defect density in our experiment and the clean doping technique into the bulk material makes us confident that we observe a " t r u e " vacancy migration temperature not affected by any residual impurities in the Ta lattice. However, one has to keep in mind, that the dose dependence will shift the trapping to higher temperatures for lower doses. As we know from the comparison between e - and heavy ion irradiation in Nb [6,7], trapping of defects occurred within about 20-30 K in both cases. Similarly, trapping in Ta after electron irradiation should occur within this temperature margin [10]. We point out, that a thermally activated migration of defects need only be assumed for the monovacancy (defect 1). Defects 2 and 3 could in principle have been formed by multiple trapping of defect 1 [7]. Similar conclusions, namely that vacancies in Ta migrate in stage III around 270 K come from recent positron annihilation experiments by Hautoj~irvi et al. [10]. The striking similarity between Nb and Ta finds its completion, when we add the PAC results recently obtained for the group-VI elements Mo [11] and W [12]. In the latter metals the vacancy :~2 A detailed analysis of the single crystal experiment and recent electron irradiation experiments corroborating the vacancy migration temperature are planned to be published in a forthcoming paper. 89
Volume 117, number 2
PHYSICS LETTERS A
28 July 1986
Table 1 Hyperfine parameters of defects trapped at 111in/lmlCd in the bcc refractory metals of group V (Nb [6], Ta this work) and group VI (Mo [11], W [12]). pQ= e2qQ/h is the quadrupole interaction frequency; ~1 is the asymmetry parameter of the electric field gradient tensor; (hkl) describes the orientation of the largest component of the electric field gradient tensor. T v is the temperature of monovacancy trapping. The other defects may have been formed by multiple trapping of defect 1. Config. gives the interpretation of the defects formed.
group V
group VI
Trapped defect
~O (MHz)
*/
(hkl)
TF (K)
Config.
Nb
1 2 3
87 (1) 105 (2) 177 (2)
0 0.65(3) - 0
(111 ) (110) unknown
250
monovac. divacancy vac. cluster
Ta
1 2 3
91 (1) 120 (2) 201 (2)
0 0.63(3) 0.18(3)
(111) unknown unknown
270
monovac. divacancy vac. cluster
Mo
1 2 3
125 (1) 155 (1) 98 (1)
0 1 0
(111) unknown (100)
450
monovac. divacancy vac. cluster
W
1 2 3
142 (2) 181 (5) 263 (5)
0 1 0
(111) (110)? (100)
650
monovac. divacancy vac. cluster
c h a r a c t e r of stage I I I a n n e a l i n g could be p r o v e d b y quenching e x p e r i m e n t s [13,14]. T a b l e 1 (right p a r t ) shows the P A C d a t a o b t a i n e d from t r a p p i n g at 11~In in stage I I I along with the t e m p e r a t u r e s of the t r a p p i n g process. Like T a a n d T b b o t h M o a n d W show three defect c o m p o n e n t s which disp l a y strong similarities with their c o u n t e r p a r t s of the g r o u p - V elements. T h e smallest frequency has = 0 with a (111) s y m m e t r y axis required for the nearest n e i g h b o r m o n o v a c a n c y in a bcc lattice. T h e frequency of the second c o m p o n e n t is a b o u t 25% higher a n d has ~/= 1 for b o t h M o a n d W ( N b a n d T a -- 0.6). T h e a u t h o r s assign this c o m p o n e n t to a d i v a c a n c y with b o t h vacancies being n e a r e s t neighbors to the p r o b e a t o m b u t next nearest n e i g h b o r s to each other. The a s y m m e t r y p a r a m e ter 7/= 1 is expected for this c o n f i g u r a t i o n if the I n a t o m as well as the vacancies are l o c a t e d on regular lattice sites. As m e n t i o n e d above, in T a a n d N b this defect c o n f i g u r a t i o n shows an asymm e t r y p a r a m e t e r 71- 0.6, i n d i c a t i n g a strong rel a x a t i o n of the Hi In p r o b e t o w a r d s the vacancies. This r e l a x a t i o n might be due to the m o r e o p e n lattice structure of T a a n d N b c o m p a r e d to M o a n d W. T h e third c o m p o n e n t consists of m o r e t h a n two vacancies as a l r e a d y discussed for N b 90
a n d Ta. In Mo, however, this c o m p o n e n t has a very low frequency. W e l d i n g e r et al. try to correlate it with a m o r e c o m p l e x c o n f i g u r a t i o n [11]. In conclusion, o u r e x p e r i m e n t s o n h i g h - p u r i t y T a show that stage I I I recovery is l o c a t e d a r o u n d 270 K a n d b y w a y of c o m p a r i s o n can be assigned to v a c a n c y migration. A former P A C e x p e r i m e n t finding this stage at c o n s i d e r a b l y higher t e m p e r a ture is p r e s u m a b l y t r o u b l e d b y i m p u r i t i e s [15]. A n e x p l a n a t i o n for such a t e m p e r a t u r e shift has been suggested b y Schultz [2]. T a k i n g all e x p e r i m e n t a l facts together, the P A C d a t a in the g r o u p - V a n d g r o u p - V I r e f r a c t o r y metals show in an impressive w a y that the s a m e species of defects in very similar c o n f i g u r a t i o n s are t r a p p e d in recovery stage III. The m i c r o s c o p i c p a r a m e t e r s (table 1) revealed b y the P A C m e t h o d thus p r o v i d e the hitherto missing link for the i n t e r p r e t a t i o n of stage ' ~ recovery in T a ( a n d N b ) , since the v a c a n c y character of this stage is k n o w n for W a n d M o from successful q u e n c h i n g experiments. R e m a r k a b l e is the very different t e m p e r a t u r e of v a c a n c y m i g r a tion in g r o u p - V a n d g r o u p - V I metals despite similar melting t e m p e r a t u r e s . A n e x p l a n a t i o n of this fact in terms of a relation between the elastic c o n s t a n t s a n d the activation e n t h a l p y of v a c a n c y
Volume 117, number 2
PHYSICS LETTERS A
migration has been suggested by Flynn [16] and Schultz [17]. W e t h a n k H. S c h u l t z a n d c o w o r k e r s a t t h e M a x - P l a n c k - I n s t i t u t f'tir M e t a l l f o r s c h u n g in S t u t t gart for preparing the high-purity tantalum samples and many stimulating discussions. Financial support of the Deutsche Forschungsgemeinschaft ( S f b 161) is g r a t e f u l l y a c k n o w l e d g e d .
References [1] W. Frank, in: Point defects and defect interaction in metals, eds. J. Takamura, M. Doyama and M. Kiritani (Univ. of Tokyo Press, Tokyo, 1982) p. 203. [2] H. Schultz, in: Point defects and defect interaction in metals, eds. J. Takamura, M. Doyama and M. Kiritani (Univ. of Tokyo Press, Tokyo, 1982) p. 183. [3] K. Faber, J. Schweikhardt and H. Schultz, Scr. Metall. 8 (1974) 713; J. Schweikhardt, Dissertation, Stuttgart (1977), unpublished. [4] M. Tietze, S. Takaki, I.A. Schwirtlich and H. Schultz, in: Point defects and defect interaction in metals, eds. J. Takamura, M. Doyama and M. Kiritani (Univ. of Tokyo Press. Tokyo, 1982) p. 265. [5] K. Maier, M. Peo, B. Saile, H.E. Sch~tfer and A. Seeger, Philos. Mag. A 40 (1979) 701.
28 July 1986
[61 R. Sielemann, H. Metzner, R. Butt, S. Klaumi~nzer, H. Haas and G. Vogel, Phys. Rev. B 25 (1982) 5555. [7] H. Metzner, R. Sielemann, S. Klaumianzer, R. Butt and W. Semmler, Z. Phys. B 61 (1985) 267. [8] E. Recknagel, G. Schatz and Th. Wichert, in: Hyperfine interactions in radioactive nuclei, ed. J. Christiansen (Springer, Berlin, 1983) p. 133. [9] H. Metzner et al., in: Hyperfine interactions VI, eds. L. Niesen, F. Pleiter and H. de Waard (Baltzer, Basel, 1983) p. 413. [10] P. Hautoj~irvi, in: Hyperfine interactions VI, eds. L. Niesen, F. Pleiter and H. de Waard (Baltzer, Basel, 1983) p. 357.. [11] A. Weidinger, R. Wessner, Th. Wichert and E. Recknagel, Phys. Lett. A 72 (1979) 369. [12] U. Pi~tz, A. Hoffmann, H.J. Rudolph and R. Vianden, Z. Phys. B 46 (1982) 107; G.J. van der Kolk, K. Post, A. van Veen, F. Pleiter and J.Th.M. de Hosson, Radiat. Eft. 84 (1985) 131. [13] K.-D. Rasch, R.W. Siegel and H. Schultz, Philos. Mag. A 41 (1980) 91. [14] I.A. Schwirtlich and H. Schultz, Philos. Mag. A 42 (1980) 601. [15] A. Weidinger, M. Deicher and J. Busse, in: Point defects and defect interaction in metals, eds. J. Takamura, M. Doyama and M. Kiritani (Univ. of Tokyo Press, Tokyo, 1982) p. 203. [16] C.P. Flynn, Phys. Rev. 171 (1968) 682. [17] H. Schultz, Scr. Metall. 8 (1974) 721.
91
PHYSICS LETTERS A
28 July 1986
MIGRATION OF VACANCIES IN TANTALUM AND STAGE III RECOVERY IN REFRACTORY bcc METALS O B S E R V E D BY PAC
R. SIELEMANN 1, H. METZNER, E. H U N G E R and S. K L A U M O N Z E R Hahn-Meitner-lnstitut fftr Kernforschung Berlin GmbH, Bereich Kern- and Strahlenpt~vsik, and Freie Universiti~t Berlin, Fachbereich Physik, D-IO00 Berlin, Germany' Received 2 April 1986, accepted for publication 19 May 1986
Annealing of radiation-induced defects in high-purity tantalum was studied by perturbed angular correlation (PAC) between 77 and 380 K. Vacancy migration around 270 K is deduced. From comparison with PAC studies on niobium, molybdenum, and tungsten a highly systematic picture of stage III defects in l llln traps in the refractory bcc metals evolves.
One of the still strongly disputed problems concerning point defects in metals is the temperature (or activation enthalpy) of vacancy migration in several refractory bcc metals, in particular for the group-V metals, V, Nb, Ta, and for Fe. The strong tendency of these metals to dissolve impurities (H, C, N, O) and the "blindness" of many experimental methods to distinguish between selfinterstitials and vacancies are major reasons for this dispute. Ta forms an outstanding example for a metal on which a large variety of experimental methods has been applied but a synopsis has not been reached [1,2]. Agreement has only developed on the position of the intrinsic annealing stage III, namely between 250 and 320 K (though shifts to 380 K have occurred under certain experimental conditions [3]). No agreement exists, however, whether this stage is due to vacancy migration. A recent quenching experiment by Tietze and coworkers [4] is compatible with vacancy migration below 300 K. However, a deduction of the vacancy migration enthalpy HI~ involving the formation enthalpy and self-diffusion data leads Maier et al. to conclude, that vacancies cannot migrate below 380 K [5,1]. In this situation experimental methods should favorably be applied which can supply information on the nature and microscopic structure of i Presently at Technische Hochschule Darmstadt, FRG.
the defects involved. In a recent study on Nb we used the PAC technique (perturbed angular correlation of 7 rays) in a large variety of experimental conditions that enabled us to identify defect migration around 250 K as vacancy migration [6,7]. To solve the intricate problem of doping the high-purity Nb material with radioactive tracers (the PAC probes) without spoiling the purity of the sample, ~11In probes were directly produced inside Nb via heavy-ion nuclear reactions [6]. Since l~lIn cannot be produced in Ta in that way, we used a recoil implantation technique which gives us complete freedom in the choice of the host material. A 2 rtm Nb foil was positioned in front of the high-purity polycrystalline Ta sample (90 ~tm thick, residual resistivity ratio 3000-5000) and ~ I n was produced by the reaction 93Nb(ZZNe,
4n)
111
75 s
35 min
Sb ---, 111Sn ~
1~1In,
employing a 22Ne beam of 90 MeV from the VICKSI heavy-ion accelerator. The nuclear reaction imparts energies up to 18 MeV to the resulting 111Sb ions which thereby leave the Nb foil and recoil into Ta (where they decay to ~lIn with the half-lives indicated). By that technique the PAC probes become distributed within a Ta layer of about 4 ~tm at an extremely low concentration ( < 1 at. ppm). The primary beam (22Ne) is stopped far (about 15 p,m) from the implanted activity.
0375-9601/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
87
Volume 117, number 2
PHYSICS LETTERS A
During irradiation the sample is cooled by a stream of liquid N 2. Simultaneously with the doping procedure defects are produced by the 22Ne ions (and also by the recoiling probe atoms). With a typical irradiation dose of 1 × 1017 Ne ions/cm 2 we estimate a defect density in Ta close to saturation. Following the irradiation an isochronal annealing program was performed. After each annealing step (10 min) a PAC spectrum employing the decay lllln ~ m C d ( T 1 / 2 - - - 2 . 8 d) was measured at 77 K. Fig. 1 shows two PAC spectra thus obtained. For details of the experimental technique and data evaluation see refs. [7,8], for details of the PAC method see ref. [8]. From the annealing sequence we obtain the following results: (i) At the irradiation temperature (around 77 K) the PAC time spectrum shows a damping of the amplitude, indicative of a distribution of electric field gradients due to an overall lattice damage. However, no distinct frequency can be detected. (ii) Starting at an annealing temperature of 200 K, a well-pronounced quadrupole precession pattern develops, showing that a fraction of the PAC probes has trapped migrating defects. The maximum of trapping occurs around 270 K. Fourier analysis very clearly reveals three distinct configurations and least squares fits to the PAC time spectra yield the following hyperfine parameters ~1. ~,Q~ = 91(1) MHz, ~/= 0, uo2 = 120(2) MHz,
, / = 0.63(3),
UQ2= 201(2) MHz,
~/= 0.18(3).
is the quadrupole interaction frequency and v/ the asymmetry parameter [7,8]. Fig. 2 shows the corresponding fractions f, (i = 1-3) of the r a i n probe atoms which have trapped defect i as a function of annealing temperature. Though the fractions ~ all appear at roughly the same temperature, closer inspection of fig. 2 shows, that defect 3 shows up at higher temperature than defects 1 and 2. On the high-temperature side, defect 1 disappears first as can be seen from the data at 348 K. VQ = e Z Q q / h
~1 First results of the measurements in Ta were communicated to the International Conference on Hyperfine interactions, Groningen, The Netherlands, 1983.
88
28 July 1986
R(t)
1111n in Tantalum
--0.05
no ~i
Traces = 77 K t ~
-o.10
-o. 4
.t
_o, --0.14~ . . . . . . . . . . . 0 1O0 200 time [ns]
,_,
.dJi f'
I"'
..500
Fig. 1. PAC spectra of 1111n/lllCd in Ta after heavy-ion irradiation at 77 K and after annealing at 292 K. Measuring temperature of the spectra is 77 K.
It is highly instructive to compare the results with those we recently obtained from an extensive PAC study on Nb. In that case electron and heavy-ion irradiations were performed on polycrystalline and single crystal Nb of high purity. Two different PAC probes, l°°Pd and rain, were employed and the following conclusions could be drawn [6,7]: (i) Trapping of thermally activated defects in three different configurations occurred at the Hi In probes around 250 K. All configurations 1-3 can be assigned to trapped vacancies, based on the trapping behavior at Pd and In and on defect geometries obtained from single crystal measurements. (ii) Configuration 1 is due to trapping of monovacancies. The argument is based on the dominant occurrence of this defect after e - irradiation and its (111) symmetry axis. From similar arguments configuration 2 can be ascribed to a trapped divacancy. In this case the single crystal measurements agree with a configuration where both vacancies are nearest neighbors to the probe atom but next nearest neighbors to each other. The asymmetry parameter ~ = 0.65 indicates a strong relaxation of the In-probe toward the
Volume 117, number 2
PHYSICS LETTERS A
10
fi [% ] 5
0
-G]-
10'
+ f2
0 i -9
.
.
.
.
.
10 fl 5
o
-q~-/-80
r~- r 200
300
t~O0
TAtK] Fig. 2. Fraction ~ of H q n / U l C d probe atoms which have
trapped defects of type pQi as a function of annealing temperature TA. Different symbols indicate 3 different samples.
vacancies. For configuration 3 a small vacancy cluster is suggested. (iii) 250 K is the temperature of free migration for at least the monovacancy proven by e - irradiation results. A remarkable similarity to the present Ta experiment is obvious. Defect trapping in both metals occurs in stage III which is almost identical in temperature for Ta and Nb. Furthermore, the same number of p r o b e - d e f e c t complexes is formed. The analogy becomes even more striking when the defect induced hyperfine parameters of these complexes are compared (see table 1), Each component in Ta has a counterpart in N b with respect to the quadrupole frequency UQ and asym-
28 July 1986
metry parameter 7. This correspondence clearly prompts an explanation that one observes analogous defects in Ta and Nb. Therefore we propose for Ta the same defect configuration as we have already described for Nb. Defect site 1: This is the trapped monovacancy in Ta. A single crystal experiment, yielding (111) symmetry for this defect gives further confirmation to this assignment ~2 Defect site 2: We propose a divacancy configuration as it is shortly discussed above for the respective defect in Nb. A detailed discussion is given in ref. [7]. The occurrence of an asymmetry parameter smaller than one (~/= 0.63) again as in the case of Nb points to a relaxation of the ~11In probe towards the vacancies. Defect site 3: This is very likely a small vacancy cluster as in Nb. The high defect density in our experiment and the clean doping technique into the bulk material makes us confident that we observe a " t r u e " vacancy migration temperature not affected by any residual impurities in the Ta lattice. However, one has to keep in mind, that the dose dependence will shift the trapping to higher temperatures for lower doses. As we know from the comparison between e - and heavy ion irradiation in Nb [6,7], trapping of defects occurred within about 20-30 K in both cases. Similarly, trapping in Ta after electron irradiation should occur within this temperature margin [10]. We point out, that a thermally activated migration of defects need only be assumed for the monovacancy (defect 1). Defects 2 and 3 could in principle have been formed by multiple trapping of defect 1 [7]. Similar conclusions, namely that vacancies in Ta migrate in stage III around 270 K come from recent positron annihilation experiments by Hautoj~irvi et al. [10]. The striking similarity between Nb and Ta finds its completion, when we add the PAC results recently obtained for the group-VI elements Mo [11] and W [12]. In the latter metals the vacancy :~2 A detailed analysis of the single crystal experiment and recent electron irradiation experiments corroborating the vacancy migration temperature are planned to be published in a forthcoming paper. 89
Volume 117, number 2
PHYSICS LETTERS A
28 July 1986
Table 1 Hyperfine parameters of defects trapped at 111in/lmlCd in the bcc refractory metals of group V (Nb [6], Ta this work) and group VI (Mo [11], W [12]). pQ= e2qQ/h is the quadrupole interaction frequency; ~1 is the asymmetry parameter of the electric field gradient tensor; (hkl) describes the orientation of the largest component of the electric field gradient tensor. T v is the temperature of monovacancy trapping. The other defects may have been formed by multiple trapping of defect 1. Config. gives the interpretation of the defects formed.
group V
group VI
Trapped defect
~O (MHz)
*/
(hkl)
TF (K)
Config.
Nb
1 2 3
87 (1) 105 (2) 177 (2)
0 0.65(3) - 0
(111 ) (110) unknown
250
monovac. divacancy vac. cluster
Ta
1 2 3
91 (1) 120 (2) 201 (2)
0 0.63(3) 0.18(3)
(111) unknown unknown
270
monovac. divacancy vac. cluster
Mo
1 2 3
125 (1) 155 (1) 98 (1)
0 1 0
(111) unknown (100)
450
monovac. divacancy vac. cluster
W
1 2 3
142 (2) 181 (5) 263 (5)
0 1 0
(111) (110)? (100)
650
monovac. divacancy vac. cluster
c h a r a c t e r of stage I I I a n n e a l i n g could be p r o v e d b y quenching e x p e r i m e n t s [13,14]. T a b l e 1 (right p a r t ) shows the P A C d a t a o b t a i n e d from t r a p p i n g at 11~In in stage I I I along with the t e m p e r a t u r e s of the t r a p p i n g process. Like T a a n d T b b o t h M o a n d W show three defect c o m p o n e n t s which disp l a y strong similarities with their c o u n t e r p a r t s of the g r o u p - V elements. T h e smallest frequency has = 0 with a (111) s y m m e t r y axis required for the nearest n e i g h b o r m o n o v a c a n c y in a bcc lattice. T h e frequency of the second c o m p o n e n t is a b o u t 25% higher a n d has ~/= 1 for b o t h M o a n d W ( N b a n d T a -- 0.6). T h e a u t h o r s assign this c o m p o n e n t to a d i v a c a n c y with b o t h vacancies being n e a r e s t neighbors to the p r o b e a t o m b u t next nearest n e i g h b o r s to each other. The a s y m m e t r y p a r a m e ter 7/= 1 is expected for this c o n f i g u r a t i o n if the I n a t o m as well as the vacancies are l o c a t e d on regular lattice sites. As m e n t i o n e d above, in T a a n d N b this defect c o n f i g u r a t i o n shows an asymm e t r y p a r a m e t e r 71- 0.6, i n d i c a t i n g a strong rel a x a t i o n of the Hi In p r o b e t o w a r d s the vacancies. This r e l a x a t i o n might be due to the m o r e o p e n lattice structure of T a a n d N b c o m p a r e d to M o a n d W. T h e third c o m p o n e n t consists of m o r e t h a n two vacancies as a l r e a d y discussed for N b 90
a n d Ta. In Mo, however, this c o m p o n e n t has a very low frequency. W e l d i n g e r et al. try to correlate it with a m o r e c o m p l e x c o n f i g u r a t i o n [11]. In conclusion, o u r e x p e r i m e n t s o n h i g h - p u r i t y T a show that stage I I I recovery is l o c a t e d a r o u n d 270 K a n d b y w a y of c o m p a r i s o n can be assigned to v a c a n c y migration. A former P A C e x p e r i m e n t finding this stage at c o n s i d e r a b l y higher t e m p e r a ture is p r e s u m a b l y t r o u b l e d b y i m p u r i t i e s [15]. A n e x p l a n a t i o n for such a t e m p e r a t u r e shift has been suggested b y Schultz [2]. T a k i n g all e x p e r i m e n t a l facts together, the P A C d a t a in the g r o u p - V a n d g r o u p - V I r e f r a c t o r y metals show in an impressive w a y that the s a m e species of defects in very similar c o n f i g u r a t i o n s are t r a p p e d in recovery stage III. The m i c r o s c o p i c p a r a m e t e r s (table 1) revealed b y the P A C m e t h o d thus p r o v i d e the hitherto missing link for the i n t e r p r e t a t i o n of stage ' ~ recovery in T a ( a n d N b ) , since the v a c a n c y character of this stage is k n o w n for W a n d M o from successful q u e n c h i n g experiments. R e m a r k a b l e is the very different t e m p e r a t u r e of v a c a n c y m i g r a tion in g r o u p - V a n d g r o u p - V I metals despite similar melting t e m p e r a t u r e s . A n e x p l a n a t i o n of this fact in terms of a relation between the elastic c o n s t a n t s a n d the activation e n t h a l p y of v a c a n c y
Volume 117, number 2
PHYSICS LETTERS A
migration has been suggested by Flynn [16] and Schultz [17]. W e t h a n k H. S c h u l t z a n d c o w o r k e r s a t t h e M a x - P l a n c k - I n s t i t u t f'tir M e t a l l f o r s c h u n g in S t u t t gart for preparing the high-purity tantalum samples and many stimulating discussions. Financial support of the Deutsche Forschungsgemeinschaft ( S f b 161) is g r a t e f u l l y a c k n o w l e d g e d .
References [1] W. Frank, in: Point defects and defect interaction in metals, eds. J. Takamura, M. Doyama and M. Kiritani (Univ. of Tokyo Press, Tokyo, 1982) p. 203. [2] H. Schultz, in: Point defects and defect interaction in metals, eds. J. Takamura, M. Doyama and M. Kiritani (Univ. of Tokyo Press, Tokyo, 1982) p. 183. [3] K. Faber, J. Schweikhardt and H. Schultz, Scr. Metall. 8 (1974) 713; J. Schweikhardt, Dissertation, Stuttgart (1977), unpublished. [4] M. Tietze, S. Takaki, I.A. Schwirtlich and H. Schultz, in: Point defects and defect interaction in metals, eds. J. Takamura, M. Doyama and M. Kiritani (Univ. of Tokyo Press. Tokyo, 1982) p. 265. [5] K. Maier, M. Peo, B. Saile, H.E. Sch~tfer and A. Seeger, Philos. Mag. A 40 (1979) 701.
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[61 R. Sielemann, H. Metzner, R. Butt, S. Klaumi~nzer, H. Haas and G. Vogel, Phys. Rev. B 25 (1982) 5555. [7] H. Metzner, R. Sielemann, S. Klaumianzer, R. Butt and W. Semmler, Z. Phys. B 61 (1985) 267. [8] E. Recknagel, G. Schatz and Th. Wichert, in: Hyperfine interactions in radioactive nuclei, ed. J. Christiansen (Springer, Berlin, 1983) p. 133. [9] H. Metzner et al., in: Hyperfine interactions VI, eds. L. Niesen, F. Pleiter and H. de Waard (Baltzer, Basel, 1983) p. 413. [10] P. Hautoj~irvi, in: Hyperfine interactions VI, eds. L. Niesen, F. Pleiter and H. de Waard (Baltzer, Basel, 1983) p. 357.. [11] A. Weidinger, R. Wessner, Th. Wichert and E. Recknagel, Phys. Lett. A 72 (1979) 369. [12] U. Pi~tz, A. Hoffmann, H.J. Rudolph and R. Vianden, Z. Phys. B 46 (1982) 107; G.J. van der Kolk, K. Post, A. van Veen, F. Pleiter and J.Th.M. de Hosson, Radiat. Eft. 84 (1985) 131. [13] K.-D. Rasch, R.W. Siegel and H. Schultz, Philos. Mag. A 41 (1980) 91. [14] I.A. Schwirtlich and H. Schultz, Philos. Mag. A 42 (1980) 601. [15] A. Weidinger, M. Deicher and J. Busse, in: Point defects and defect interaction in metals, eds. J. Takamura, M. Doyama and M. Kiritani (Univ. of Tokyo Press, Tokyo, 1982) p. 203. [16] C.P. Flynn, Phys. Rev. 171 (1968) 682. [17] H. Schultz, Scr. Metall. 8 (1974) 721.
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