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Fast oxygen mobility in tetragonal YBa2Cu3O7-x by anelastic relaxation measurements
~
Solid State Communications, Vol. 77, No. 6, p p . 4 2 9 - 4 3 1 , 1991. Printed in Great Britain.
0038-1098/9153.00+.00 Pergamon Press plc
FAST OXYGEN MOBILITY IN TETRAGONAL YBa2CuaOT_= BY ANELASTIC RELAXATION MEASUREMENTS
G.Cannelli 1, R.Cantelli 2, F.Cordero s, M. Ferretti 4 and F. Trequattrini a (1) Universit/L di Perugia, Dipartimento di Fisica, 1-06100 Perugia, Italy (2) II Universit~t di Roma, Dipartimento di Fisica Via E. Carnevale, 1-00173 Roma, Italy (3) CNR, Istituto di Acustica "O.M. Corbino', Via Cassia 1216, 1-00189 Roma, Italy (4) Universit~ di Genova, Istituto di Chimica Fisica, 1-16132 Genova, Italy
(Received 8.11.90 by E. Molinari)
ABSTRACT Evidence of a very high mobility of the isolated O atoms in the Cu(1)-O basal planes of tetragonal YBa2CuaOe.2 is reported. The jumps of the isolated O atoms between the O(4) and O(5) positions are expected to give rise to a well detectable anelastic relaxation process, and a recently reported elastic energy dissipation peak in YBa~CusOe.2 (63 K, 1.1 kHz) was attributed to this mechanism, resulting in an O diffusion coefficient in the basal plane of 4 × 10-4exp(-0.11 eV/kT) cm2/sec. Measurements are produced here which show that all anelastic processes above room temperature in orthorhombic YBa2CusOz-= disappear in YBa2CusOs.2, confirming that the low temperature peak is indeed due to the diffusion of the free oxygen.
1. INTRODUCTION
it is possible to evaluate the parameters characterizing the atomic jump. Indeed, anelastic relaxation experiments in orthorhombic YBa2CusOr-= at high temperature reveal some peaks above 400 K with activation energies around 1 eV 5-1°, which are attributed to jumps of O atoms which involve breaking and reconstruction of the Cu(1)-O(4) chains. When the O stoichiometry is decreased below 6.3, the material exhibits an energy loss peak around 63 K at about 1 kHz (labelled as P2), which has been attributed to the jumps of the isolated O atoms in the basal planes; the resulting oxygen diffusion coefficient in the basal planes is exceptionally high 11, even comparable to that of H in the transition metals. To test our hypothesis, we carried out measurements above room temperature, to see whether other processes are present in YBa2Cu30,~; if so, the jumps of the isolated O atoms could be associated to them, rather than to P2, resulting in a lower O diffusion coefficient.
At present, the dynamics of the O atoms in the basal planes of YBa2CusOT-, is a controversial subject; most of the papers report a very low O diffusivity, and the measured values of its activation energy range from 0.4 to 1.T e V (see e.g. Refs. I-3). Most of the experiments aimed at measuring the O diffusion coefficient make use of permeation methods, and the surface limiting processes during the introduction and extraction of O can decrease the measured diffusivities of several orders of magnitude. In addition, the imposed O concentration gradients can give rise to the formation of different phases where the O mobility is inhibited; therefore, the measured diffusivities are controlled by the slowest process involved in the O long range diffusion. The acoustic measurements, i.e. elastic energy dissipation and elastic constants measurements, are a powerful tool for studying the jumps of atoms in solids; in fact, when an atomic jump causes a change A,~ in the local elastic distortion (elastic dipole) it also gives rise to a peak in the elastic energy loss Q - l as a function of temperature4: Q-I =
/'cv0(AA} 2M wr kT 1 + (¢or) 2
2. EXPERIMENTAL AND RESULTS Elastic energy loss and resonant frequency measurements have been performed from LN temperature up to 750 K on two polycrystalline rectangular bars of YBa2CusO7-= prepared according to a procedure described in Ref. 12. Sample 1 was the same sample 1 used in Ref. 11, whilst sample 2 came from a different batch, but had similar characteristics in the as prepared state (7 - = = 6.94 4- 0.05); in particular they exhibited the same anelastic relaxation spectrum below room temperature. Figure 1 shows the elastic energy loss and resonant frequency of sample 2 from room temperature to 760 K; the
(1)
where f is a factor of the order of 1/2 depending on the geometry of the jump and sample vibration, e the molar concentration of the jumping atoms, vo the cell volume, M the elastic modulus, w the angular vibration frequency of the sample, and r the relaxation time, which is close to the mean time between two subsequent jumps; r generally follows the Arrhenius law r = roeW/kr. From the peak intensity, shape and shift in temperature at different vibration frequencies, 429
430
FAST OXYGEN M O B I L I T Y
IN TETRAGONAL Y B a 2 C u 3 0 7 _ x
Vol. 77, No. 6
o -0.05:
YBa2CUaOT-x
sample 2
,
r
YBa2Cu306.2
'
x 10-2 o
1 5
~- -o. 10
'
#2
~
~ -2
-0.15
1.0i
-3
4
O. 07 0.~
O. 06
% O. O~
O. 04
°
B
0.02
0.01
0~-,,.,,,,,,-""~ ~ - 300 400
,
,
500
500
f
700
800
T (K)
Figure 1: Elastic energy dissipation and relative resonant frequency variation of the originally YBa2 CuzOs.94 sample measured in vacuum during a first heating (1) and cooling (~) and a subsequent heating (8).
measurements were performed in vacuum (better than 10- s mbar), and therefore O loss occurred above 600 K. The smaller dissipation maximum centred at 700 K (here labelled as B) was obtained during the first heating (curve 1) and is presumably part of a higher peak at higher temperature which is suppressed by the O loss. Such a peak was replaced by another one at 560 K (labelled as A), which was traced during cooling to room temperature (curve 2) and a subsequent heating (curve 3) which should not have caused additional loss of O. The resonant frequency is proportional to the square root of the elastic modulus M, which exhibits a jump (modulus defect) in correspondence of the Q-1 peak; for a single time relaxation process is
dM(T)/M = - Q-I(T)/wr
:
,
,
L
100
~-;222L~_
20o
,
,
300
40o
T {K}
,
,
500
60o
|
7 0o
Figure 2: Elastic energy dissipation and relative resonant frequency variation of sample 1 degassed to YBa2Cu30&2 at the two vibration frequencies of 400 Hz (x) and 5.1 kHz (o).
3
T
~
(2)
where Q-x and wr are given in eq. (1) 4. It is difficult to make a quantitative analysis in terms of eq. (2), because the measurements were not extended to the the end of the step in frequency, in order to not loose too much O, and because part of the softening can be attributed to the O loss itself. After these high temperature measurements, the O content was roughly estimated to be between 06.4 and O6.5 by the resistivity increase around room temperature with respect to the completely oxygenated state. Figure 2 presents the elastic energy loss and resonant frequency of sample 1 after an outgassing treatment of 1 h
at 640 °C in a vacuum better than 10 -6 mbar, which reduced the O content to about Oe.2, as evaluated by X-ray diffraction11. The measurement below room temperature is that of Ref. 11, and shows peak P2 at 63 K, which is only 25 % broader than a single Debye peak (eq. 1) with r0 = 2.3 × 10-13 sec and W = 0.11 eV; the concomitant frequency jump satisfies eq. (2), when the broadening of the peak due to a spectrum of relaxation times is taken into account 11. Such a peak has subsequently been observed by other authors 13-1s and the data are consistent. The main result here is that above room temperature only an increasing background of the Q-1 is present, but neither the peaks nor the modulus steps of the partially outgassed sample 2 are observed. The measurement above room temperature was conducted at two vibration frequencies, 400 Hz (x) and 5.1 kHz (circles). After the high temperature measurement of Fig. 2 it was checked that P2 was still present in sample 1. 3. DISCUSSION Anelastic relaxation processes above room temperature have already been reported both by pendulum measurements between 0.01 and 10 Hz 5,7,9 and at higher frequenciess-l°. Such effects have activation energies of the order of 1 eV, and are thought to be due to the O mobility in the basal plane. Because they are observed at high O contents (7 - x > 6.5) when the material is orthorhombic, they should involve jumps out of or into the chains or fragments of chain, and their activation energies include the binding energy of the O atom to the chain. These jumps constitute the elementary steps of the long range diffusion of the O atoms in the Cu(I)-O planes, and in fact both diffusion and anelastic processes have comparable activation energies in the orthorhombic phase l-3's'T-l°. A comparison in an Arrhenius plot of the present high temperature peaks with those reported in literature indicates that peak B might be identified with the peak labelled as V in Ref. 5, and with those of Refs. 7-10, whereas peak A could be the shoulder of peak V in Ref. 5. If that is true, the evolution of peaks A and B can be described as follows. At 7 - x ~ 6.9 only peak B is present; when the O content is gradually reduced, its height first increasess and then decreases up to its complete suppression (Fig. 1), while peak
Vol. 77, No. 6
FAST OXYGEN MOBILITY
A develops. Eventually ( 7 - x < 6.2) both peaks are absent. At low enough O content, 7 - x < 6.3, when the high temperature peaks have disappeared, only the low temperature peak P2 is present, as shown in Fig. 2. This result reinforces our hypothesis that P2 is due to the jumps of the isolated O atoms between the equivalent 0(4) and O(5) positions, because they are expected to give rise to a well detectable dissipation maximum. In fact, a molar concentration c of O atoms in 0(4) position contributes to the e®z strain with an anelastic strain cA I and to the %v strain withcA2, w i t h A A -- AI - A2 > 0 , and vice versa for the atoms in 0(5) position; ezz is the same in both cases. The application of a shear stress in the ab plane (the vibration of the sample) perturbs the energies of the sites O(4) and 0(5) proportionally to 1 and 2, and their populations relax toward the new thermal equilibrium distribution, giving rise to elastic energy dissipation according to eq. (1) with M (Cli - c12) / 2 (Snoek effect4). Even assuming a low value of A A N 0.1 Is, a concentration of effectively isolated O atoms c ~ 0.05, a shear modulus of the order of 8 × 10 II erg/cm s (Ref. 17), and f ~ 1/5, one expects from eq. (1) a peak height of about 0.08 around 600 K and ten times higher at 60 K. The fact that P2 is much lower than this value may
IN TETRAGONAL
YBa2Cu307_ x
431
be due to a small value of AA or rather to the fact that, on lowering temperature, the concentration of O atoms actually isolated is significantly reduced by the formation of other ordered phases, as suggested by the small effect around 220 K in Fig. 211'is. Other mechanisms could give rise to a peak like P2, but any alternative supposition should also explain why an intense peak due to the O(4)-O(5) jumps of free oxygens is not observed. Concluding, we report high temperature acoustic measurements which show that P2 is the only anelastic relaxation process in the nearly O depleted YBa2Cu3OT-z; therefore we confirm our hypothesis that the low temperature acoustic loss peak in the tetragonal phase is due to 0(4)O(5) jumps of isolated O atoms (Snoek effect), and the activation energy for such jumps is 0.1 eV.
Acknowledgement - The authors wish to thank Mr. S. D'Angelo and Mr. F. Corvasce for their technical assistance. This work has been supported by the National Research Council of Italy under the Progetto Finalizzato "Superconductive and Cryogenic Technologies".
REFERENCES
1. K.N. Tu, N.C. Yeh, S.I. Park, and C.C. Tsuei, Phys. Rev. B 39, 304 (1989).
11. G. Cannelli, R. Cantelli and F. Cordero, Phys. Rev. B 38, 7200 (1988).
2. G. Ottaviani, C. Nobili, F. Nays, M. Affronte, T. Manfredini, F.C. Matacotta, and E. Galli, Phys. Rev. B 39, 9060 (1989).
12. G.A. Costa, M. Ferretti, M.L. Fornasini, and G.L. O1cese, Solid State Comm. 65,469 (1988).
3. D.J. Vischjager, P.J. van Der Put, J. Schram, and J. Schoonman, Sol. State Ionies 27, 199 (1988). 4. A.S. Nowick and B.S. Berry, "Anelastic Relaxation in Crystalline Solids" (Academic Press, London, 1972). 5. T. Chen, J. Zhang, J. Huang, Y. Chen, M. Yang, X. Xie, T. Li, and L. Xie, Chinese Phys. Lett. 5, 185 (1988). 6. X.M. Xie, T.G. Chen, and Z.L. Wu, Phys. Rev. B 40, 4549 (1989).
7. J.X. Zhang, G.M. Lin, Z.C. Lin, K.F. Liang, P.C.W. Fung, and G.G. Siu, J. Phys.: Condens. Matter 1, 6939 (1989). 8. J.L. Tallon, A.H. Sehuitema, and N.E. Tapp, Appl. Phys. Lett. 52,507 (1988). 9. J.R. Cost, and J.T. Stanley, submitted to J. of Materials Research. 10. E. Bonetti, E.G. Campari, A. Casagrande, and S. Mantovani, Proc. of the 3rd National Meeting "High Temperature Superconductivity", Genoa, Italy, ed. by C. Ferdeghini and A.S. Siri (World Scientific, Singapore, 1990).
13. D.P. Almond, Qingxian Wang, J. Freestone, E.F. Lambson, B. Chapman, and G.A. Saunders, J. Phys.: Condens. Matter 1, 6853 (1989). 14. V.D. Natsik, P.P. Pal-Val, J. Engert, and H.-J. Kaufmann, Proc. of the 9th Int. Conf. on Internal Friction and Ultrasonic Attenuation in Solids (Int. Academic Publishers, Beijing, China, 1989). 15. S. de Brion, J.Y. Henry, R. Calemczuk, and E. Bonjour, Europhys. Lett. 12,281 (1990). 16. The Snoek effect due to O, N and C atoms is observed in bcc metals, where such interstitials sit in sites with tetrahedral symmetry and the associated elastic dipoles have AA N 0.5 -- 1 (see Ref. 4). 17. M. Saint-Paul, and J.Y. Henry, Solid State Comm. 72, 685 (1989).
18. G. Cannelli, R. Cantelli, F. Cordero, G.A. Costa, M. Ferretti, and G.L. Olcese, Europhys. (1988).
Lett. 6, 271
Solid State Communications, Vol. 77, No. 6, p p . 4 2 9 - 4 3 1 , 1991. Printed in Great Britain.
0038-1098/9153.00+.00 Pergamon Press plc
FAST OXYGEN MOBILITY IN TETRAGONAL YBa2CuaOT_= BY ANELASTIC RELAXATION MEASUREMENTS
G.Cannelli 1, R.Cantelli 2, F.Cordero s, M. Ferretti 4 and F. Trequattrini a (1) Universit/L di Perugia, Dipartimento di Fisica, 1-06100 Perugia, Italy (2) II Universit~t di Roma, Dipartimento di Fisica Via E. Carnevale, 1-00173 Roma, Italy (3) CNR, Istituto di Acustica "O.M. Corbino', Via Cassia 1216, 1-00189 Roma, Italy (4) Universit~ di Genova, Istituto di Chimica Fisica, 1-16132 Genova, Italy
(Received 8.11.90 by E. Molinari)
ABSTRACT Evidence of a very high mobility of the isolated O atoms in the Cu(1)-O basal planes of tetragonal YBa2CuaOe.2 is reported. The jumps of the isolated O atoms between the O(4) and O(5) positions are expected to give rise to a well detectable anelastic relaxation process, and a recently reported elastic energy dissipation peak in YBa~CusOe.2 (63 K, 1.1 kHz) was attributed to this mechanism, resulting in an O diffusion coefficient in the basal plane of 4 × 10-4exp(-0.11 eV/kT) cm2/sec. Measurements are produced here which show that all anelastic processes above room temperature in orthorhombic YBa2CusOz-= disappear in YBa2CusOs.2, confirming that the low temperature peak is indeed due to the diffusion of the free oxygen.
1. INTRODUCTION
it is possible to evaluate the parameters characterizing the atomic jump. Indeed, anelastic relaxation experiments in orthorhombic YBa2CusOr-= at high temperature reveal some peaks above 400 K with activation energies around 1 eV 5-1°, which are attributed to jumps of O atoms which involve breaking and reconstruction of the Cu(1)-O(4) chains. When the O stoichiometry is decreased below 6.3, the material exhibits an energy loss peak around 63 K at about 1 kHz (labelled as P2), which has been attributed to the jumps of the isolated O atoms in the basal planes; the resulting oxygen diffusion coefficient in the basal planes is exceptionally high 11, even comparable to that of H in the transition metals. To test our hypothesis, we carried out measurements above room temperature, to see whether other processes are present in YBa2Cu30,~; if so, the jumps of the isolated O atoms could be associated to them, rather than to P2, resulting in a lower O diffusion coefficient.
At present, the dynamics of the O atoms in the basal planes of YBa2CusOT-, is a controversial subject; most of the papers report a very low O diffusivity, and the measured values of its activation energy range from 0.4 to 1.T e V (see e.g. Refs. I-3). Most of the experiments aimed at measuring the O diffusion coefficient make use of permeation methods, and the surface limiting processes during the introduction and extraction of O can decrease the measured diffusivities of several orders of magnitude. In addition, the imposed O concentration gradients can give rise to the formation of different phases where the O mobility is inhibited; therefore, the measured diffusivities are controlled by the slowest process involved in the O long range diffusion. The acoustic measurements, i.e. elastic energy dissipation and elastic constants measurements, are a powerful tool for studying the jumps of atoms in solids; in fact, when an atomic jump causes a change A,~ in the local elastic distortion (elastic dipole) it also gives rise to a peak in the elastic energy loss Q - l as a function of temperature4: Q-I =
/'cv0(AA} 2M wr kT 1 + (¢or) 2
2. EXPERIMENTAL AND RESULTS Elastic energy loss and resonant frequency measurements have been performed from LN temperature up to 750 K on two polycrystalline rectangular bars of YBa2CusO7-= prepared according to a procedure described in Ref. 12. Sample 1 was the same sample 1 used in Ref. 11, whilst sample 2 came from a different batch, but had similar characteristics in the as prepared state (7 - = = 6.94 4- 0.05); in particular they exhibited the same anelastic relaxation spectrum below room temperature. Figure 1 shows the elastic energy loss and resonant frequency of sample 2 from room temperature to 760 K; the
(1)
where f is a factor of the order of 1/2 depending on the geometry of the jump and sample vibration, e the molar concentration of the jumping atoms, vo the cell volume, M the elastic modulus, w the angular vibration frequency of the sample, and r the relaxation time, which is close to the mean time between two subsequent jumps; r generally follows the Arrhenius law r = roeW/kr. From the peak intensity, shape and shift in temperature at different vibration frequencies, 429
430
FAST OXYGEN M O B I L I T Y
IN TETRAGONAL Y B a 2 C u 3 0 7 _ x
Vol. 77, No. 6
o -0.05:
YBa2CUaOT-x
sample 2
,
r
YBa2Cu306.2
'
x 10-2 o
1 5
~- -o. 10
'
#2
~
~ -2
-0.15
1.0i
-3
4
O. 07 0.~
O. 06
% O. O~
O. 04
°
B
0.02
0.01
0~-,,.,,,,,,-""~ ~ - 300 400
,
,
500
500
f
700
800
T (K)
Figure 1: Elastic energy dissipation and relative resonant frequency variation of the originally YBa2 CuzOs.94 sample measured in vacuum during a first heating (1) and cooling (~) and a subsequent heating (8).
measurements were performed in vacuum (better than 10- s mbar), and therefore O loss occurred above 600 K. The smaller dissipation maximum centred at 700 K (here labelled as B) was obtained during the first heating (curve 1) and is presumably part of a higher peak at higher temperature which is suppressed by the O loss. Such a peak was replaced by another one at 560 K (labelled as A), which was traced during cooling to room temperature (curve 2) and a subsequent heating (curve 3) which should not have caused additional loss of O. The resonant frequency is proportional to the square root of the elastic modulus M, which exhibits a jump (modulus defect) in correspondence of the Q-1 peak; for a single time relaxation process is
dM(T)/M = - Q-I(T)/wr
:
,
,
L
100
~-;222L~_
20o
,
,
300
40o
T {K}
,
,
500
60o
|
7 0o
Figure 2: Elastic energy dissipation and relative resonant frequency variation of sample 1 degassed to YBa2Cu30&2 at the two vibration frequencies of 400 Hz (x) and 5.1 kHz (o).
3
T
~
(2)
where Q-x and wr are given in eq. (1) 4. It is difficult to make a quantitative analysis in terms of eq. (2), because the measurements were not extended to the the end of the step in frequency, in order to not loose too much O, and because part of the softening can be attributed to the O loss itself. After these high temperature measurements, the O content was roughly estimated to be between 06.4 and O6.5 by the resistivity increase around room temperature with respect to the completely oxygenated state. Figure 2 presents the elastic energy loss and resonant frequency of sample 1 after an outgassing treatment of 1 h
at 640 °C in a vacuum better than 10 -6 mbar, which reduced the O content to about Oe.2, as evaluated by X-ray diffraction11. The measurement below room temperature is that of Ref. 11, and shows peak P2 at 63 K, which is only 25 % broader than a single Debye peak (eq. 1) with r0 = 2.3 × 10-13 sec and W = 0.11 eV; the concomitant frequency jump satisfies eq. (2), when the broadening of the peak due to a spectrum of relaxation times is taken into account 11. Such a peak has subsequently been observed by other authors 13-1s and the data are consistent. The main result here is that above room temperature only an increasing background of the Q-1 is present, but neither the peaks nor the modulus steps of the partially outgassed sample 2 are observed. The measurement above room temperature was conducted at two vibration frequencies, 400 Hz (x) and 5.1 kHz (circles). After the high temperature measurement of Fig. 2 it was checked that P2 was still present in sample 1. 3. DISCUSSION Anelastic relaxation processes above room temperature have already been reported both by pendulum measurements between 0.01 and 10 Hz 5,7,9 and at higher frequenciess-l°. Such effects have activation energies of the order of 1 eV, and are thought to be due to the O mobility in the basal plane. Because they are observed at high O contents (7 - x > 6.5) when the material is orthorhombic, they should involve jumps out of or into the chains or fragments of chain, and their activation energies include the binding energy of the O atom to the chain. These jumps constitute the elementary steps of the long range diffusion of the O atoms in the Cu(I)-O planes, and in fact both diffusion and anelastic processes have comparable activation energies in the orthorhombic phase l-3's'T-l°. A comparison in an Arrhenius plot of the present high temperature peaks with those reported in literature indicates that peak B might be identified with the peak labelled as V in Ref. 5, and with those of Refs. 7-10, whereas peak A could be the shoulder of peak V in Ref. 5. If that is true, the evolution of peaks A and B can be described as follows. At 7 - x ~ 6.9 only peak B is present; when the O content is gradually reduced, its height first increasess and then decreases up to its complete suppression (Fig. 1), while peak
Vol. 77, No. 6
FAST OXYGEN MOBILITY
A develops. Eventually ( 7 - x < 6.2) both peaks are absent. At low enough O content, 7 - x < 6.3, when the high temperature peaks have disappeared, only the low temperature peak P2 is present, as shown in Fig. 2. This result reinforces our hypothesis that P2 is due to the jumps of the isolated O atoms between the equivalent 0(4) and O(5) positions, because they are expected to give rise to a well detectable dissipation maximum. In fact, a molar concentration c of O atoms in 0(4) position contributes to the e®z strain with an anelastic strain cA I and to the %v strain withcA2, w i t h A A -- AI - A2 > 0 , and vice versa for the atoms in 0(5) position; ezz is the same in both cases. The application of a shear stress in the ab plane (the vibration of the sample) perturbs the energies of the sites O(4) and 0(5) proportionally to 1 and 2, and their populations relax toward the new thermal equilibrium distribution, giving rise to elastic energy dissipation according to eq. (1) with M (Cli - c12) / 2 (Snoek effect4). Even assuming a low value of A A N 0.1 Is, a concentration of effectively isolated O atoms c ~ 0.05, a shear modulus of the order of 8 × 10 II erg/cm s (Ref. 17), and f ~ 1/5, one expects from eq. (1) a peak height of about 0.08 around 600 K and ten times higher at 60 K. The fact that P2 is much lower than this value may
IN TETRAGONAL
YBa2Cu307_ x
431
be due to a small value of AA or rather to the fact that, on lowering temperature, the concentration of O atoms actually isolated is significantly reduced by the formation of other ordered phases, as suggested by the small effect around 220 K in Fig. 211'is. Other mechanisms could give rise to a peak like P2, but any alternative supposition should also explain why an intense peak due to the O(4)-O(5) jumps of free oxygens is not observed. Concluding, we report high temperature acoustic measurements which show that P2 is the only anelastic relaxation process in the nearly O depleted YBa2Cu3OT-z; therefore we confirm our hypothesis that the low temperature acoustic loss peak in the tetragonal phase is due to 0(4)O(5) jumps of isolated O atoms (Snoek effect), and the activation energy for such jumps is 0.1 eV.
Acknowledgement - The authors wish to thank Mr. S. D'Angelo and Mr. F. Corvasce for their technical assistance. This work has been supported by the National Research Council of Italy under the Progetto Finalizzato "Superconductive and Cryogenic Technologies".
REFERENCES
1. K.N. Tu, N.C. Yeh, S.I. Park, and C.C. Tsuei, Phys. Rev. B 39, 304 (1989).
11. G. Cannelli, R. Cantelli and F. Cordero, Phys. Rev. B 38, 7200 (1988).
2. G. Ottaviani, C. Nobili, F. Nays, M. Affronte, T. Manfredini, F.C. Matacotta, and E. Galli, Phys. Rev. B 39, 9060 (1989).
12. G.A. Costa, M. Ferretti, M.L. Fornasini, and G.L. O1cese, Solid State Comm. 65,469 (1988).
3. D.J. Vischjager, P.J. van Der Put, J. Schram, and J. Schoonman, Sol. State Ionies 27, 199 (1988). 4. A.S. Nowick and B.S. Berry, "Anelastic Relaxation in Crystalline Solids" (Academic Press, London, 1972). 5. T. Chen, J. Zhang, J. Huang, Y. Chen, M. Yang, X. Xie, T. Li, and L. Xie, Chinese Phys. Lett. 5, 185 (1988). 6. X.M. Xie, T.G. Chen, and Z.L. Wu, Phys. Rev. B 40, 4549 (1989).
7. J.X. Zhang, G.M. Lin, Z.C. Lin, K.F. Liang, P.C.W. Fung, and G.G. Siu, J. Phys.: Condens. Matter 1, 6939 (1989). 8. J.L. Tallon, A.H. Sehuitema, and N.E. Tapp, Appl. Phys. Lett. 52,507 (1988). 9. J.R. Cost, and J.T. Stanley, submitted to J. of Materials Research. 10. E. Bonetti, E.G. Campari, A. Casagrande, and S. Mantovani, Proc. of the 3rd National Meeting "High Temperature Superconductivity", Genoa, Italy, ed. by C. Ferdeghini and A.S. Siri (World Scientific, Singapore, 1990).
13. D.P. Almond, Qingxian Wang, J. Freestone, E.F. Lambson, B. Chapman, and G.A. Saunders, J. Phys.: Condens. Matter 1, 6853 (1989). 14. V.D. Natsik, P.P. Pal-Val, J. Engert, and H.-J. Kaufmann, Proc. of the 9th Int. Conf. on Internal Friction and Ultrasonic Attenuation in Solids (Int. Academic Publishers, Beijing, China, 1989). 15. S. de Brion, J.Y. Henry, R. Calemczuk, and E. Bonjour, Europhys. Lett. 12,281 (1990). 16. The Snoek effect due to O, N and C atoms is observed in bcc metals, where such interstitials sit in sites with tetrahedral symmetry and the associated elastic dipoles have AA N 0.5 -- 1 (see Ref. 4). 17. M. Saint-Paul, and J.Y. Henry, Solid State Comm. 72, 685 (1989).
18. G. Cannelli, R. Cantelli, F. Cordero, G.A. Costa, M. Ferretti, and G.L. Olcese, Europhys. (1988).
Lett. 6, 271