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Mobility and short-range ordering of oxygen in ifRrmBain2Cuin3Oinrm6+x by anelastic relaxation and possible correlation with the 90 K and 60 K superconducting phases
•~ )
Solid State Communications, Vol. 82, No. 6, PP. 433-436, 1992. Printed in Great Britain.
0038-109819255.00+.00 Pergamon Press Ltd
M O B I L I T Y A N D S H O R T - R A N G E O R D E R I N G O F O X Y G E N IN RBaICusOs+, B Y A N E L A S T I C R E L A X A T I O N A N D POSSIBLE C O R R E L A T I O N W I T H T H E 90 K A N D 60 K S U P E R C O N D U C T I N G PHASES
G.Cannelli,1 R.Cantelli, 1 F.Cordero, s F. Trequattrini,2, and M. Ferrettis (1) Universitb di Roma "La Sapienza~, Dip. di Energetica, Via Scarps 14, 1-00161 Rome, Italy (2) CNR, letituto di Acustica "O.M. Corbiuo", Via Cassia 1216, 1-00189 Rome, Italy (3) Universith di Genova, Istituto di Chimica Fisica, 1-16132 Genova, Italy (Received 9 March 1992 by E. Molinari)
The evolution of two anelastic relaxation processes in RBalCusOe+s (R = Y, Eu) above room temperature has been investigated and discussed over the whole range of the O stoichiometry. The procmmm seem to be associated with the O jumps in the O-! and 0-II domains, and could provide the tool for evaluating the fractions of the 90 and 60 K superconducting phases.
The RBa~CusOe+, (R = Y,Eu) samples were rectangu. ler bars prepared as explained in Ref. 4 which were elactrmtatically excited on their flexurnl modes at frequencies included between '0.4 and 20 kHs; both the Y and Eu based materials exhibited the same phenomenology. The l i t and 5th modes (with a ratio of the frequencies equal to 13) were excited during the same run, so that the thermally activated processes could be readily discerned from those associated to phase transformations. Measurements were carried out in vacuum (10-s tuber), and therefore O loss occurred above 600 K. Above room temperature two thermally activated relaxation processes are observed, and figure 1 presents the evolution of the corresponding Q - I peaks PHI and PH2 with decrease of the O stoichiometry, starting from z --0.9. The dashed curve 2 represents several thermal cycles during which PH2 ~ up to curve $, due to O loss; the fact that initially its height is an increasing function of the O vacancy concentration, is in accord with what found by Xieet eL.s This circumstance led those authors to formulate the hypothesis that in the O-/phese (orthorh0mbic with all Cu(1)-O(1) chains filled) O can jump out era chain only into a 0(5) site of the same Cues plane which is nelshbour to a vacancy in the next chain. Peak PH2 is ~dentiflad with that observed by Xie et ~ and we adopt their interpretation. Proceeding with the O outgassing, PH2 disappears together with the O-lphaee. At a temperature lower than that of PH2 another peak, PHI, appears, which develops at lower values of z, and reaches its maximum intensity around z -~0.4. While z decreases, it shifts from -~500 to --600 K,e and correspondingly its activation energy varies from .~1.1 eV to ,-1.$ eV, as estimated from the temperature shift with frequency. The peak is significantly broader than a single time process. The first stages of the evolution of PH1 in YBaICusOe+l appear in Fig. 1, while its further evolution is shown in Fig. 2, for EuBaICusOe~.,; both types of samples give very similar re-
The RBalCusOs+, superconductors have cristallo~'aphic planes which can accommodate a variable O etoichiometry from empty to half filled. As a result, they have a complex and mostly unexplored z, T phase diagram and the 0 mobility in their C u e . plane is certainly much higher than in other oxides. Detailed knowledge of the phase diagram and O mobility are of great interest, also because the physical properties of RBasCusOe+s, and notably the electric ones, depend on the type of ordering o r e in the Cues planes, and not only on its concentration.1 A striking example of that is the increase of TG of tens of kelvin in quenched O deficient single crystals, simply upon aging at room temperature.2 Both the mobility and orderingof oxygen can be effectivelystudied by anelastic relaxation measurements (elastic energy dissipation and modulus). In fact, when an oxygen jumps between two different portions 1 and 2, a change ~A in the local elastic distortion (elastic dipole) occurs, and gives rise to a peak in the elastic energy loss, Q-I, as function of temperature:s q~l;l"
I + Coot)i
(1)
where c is the concentration of the jumping atoms, nl and n2 their equilibrium fractions in sites 1 and 2, w is the angular vibration frequency of the sample; r(T) is the relaxation
time given by
II, = l/n,, + l/rii,
(2)
where i/r,y is the jump rate from i to j, which generally follows the Arrhenins law rdi = I"0exp(Wil/IcT). The maximum of Q - I occurs when cot ,,, I, i.e. at a temperature such that the atomic jump frequency is close to the an&~.tlar vibration frequency of the semple. In the present paper we report on now measurements above room temperature for z varying from Imm than 0.2 to 0.9. The results can be interpreted in terms of the motion and short-range ordering of 0 in the Cue ! plane of RBa2CusOe+,. 433
434
OXYGEN IN RBa2Cu306+ x 7
i
6
i
i
i
i
i
Vol. 82, No; 6 T
]
PH2
2~,
YBa2Cu306+x
•
5
0-II
./.
2o../,. •
•
O-I
•
•
•
• •
L •
•
4 • aO-'a-4° e e e •
P2
2
1o. t I0-~
ev}
• •
PHi
1.I.1 ev)
•
0• •
e
PH2
t-4 ev)
I .r-. O
400
500
600 T
700
(K)
_I0-2 Fig. 1 Elastic energy lca of YBa~Cu3Os+® measured in vacuum starting from z ~0.9 at a vibration frequency of 1.8 kHz. suits. The overall phenomenology is sketched in Fig. 3, where the heights of PHI end PH2 are plotted against z, together with that of P2. 7 Peak P2 appears around 60 K (1 kHz) end is characterized by a nearly single relaxation time with an activation energy W = 0.11 eV; it is the only stable peak at the lowest O contents and should therefore be associated to the jumps of free oxygen.~,s The resulting mobility of free 0 in the CuO# planes is extraordinarily high, what can be accepted considering the unique properties of such planes.
6
i. 7 kHz\I
5
1
EuBa2Cu3Os÷x 1 .
4
22 kHz
7o
2
I
bO
500
600
700
800
T (K)
Fig. 2 Elastic energy loamof EuBa2CusOe+, meesured in vacuum starting from z <0,5; meseurJments were concomitantly made at a vibration frequency of 1.7 kHz (continuous line) and 22 kHz (broken line). On proceeding with the O outga~ing, peak PH1 deerea~m and shifts to higher temperature. Measurements are performed both on cooling (o) and heating (+).
I0-3 0
0.5 X
Fig. 3 Lower part: intensity of P2, PHI and PH2 at ,~I kHz as a function of the O stoichiometry. Upper part: CuO, planes and O jumps which are supposed to give rke to P2, PH1 and PH2. Small circles: Cu(1); large closed circles: 0 ( 1 ) and 0 ( 5 ) .
An explanation of the possible large di~erence in activation energy between the jumps of the isolated and aggregated oxygens may be that in the first case the chemical state does not change, while in second case does. This can be seen in the framework of a simple model for the mech, h i m of charge trander between the CuO, and the CuO2 planes.° According to that interpretation, the Cu(l) atoms which are coordinated only with the two apical 0(4) atoms have valence +1, while thou which are also coordinated with one or two O(1) atoms have valence +2. When the jump of an oxygen involves its aggregation to a chain fragment or to another oxygen to form a pair, the total valence of the Cu(1) atoms changes of -1, Which implies the formation of a bond of chemical nature. Instead, no variation of the wdences is involved in the jump of an isolated oxygen. The upper part of Fig. $ schematically represents the CuO= plane for 0
'Vol. '82, No. 6
EuBa2Cu306+x ~i 1 i
6
OXYGEN IN RBa2Cu306+ x l
l
i
l
l
~
435
l
•
•
' " •
O/O~3
ATf 2, B,,
' "
150!i141 0
t:
I1041
~4
T
C)
~3
~"V~1~1412~n.~12 ~"VM21 123 14 400 I
I
500 l
I
T
600 (K) I
I
700 I
,l
800
Elastic
Fig. 4 energy lose of EuBa2CusO6+, (= -+0.3) after 2 h in vacuum at 990 K followed by quenching to room temperature.
the O stoichiometry z, while PHI and PH2 are. An additional mechanism which would be correlated with is the motion of twins; however, when z <0.4 the twin density should be very low, but PH1 is still intense, and we suppose that PH1 is due to O jumps not necessarily involving twin motion. We show now that the jumps of the free oxygens are not included among those giving rise to PHI. This is deduced from Fig. 4, which shows the Q - l o f a EuBa2CuaO~.6.s sample after aging for 2 h at 990 K followed by quenching to room temperature. A few hours after quenching only a small trace of PH1 is present (curve 1), which develops during the subsequent measurements (curves 2 and 3). Its growth with aging at high temperature, without absorption of additional O, excludes that it is due to free O; indeed, the concentration of isolated oxygens is expected to be maximum just after quenching from the highest temperature. The behavior of PHI can be explained as foilows. Above 700 K and with z <0.3 most of the O atoms are expected to be free, and the quenching to room temperature freezes a highly disordered state. This is not in contradiction with the supposed high mobility of the free oxygens (W -- 0.U eV); in fact, during quenching such atoms can form pairs, whose dissociation would require an activation energy comparable to that for an O atom to leave a chain (-- 1 eV); therefore, the pairs would be immobile at room temperature on the time scale of hours or days. The O-O complexes would not give rise to observable anelastic relaxation, as will be discussed below; consequently, after quenching, only a small fraction of oxygens has formed the domains which contribute to PH1. The aggregation in ordered domains (presumably O-II phase) takes place at an observable rate while cycling the sample at temperatures higher than 400-500 K but below the boundary line to the completely disordered tetragonal phase. Peak PH1 is included between these temperatures, at the frequencies of our experiments, and therefore it is observed to grow after quenching from the disordered state (Fig. 4). The dissipation peak at 480 K is frequency independent and hence associated to a phase transformation, paesihiy implying O ordering. 10
z
(a)
(b)
I.
WI~ E14 14 (c)
Fig. 5 Dissociation (a), reorientation (b) of an O pair and diseociation of a chain in the 0-II phase (c) with the respective potential profiles. Small circles: Cu(1); large clceed circles: O(1) and O(5).
We now discuss why a dissipation peak due to 0 pairs should not be observable. An 0 pair can give rile to anelastic relaxation in two ways: by reorienting itself or by dissociating and reforming under the applied cyclic strew. The case of the dissociation and formation of the pair is represented in Fig. 5(a); in the lower part is drawn the potential profile seen by atom A when it jumps between positions 1, 2 and 3, with atom B fixed. Comddaring that the activation energies for the jumps of the c0cygens (chemically) bound in chains are of the order of 1 eV or more (PHI and PH2) and that the activation energy for the jumps of free oxygen is 0.11 eV (P2), we conclude that the difference in site energy between sites 1 and 2 should be E12 "~ 1 eV. Consequently, the activation energy for dissociation is Wll 1 eV and that for the pair formation is W21 N 0.I eV. The elastic energy Ices peak is given by eq. (I) with n l n 2 / / ~ (proportional to [kTcceh2(El2/2kT)] -1, as derived from Boltsmann statistics) which is maximum at kT N and with 1/r = 1/raz + 1/rx2 .~ !/T21 close to the relaxation rate of free O. The Q - I curve is the product of n l n s / l ~ , which has the maxhnum at several thoummdm of kelvin, and of the Dehye curve, which is max/mum for wr - 1; the latter condition is satisfied at a temperature close to that of P2, which has a comparable rand is located around 60 K. Then, the dissipation turns out to be unolmervable at all temperatures, because nlns is finite where the Debye part is negligible and vice versa. The rsorientation of a pair of oxygens is represented in Fig. 5(b), and it occurs in two steps: first atom A jumps to the pceition 4, and then atom B moves to polition 5. If during the first step atom A saw a potential profile similar to that seen when it jumps out of s chain in the O-Hphase (Fig. 5(c)), then the reorientation of the pair would give rise to a dissipation peak close to PH1. Instead, we suppose that the ionic charges of the O atoms are less screened when they are either isolated or paired or in very short chains, than when they belong to an ordered conducting phase. This implies that the electrostatic repulsion between atoms A and B is larger in the case of the pa/r, whenever they are not screened by the Cu atom; hence, both the activation energy W14 for jumping from 1 to 4 and the site energy difference
0.65~.2,
436
OXYGEN IN RBa2Cu306+ x
Eli between the two positions are larger in the pair than in the ordered chain (Fig. 5(b,c)). Again, the large difference in site energy El, would cause the conditions ~oT .- 1 and k T ~ 0.65E14 to occur at well separated temperatures, and therefore the Q-1 to have negligible intensity. The above considerations will apply to the case of short chains as well, if the electrostatic repulsion is larger than in the ordered domains; and the observed shift to higher temperature and increase of activation energy of PH1 on lowering z could be due to the change of the O potential profile with varying the level of screening. We come to the conclusion that PH1 is due to the jumps of the oxygens within the 0-II phase or ordered domains where the electrostatic interactions between ions are screened enough. Such domains could coincide with those Poulasn et al. hypothesized to contribute to the charge transfer by creating holes in the superconducting CuOs planes. If so, it is possible to experimentally evaluate the fractions of the 90 K phase and 60 K phase hypothesized by Ponleen eta/. 1 by measuring the intensity of the relaxation processes PH2 and PHI, respectively. The fractions of these domains cannot be measured by other techniques, like diffraction experiments, because the O ordering occurs on a too short length scale. In conclusion, in RBasCuzOe+= there are three anelastic relaxation processes which can be due to O(1)-O(5) jumps of O in the CuO8 plane: peak P2 (with an activation energy of 0.1 eV), PHI and PH2 (1-1.3 eV). The evolution of the latter two processes has been studied as a function of the O content, and has been put in connection with the possible types of O jumps. Peak PH2 has been associated with O vacancies in the O.lphase, as deduced from the dependence of its intensity on z, and PHI with O jumps in domains of other phases. From the dependence of PH1 on quenching, it has been argued that isolated or paired oxygens do not contribute to PH1, in accord with a previous assignment of free
Vol. 82, No~ 6"
O motion to P2. From a discussion of the possible types of potential energy profiles felt by oxygen in its different states of aggregation, it has been argued that only jumps within the 0.II phase are likely to contribute to PHI. According to the above analysis, the measurement of the intensity of PH1 and PH2 provides information on the fractions of 0 . I and 0.II phases, regardless of their domain size. ACKNOWLEDGEMENT: This work has been supported by the National Research Council of Italy under the Progetto Finalizzato nSuperconductive and Cryogenic Technologies". REFERENCES 1. H.F. Pouleen, N.H. Andersen, J.V. Andersen, H. Bohr, and O.G. Mouritmn, Nature 349, 594 (1991). 2. B.W. Veal, A~.P. Paulikas, Hoydoo You, Hao Shi, Y. Fang, and J.W. Downey, Phys. Rev. B 42, 6305 (1990). 3. A.S. Nowick and B.S. Berry, Anelastic ReIazation in Crgetalline 8elide (Acadendc Press, New York, 1972). 4. G.A. Custa, M. Ferretti, M.L. Fornasini, and G.L. Olcese, Solid State Commun. 65, 469 (1988). 5. X.M. Xie, T.G. Chen, and Z.L. Wu, Phys. Rev. B 40, 4549 (1089). 6. M. Canali, G. Cannelli, R. Cantelli, F. Cordero, M. Ferretti, and F. Tmquattrini, Physics C. ISf-lS9, 897 (1991). 7. G. CanneUi, R. Cantelli and F. Cordero, Phys. Rev. B 38, 7200 (1988). 8. G. Cannelli, R. Cantelli, F. Cordero, M. Fsrretti, and F. Trequattrini, Solid State Commun. 7T, 429 (1991). 9. G. Ceder, M. Asta, and D. de Fontaine, Physics C 177, 106 (1991). 10. G. Cannel]i, R. Cantalli, F. Cordaro, F. Trequattrini, S. Ferraro and M. Ferretti, Solid State Commun. 80, 715 (1991).
Solid State Communications, Vol. 82, No. 6, PP. 433-436, 1992. Printed in Great Britain.
0038-109819255.00+.00 Pergamon Press Ltd
M O B I L I T Y A N D S H O R T - R A N G E O R D E R I N G O F O X Y G E N IN RBaICusOs+, B Y A N E L A S T I C R E L A X A T I O N A N D POSSIBLE C O R R E L A T I O N W I T H T H E 90 K A N D 60 K S U P E R C O N D U C T I N G PHASES
G.Cannelli,1 R.Cantelli, 1 F.Cordero, s F. Trequattrini,2, and M. Ferrettis (1) Universitb di Roma "La Sapienza~, Dip. di Energetica, Via Scarps 14, 1-00161 Rome, Italy (2) CNR, letituto di Acustica "O.M. Corbiuo", Via Cassia 1216, 1-00189 Rome, Italy (3) Universith di Genova, Istituto di Chimica Fisica, 1-16132 Genova, Italy (Received 9 March 1992 by E. Molinari)
The evolution of two anelastic relaxation processes in RBalCusOe+s (R = Y, Eu) above room temperature has been investigated and discussed over the whole range of the O stoichiometry. The procmmm seem to be associated with the O jumps in the O-! and 0-II domains, and could provide the tool for evaluating the fractions of the 90 and 60 K superconducting phases.
The RBa~CusOe+, (R = Y,Eu) samples were rectangu. ler bars prepared as explained in Ref. 4 which were elactrmtatically excited on their flexurnl modes at frequencies included between '0.4 and 20 kHs; both the Y and Eu based materials exhibited the same phenomenology. The l i t and 5th modes (with a ratio of the frequencies equal to 13) were excited during the same run, so that the thermally activated processes could be readily discerned from those associated to phase transformations. Measurements were carried out in vacuum (10-s tuber), and therefore O loss occurred above 600 K. Above room temperature two thermally activated relaxation processes are observed, and figure 1 presents the evolution of the corresponding Q - I peaks PHI and PH2 with decrease of the O stoichiometry, starting from z --0.9. The dashed curve 2 represents several thermal cycles during which PH2 ~ up to curve $, due to O loss; the fact that initially its height is an increasing function of the O vacancy concentration, is in accord with what found by Xieet eL.s This circumstance led those authors to formulate the hypothesis that in the O-/phese (orthorh0mbic with all Cu(1)-O(1) chains filled) O can jump out era chain only into a 0(5) site of the same Cues plane which is nelshbour to a vacancy in the next chain. Peak PH2 is ~dentiflad with that observed by Xie et ~ and we adopt their interpretation. Proceeding with the O outgassing, PH2 disappears together with the O-lphaee. At a temperature lower than that of PH2 another peak, PHI, appears, which develops at lower values of z, and reaches its maximum intensity around z -~0.4. While z decreases, it shifts from -~500 to --600 K,e and correspondingly its activation energy varies from .~1.1 eV to ,-1.$ eV, as estimated from the temperature shift with frequency. The peak is significantly broader than a single time process. The first stages of the evolution of PH1 in YBaICusOe+l appear in Fig. 1, while its further evolution is shown in Fig. 2, for EuBaICusOe~.,; both types of samples give very similar re-
The RBalCusOs+, superconductors have cristallo~'aphic planes which can accommodate a variable O etoichiometry from empty to half filled. As a result, they have a complex and mostly unexplored z, T phase diagram and the 0 mobility in their C u e . plane is certainly much higher than in other oxides. Detailed knowledge of the phase diagram and O mobility are of great interest, also because the physical properties of RBasCusOe+s, and notably the electric ones, depend on the type of ordering o r e in the Cues planes, and not only on its concentration.1 A striking example of that is the increase of TG of tens of kelvin in quenched O deficient single crystals, simply upon aging at room temperature.2 Both the mobility and orderingof oxygen can be effectivelystudied by anelastic relaxation measurements (elastic energy dissipation and modulus). In fact, when an oxygen jumps between two different portions 1 and 2, a change ~A in the local elastic distortion (elastic dipole) occurs, and gives rise to a peak in the elastic energy loss, Q-I, as function of temperature:s q~l;l"
I + Coot)i
(1)
where c is the concentration of the jumping atoms, nl and n2 their equilibrium fractions in sites 1 and 2, w is the angular vibration frequency of the sample; r(T) is the relaxation
time given by
II, = l/n,, + l/rii,
(2)
where i/r,y is the jump rate from i to j, which generally follows the Arrhenins law rdi = I"0exp(Wil/IcT). The maximum of Q - I occurs when cot ,,, I, i.e. at a temperature such that the atomic jump frequency is close to the an&~.tlar vibration frequency of the semple. In the present paper we report on now measurements above room temperature for z varying from Imm than 0.2 to 0.9. The results can be interpreted in terms of the motion and short-range ordering of 0 in the Cue ! plane of RBa2CusOe+,. 433
434
OXYGEN IN RBa2Cu306+ x 7
i
6
i
i
i
i
i
Vol. 82, No; 6 T
]
PH2
2~,
YBa2Cu306+x
•
5
0-II
./.
2o../,. •
•
O-I
•
•
•
• •
L •
•
4 • aO-'a-4° e e e •
P2
2
1o. t I0-~
ev}
• •
PHi
1.I.1 ev)
•
0• •
e
PH2
t-4 ev)
I .r-. O
400
500
600 T
700
(K)
_I0-2 Fig. 1 Elastic energy lca of YBa~Cu3Os+® measured in vacuum starting from z ~0.9 at a vibration frequency of 1.8 kHz. suits. The overall phenomenology is sketched in Fig. 3, where the heights of PHI end PH2 are plotted against z, together with that of P2. 7 Peak P2 appears around 60 K (1 kHz) end is characterized by a nearly single relaxation time with an activation energy W = 0.11 eV; it is the only stable peak at the lowest O contents and should therefore be associated to the jumps of free oxygen.~,s The resulting mobility of free 0 in the CuO# planes is extraordinarily high, what can be accepted considering the unique properties of such planes.
6
i. 7 kHz\I
5
1
EuBa2Cu3Os÷x 1 .
4
22 kHz
7o
2
I
bO
500
600
700
800
T (K)
Fig. 2 Elastic energy loamof EuBa2CusOe+, meesured in vacuum starting from z <0,5; meseurJments were concomitantly made at a vibration frequency of 1.7 kHz (continuous line) and 22 kHz (broken line). On proceeding with the O outga~ing, peak PH1 deerea~m and shifts to higher temperature. Measurements are performed both on cooling (o) and heating (+).
I0-3 0
0.5 X
Fig. 3 Lower part: intensity of P2, PHI and PH2 at ,~I kHz as a function of the O stoichiometry. Upper part: CuO, planes and O jumps which are supposed to give rke to P2, PH1 and PH2. Small circles: Cu(1); large closed circles: 0 ( 1 ) and 0 ( 5 ) .
An explanation of the possible large di~erence in activation energy between the jumps of the isolated and aggregated oxygens may be that in the first case the chemical state does not change, while in second case does. This can be seen in the framework of a simple model for the mech, h i m of charge trander between the CuO, and the CuO2 planes.° According to that interpretation, the Cu(l) atoms which are coordinated only with the two apical 0(4) atoms have valence +1, while thou which are also coordinated with one or two O(1) atoms have valence +2. When the jump of an oxygen involves its aggregation to a chain fragment or to another oxygen to form a pair, the total valence of the Cu(1) atoms changes of -1, Which implies the formation of a bond of chemical nature. Instead, no variation of the wdences is involved in the jump of an isolated oxygen. The upper part of Fig. $ schematically represents the CuO= plane for 0
'Vol. '82, No. 6
EuBa2Cu306+x ~i 1 i
6
OXYGEN IN RBa2Cu306+ x l
l
i
l
l
~
435
l
•
•
' " •
O/O~3
ATf 2, B,,
' "
150!i141 0
t:
I1041
~4
T
C)
~3
~"V~1~1412~n.~12 ~"VM21 123 14 400 I
I
500 l
I
T
600 (K) I
I
700 I
,l
800
Elastic
Fig. 4 energy lose of EuBa2CusO6+, (= -+0.3) after 2 h in vacuum at 990 K followed by quenching to room temperature.
the O stoichiometry z, while PHI and PH2 are. An additional mechanism which would be correlated with is the motion of twins; however, when z <0.4 the twin density should be very low, but PH1 is still intense, and we suppose that PH1 is due to O jumps not necessarily involving twin motion. We show now that the jumps of the free oxygens are not included among those giving rise to PHI. This is deduced from Fig. 4, which shows the Q - l o f a EuBa2CuaO~.6.s sample after aging for 2 h at 990 K followed by quenching to room temperature. A few hours after quenching only a small trace of PH1 is present (curve 1), which develops during the subsequent measurements (curves 2 and 3). Its growth with aging at high temperature, without absorption of additional O, excludes that it is due to free O; indeed, the concentration of isolated oxygens is expected to be maximum just after quenching from the highest temperature. The behavior of PHI can be explained as foilows. Above 700 K and with z <0.3 most of the O atoms are expected to be free, and the quenching to room temperature freezes a highly disordered state. This is not in contradiction with the supposed high mobility of the free oxygens (W -- 0.U eV); in fact, during quenching such atoms can form pairs, whose dissociation would require an activation energy comparable to that for an O atom to leave a chain (-- 1 eV); therefore, the pairs would be immobile at room temperature on the time scale of hours or days. The O-O complexes would not give rise to observable anelastic relaxation, as will be discussed below; consequently, after quenching, only a small fraction of oxygens has formed the domains which contribute to PH1. The aggregation in ordered domains (presumably O-II phase) takes place at an observable rate while cycling the sample at temperatures higher than 400-500 K but below the boundary line to the completely disordered tetragonal phase. Peak PH1 is included between these temperatures, at the frequencies of our experiments, and therefore it is observed to grow after quenching from the disordered state (Fig. 4). The dissipation peak at 480 K is frequency independent and hence associated to a phase transformation, paesihiy implying O ordering. 10
z
(a)
(b)
I.
WI~ E14 14 (c)
Fig. 5 Dissociation (a), reorientation (b) of an O pair and diseociation of a chain in the 0-II phase (c) with the respective potential profiles. Small circles: Cu(1); large clceed circles: O(1) and O(5).
We now discuss why a dissipation peak due to 0 pairs should not be observable. An 0 pair can give rile to anelastic relaxation in two ways: by reorienting itself or by dissociating and reforming under the applied cyclic strew. The case of the dissociation and formation of the pair is represented in Fig. 5(a); in the lower part is drawn the potential profile seen by atom A when it jumps between positions 1, 2 and 3, with atom B fixed. Comddaring that the activation energies for the jumps of the c0cygens (chemically) bound in chains are of the order of 1 eV or more (PHI and PH2) and that the activation energy for the jumps of free oxygen is 0.11 eV (P2), we conclude that the difference in site energy between sites 1 and 2 should be E12 "~ 1 eV. Consequently, the activation energy for dissociation is Wll 1 eV and that for the pair formation is W21 N 0.I eV. The elastic energy Ices peak is given by eq. (I) with n l n 2 / / ~ (proportional to [kTcceh2(El2/2kT)] -1, as derived from Boltsmann statistics) which is maximum at kT N and with 1/r = 1/raz + 1/rx2 .~ !/T21 close to the relaxation rate of free O. The Q - I curve is the product of n l n s / l ~ , which has the maxhnum at several thoummdm of kelvin, and of the Dehye curve, which is max/mum for wr - 1; the latter condition is satisfied at a temperature close to that of P2, which has a comparable rand is located around 60 K. Then, the dissipation turns out to be unolmervable at all temperatures, because nlns is finite where the Debye part is negligible and vice versa. The rsorientation of a pair of oxygens is represented in Fig. 5(b), and it occurs in two steps: first atom A jumps to the pceition 4, and then atom B moves to polition 5. If during the first step atom A saw a potential profile similar to that seen when it jumps out of s chain in the O-Hphase (Fig. 5(c)), then the reorientation of the pair would give rise to a dissipation peak close to PH1. Instead, we suppose that the ionic charges of the O atoms are less screened when they are either isolated or paired or in very short chains, than when they belong to an ordered conducting phase. This implies that the electrostatic repulsion between atoms A and B is larger in the case of the pa/r, whenever they are not screened by the Cu atom; hence, both the activation energy W14 for jumping from 1 to 4 and the site energy difference
0.65~.2,
436
OXYGEN IN RBa2Cu306+ x
Eli between the two positions are larger in the pair than in the ordered chain (Fig. 5(b,c)). Again, the large difference in site energy El, would cause the conditions ~oT .- 1 and k T ~ 0.65E14 to occur at well separated temperatures, and therefore the Q-1 to have negligible intensity. The above considerations will apply to the case of short chains as well, if the electrostatic repulsion is larger than in the ordered domains; and the observed shift to higher temperature and increase of activation energy of PH1 on lowering z could be due to the change of the O potential profile with varying the level of screening. We come to the conclusion that PH1 is due to the jumps of the oxygens within the 0-II phase or ordered domains where the electrostatic interactions between ions are screened enough. Such domains could coincide with those Poulasn et al. hypothesized to contribute to the charge transfer by creating holes in the superconducting CuOs planes. If so, it is possible to experimentally evaluate the fractions of the 90 K phase and 60 K phase hypothesized by Ponleen eta/. 1 by measuring the intensity of the relaxation processes PH2 and PHI, respectively. The fractions of these domains cannot be measured by other techniques, like diffraction experiments, because the O ordering occurs on a too short length scale. In conclusion, in RBasCuzOe+= there are three anelastic relaxation processes which can be due to O(1)-O(5) jumps of O in the CuO8 plane: peak P2 (with an activation energy of 0.1 eV), PHI and PH2 (1-1.3 eV). The evolution of the latter two processes has been studied as a function of the O content, and has been put in connection with the possible types of O jumps. Peak PH2 has been associated with O vacancies in the O.lphase, as deduced from the dependence of its intensity on z, and PHI with O jumps in domains of other phases. From the dependence of PH1 on quenching, it has been argued that isolated or paired oxygens do not contribute to PH1, in accord with a previous assignment of free
Vol. 82, No~ 6"
O motion to P2. From a discussion of the possible types of potential energy profiles felt by oxygen in its different states of aggregation, it has been argued that only jumps within the 0.II phase are likely to contribute to PHI. According to the above analysis, the measurement of the intensity of PH1 and PH2 provides information on the fractions of 0 . I and 0.II phases, regardless of their domain size. ACKNOWLEDGEMENT: This work has been supported by the National Research Council of Italy under the Progetto Finalizzato nSuperconductive and Cryogenic Technologies". REFERENCES 1. H.F. Pouleen, N.H. Andersen, J.V. Andersen, H. Bohr, and O.G. Mouritmn, Nature 349, 594 (1991). 2. B.W. Veal, A~.P. Paulikas, Hoydoo You, Hao Shi, Y. Fang, and J.W. Downey, Phys. Rev. B 42, 6305 (1990). 3. A.S. Nowick and B.S. Berry, Anelastic ReIazation in Crgetalline 8elide (Acadendc Press, New York, 1972). 4. G.A. Custa, M. Ferretti, M.L. Fornasini, and G.L. Olcese, Solid State Commun. 65, 469 (1988). 5. X.M. Xie, T.G. Chen, and Z.L. Wu, Phys. Rev. B 40, 4549 (1089). 6. M. Canali, G. Cannelli, R. Cantelli, F. Cordero, M. Ferretti, and F. Tmquattrini, Physics C. ISf-lS9, 897 (1991). 7. G. CanneUi, R. Cantelli and F. Cordero, Phys. Rev. B 38, 7200 (1988). 8. G. Cannelli, R. Cantelli, F. Cordero, M. Fsrretti, and F. Trequattrini, Solid State Commun. 7T, 429 (1991). 9. G. Ceder, M. Asta, and D. de Fontaine, Physics C 177, 106 (1991). 10. G. Cannel]i, R. Cantalli, F. Cordaro, F. Trequattrini, S. Ferraro and M. Ferretti, Solid State Commun. 80, 715 (1991).