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Oxygen disordering in tetragonal Y1Ba2Cu3O6+ε
Physica C 168 (1990) 549-555 North-Holland
O X Y G E N D I S O R D E R I N G I N T E T R A G O N A L YIBa2Cu306+ ~
A time dependent study C. NOBILI, G. O T T A V I A N I and M.C. ROSSI Physics Department, Modena University, Via Campi 213/,2, 41100 Modena, Italy
M. S P A R P A G L I O N E Functional Materials Lab., Enichem, Via S. Salvo 1, 20097San Donato, Milano, Italy
Received 2 May 1990
In situ sheet resistance and weight measurements have been performed on tetragonal YIBa2Cu306+,oxide in an Ar atmosphere in the 550-650 °C temperature range. The starting material was orthorhombic Y tBa2Cu306.9o.The isothermal annealing produces a decrease in weight due to oxygen losses, quickly transforms the material from orthorhombic to tetragonal, and induces an increase in the sheet resistance value. While the variation in weight ends after 200 rain, longer times, about two orders of magnitude more, are necessary to stabilize the value of the resistance. The resistance increases exponentially with a single time constant, and the Arrhenius plot of the time constants measured after three isothermal treatments suggests a process thermally activated with an activation energy of 0.9 + 0.1 eV. The data are consistent with a microscopic process where ultrafine ordered domains, constituted by short chains in the Cu-O planes embedded in a tetragonal matrix, evolve in a uniformly disordered sample. The homogenization affects the electrical transport properties of the material. 1. Introduction The physical properties o f the high-Tc superconductor oxide, YiBa2Cu306+o are strongly affected by the oxygen content and its ordering [ 1-6 ]. It has been shown that oxygen either determines the electronic properties o f the material, modifying, for instance, the transition superconductivity temperature or inducing structural changes, such as the orthorhombic tetragonal transformation. Oxygen is quite mobile in the oxide: in- and out-diffusion studies [ 7 12 ] have shown that it can be transported at temperatures as low as 300°C. The structure o f the orthorhombic phase can be represented by a stacking in the c-axis direction o f a squence o f layers ...CuO, BaO, CuO2, Y, CuO2, BaO, C u O .... The structure o f the CuO planes m a y be regarded as consisting o f Cu atoms, oxygen atoms and oxygen vacancies [ 1-6,13 ]. In the orthorhombic phase the oxygen atoms and vacancies are ordered so that there are linear C u - O chains parallel to the b-axis and C u - V chains parallel to the a-axis. The charge carriers, at least in the metallic phase, 0921-4534/90/$03.50 © Elsevier Science Publishers B.V. ( North-Holland )
are holes [ 14-15 ] likely in a valence band o f the CuO2 planes. The ordering o f the oxygen atoms in the C u - O linear chains, rather than the total quantity o f oxygen, is believed to control the density o f holes in the CuO2 planes; consequently the electrical properties o f the material are mostly affected by the ordering [ 1,2]. At present it is unknown how long a chain should be in order to affect the carrier concentration. The ordering also affects the structural properties o f the material. The value o f E reported in the literature for the orthorhombic-tetragonal transformation is not unique. A large spread o f data can be found, and generally they lie around e=0.4. Recently, an orthorhombic oxide has been produced with an oxygen concentration as low as e = 0 . 2 [ 16,17 ]. The reason for the large spread o f the values o f e is not clear, but one possibility is that the transformations are kinetically controlled, and that the samples analyzed were probably not in thermodynamic equilibrium. F r o m the analysis o f data obtained by performing in-diffusion experiments [7,8 ] we suggest that oxygen atoms, randomly distributed in a tetragonal
550
c. Nobili et al. / Oxygen disordering in tetragonal YBCO
structure, tend to cluster in ordered C u - O chain segments. We propose the presence of ultrafine ordered domains, in a tetragonal matrix and characterized by a high resistivity, which induce high carrier concentrations. The formation of the domains occurs in times of the order of minutes at temperatures as low as 250°C and there is no appreciable uptake of oxygen. Several other authors [ 18-20 ] have been compelled to assume a dishomogeneous material in order to explain some apparent contradictory experimental findings. For instance, Rebelsky et al. [ 19 ] and Petitgrand et al. [ 20 ] have detected, in the same sample, an antiferromagnetic state typical of the tetragonal structure, together with a superconducting transition temperature typical of the orthorhombic structure. The purpose of the present paper is to perform outdiffusion experiments at relatively low temperatures, to see if the domains or their effects can be detected, and to attempt to investigate the time evolution of the domains. Similarly to our previous approach, we will use mostly in situ resistivity and weight loss measurements.
resistivity measurements were performed with spring loaded contacts on small gold dots deposited by sputtering onto the oxide. The measurement procedure was to heat the sample, at a rate of 5°C/min, in oxygen up to the desired temperature To; then the gas was changed to argon and the sample ws left at To for as long as was necessary. The cooling down, always in an Ar atmosphere, was performed at a rate of 1.5°C/min. The sample was weighed before and after each heat treatment. The in situ gravimetric measurements were performed with the same procedure, using a Netzsch STA 409 system. The quantity of the material involved in each thermogravimetric measurement was about 100 mg and the sensitivity of the apparatus is 10-2 mg. Again, before and after each treatment, the sample was weighed in an independent system; consistent results were obtained.
3. Results Figure 1 shows the variation of the sheet resistance R / R o , measured as a function of the annealing time
2. Experimental The samples used, YtBa2Cu306.9o, were obtained starting from a stoichiometric composition of Y203, BaCO3 and CuO powder oxides. The starting materials were reacted at 950°C in air, ground and reacted again at the same temperature. In order to obtain a homogeneous set of data, we have used samples from the same batch. Pellets 12 m m in diameter and 2.5 m m in thickness were sintered at 950°C for 16 h and annealed in flowing oxygen at 500°C for 12 h. The material is polycrystalline and the grains present a large distribution in size; parallel to the c-axis the grains have thicknesses in the 10-20 lxm range. A much larger distribution is present parallel to the (a, b)-axis, where the grain size ranges from a few tens up to a few hundreds of microns. Holes are present and the sample average density is about 75% of bulk density. The samples have a resistivity of 1-3 mr2 cm at room temperature. The superconducting transition temperature, obtained by sheet resistance measurements, is 91.5 K with A T = 2 K. The in situ
at three different temperatures. The resistances have been normalized to the value Ro, measured at room temperature before any heat treatment. The starting time corresponds to the change from oxygen to argon, which occurs when To, the selected temperature for the measurements, has been reached. After an initial linear increase, not observable in the scale used, the resistance increases in a sublinear mode until it reaches a saturation value. Longer times are necessary at lower temperatures to reach a steady state condition, and the saturation value increases with the temperature. The normalized saturation value Rs/Ro of the sheet resistance is reported in table I; by increasing the heat treatment temperature from 550 to 650°C, this value increases by about a factor of 4. The inset in fig. 1 shows the relative weight loss measured for samples similar to those used in the sheet resistance measurement. Again, t=O corresponds to the time when the atmosphere was changed from oxygen to argon; the scale on the right hand side reports the oxygen quantity per formula unit. Heating in oxygen produces a weight loss which depends upon the temperature To; the oxygen contents measured at this point, indicated by ~in, are re-
551
C. Nobili et al. / Oxygen disordering in tetragonal YBCO
o
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Fig. 1. Normalized sheet resistance as a function of the annealing time. The normalization has been performed with the sheet resistance value, Ro, measured at room temperature before any heat treatments. The inset shows the normalized weight loss as a function of the time for similar samples.
Table I Summary of some experimental results obtained at the three different heating temperatures.
To
~.i.
~
R./Ro
(°C) 556 606 653
0.0 0.79 0.66
0.26 0.10 0.01
300 900 1100
z
13,
(rain)
(eV)
4000 1890 1000
0.07 0.12 0.14
ported in table I. In the presence of argon, the major variation occurs in the first 20 min; later the variation is much slower and after 200 min there are no more detectable losses. The slight decrease of the curve at 653°C is not a real effect, but is attributed to a drift of the experimental apparatus. Also reported in table I, indicated by ~, are the final values of the oxygen contents. Weight measurements performed at room temperature before and after the heat treatment on the samples used for the sheet resistivity measurements give the same value for ¢ as measured during in situ weight measurements. The values of (Einare comparable to the published results [ 21-
23] obtained after heat treatments in similar conditions. The incremental loss of oxygen, Ac = e-ein, is the same for all three samples and is around 0.65. This value is consistent with measurements of oxygen content [ 23 ] performed at various temperatures and oxygen partial pressures. X-ray diffraction analysis, performed in the samples after heat treatment, shows the tetragonal nature of our samples with a = b = 3 . 8 6 1 ~,, c = 11.810 A. The sheet resistance curves shown in fig. 1 suggest an exponential time dependence. As shown in fig. 2, the experimental data can be described by R I R o = R J R o { 1 - exp [ - t l z ] } + 1 ,
where z is the time constant necessary to describe the process. The values of z obtained at the different temperatures are reported in table I and plotted in the inset in fig. 2, in an Arrhenius mode. The data can be fitted with a straight line allowing an activation energy of 0.9_+ 0.1 eV to be associated with the process. The cooling at room temperature, performed in an argon atmosphere, does not produce a weight vari-
C. Nobili et al. / Oxygen disordering in tetragonal YBCO
552
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Fig. 2. Log plot of the sheet resistance data reported in fig. 1 as a function of the ratio t/r. The data exhibit a linear behaviour. The inset shows an Arrhenius plot of the values of 3.
Fig. 3. Arrhenius plot of the normalized sheet resistance data measured in the sample annealed at 653°C and cooled after 300 and 3300 min. The inset shows the data on a linear scale.
ation or any detectable metallurgical transformation. The inset in fig. 3 shows the R/Ro curve taken from the sample annealed at 653°C, as a function of the temperature during the cooling down procedure. Similar results are obtained by reheating the samples, confirming that no metallurgical transformations capable of changing the sheet resistance have occurred. Two curves are presented in fig. 3: one obtained after 300 min of isothermal heating and the other after 3300 min. The data can be described by a function of the type
tures, 653, 606 and 656 °C, can be also described with an exponential function: table I lists the values of Eg measured. It should be noted that E s increases with decreasing oxygen content.
R/Ro = const. X exp [Eg/kT] , as shown in the Arrhenius plot. The measured slopes are 0.07 and 0.14 eV for the isothermal annealing after 300 and 3300 rain, respectively. Weight measurements performed at room temperature indicate that both samples have the same oxygen content. The cooling down curves obtained from samples heat treated for long times at three different tempera-
4. Discussion and conclusion
The relative increase in resistance by a factor of up to 1000, as shown in fig. 1, in conditions where there is no appreciable loss of oxygen is the most relevant result presented here. Although the data reported here are for samples from the same batch, many types of samples from different sources and prepared at different times in slightly different conditions have been analyzed. The resistance increases exponentially with time in all the samples, irrespective of the preparation procedures, the annealing path followed, the grain size distribution and the surface morphology. The time constants and saturation resistances range
C. Nobili et al. / Oxygen disordering in tetragonal YBCO over a large set of values and at present we are unable to correlate them with any morphological quantity or experimental observations. The same activation energy has been obtained for the various sets of samples. In the presence of a kinetic process and a polycrystalline material, the question of whether the measured changes in resistivity come from the bulk or from the grain boundaries is crucial for understanding the microscopic aspects of the diffusion. Grain boundaries seem to be unlikely since similar results have been obtained for samples with different grain size distributions. Moreover, a qualitative argument against grain boundaries is that we are considering processes which occur at relatively high temperatures for times which are of the order of several thousands of minutes. Our analysis will be based on the assumption that bulk phenomena control the process. In order to identify the mechanisms responsible for the exponential behaviour, the basic assumption is that the resistance directly reflects the microscopic changes. We also assume, similarly to other researchers involved in kinetics studies [ 7-12 ], that oxygen loss and disorder only occur on CuO planes and that the CuO2, BaO and Y layers are inert. Various microscopic processes can lead to the resistance change, with the most likely of these being: - oxygen losses: such losses should be very small, below the sensitivity of our experimental apparatus but still capable of affecting the conductivity; an order-disorder transformation, such as the disappearance of ordered domains. In both cases, the process can be described [ 24-26 ] in the limit of the classical approximation by: -
d N / d t = (No - N ) / z , and if a free energy barrier AG controls the process: 1 / z = poexp ( - A G / k T ) , where/1o is the frequency factor, No is the initial oxygen (or domain) concentration in the unit cell and N is the oxygen (or domain) concentration actually evolved. This relation applies either if a classical desorbtion process occurs, or if domains with ordered oxygen structures embedded in disordered material are present and an ordered state goes toward the dis-
553
ordered one. The exponential solution of this equation justifies the experimental results, and the activation energy determined, 0.9 _+0.1 eV, assumes the meaning of a free energy barrier either for the transport of oxygen or the dissolution of the domains. According to the data reported in the literature [ 712,25,26], 1-1.3 eV is generally the energy associated with the diffusion of oxygen in the orthorhombic form, while for tetragonal materials the energy is higher, 1.3-2 eV. A large spread of data exists: an experiment [25,26] performed with the same material using the same experimental apparatus gives 1 eV for the diffusion in orthorhombic material and a higher value, 1.26 eV, for tetragonal material. Our findings refer to a process which occurs in a material which according to X-ray diffraction is tetragonal, and we expect to find a high value for the activation energy, of the order of 1.26 eV or higher. The fact that we obtain a value of 0.9 eV strongly suggests transport in an orthorhombic material. These two contradictory findings, a tetragonal material and an activation energy typical of an orthorhombic structure, can be reconciled by assuming that the starting material has ultrafine ordered orthorhombic domains embedded in a tetragonal matrix. The destruction of these domains is responsible for the observed increase in resistance. At present it is difficult to say whether the oxygen in the domains is really lost or if it goes to some other part of the material, creating a distorted tetragonal structure, but all our experimental findings are in favour of a redistribution process. In fact, according to the data reported in the literature [ 7-12,27 ] only hundreds of minutes are necessary for the oxygen to diffuse out of the YBCO grains in our experimental conditions: thus diffusion processes, at least of the kind considered in the literature, cannot be responsible for the phenomena which we have observed. This result is complementary to our in-diffusion experiments where ordered domains are created by exposing a tetragonal structure to an oxygen flux. Recently Tu et al. [ 12 ] have given a detailed analysis of oxygen transport in the C u - O plane of the oxide. They discuss two models: one nonconservative which results in a net flux of matter associated with the oxygen vacancies and another one, conservative, which is via a twinning process and requires the formation of four disordered C u - O bonds for an estimated activation en-
554
C. Nobili et al. / Oxygen disordering in tetragonal YBCO
ergy of 0.8 eV [ 12,13]. The former model does not hold in our case since it applies to processes where oxygen is effectively lost; the latter mechanism, where modifications occur without an oxygen loss is applicable to our experimental conditions. Moreover, the predicted value of the activation energy is very similar to the one which we have measured. The twinning mechanism could be the microscopic process responsible for the order-disorder transformation. From the above discussion it is clear that the microstructure of a material is important and it depends upon the degree to which thermal equilibrium is achieved. Therefore, we expect such a process to be extremely sensitive to the preparation conditions. When equilibrium has been reached, the samples exhibit electronic conductivity which decreases with decreasing temperature, with a behaviour characterized by a single activation energy, in a temperature range where most of the data reported in the literature [ 1,2] exhibit mixed metallic and semiconducting conduction. The discussion of these results is beyond the purpose of the present paper; here we have only reported the effect of homogenization on the electronic transport properties. In conclusion, we have presented a process which requires several thousands of minutes to reach steady state conditions. This process, which occurs at a fixed temperature and depends exponentially on time, is characterized by a change in sheet resistance but not by a change in weight. These data are interpreted by assuming that the sample is composed of ultrafine ordered domains embedded in a tetragonal matrix. We believe that the disappearance of these domains, a transformation of the order-disorder type, is reponsible for the observed behaviour.The process is thermally activated with an activation energy of 0.9+_0.1 eV.
Acknowledgements This work was partially supported by the National Research Council of Italy (CAR) under the Progetto Finalizzato "Superconductive and Cryogenic Technologies" and the Istituto Nazionale di Fisica Nucleare ( I N F N ) . One of us (G.O.) wants to thank
Prof. M. Monteleone for providing the stimulus necessary to write the paper.
References [ 1 ] J.C. Phillips, Physics of High-To Superconductors (Academic Press, San Diego, 1989). [2] C.P. Poolc Jr., T. Datta and H.A. Farach, Copper Oxide Superconductors (Wiley, New York, 1988). [3]A.W. Hewat, E.A. Hewal, P. Bordet, J.-J. Capponi, C. Chaillout, J. Chenavas, J.L Hodeau, M. Marezio, P. Strobel, M. Francois, K. Yvon, P. Fischer and J.L. Tholence, IBM J. Res. Dev. 33 (1989) 220. [ 4 ] P. Meuffles, B. Rupp and E. Porschke, Physica C 156 (1988) 441. [5]R.J. Cava, B. Batlogg, C.H. Chen, E.A. Rietman, S.M. Zahurak and D. Werder, Phys. Rev. B36 ( 1987 ) 5719. [6] J.D. Jorgensen, M.A. Beno, D.G. Hinks, L. Soderholm, K.J. Volin, R.L. Hitterman, J.D. Grace, I.K. Schuller, C.U. Segre, K. Zhang and M.S. Kleefisch, Phys. Rev. B36 (1987) 3608. [7]G. Ottaviani, C. Nobili, F. Nava, M. Affronte, T. Manfredini, F.C. Matacotta and E. Galli, J. Less-Comm. Met. 150 (1989) 177. [8]G. Ottaviani, C. Nobili, F. Nava, M. Affronte, T. Manfredini, F.C. MatacoUa and E. Galli, Phys. Rev. B39 (1989) 9069. [9] K.N. Tu, S.I. Park and C.C. Tsuei, Appl. Phys. Lett. 51 (1987) 2158. [ 10] K.N. Tu, N.C. Yeh, S.I. Park and C.C. Tsuei, Phys. Rev. B38 (1988) 5118. [ 11 ] K.N. Tu, C.C. Tsuei, S.I. Park and A. Levi, Phys. Rev. B38 (1988) 772. [12] K.N. Tu, N.C. Yeh, S.I. Park and C.C. Tsuei, Phys. Rev. B39 (1989) 304. [13] H. Bakker, D.O Welch and O.W. Lazareth Jr., Solid State Commun. 64 (1987) 237. [14] J.M. Tranquada, S.M. Heald, A.R. Moodenbaugh and Y. Xu, Phys. Rev. B38 (1988) 8893. [ 15 ] K. Kitazawa, IBM J. Res. Dev. 33 (1989) 201. [ 16 ] Y. Nakazawa and M. Ishikawa, Physica C 158 (1989) 381. [ 17 ] Y. Nakazawa and M. Ishikawa, Physica C 162-164 (1989) 83. [ 18] A.G. Khachaturyan and J.W. Morris Jr., Phys. Rev. Lett. 61 (1988) 215. [ 19 ] L. Rebelsky, J.M. Tranquada, G. Shirane, Y. Nakazawa and M. Ishikawa, Report BNL-43025 (1989). [20] D. Petitgrand, G. Collin, P. Schweiss, S. Hadjoudj and S. Senoussi, J. Phys. (Paris)49 (1988) 1815. [21 ] H. Verweij and L.F. Feiner, Phillips J. Res. 44 (1989) 99. [22] H. Verweij and W.H.M. Bruggink, J. Phys. Chem. Sol. 50 (1989) 75. [23] K. Kishio, J. Shimoyama, T. Hasegawa, K. Kitazawa and K. Fueki, Jpn. J. Appl. Phys. 26 (1987) L1228. [24] S. Oguz and M.A. Paesler, Phys. Rev. B22 (1980) 22.
C. Nobili et al. / Oxygen disordering in tetragonal YBCO [25]H. Strauven, J.P. Locquet, O.B. Verbeke and Y. Bruynseraede, Solid State Commun. 65 ( 1988 ) 293. [26]J.P. Locquet, H. Strauven, B. Wuyts, O.B. Verbeke, K. Zhang, I.K. Schuller, J. Vanacken, C. van Haesendonck and
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Y. Bruynscraede, Physica C 153 ( 1988 ) 822. [27] K. Kishio, J. Shimoyama, T. Hasegawa, K. Kitazawa and K. Fueki, Jpn. J. Appl. Phys. 26 (1987) L1228.
O X Y G E N D I S O R D E R I N G I N T E T R A G O N A L YIBa2Cu306+ ~
A time dependent study C. NOBILI, G. O T T A V I A N I and M.C. ROSSI Physics Department, Modena University, Via Campi 213/,2, 41100 Modena, Italy
M. S P A R P A G L I O N E Functional Materials Lab., Enichem, Via S. Salvo 1, 20097San Donato, Milano, Italy
Received 2 May 1990
In situ sheet resistance and weight measurements have been performed on tetragonal YIBa2Cu306+,oxide in an Ar atmosphere in the 550-650 °C temperature range. The starting material was orthorhombic Y tBa2Cu306.9o.The isothermal annealing produces a decrease in weight due to oxygen losses, quickly transforms the material from orthorhombic to tetragonal, and induces an increase in the sheet resistance value. While the variation in weight ends after 200 rain, longer times, about two orders of magnitude more, are necessary to stabilize the value of the resistance. The resistance increases exponentially with a single time constant, and the Arrhenius plot of the time constants measured after three isothermal treatments suggests a process thermally activated with an activation energy of 0.9 + 0.1 eV. The data are consistent with a microscopic process where ultrafine ordered domains, constituted by short chains in the Cu-O planes embedded in a tetragonal matrix, evolve in a uniformly disordered sample. The homogenization affects the electrical transport properties of the material. 1. Introduction The physical properties o f the high-Tc superconductor oxide, YiBa2Cu306+o are strongly affected by the oxygen content and its ordering [ 1-6 ]. It has been shown that oxygen either determines the electronic properties o f the material, modifying, for instance, the transition superconductivity temperature or inducing structural changes, such as the orthorhombic tetragonal transformation. Oxygen is quite mobile in the oxide: in- and out-diffusion studies [ 7 12 ] have shown that it can be transported at temperatures as low as 300°C. The structure o f the orthorhombic phase can be represented by a stacking in the c-axis direction o f a squence o f layers ...CuO, BaO, CuO2, Y, CuO2, BaO, C u O .... The structure o f the CuO planes m a y be regarded as consisting o f Cu atoms, oxygen atoms and oxygen vacancies [ 1-6,13 ]. In the orthorhombic phase the oxygen atoms and vacancies are ordered so that there are linear C u - O chains parallel to the b-axis and C u - V chains parallel to the a-axis. The charge carriers, at least in the metallic phase, 0921-4534/90/$03.50 © Elsevier Science Publishers B.V. ( North-Holland )
are holes [ 14-15 ] likely in a valence band o f the CuO2 planes. The ordering o f the oxygen atoms in the C u - O linear chains, rather than the total quantity o f oxygen, is believed to control the density o f holes in the CuO2 planes; consequently the electrical properties o f the material are mostly affected by the ordering [ 1,2]. At present it is unknown how long a chain should be in order to affect the carrier concentration. The ordering also affects the structural properties o f the material. The value o f E reported in the literature for the orthorhombic-tetragonal transformation is not unique. A large spread o f data can be found, and generally they lie around e=0.4. Recently, an orthorhombic oxide has been produced with an oxygen concentration as low as e = 0 . 2 [ 16,17 ]. The reason for the large spread o f the values o f e is not clear, but one possibility is that the transformations are kinetically controlled, and that the samples analyzed were probably not in thermodynamic equilibrium. F r o m the analysis o f data obtained by performing in-diffusion experiments [7,8 ] we suggest that oxygen atoms, randomly distributed in a tetragonal
550
c. Nobili et al. / Oxygen disordering in tetragonal YBCO
structure, tend to cluster in ordered C u - O chain segments. We propose the presence of ultrafine ordered domains, in a tetragonal matrix and characterized by a high resistivity, which induce high carrier concentrations. The formation of the domains occurs in times of the order of minutes at temperatures as low as 250°C and there is no appreciable uptake of oxygen. Several other authors [ 18-20 ] have been compelled to assume a dishomogeneous material in order to explain some apparent contradictory experimental findings. For instance, Rebelsky et al. [ 19 ] and Petitgrand et al. [ 20 ] have detected, in the same sample, an antiferromagnetic state typical of the tetragonal structure, together with a superconducting transition temperature typical of the orthorhombic structure. The purpose of the present paper is to perform outdiffusion experiments at relatively low temperatures, to see if the domains or their effects can be detected, and to attempt to investigate the time evolution of the domains. Similarly to our previous approach, we will use mostly in situ resistivity and weight loss measurements.
resistivity measurements were performed with spring loaded contacts on small gold dots deposited by sputtering onto the oxide. The measurement procedure was to heat the sample, at a rate of 5°C/min, in oxygen up to the desired temperature To; then the gas was changed to argon and the sample ws left at To for as long as was necessary. The cooling down, always in an Ar atmosphere, was performed at a rate of 1.5°C/min. The sample was weighed before and after each heat treatment. The in situ gravimetric measurements were performed with the same procedure, using a Netzsch STA 409 system. The quantity of the material involved in each thermogravimetric measurement was about 100 mg and the sensitivity of the apparatus is 10-2 mg. Again, before and after each treatment, the sample was weighed in an independent system; consistent results were obtained.
3. Results Figure 1 shows the variation of the sheet resistance R / R o , measured as a function of the annealing time
2. Experimental The samples used, YtBa2Cu306.9o, were obtained starting from a stoichiometric composition of Y203, BaCO3 and CuO powder oxides. The starting materials were reacted at 950°C in air, ground and reacted again at the same temperature. In order to obtain a homogeneous set of data, we have used samples from the same batch. Pellets 12 m m in diameter and 2.5 m m in thickness were sintered at 950°C for 16 h and annealed in flowing oxygen at 500°C for 12 h. The material is polycrystalline and the grains present a large distribution in size; parallel to the c-axis the grains have thicknesses in the 10-20 lxm range. A much larger distribution is present parallel to the (a, b)-axis, where the grain size ranges from a few tens up to a few hundreds of microns. Holes are present and the sample average density is about 75% of bulk density. The samples have a resistivity of 1-3 mr2 cm at room temperature. The superconducting transition temperature, obtained by sheet resistance measurements, is 91.5 K with A T = 2 K. The in situ
at three different temperatures. The resistances have been normalized to the value Ro, measured at room temperature before any heat treatment. The starting time corresponds to the change from oxygen to argon, which occurs when To, the selected temperature for the measurements, has been reached. After an initial linear increase, not observable in the scale used, the resistance increases in a sublinear mode until it reaches a saturation value. Longer times are necessary at lower temperatures to reach a steady state condition, and the saturation value increases with the temperature. The normalized saturation value Rs/Ro of the sheet resistance is reported in table I; by increasing the heat treatment temperature from 550 to 650°C, this value increases by about a factor of 4. The inset in fig. 1 shows the relative weight loss measured for samples similar to those used in the sheet resistance measurement. Again, t=O corresponds to the time when the atmosphere was changed from oxygen to argon; the scale on the right hand side reports the oxygen quantity per formula unit. Heating in oxygen produces a weight loss which depends upon the temperature To; the oxygen contents measured at this point, indicated by ~in, are re-
551
C. Nobili et al. / Oxygen disordering in tetragonal YBCO
o
Ar
1500
q
,
u
,
,
6.9
-I ,0
6,5
-t. 5
o
5 5 6 °C
-2.0
6.3
e 0 6 °C
-2.51 0
I
,
100
200
6.1
~
5.,
300
400
500
TIME ( mln
0 ¢Y
n..
/
~ !
t
I
0
~
~ I
556 °C
I
,I
TIME ( m i n )
I
I
I
I
12000
Fig. 1. Normalized sheet resistance as a function of the annealing time. The normalization has been performed with the sheet resistance value, Ro, measured at room temperature before any heat treatments. The inset shows the normalized weight loss as a function of the time for similar samples.
Table I Summary of some experimental results obtained at the three different heating temperatures.
To
~.i.
~
R./Ro
(°C) 556 606 653
0.0 0.79 0.66
0.26 0.10 0.01
300 900 1100
z
13,
(rain)
(eV)
4000 1890 1000
0.07 0.12 0.14
ported in table I. In the presence of argon, the major variation occurs in the first 20 min; later the variation is much slower and after 200 min there are no more detectable losses. The slight decrease of the curve at 653°C is not a real effect, but is attributed to a drift of the experimental apparatus. Also reported in table I, indicated by ~, are the final values of the oxygen contents. Weight measurements performed at room temperature before and after the heat treatment on the samples used for the sheet resistivity measurements give the same value for ¢ as measured during in situ weight measurements. The values of (Einare comparable to the published results [ 21-
23] obtained after heat treatments in similar conditions. The incremental loss of oxygen, Ac = e-ein, is the same for all three samples and is around 0.65. This value is consistent with measurements of oxygen content [ 23 ] performed at various temperatures and oxygen partial pressures. X-ray diffraction analysis, performed in the samples after heat treatment, shows the tetragonal nature of our samples with a = b = 3 . 8 6 1 ~,, c = 11.810 A. The sheet resistance curves shown in fig. 1 suggest an exponential time dependence. As shown in fig. 2, the experimental data can be described by R I R o = R J R o { 1 - exp [ - t l z ] } + 1 ,
where z is the time constant necessary to describe the process. The values of z obtained at the different temperatures are reported in table I and plotted in the inset in fig. 2, in an Arrhenius mode. The data can be fitted with a straight line allowing an activation energy of 0.9_+ 0.1 eV to be associated with the process. The cooling at room temperature, performed in an argon atmosphere, does not produce a weight vari-
C. Nobili et al. / Oxygen disordering in tetragonal YBCO
552
12
10°~
I
I
I
I ~ .°
•
.% .S' ¢'r
@d Od
z~ 606 °c 556 °C
°"~-~.~
o.
~ , Z "
10
I
o 655 °C
]
11
I
9
I
¢J
%C
1 0 -1
O! .
4,
8
]
r
,
. . . .
,
. . . .
,
.
a.5 Et = 0 9 .t o I eV
g o
7.5
653 °C 7.o
1.1o
6k lO
1.15
IO00/T
2O 1/kT
50 (eV)-'
40
1 0 -2
0.0
1.2o
(K -I)
I
I
I
I
I
0.5
1.0
1.5
2.0
2.5
3.0
t/T
Fig. 2. Log plot of the sheet resistance data reported in fig. 1 as a function of the ratio t/r. The data exhibit a linear behaviour. The inset shows an Arrhenius plot of the values of 3.
Fig. 3. Arrhenius plot of the normalized sheet resistance data measured in the sample annealed at 653°C and cooled after 300 and 3300 min. The inset shows the data on a linear scale.
ation or any detectable metallurgical transformation. The inset in fig. 3 shows the R/Ro curve taken from the sample annealed at 653°C, as a function of the temperature during the cooling down procedure. Similar results are obtained by reheating the samples, confirming that no metallurgical transformations capable of changing the sheet resistance have occurred. Two curves are presented in fig. 3: one obtained after 300 min of isothermal heating and the other after 3300 min. The data can be described by a function of the type
tures, 653, 606 and 656 °C, can be also described with an exponential function: table I lists the values of Eg measured. It should be noted that E s increases with decreasing oxygen content.
R/Ro = const. X exp [Eg/kT] , as shown in the Arrhenius plot. The measured slopes are 0.07 and 0.14 eV for the isothermal annealing after 300 and 3300 rain, respectively. Weight measurements performed at room temperature indicate that both samples have the same oxygen content. The cooling down curves obtained from samples heat treated for long times at three different tempera-
4. Discussion and conclusion
The relative increase in resistance by a factor of up to 1000, as shown in fig. 1, in conditions where there is no appreciable loss of oxygen is the most relevant result presented here. Although the data reported here are for samples from the same batch, many types of samples from different sources and prepared at different times in slightly different conditions have been analyzed. The resistance increases exponentially with time in all the samples, irrespective of the preparation procedures, the annealing path followed, the grain size distribution and the surface morphology. The time constants and saturation resistances range
C. Nobili et al. / Oxygen disordering in tetragonal YBCO over a large set of values and at present we are unable to correlate them with any morphological quantity or experimental observations. The same activation energy has been obtained for the various sets of samples. In the presence of a kinetic process and a polycrystalline material, the question of whether the measured changes in resistivity come from the bulk or from the grain boundaries is crucial for understanding the microscopic aspects of the diffusion. Grain boundaries seem to be unlikely since similar results have been obtained for samples with different grain size distributions. Moreover, a qualitative argument against grain boundaries is that we are considering processes which occur at relatively high temperatures for times which are of the order of several thousands of minutes. Our analysis will be based on the assumption that bulk phenomena control the process. In order to identify the mechanisms responsible for the exponential behaviour, the basic assumption is that the resistance directly reflects the microscopic changes. We also assume, similarly to other researchers involved in kinetics studies [ 7-12 ], that oxygen loss and disorder only occur on CuO planes and that the CuO2, BaO and Y layers are inert. Various microscopic processes can lead to the resistance change, with the most likely of these being: - oxygen losses: such losses should be very small, below the sensitivity of our experimental apparatus but still capable of affecting the conductivity; an order-disorder transformation, such as the disappearance of ordered domains. In both cases, the process can be described [ 24-26 ] in the limit of the classical approximation by: -
d N / d t = (No - N ) / z , and if a free energy barrier AG controls the process: 1 / z = poexp ( - A G / k T ) , where/1o is the frequency factor, No is the initial oxygen (or domain) concentration in the unit cell and N is the oxygen (or domain) concentration actually evolved. This relation applies either if a classical desorbtion process occurs, or if domains with ordered oxygen structures embedded in disordered material are present and an ordered state goes toward the dis-
553
ordered one. The exponential solution of this equation justifies the experimental results, and the activation energy determined, 0.9 _+0.1 eV, assumes the meaning of a free energy barrier either for the transport of oxygen or the dissolution of the domains. According to the data reported in the literature [ 712,25,26], 1-1.3 eV is generally the energy associated with the diffusion of oxygen in the orthorhombic form, while for tetragonal materials the energy is higher, 1.3-2 eV. A large spread of data exists: an experiment [25,26] performed with the same material using the same experimental apparatus gives 1 eV for the diffusion in orthorhombic material and a higher value, 1.26 eV, for tetragonal material. Our findings refer to a process which occurs in a material which according to X-ray diffraction is tetragonal, and we expect to find a high value for the activation energy, of the order of 1.26 eV or higher. The fact that we obtain a value of 0.9 eV strongly suggests transport in an orthorhombic material. These two contradictory findings, a tetragonal material and an activation energy typical of an orthorhombic structure, can be reconciled by assuming that the starting material has ultrafine ordered orthorhombic domains embedded in a tetragonal matrix. The destruction of these domains is responsible for the observed increase in resistance. At present it is difficult to say whether the oxygen in the domains is really lost or if it goes to some other part of the material, creating a distorted tetragonal structure, but all our experimental findings are in favour of a redistribution process. In fact, according to the data reported in the literature [ 7-12,27 ] only hundreds of minutes are necessary for the oxygen to diffuse out of the YBCO grains in our experimental conditions: thus diffusion processes, at least of the kind considered in the literature, cannot be responsible for the phenomena which we have observed. This result is complementary to our in-diffusion experiments where ordered domains are created by exposing a tetragonal structure to an oxygen flux. Recently Tu et al. [ 12 ] have given a detailed analysis of oxygen transport in the C u - O plane of the oxide. They discuss two models: one nonconservative which results in a net flux of matter associated with the oxygen vacancies and another one, conservative, which is via a twinning process and requires the formation of four disordered C u - O bonds for an estimated activation en-
554
C. Nobili et al. / Oxygen disordering in tetragonal YBCO
ergy of 0.8 eV [ 12,13]. The former model does not hold in our case since it applies to processes where oxygen is effectively lost; the latter mechanism, where modifications occur without an oxygen loss is applicable to our experimental conditions. Moreover, the predicted value of the activation energy is very similar to the one which we have measured. The twinning mechanism could be the microscopic process responsible for the order-disorder transformation. From the above discussion it is clear that the microstructure of a material is important and it depends upon the degree to which thermal equilibrium is achieved. Therefore, we expect such a process to be extremely sensitive to the preparation conditions. When equilibrium has been reached, the samples exhibit electronic conductivity which decreases with decreasing temperature, with a behaviour characterized by a single activation energy, in a temperature range where most of the data reported in the literature [ 1,2] exhibit mixed metallic and semiconducting conduction. The discussion of these results is beyond the purpose of the present paper; here we have only reported the effect of homogenization on the electronic transport properties. In conclusion, we have presented a process which requires several thousands of minutes to reach steady state conditions. This process, which occurs at a fixed temperature and depends exponentially on time, is characterized by a change in sheet resistance but not by a change in weight. These data are interpreted by assuming that the sample is composed of ultrafine ordered domains embedded in a tetragonal matrix. We believe that the disappearance of these domains, a transformation of the order-disorder type, is reponsible for the observed behaviour.The process is thermally activated with an activation energy of 0.9+_0.1 eV.
Acknowledgements This work was partially supported by the National Research Council of Italy (CAR) under the Progetto Finalizzato "Superconductive and Cryogenic Technologies" and the Istituto Nazionale di Fisica Nucleare ( I N F N ) . One of us (G.O.) wants to thank
Prof. M. Monteleone for providing the stimulus necessary to write the paper.
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