Oxygen evolution from YBa2Cu3O6.85high Tcsuperconductors

~i~#"°'~°',~_PrintedS°lid ,~ StateinGreatC°mmunicati°nS'Britain.Vol.65,No.4,

pp.293-296,

OXYGEN EVOLUTION FROM YBa2Cu306.85 H. Strauven

, J.P. Locquet

0038-1098/88 $3.00 + .00 Pergamon Journals Ltd.

1988.

HIGH T c SUPERCONDUCTORS

, O.B. Verbeke

and Y. Bruynseraede

Laboratorium voor Vaste Stof en Hoge Druk Fysika Laboratorium voor Vaste Stof-Fysika en Magnetisme Katholieke Universiteit Leuven, Departement Natuurkunde Celestijnenlaan 200 D, B-3030 Leuven, Belgium (Received by S. Amelinckx

- October 20, 1987)

Neutron and X-ray diffraction experiments clearly showed the relation between the oxygen content and the structural phase transition in high temperature superconductors. We report on measurements of the oxygen desorption in YBa2Cu306.85 using high temperature oxygen evolution techniques, and find quantitative agreement with neutron diffraction data. Below 800°C the expansion coefficient is clearly related to the evolution rate. Using desorption relations we assign a value of 0.16 eV to the oxygenoxygen repulsion energy, responsible for the order-disorder transition in the linear Cu-O chains. Above 800°C several sharp oxygen escape peaks are observed.

2. EXPERIMENTAL METHOD

i. INTRODUCTION

There is a strong relation between the critical temperature, the structural phase and the oxygen content in the recently discovered high temperature superconductors YBa2Cu306.85 [1-4]. Recent x-ray [5,6] and

Stoichiometric

samples

of

YBa2Cu306.85

were prepared using standard powder metallurgical techniques. After mixing and grinding of well-dried Y203 , CuO and BaCO 3 powders, the samples were cold pressed in disk shaped pellets (2mm thick and 5mm diameter), sintered in a pure oxygen flow for 10 hours at 975°C, 10 hours at 650°C and then furnace cooled (100°C / hour). This procedure yields 95% single-phase compounds ( determined by X-ray diffraction ) with sharp transition (AT = IK) and midpoint T • C values of 93K. The oxygen evolution experlments are performed by furnace annealing the samples, placed on a sapphire holder, in the tip of a

Neutron Diffraction (ND) experiments [7] as well as TEM studies [8] clearly showed a structural phase transition near 750°C, together with a decrease in oxygen stoichiomerry as the temperature is raised. Between room temperature and 900°C, the oxygen content per unit cell, 6.85-Ax, measured in a flowing oxygen atmosphere, varied monotonically from 6.85 to 6.1. This oxygen is almost entirely evolving from the Ol (05) position, i.e. from the linear Cu-O chains as unambigiously determined by ND [7]. The structural phase transition from orthorhombic with P m ~ symmetry to tetragonal with P4/rm~m symmetry occurs for an oxygen content of 6.5, precisely the content for

vacuum pumped closed quartz tube (i0-4m3). During annealing, the pressure measured by a capacitance manometer ( 1% accuracy ) is continuously recorded by fast computer-aided sampling in order to calculate the time-derivative of the oxygen evolution. An alumel-chromel thermocouple (2% accuracy) in thermal contact with the quartz volume is used for monitoring the temperature. The quartz volume can be connected (by a leak valve) to a mass spectrometer chamber in order to analyse the amount and the nature of the gases evolved from the specimen. This set-up was previously used to study hydrogen evolution from a-Si:H thin layers [i0]. Our experiments clearly indicated that degassing of the environment (quartz tube, sapphire holder) is negligible.

which the presence of Cu 3+ is expected to vanish. The critical temperature T is also a monotonous function of the oxygen c content and decreases from T = 93 K (6.85-Ax = 6.85) to T = 0 K (6.85-Ax ~ 6.5). CIn this paper we report oxygen evolution experiments in agreement with neutron diffraction data and determine the activation energy for desorption. To our knowledge only one group [9] has reported similar measurements on high T superconductors Those authors observed c a spontaneous oxygen reabsorption around 600°C for a large sample placed in a relatively small volume which may be responsible for a building-up of a substantial oxygen pressure.

3. EXPERIMENTAL RESULTS

gen

293

Fig.l, curve c shows the measured oxyevolution, from a 17.245 mg YBa2Cu306.85

294

OXYGEN EVOLUTION FROM YBa2Cu306.85 4

HIGH T

c

SUPERCONDUCTORS

Vol. 65, No. 4

N = N O (l-exp(-~ola J~ exp(-~G/kBT ) dT))

~o

~3

With multiple desorption this equation becomes :

).8

N = E. Noi(l-exp(-~oi/af~ l

processes

[21

present,

exp(-AGi/kBT ) dT)) [3]

• 96 94 D.2

0

I

2OO

0

D

io

400

600

800

~00 T ('C)

Fig. 1

Left legend : evolved oxygen pressure versus temperature as measured by the capacitance manometer for : a) the tube without sample ; b) the 17.245 mg sample taking into account the temperature correction ; c) the 17.245 mg sample without any corrections ; d) the tube without sample filled at room temperature with 2 Torr oxygen. Right legend : the oxygen deficiency Ax from YBa2Cu306.85_A x. The squares present the oxygen deficiency diffraction data [7].

from

ture clearly shows the presence of at least five distinct desorption processes above 500°C (Fig.2). Classically, first order desorption rate experiments are interpreted using the relation [ii] :

is the frequency

evolution from a-Si:H layers

is given by kBT/~

= 1013 s -I [ii]. This discrepancy indicates that the oxygen escape proceeds by hoppingdiffusion along vacancies in the Cu-O chains. In this case many identical absorption and desorption processes are involved before the final escape of one atom. Table I Desorption parameters i

( 10 -6 Tort ). Subsequent annealing of the sample indicates no oxygen evolution, as can be expected from a complete desorption process. The reversibility of the whole procedure was checked by a second furnace cooling in oxygen atmosphere. The desorption rate APo2/At versus tempera-

where ~o

104 s -I This frequency is orders of magnitude smaller compared to regular desorption experiments. For example, ~o for hydrogen

neutron

sample between room temperature and 1000°C, with a typical temperature ramp of 10°C/min. The pressure increase due to degassing from the empty tube and the sapphire plate is negligible ( Fig.l, curve a ). Fig.l, curve d shows the pressure increase due to heating of 2 Torr oxygen in the empty tube. If this correction is taken into account a curve identical to curve c but with a smaller amplitude is obtained (Fig.l, curve b). The measured oxygen pressure can now be directly related to the oxygen deficiency Ax (right axis), using the ideal gas law and the sample mass. After this oxygen evolution experiment, the sample is furnace cooled under vacuum

dN/dt = ~o (No-N) exp(-AG/kBT)

This relation was used to fit the corrected evolution data ( Fig.l, curve b ) up to 950°C yielding the parameters listed in Table I. Up to 800°C only two desorption processes (labeled 1 and 2 in Fig. 2) can be observed, corresponding to frequency factors ~oi and ~o2 of about

[i]

factor, N O the ini-

tial oxygen content in one unit cell, N the content evolved from one unit cell and AG a free energy barrier. After integration we obtain, for an isochronal anneal [12] with a temperature ramp AT = a At

Noi oxygen atoms per unit cell

~oi/a I/K

AG i eV

0.95 ± 0.04

1

0.31

± 0.02

6.2 103*

2

0.16

± 0.02

5.4 104*

1.27 ± 0.01

3

0.03

± 0.02

1023**

5.4

4

0.13

± 0.02

5

0.050 ± 0.003

± 0.I

1023**

5.52 ± 0.01

1025**

6.42 ± 0.02

The error on the frequency factor is of the order of the obtained value. These parameters were fixed during the fitting procedure. The relationship between the desorption process and the structural changes, observed by ND is also illustrated in Figure 2. The shape and peak position ( labeled 1 and 2 ) of the desorption rate at low temperature are in excellent agreement with the expansion coefficient data (AV/AT, V = unit cell volume) obtained from ND experiments [7]. The maximum of APo2/At occurs at a temperature T = 650 °C at which, apart from the linear but already oxygen deficient chains, the 05 site becomes occupied. For the higher temperature peaks ( labeled 3, 4, 5 ) very large values for ~oi ( Table I ) are that fast. sion

obtained using relation [3], indicating these desorption processes are extremely No correlation with the structural expancoefficient is observed. A detailed comparison of the oxygen deficiency Ax obtained from ND experiments in equilibrium flow conditions, and our oxygen evolution experiments is also presented in

Vol. 65, No. 4

OXYGEN EVOLUTION FROM YBa2Cu306,85

°o

~"

°

13~

°

D°

"

I

~1.5

I

10

'0

0.5 0

Fig. 2

200

400

600

800

1000 T (°C)

Left legend : evolution rate versus temperature showing clearly five distinct peaks. Right legend : the lattice expansion coefficient versus temperature.

Figure i. The ND data, taken in 100% oxygen atmosphere, were obtained at discrete temperatures. Continuous oxygen evolution experiments, conducted with a temperature rate of 10°C/min, are within the spread of the experimental ND data up to high temperatures.

4. DISCUSSION

Neutron diffraction experiments [7] have shown that between room temperature and 720°C, the oxygen occupancy of the Ol site changes from 0.85 to 0.25 while the occupancy of the 05 site increases from 0 to 0.25. The difference Ax = 0.35 is evolved from the material. This is accompanied by a non-linear lattice expansion having a maximum at about 650°C. At 720°C the Ol and the 05 occupancy becomes equal to 0.25 and the structure changes from orthorhombic to tetragonal. In our evolution experiment the first desorption process (Figure 2) stops at about 720oC for a desorbed amount of Ax = 0.31 ± 0.02 and can therefore be ascribed to desorption from the orthorhombic structure. Using eq.(3) the activation energy for desorption from the Ol site, AGI, is 0.95 eV. The temperature which delimits the two broad peaks (Fig. 2) in the evolution rate experiment is equal to the structural phase transition temperature (720°C). This was also derived from thermogravimetric measurements [13]. Above 720°C in the tetragonal phase, the Ol and 05 positions are degenerate. We assign the second peak in Figure 2 to desorption from these sites and obtain, using eq.(3) for the activation energy AG 2 = 1.27 eV. We emphasize that the activation energy for desorption from the ordered Cu-O chains ( Ol position, orthorhombic phase ) is smaller than for desorption from degenerate Ol and 05 positions (tetragonal phase). An oxygen-oxygen repulsion mechanism may be responsible for this order-disorder reaction.

HIGH T c SUPERCONDUCTORS

295

For an oxygen content 6.85-Ax larger than 6.5 (below 720°C) repulsing oxygen-oxygen neighbours are always present in the linear chains. For a lower content the remaining oxygens at the Ol or 05 position can always be arranged in such a configuration that no oxygen-oxygen neighbours occur. The difference in activation energy for desorption from the orthorhombic (0.95 eV) and the tetragonal (1.27 eV) phase can therefore be identified as twice the oxygen-oxygen repulsion energy (0.16 eV). A theoretical model [14] has recently been applied to this phase transition. This order-disorder model calculation of the phase transition, using a fit to the experimental x-ray diffraction data [5], yields the heat of solution per atom ( 1.2 eV), and the oxygenoxygen repulsion energy (0.22 eV). Our experimental data (0.16 eV ± 0.04) support this model. The high temperature desorption curve is drastically different from the low temperature part. The sharpness of the desorption process starting at about 830°C is difficult to fit by equation [3], due to the extremely high frequency factor. This suggest that a phase transition may be involved. On the other hand there has been no evidence for any additional phase transition from structural work. Neutron diffraction experiments up to 900°C on a sample with an oxygen content of 6.1 [7] and on a quenched sample with an oxygen content of 5.9 [15] showed that above 700°C oxygen vacancies start to grow around the copper planes (02 position), while the O1-05 sites are loosing additional oxygen. Our results are in agreement with these observations. The third and the fourth peak in Fig. 2 can therefore be related to desorption from these sites. The fifth desorption peak above 950°C can tentatively be attributed to a decomposition process [16].

5.CONCLUSIONS Detailed oxygen evolution experiments performed on YBa2Cu306.85 superconductors, are in good agreement with neutron diffraction experiments. Using desorption relations, values for the activation energies of the different desorption processes are obtained. In particular, the oxygen-oxygen repulsion energy responsible for the order-disorder transition, is estimated to be 0.16 eV.

&e~le~eMe~t -We are indebted to A. Persoons (Laboratorium voor Chemische en Biologische Dynamika, K.U. Leuven) and W. Sevenhans for preparing the samples and to W. Boon and C. Van Haesendonck for stimulating discussions. The x-ray experiments were performed by W. Mortier (Laboratorium voor Oppervlakte Chemie, K.U. Leuven). We also thank the Belgian Interuniversitair Instituut voor Kernwetenschappen (I.I.K.W.) for financial support. One of us (JPL) is a research fellow of the I.I.K.W.

296

OXYGEN EVOLUTION FROM YBa2Cu306.85

HIGH T

c

SUPERCONDUCTORS

Vol. 65, No. 4

References

i. J.G. Bednorz and K.A Muller, Z. Phys. B6___44, 189 (1986). 2. M.K.Wu, J°R. Ashburn, C.J. Torng, P.H. Hot, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. Lett. 58, 908, (1987). 3 D.G. Hinks, L. Soderholm, D.W. Capone II, J.D. Jorgensen, Ivan K. Schuller, C.U. Segre, K. Zhang and J.D. Grace, Appl. Phys. Lett. 50, 1688 (1987) 4. M.A. Beno, L. Soderholm, D.W. Capone II, D.G. Hinks, J.D. Jorgensen, J.D. Grace, Ivan K. Schuller, C.U. Segre and K. Zhang, Appl. Phys. Lett. 51, (1987) 5. Ivan K. Schuller, D.G. Hinks, M.A. Beno, D.W. Capone II, L. Soderholm, J.-P. Locquet, Y. Bruynseraede, C.U. Segre, and K. Zhang, Solid State Comm., 63, 385, (1987) 6. K. Yukino, et al., Jpn. J. AppI. Phys. 26, L869, (1987). 7. J.D. Jorgensen, M.A. Beno, D.G. Hinks, L. Soderholm, K.J. Volin, R.L. Hitterman, J.D. Grace, Ivan K. Schuller, C.U. Segre, K. Zhang, and M.S. Kleefisch, Phys. Rev. B (in press). 8. G. Van Tendeloo, H.W. Zandbergen, and S. Amelinckx, Solid State Comm., 63, 385, (1987)

G. Van Tendeloo, H.W. Zandbergen, and S. Amelinckx, Solid State Comm., 63, 603, (1987) 9. J.P. Burger, J. Lesueur, M. Nicolas, J.N. Daou, L. Dumoulin, P. Vajda, J. Physique, 48, 9, (1987) 10. H. Strauven, A. Stesmans, J. Winters, J. Spinnewijn, and O.B. Verbeke, submitted J. Mat. Res. ii. S. Oguz and M.A. Paesler, Phys. Rev. B22, 12, 6213, (1980). 12. W. Primak Phys. Rev. I00, 6, 1677, (1955). 13. J.F. Marucco, C. Noguera, P. Garoche, and G. Collin, Orsay High Tc Preprints, ~, (1987). 14. H. Bakk~r, D.O. Welch, and O.W. Lazareth Jr., Solid. State. Comm. (in press). 15. S. Katano, S. Funahashi, T. Hatano, A. Matsushita, K. Nakamura, T. Matsumoto, K. Ogawa, Jpn. Jn. Appl. Phys. 26, LI049, (1987). 16. M.P.A. Viegers, D.M. de Leeuw, C.A.H.A. Mutsaers, H.C.A. Smoorenburg, J.H.T. Hengst, J.W.C. de Vries, and P.C. Zalm, preprint