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Dynamics of Oxygen ordering in YBa2Cu3O6+x studied by neutron and high-energy synchrotron x-ray diffraction
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PHYSICA ©
~
Physica C 282-287 (1997) 1089-1090
ELSEVIER
Dynamics of Oxygen Ordering in YBa2Cu306+x Studied by Neutron and High-Energy Synchrotron X-ray Diffraction. T. Frello·, N.H. Andersen-, J. Madsen-, M. KAne, M. von Ziromennannb, O. Schmidt-, H.F. Poulsen-, J.R Schneiderb and Th. Wolf -rusa National Laboratory, OK-4000 Roskilde, Oenmark ~SYLAB, Notkestra6e 85, 0-22603 Hamburg, Gennany
CPorschungszentrum Karlsruhe, 0-76021, Gennany The ~cs of the ortho-ll oxygen structure in a high purity YBa2CU306+x single crystal with x=0.50 has been studied by neutron and by X-ray diffraction with a photon energy of 100 keY. Our data show that the oxygen order develops on two different time-scales, one of the order of seconds and a much slower of the order of weeks and months. The mechanism dominating the slow time-scale is related to oxygen diffusion, while the fast mechanism may result from a temperature-dependent change in the average oxygen chain length.
1. Introduction The close relation between oxygen ordering and superconductivity in YBa2CU306+x with 0~1 has stimulated many structural studies of this material. Of particular interest has been the oithorhombic double cell structure, called ortho-ll, which is found for oxygen stoichiometries 0.35~0.67 and leads to a plateau of Tc 60 K. It results from oxygen ordering in CuO chains along the b axis in the basal plane of the unit cell with the oxygen being preferentially on every second chain. Among the unsettled structural properties are the reasons why the ortho-ll structure does not develop long range order, and why the' single cell ortho-I structure remains stable down to temperatures that are significantly lower than predicted by most theoretical models.
=
2. Experimental The sample studied was a high-purity naturally twinned single crystal of YBCO of dimensions 2.5x4.0xO.25 rom3 weighing 17 mg. It was prepared to a oxygen stoichiometry of x=0.50 by a highprecision gas-volumetric method described in Ref. 1. The ortho-ll structure gives rise to a doubling of the unit cell along the a axis, which can be detected 0921-4534/97/$17.00 © Elsevier Science B.V. All rights reserved. PH S0921-4534(97)00649-7
in diffraction studies as weak superstructure reflections (10-4 of a Bragg peak) at half-integer values of the Miller indices along the a-axis (h+%,k,/). The development of the ortho-ll structure was measured by neutron and X-ray diffraction by centering on a superstructure reflection and monitoring the development of the diffracted intensity as function of time and temperature. Neutron diffraction was performed at Riso National Laboratory using a neutron energy of 5 meV. The X-ray diffraction was performed at the dedicated high-energy wiggler beamline BW5 at HASYLAB. The photon energy was 100 keY with a penetration length of = 1.35 rom in YBCO, meaning that both neutron and X-ray experiments could be performed in a transmission geometry probing the bulk of the sample. All slits were opened to integrate over the vertical and horizontal scattered intensity. The crystal was mounted in a furnace where the temperature could be varied from room temperature to 200°C. The ortho-II structure decreases with increasing temperature2 and disappears above 160°C. By a fast temperature quench from 170 °C or above the oxygen will freeze out in a partly disordered state, away from thermal equilibrium. The oxygen then orders in ortho-II structure on a macroscopically slow scale.
=
1090
T. FreUo et al.lPhysica C 282-287 (1997) 1089-1090
2600
~ 2200
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Time (hours)
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Time (min)
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Figure 1. Ortho-II development measured by neutron diffraction of the (¥Z,O,O) reflection. The temperature was quenched from 170°C to 70°C at t=0. Temperature changes are indicated on the figure by vertical arrows. The solid lines are guides to the eye.
Figure 2. Ortho-II development measured by X-ray diffraction of the (5/2,0,5) reflection. Here the temperature was quenched from 180°C to 60 °C at t=O. See also caption to Fig. 1.
In order to get reasonable statistics the time resolution of the neutron measurements was limited to one hour per data point. The much higher flux from the synchrotron permitted a time resolution of 11:11 second.
The ordering process clearly consists of two mechanisms with widely different time scales. We relate the slow ortho-II growth, which gives a concomitant increase in Te, to oxygen diffusion3,4. The fast ordering has to our knowledge not been observed before, and its origin is not as easily established. Theoretical calculations by Uimins suggest that the oxygen chain length is strongly temperature dependent. Any temperature increase will immediately reduce the average chain length and thereby reduce the number of oxygen atoms contributing to the diffracted superstructure signal. The reverse is true when the temperature is lowered. For a given finite temperature there is a maximum in the average chain length. Since the oxygen mobility is decreasing with decreasing temperature, the temperature at which the chain length is sufficiently large to make long-range order is so low that the oxygen mobility is prohibitively small. This may explain why long-range order is never observed for the ortho-II structure.
3. Results The neutron data in Fig. 1 show a logarithmic growth of the ortho-II structure at constant temperature after the quench. By lowering the temperature to 60 °C there is a small instantaneous increase in the intensity, and the slope of the curve becomes essentially zero. When the temperature is increased from 75°C to 100 °C in steps of 5°, there is always an instantaneous decrease in intensity and a change in slope. Around 100 °C the slope becomes negative, and the optimum temperature for developing ortho-II is 80 - 85°C. The X-ray data in Fig. 2 clearly show that the instantaneous jump in intensity upon change in temperature really occurs within a few seconds. A temperature raise gives an instantaneous decrease in the scattered intensity, most clearly seen for 90°C and 100 °C, and a temperature decrease gives an instantaneous increase in intensity. When comparing Fig. 1 and 2, one must bear in mind the different time scales on the two figures; the time span of Fig. 2 corresponds to two points only on Fig. 1.
4. Discussion
REFERENCES N.H. Andersen et al., Physica C 172 (1990) 31 P. Scbleger et 01., Phys. Rev. Lett. 74 (1995) 1446 3 J.~. Jorgensen et 01., Physica C 167 (1990) 571 4 H.F. Poulsen et al., Phys. Rev. Lett. 66 (1991) 465; H.F. Poulsen et al., Nature 349, 594 (1991) S G. Uimin, Phys. Rev. B, 50 (1994) 9531 1
2
PHYSICA ©
~
Physica C 282-287 (1997) 1089-1090
ELSEVIER
Dynamics of Oxygen Ordering in YBa2Cu306+x Studied by Neutron and High-Energy Synchrotron X-ray Diffraction. T. Frello·, N.H. Andersen-, J. Madsen-, M. KAne, M. von Ziromennannb, O. Schmidt-, H.F. Poulsen-, J.R Schneiderb and Th. Wolf -rusa National Laboratory, OK-4000 Roskilde, Oenmark ~SYLAB, Notkestra6e 85, 0-22603 Hamburg, Gennany
CPorschungszentrum Karlsruhe, 0-76021, Gennany The ~cs of the ortho-ll oxygen structure in a high purity YBa2CU306+x single crystal with x=0.50 has been studied by neutron and by X-ray diffraction with a photon energy of 100 keY. Our data show that the oxygen order develops on two different time-scales, one of the order of seconds and a much slower of the order of weeks and months. The mechanism dominating the slow time-scale is related to oxygen diffusion, while the fast mechanism may result from a temperature-dependent change in the average oxygen chain length.
1. Introduction The close relation between oxygen ordering and superconductivity in YBa2CU306+x with 0~1 has stimulated many structural studies of this material. Of particular interest has been the oithorhombic double cell structure, called ortho-ll, which is found for oxygen stoichiometries 0.35~0.67 and leads to a plateau of Tc 60 K. It results from oxygen ordering in CuO chains along the b axis in the basal plane of the unit cell with the oxygen being preferentially on every second chain. Among the unsettled structural properties are the reasons why the ortho-ll structure does not develop long range order, and why the' single cell ortho-I structure remains stable down to temperatures that are significantly lower than predicted by most theoretical models.
=
2. Experimental The sample studied was a high-purity naturally twinned single crystal of YBCO of dimensions 2.5x4.0xO.25 rom3 weighing 17 mg. It was prepared to a oxygen stoichiometry of x=0.50 by a highprecision gas-volumetric method described in Ref. 1. The ortho-ll structure gives rise to a doubling of the unit cell along the a axis, which can be detected 0921-4534/97/$17.00 © Elsevier Science B.V. All rights reserved. PH S0921-4534(97)00649-7
in diffraction studies as weak superstructure reflections (10-4 of a Bragg peak) at half-integer values of the Miller indices along the a-axis (h+%,k,/). The development of the ortho-ll structure was measured by neutron and X-ray diffraction by centering on a superstructure reflection and monitoring the development of the diffracted intensity as function of time and temperature. Neutron diffraction was performed at Riso National Laboratory using a neutron energy of 5 meV. The X-ray diffraction was performed at the dedicated high-energy wiggler beamline BW5 at HASYLAB. The photon energy was 100 keY with a penetration length of = 1.35 rom in YBCO, meaning that both neutron and X-ray experiments could be performed in a transmission geometry probing the bulk of the sample. All slits were opened to integrate over the vertical and horizontal scattered intensity. The crystal was mounted in a furnace where the temperature could be varied from room temperature to 200°C. The ortho-II structure decreases with increasing temperature2 and disappears above 160°C. By a fast temperature quench from 170 °C or above the oxygen will freeze out in a partly disordered state, away from thermal equilibrium. The oxygen then orders in ortho-II structure on a macroscopically slow scale.
=
1090
T. FreUo et al.lPhysica C 282-287 (1997) 1089-1090
2600
~ 2200
I it 1..~
orr r
0
~ ..... ~ ~
0
U
1800
o
1400
2000
i
0
E)O 16
roO
gO gf!
u
r
0
\00
~ 1500
1
1000
o
u
500
0
1000
0
100
200
300
o
400
Time (hours)
20
40
60
80
Time (min)
100 120
Figure 1. Ortho-II development measured by neutron diffraction of the (¥Z,O,O) reflection. The temperature was quenched from 170°C to 70°C at t=0. Temperature changes are indicated on the figure by vertical arrows. The solid lines are guides to the eye.
Figure 2. Ortho-II development measured by X-ray diffraction of the (5/2,0,5) reflection. Here the temperature was quenched from 180°C to 60 °C at t=O. See also caption to Fig. 1.
In order to get reasonable statistics the time resolution of the neutron measurements was limited to one hour per data point. The much higher flux from the synchrotron permitted a time resolution of 11:11 second.
The ordering process clearly consists of two mechanisms with widely different time scales. We relate the slow ortho-II growth, which gives a concomitant increase in Te, to oxygen diffusion3,4. The fast ordering has to our knowledge not been observed before, and its origin is not as easily established. Theoretical calculations by Uimins suggest that the oxygen chain length is strongly temperature dependent. Any temperature increase will immediately reduce the average chain length and thereby reduce the number of oxygen atoms contributing to the diffracted superstructure signal. The reverse is true when the temperature is lowered. For a given finite temperature there is a maximum in the average chain length. Since the oxygen mobility is decreasing with decreasing temperature, the temperature at which the chain length is sufficiently large to make long-range order is so low that the oxygen mobility is prohibitively small. This may explain why long-range order is never observed for the ortho-II structure.
3. Results The neutron data in Fig. 1 show a logarithmic growth of the ortho-II structure at constant temperature after the quench. By lowering the temperature to 60 °C there is a small instantaneous increase in the intensity, and the slope of the curve becomes essentially zero. When the temperature is increased from 75°C to 100 °C in steps of 5°, there is always an instantaneous decrease in intensity and a change in slope. Around 100 °C the slope becomes negative, and the optimum temperature for developing ortho-II is 80 - 85°C. The X-ray data in Fig. 2 clearly show that the instantaneous jump in intensity upon change in temperature really occurs within a few seconds. A temperature raise gives an instantaneous decrease in the scattered intensity, most clearly seen for 90°C and 100 °C, and a temperature decrease gives an instantaneous increase in intensity. When comparing Fig. 1 and 2, one must bear in mind the different time scales on the two figures; the time span of Fig. 2 corresponds to two points only on Fig. 1.
4. Discussion
REFERENCES N.H. Andersen et al., Physica C 172 (1990) 31 P. Scbleger et 01., Phys. Rev. Lett. 74 (1995) 1446 3 J.~. Jorgensen et 01., Physica C 167 (1990) 571 4 H.F. Poulsen et al., Phys. Rev. Lett. 66 (1991) 465; H.F. Poulsen et al., Nature 349, 594 (1991) S G. Uimin, Phys. Rev. B, 50 (1994) 9531 1
2