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Evolution of the orthorhombic phase and superconductivity in YBa2Cu3O7-x
0038-1098188
+ Solid State Communicatins,Vol. 68, No. 7, pp. 639-641, 1988. Printed in Great Britain.
EVOLUTION
OF THE ORTHORHOMBIC
PHASE AND SUPERCONDUCTIVITY
$3.00 + .OO
Pergamon Press plc
IN Yda2CU3O7_x
G.M. Lin, Q.Z. Huang and J.X. Zhang Department
Department
of Physics,
of Applied
Zhongshan
University,
G.G. Siu, M.J. Stokes Science, City Polytechnic
Guangzhou,
China
of Hong Kong, HK
( Received 30 July, 1988 by W.Y. Kuan ) The time evolution of the orthorhombic phase from starting materials of YBa$u307_x compound in the sintering process is examined using in situ hlqh temoerature X-rav diffraction analvsis. Two transition stages have been discovered: the first stage-occurs in the high temperature (95OoC) region, where the tetragonal perovskite like layer structure undergoes a reorganization in lattice parameter; the sesond transition takes place at about 700°C during the cooling process. This transition concern the change of oxygen content and vacancy state. The effect of first transition on the superconductivity has been studied.
INTRODUCTION
solid state reaction and their transition can be investigated. The pellet is placed on a sample stand, (a high temperature accessory of the diffractometer D/min 3A, Rigaku). Diffraction patterns are recorded at certain temperature over selected time invervals covering the Particular experimental condiwhold process. tions are: heating rate -lOoC/min, cooling rate -5oC/min, CuK radiation.
The YBa2Cu307_x compound is the most important superconducting phase of the Y-Ba-Cu-0 The perovskite like layer structure series. provides Cu-0 atom planes in a layered distribution, which is one of the structure conditions Orthorhombic distortion for superconduction. in the structure, is also a condition for high T, superconduction. It is hence important for development of sintering techniques and for understanding of superconduction mechanisms to investigate the formation of the YBa2Cu307-x superconducting perovskite like structure produced in the solid phase reaction, and to examine in detail the process of orthorhombic distortion. In this paper, we report a new results observed through in situ high temperature X-ray diffraction studies of the (110) diffraction peak of the YBa2Cu307-x phase (l-2-3 phase).
RESULTS
AND DISCUSSION
THE FORMATION OF TETRAGONAL PEROVSKITE LIKE STRUCTURE-From room temperature to below 8800C X-ray diffraction patterns obtained are still the mixed diffraction patterns of the primitive (Fig. la at 7500C). materials BaC03, CuO and Y20 At 88OoC, a group of new dif ? raction peaks appear superimposed on the background of the mixture paterns, which by analysis, belong to the diffraction of ordered cubic phase with a= 3.875 A, (Fig. lb, at 88OOCl. Fig. lb shows clearlv the initial staoe of the formation of the YBa2CU307_x phase. *It can be identified as a=b=c/3 tetragonal perovskite like structure. This phase is unstable and transient. In the subsequent process of further continuous heating, the (100) line splits, which shows that the (103) and (013) planes are no longer equivalent to the (110) plane, and thus the tetragonal perovskite structure with a=b#c/3 is formed.
EXPERIMENT An acid solution prepared from BaC03(99.9%) CuO(99.5%) and Y203(99.99%) powder with Y:Ba:Cu ratio of 1:2:3 is pyrolysed and precipitated. The powder mixture so obtained is then pressed to 010x1 (mm) pellet. Its DTA measurement shows a broad heat absorption peak for the solid state reaction in the range from 880°C to 950°C. The general experimental programme is that the sample is heated in air attaining 950°C, this temperature is maintained for a fixed period and finally the sample is followed by furnace cooling. The electrical resistance curve is then obtained using the usual 4-lead technique with pressed indium solidring, and the zero resistant temperature Tot is determined. In order to understand the effect of the high temperature holding period on superconductivity, in situ X-ray diffraction observations of the samples were made during the sintering process. In this way the products of
FORMATION OF ORTHORHOMBIC PHASE-As mentioned above the (110) diffraction line initially includes the diffraction effects of the (110) (103) and (013) crystal planes in the tetragonDetailed information of the tetragonal phase. al-orthorhombic transition is hence obtained from the evolution of the shape, intensity and splitting of the (110) peak while the temperature is maintained at 950°C. Fig.2 shows the change in the diffraction peaks for the crystal planes (0131,(103),(110) of the l-2-3 phase over
639
640
SUPERCONDUCTIVITY
IN YBa#~~07_~
Vol. 68, No. 7
2
II
(a)
’
1500
y203
2 Baco, 3 Cue *
YBa,Cu,O,-x
Fig.2
1
20
I
I
30
40 Two
Fig.1
The formation
I
I
50
60
I 70
The gradual change of the profile (110) line.
of
theta
of tetragonal
structure. x
the period while the temperature is maintained at 9500C and the subsequent cooling process. The (110) line is seen to split into the (103) and (110) lines of the tetragonal structure soon after reaching 950°C. During the period the intensities of these two diffraction peaks change and even develop to a point where reversal in their relative intensity ratios occurs (over the same period however the relative intensities of the (200) and (006) lines are not reversed). These changes are accompanied by any further peak shifts in the overall diffraction pattern. The continuous development of this transition while the temperature is held at highest value has importance, it is suggested, in producting the required crystal structure for superconduction. (Fig.31 According to the result of TG analysis during heating /l/, it is suggested that these changes are concerned with the process of deoxidation. The value of c is increasing relative to oxygen content. In other words, the increasing of c is associated with a loss in oxygen after formation of l-2-3 phase. During cooling to 880°C, the material structure now remains relatively stable, as shown by the invariant relative of the (110) line. However, as the cooling process from 880°C to 6OOoC, the relative intensities of the (1101 and (103) lines change irregularly, suggesting that the crystal structure reverts to a generally stronger oxygen content character. This process lasts until the (103) and (013) lines splits at about 700°C when the relative intensity ratios are reversed due to the (103) peak now being superimposed on the (110) peak. At the same time the (020) and (200) lines aplit, and the (020) line combines with (006) so that the relative intensity ratios of the (006) and (ZOO) are also reversed. This is the tetragonal orthorhombic transition caused by reordering of the oxygen atoms and the associated vacncies, which has been reported in other studies.
xx
xx 00
o”
x
oz
’
xxx xx
.
a
.O
0.
:0
l
0
. X
,, 0 Y
X
x
x
2hrs
08
o
4hrs
0.
l
6hrs
I 150
I 200
T(K) Fig.3
Resistance vs. temperature curves of samples after 2,4 and 6 hrs. holding at 9500C respectively followed by furnace cooling.
SUPERCONDUCTIVITY Fig.3 shwos the resistance vs. temperature curves corresponding to samples which are maintained for 2,4 and 6 hrs. at 9500C followed by furnace cooling. Table I. shows the relationship between Tot and the period for which the sample is held at maximum temperature. In all cases the subsequent cooling rates are the same. It can be seen that Tot gradually rises, with extension of the high temperature holding period. It is interesting to note that, the resistance vs. temperature characteristic changes from semiconducting (Fig.3, 2 hrs.) to metallic (Fig.3, 6 hrs.). The longer of high temperature holding, the more completety of high temperature tetragonal structure (Fig.2). It seems that the organization of lattice parameter in high temperature holding stage, which is associated
Relationship
holding
TABLE I between To, and holding
time at 9500C (hr.) : 6
641
SUPERCONDUCTIVITYIN YBa2Cu307_x
Vol. 68, No. 7
To
time
+,,03
d 67.7 86.4 89.8
12; 2415
2. During the‘period when the maximum temperature is maintained and subsequent cooling process, lattice parameters change continuously as oxygen content is increasing or decreasing. 3. A longer period of high temperature maintenance, which concerns a more completet development of high temperature tetragonal structure, raises the superconduction.
with change of oxygen content/l/, is one of the important condition for superconduction. CONCLUSIONS 1. In the formation of YBa2Cu307_x superconducting phase, the transient cubic phase or tetragonal structure with a=b#c/3 is formed first.
REFERENCE 1. P.K. Gallagher, 2, 995,(1987)
et al., Mater. Res. Bull.
+ Solid State Communicatins,Vol. 68, No. 7, pp. 639-641, 1988. Printed in Great Britain.
EVOLUTION
OF THE ORTHORHOMBIC
PHASE AND SUPERCONDUCTIVITY
$3.00 + .OO
Pergamon Press plc
IN Yda2CU3O7_x
G.M. Lin, Q.Z. Huang and J.X. Zhang Department
Department
of Physics,
of Applied
Zhongshan
University,
G.G. Siu, M.J. Stokes Science, City Polytechnic
Guangzhou,
China
of Hong Kong, HK
( Received 30 July, 1988 by W.Y. Kuan ) The time evolution of the orthorhombic phase from starting materials of YBa$u307_x compound in the sintering process is examined using in situ hlqh temoerature X-rav diffraction analvsis. Two transition stages have been discovered: the first stage-occurs in the high temperature (95OoC) region, where the tetragonal perovskite like layer structure undergoes a reorganization in lattice parameter; the sesond transition takes place at about 700°C during the cooling process. This transition concern the change of oxygen content and vacancy state. The effect of first transition on the superconductivity has been studied.
INTRODUCTION
solid state reaction and their transition can be investigated. The pellet is placed on a sample stand, (a high temperature accessory of the diffractometer D/min 3A, Rigaku). Diffraction patterns are recorded at certain temperature over selected time invervals covering the Particular experimental condiwhold process. tions are: heating rate -lOoC/min, cooling rate -5oC/min, CuK radiation.
The YBa2Cu307_x compound is the most important superconducting phase of the Y-Ba-Cu-0 The perovskite like layer structure series. provides Cu-0 atom planes in a layered distribution, which is one of the structure conditions Orthorhombic distortion for superconduction. in the structure, is also a condition for high T, superconduction. It is hence important for development of sintering techniques and for understanding of superconduction mechanisms to investigate the formation of the YBa2Cu307-x superconducting perovskite like structure produced in the solid phase reaction, and to examine in detail the process of orthorhombic distortion. In this paper, we report a new results observed through in situ high temperature X-ray diffraction studies of the (110) diffraction peak of the YBa2Cu307-x phase (l-2-3 phase).
RESULTS
AND DISCUSSION
THE FORMATION OF TETRAGONAL PEROVSKITE LIKE STRUCTURE-From room temperature to below 8800C X-ray diffraction patterns obtained are still the mixed diffraction patterns of the primitive (Fig. la at 7500C). materials BaC03, CuO and Y20 At 88OoC, a group of new dif ? raction peaks appear superimposed on the background of the mixture paterns, which by analysis, belong to the diffraction of ordered cubic phase with a= 3.875 A, (Fig. lb, at 88OOCl. Fig. lb shows clearlv the initial staoe of the formation of the YBa2CU307_x phase. *It can be identified as a=b=c/3 tetragonal perovskite like structure. This phase is unstable and transient. In the subsequent process of further continuous heating, the (100) line splits, which shows that the (103) and (013) planes are no longer equivalent to the (110) plane, and thus the tetragonal perovskite structure with a=b#c/3 is formed.
EXPERIMENT An acid solution prepared from BaC03(99.9%) CuO(99.5%) and Y203(99.99%) powder with Y:Ba:Cu ratio of 1:2:3 is pyrolysed and precipitated. The powder mixture so obtained is then pressed to 010x1 (mm) pellet. Its DTA measurement shows a broad heat absorption peak for the solid state reaction in the range from 880°C to 950°C. The general experimental programme is that the sample is heated in air attaining 950°C, this temperature is maintained for a fixed period and finally the sample is followed by furnace cooling. The electrical resistance curve is then obtained using the usual 4-lead technique with pressed indium solidring, and the zero resistant temperature Tot is determined. In order to understand the effect of the high temperature holding period on superconductivity, in situ X-ray diffraction observations of the samples were made during the sintering process. In this way the products of
FORMATION OF ORTHORHOMBIC PHASE-As mentioned above the (110) diffraction line initially includes the diffraction effects of the (110) (103) and (013) crystal planes in the tetragonDetailed information of the tetragonal phase. al-orthorhombic transition is hence obtained from the evolution of the shape, intensity and splitting of the (110) peak while the temperature is maintained at 950°C. Fig.2 shows the change in the diffraction peaks for the crystal planes (0131,(103),(110) of the l-2-3 phase over
639
640
SUPERCONDUCTIVITY
IN YBa#~~07_~
Vol. 68, No. 7
2
II
(a)
’
1500
y203
2 Baco, 3 Cue *
YBa,Cu,O,-x
Fig.2
1
20
I
I
30
40 Two
Fig.1
The formation
I
I
50
60
I 70
The gradual change of the profile (110) line.
of
theta
of tetragonal
structure. x
the period while the temperature is maintained at 9500C and the subsequent cooling process. The (110) line is seen to split into the (103) and (110) lines of the tetragonal structure soon after reaching 950°C. During the period the intensities of these two diffraction peaks change and even develop to a point where reversal in their relative intensity ratios occurs (over the same period however the relative intensities of the (200) and (006) lines are not reversed). These changes are accompanied by any further peak shifts in the overall diffraction pattern. The continuous development of this transition while the temperature is held at highest value has importance, it is suggested, in producting the required crystal structure for superconduction. (Fig.31 According to the result of TG analysis during heating /l/, it is suggested that these changes are concerned with the process of deoxidation. The value of c is increasing relative to oxygen content. In other words, the increasing of c is associated with a loss in oxygen after formation of l-2-3 phase. During cooling to 880°C, the material structure now remains relatively stable, as shown by the invariant relative of the (110) line. However, as the cooling process from 880°C to 6OOoC, the relative intensities of the (1101 and (103) lines change irregularly, suggesting that the crystal structure reverts to a generally stronger oxygen content character. This process lasts until the (103) and (013) lines splits at about 700°C when the relative intensity ratios are reversed due to the (103) peak now being superimposed on the (110) peak. At the same time the (020) and (200) lines aplit, and the (020) line combines with (006) so that the relative intensity ratios of the (006) and (ZOO) are also reversed. This is the tetragonal orthorhombic transition caused by reordering of the oxygen atoms and the associated vacncies, which has been reported in other studies.
xx
xx 00
o”
x
oz
’
xxx xx
.
a
.O
0.
:0
l
0
. X
,, 0 Y
X
x
x
2hrs
08
o
4hrs
0.
l
6hrs
I 150
I 200
T(K) Fig.3
Resistance vs. temperature curves of samples after 2,4 and 6 hrs. holding at 9500C respectively followed by furnace cooling.
SUPERCONDUCTIVITY Fig.3 shwos the resistance vs. temperature curves corresponding to samples which are maintained for 2,4 and 6 hrs. at 9500C followed by furnace cooling. Table I. shows the relationship between Tot and the period for which the sample is held at maximum temperature. In all cases the subsequent cooling rates are the same. It can be seen that Tot gradually rises, with extension of the high temperature holding period. It is interesting to note that, the resistance vs. temperature characteristic changes from semiconducting (Fig.3, 2 hrs.) to metallic (Fig.3, 6 hrs.). The longer of high temperature holding, the more completety of high temperature tetragonal structure (Fig.2). It seems that the organization of lattice parameter in high temperature holding stage, which is associated
Relationship
holding
TABLE I between To, and holding
time at 9500C (hr.) : 6
641
SUPERCONDUCTIVITYIN YBa2Cu307_x
Vol. 68, No. 7
To
time
+,,03
d 67.7 86.4 89.8
12; 2415
2. During the‘period when the maximum temperature is maintained and subsequent cooling process, lattice parameters change continuously as oxygen content is increasing or decreasing. 3. A longer period of high temperature maintenance, which concerns a more completet development of high temperature tetragonal structure, raises the superconduction.
with change of oxygen content/l/, is one of the important condition for superconduction. CONCLUSIONS 1. In the formation of YBa2Cu307_x superconducting phase, the transient cubic phase or tetragonal structure with a=b#c/3 is formed first.
REFERENCE 1. P.K. Gallagher, 2, 995,(1987)
et al., Mater. Res. Bull.