- Email: [email protected]
Superconductivity in the series La3Ba3CaxCu6+xOy, 0≤x≤3, an oxygen deficient tetragonal triple perovskite structure
Physica C 176 ( 1991 ) 451-456 North-Holland
Superconductivity in the series La3Ba3CaxCu6+xOy, 0_
The system La3Ba3CaxCu6+xOr was chosen for a study as a system of high temperature superconductors. The system retains its tetragonal triple layer perovskite structure over the range 0 < x < 3. The transition temperature (To) defined by the onset of the transition in the out of phase component of the AC susceptibility rises with x to 80 K for x = 1. Tc remains fixed at approximately 80 K through x= 3. The unit cell size is also strongly dependent on calcium content for x< 1. For x> 1 it is fairly constant. There is a strong correlation between the size of the unit cell and To. 1. Introduction
The B a - C u - L a - O system has proven to be very fertile in the field o f high t e m p e r a t u r e superconductivity. The c o m p o u n d La2_xBaxCuO4_6 has the K2NiF4 oxygen deficient double perovskite structure. The related c o m p o u n d La2_xSrxCuO4_6, studied by Nguyen et al. [ 1,2 ] has this same structure. Bednorz et al. [3,4] showed both c o m p o u n d s to be superconducting at T~~40 K. LaBa2Cu307_6 was found [ 5 ] to be superconducting with T~ = 92 K. The structure o f LaBaaCu3OT_6 is isomorphic to that o f YBa2Cu307_ 6 [6 ]; the latter was the first c o m p o u n d found [7] to have T~ above the boiling point o f nitrogen. The oxygen defect triple perovskite La3Ba3Cu6Ol4+6 was studied by Er Rahko et al. [ 8] for its transport and structural properties. It is very similar in its crystal structure to YBa2Cu307+6. One layer o f La replaces the Y and the rest o f the La mixes together with the Ba to fill the Ba sites. There is no particular Ba, La ordering on the Ba sites. Although La3Ba3Cu6Ol4+6 has a low r o o m t e m p e r a t u r e resistance, it was found to have the t e m p e r a t u r e dependent resistance o f a semiconductor. Furthermore, no superconductivity was found [ 9,10 ] down to liquid helium temperatures. The present study began with La3Ba3fu6Ol4+~.
Recent work by Raveau et al. [ 1 1 ] and Cava et al. [12] have emphasized the role played by intergrowths o f multiple oxygen deficient perovskite layers and "rock-salt" type layers i n the crystal structure o f high-To superconductors. Initially an a t t e m p t was m a d e to a d d a "rock-salt" layer by adding CaCuO2 to the starting compound. A single phase triple perovskite structure was clearly seen in the Xray powder pattern. In a d d i t i o n a sharp (half-width 1 K ) superconducting transition was seen in the AC susceptibility. The onset o f the transition is seen in the out o f phase c o m p o n e n t at 80.2 K. The minim u m in the out of phase voltage is more easily picked out and is 74.4 K. The size o f the unit cell and the X-ray pattern were p r o o f that the CaCuO2 was not adding a "rock-salt" layer to the unit cell but going into the oxygen deficient c o m p o u n d La3Ba3Cu6Ol4+6~ substitutionally. The above findings p r o m p t e d the a d d i t i o n o f m o r e CaCuO2 to form the c o m p o u n d s Ba3La3Ca2Cu8Oy and Ba3La3Ca3Cu9Oy. Both compounds had an onset Tc at approximately 80 K. Since the X-rays indicate a triple perovskite unit cell, the chemical formula should more a p p r o p r i a t e l y be written La(3_z)/2Ba(3_z)/2CazCu3Oy.This notation indicates the three Cu a t o m s in the unit cell. The notation used throughout the text indicates the direction taken by the line o f research. The relation between x and z is z = 3 x / ( 6 + x ) .
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
452
s. Engelsberg / Superconductivity in the serws La3Ba~Ca,Cu~,+ ,0~
No observable i m p u r i t y lines were found in the Xray spectra of these compounds. However, upon increasing the calcium content to La3Ba3Ca6Cu~20,. a large n u m b e r o f i m p u r i t y lines appeared. At this stage a review of the literature revealed that several groups [ 13-16 ] had studied the c o m p o u n d LaBaCaCu30,, which is equivalent to the case x = 3 in this work. They chose this c o m p o u n d by analogy with YBa2Cu3OT_a using the idea o f substituting Ca for Y and replacing half the Ba by La. The present study indicates the advantages o f considering the system La3Ba3Ca,Cu6+,O,.. This system retains its crystal structure as x is varied from 0 to 3, without the formation o f any i m p u r i t y phases. The unit cell d i m e n s i o n s and the transition t e m p e r a t u r e are observed to be strongly correlated, as they vary with x going from zero to one. This correlation is illustrated in fig. 1 which shows the transition t e m p e r a t u r e 7~ as a function o f t , the height o f the unit cell. The system has a very b r o a d region o f x ( 1 < x < 3) in which Tc and the cell size r e m a i n almost constant. This is the type o f system in which the correlations observed might help in clarifying the m e c h a n i s m s playing a role in the onset o f superconductivity.
2. E x p e r i m e n t a l
The starting c o m p o u n d s were high purity CaCO3, CuO, BaCO3 and La203. The first firing was done on Q
o
g
O
11 6 0
i
i
11.62
11.64
J
11 6 6 c
I
11 6 8
I
11 7 0
11
72
(I)
Fig. 1. Transition temperature, To, as a function of unit cell height, c.
a finely mixed powder at 900°C for 12 h under air. The cooling rate was set to 5 ¢ C / m i n down to 600°C. Furnace cooling then took the c o m p o u n d to about 200°C in several hours. The resulting black powder was r e m o v e d from the furnace, reground, and pressed into a disc shaped pellet under a pressure of approximately 1.7 × 108 N / m 2. The resulting pellets had a d i a m e t e r of about 12 m m and a thickness o f a p p r o x i m a t e l y 1.3 mm. The pellets were placed in an a l u m i n a boat which was placed inside a quartz firing tube. Oxygen was set to flow at a slow rate as the c o m p o u n d s were sintered for 24 h at 1000 :C. The cooling rate was the same as described earlier. The X-ray diffraction patterns were obtained after the pellets were tested for superconductivity and reground to a fine powder. The Cu Kct line was utilized in a Phillips X-ray Diffraction Analyser System 1840 to obtain the diffraction spectrum. All c o m p o u n d s synthesized showed the triple perovskite structure. For x small in La3Ba3CaxCu6+ ~O~ only the most intense X-ray lines, 0 0 1 , 0 0 5 , 2 0 5 stood out to distinguish this structure from the cubic perovskite as seen for example with SrTiO3. With increasing Ca content the 001 line d i s a p p e a r e d and a large n u m b e r o f other lines of the triple pcrovskite structure appeared. To within the accuracy o f the measurements, all lines retained their relative intensities for .r < 1. The x = 6 c o m p o u n d which had about the same cell size and T~ as the x = 3 c o m p o u n d showed several i m p u r i t y lines not seen in any o f the other compounds and some o f its lines had intensities greater than those c o m p o u n d s with x < 3. Therefore the data for x = 6 will not be included. Several reductions were m a d e on the x = 0 comp o u n d in an a t t e m p t to transform it to a superconductor. None were successful. Figure 2 shows diffractograms for three o f the c o m p o u n d s synthesized, x = 0 , 1 and 3. In addition, table 1 lists the relative intensities o f the lines appearing between 5 ~ < 2 0 < 6 5 ° for all the values o f x measured. The scattering angles, 20, given in table 1 are a p p r o x i m a t e values which vary with the cell size. They are p r o v i d e d as an aid in identifying the lines o f the pattern. The intensities observed for x = 0 were, to within the experimental uncertainty, equal to those observed by Er Rahko et al. [8].
S. Engelsberg / Superconductivity in the series La3Ba3CaxCu6+xOy
7" tad
I
I
I
t
I
1
60
50
40
50
20
I0
-~
20
Fig. 2. X-ray diffraction patterns for x = O , x = 1 and x = 3 , x=O corresponds to the pattern at the bottom and x = 3 is the pattern at the top of the figure.
The peak positions of the lines in the X-ray patterns were used to determine the average height (c) and width (a) of the unit cell as a function ofx. These results are shown in figs. 3 and 4. It is notable that
453
the major change in cell size takes place for x < 1. Thereafter the cell size remains fairly constant. The identity ( c ) = 3 ( a ) common to tetragonal triple perovskites held in almost all cases. Only x = 0 . 9 and x = 1.0 had slight deviations. Superconductivity of the compounds was detected from measurements of a mutual inductance with the sintered pellet inserted in the primary coil. The mutual inductance was measured using an EGG PAR model 5208 two phase lockin amplifier. The lockin frequency used was 10 kHz. To check for possible skin depth effects the frequency was lowered for one of the compounds to 100 Hz. The superconducting transition was essentially unchanged in both position and width. The signal characteristic of a superconducting transition is seen in both the in phase and out of phase voliages as the temperature was lowered through T¢. A typical example of the in and out of phase components is shown in figs. 5 and 6 for x = 1. The out of phase component acts somewhat like the temperature derivative of the in phase component and so is more sensitive to the onset of the transition. The out of phase component also provides a well defined position of the minimum and half width which characterize the transition. For x < 1, the half width is fairly constant at about 1 K. As x decreased from one towards zero, the width
Table 1 Relative intensities for X-ray lines of La3Ba3CaxCu6+xOy 20
indices
7.6 15.3
001 002
22.9
100 003
x=0
x=0.25
x=0.5
x=0.75
x=0.9
x= 1
x=2
x=3
1.6 1.2 t
11.3
9.8
1.8
1.8
2.6
9.5
7.8
8.3
7.9
8.6
7.9
2.0 100
2.7 100
3.7 100
2.9 100
3.9 100
J
27.7 32.7 33.75 36.4
102 103 111 112
38.6
014 005
2.0
2.0
4.1 100 4.1 1.9
4.3
4.9
6.3
7.0
6.3
8.5
7.8
7.3
13.2
11.8
11.8
11.7
12.1
12.2
11.7
9.8
27.4
27.9
30.5
30.3
30.1
26.9
31.3
35.7
2.1 5.8 2.2 39.9 4.2
2.3 5.6 1.9 38.7 4.2
2.3 4.8 2.4 39.3 4.8
2.9 4.3 2.3 37.0 4.5
2.6 5:8 1.7 39.4 4.4
3.8 5.1 2.4 37.5 3.8
100
100
1.8 t J
40.4
1t3
46.5
200 006
51.6 52.7 55.5 58.4 62.3
115 203 212 213 205
1 )
7.0
6.2
39.5 2.8
38.6 2.5
4 54
S. Engelsberg / Superconductivity in the series La3Ba3CaxCU~ +,0,.
.r O'a e,D
~g cO
J
g,
co to
I
j
o.o
I
06
I
12
18
i I
i b
I
_ _ J
I
24
65
75 8O 85 Temperature (K) In phaselockin voltage as a function oftemperature mr
3.0
x
Fig.
Fig. 3. Unit cell height, c, as a function ofx in La3Ba3CaxCu6+,O,,.
x=
5.
70
1.
C~
F
I 0 2>
CO
I
I
3 g 5--
v
I o cx/
CO
0
I
O0
o ~D I 0 0 o~ I
CO
0.0
06
I2
18
24
30
i
65
70
75 80 Temperature (K)
85
x
Fig. 4. Unit cell width, a, as a function ofx in La3Ba3Ca~Cu6+~O ,. o f the t r a n s i t i o n b r o a d e n s c o n t i n u o u s l y . T h e s e characteristics can be seen in fig. 7 which shows the o n s e t ( • ) o f the t r a n s i t i o n for each v a l u e o f x. T h e l o w e r s y m b o l ( o ) at each x i n d i c a t e s the t e m p e r a t u r e o f the m i n i m u m seen in the out o f phase voltage. At the lowest x, x = 0 . 2 5 , for w h i c h a t r a n s i t i o n was obs e r v e d the o n s e t t e m p e r a t u r e was T ~ = 5 . 7 K. T h e m i n i m u m in the out o f phase voltage d i d not a p p e a r at the lowest t e m p e r a t u r e s accessible in this experi m e n t , n a m e l y T ~ 4 . 2 K. T h e r e t r a c k i n g o f the signal seen at a p p r o x i m a t e l y 72 K in b o t h fig. 4 a n d fig. 5 is a result o f a slight w a r m i n g a n d t h e n a r e c o o l i n g o f the c o m p o u n d to much lower temperatures. This happens occasion-
Fig. 6. Out of phase lockin voltage as a function of temperature lbrx= 1. ally due to a slight, short d u r a t i o n p r e s s u r i z a t i o n o f the H e d e w a r f r o m the H e gas line. T h e small closed l o o p o b s e r v e d in the figures is a g o o d i n d i c a t i o n that the m e a s u r e m e n t s are taken close to t h e r m a l equilibrium. T h e change in b e h a v i o r f r o m x less than o n e to x greater t h a n one is rather striking. T h e r e does not s e e m to be an o b v i o u s reason f o r x = l to be the onset o f saturation. T h u s the reason for the c h a n g e o v e r f r o m a d y n a m i c region to a static one is not clear.
S. Engelsberg / Superconductivity in the series La3Ba~CaxCu6+xOy 0
.• o @
0
0
v~"J
0
O
0.0
J
L
i
i
06
12
IB
2.4
3.0
x
Fig. 7. Transition temperatures defined by the onset (•) and minimum ( o ) in the out of phase lockin voltage as a function of x in La3Ba3CaxCu6+xOy.
3. Discussion The La3Ba3CaxCu6+xO.v system clearly merits study. It provides a single crystallographic array which can be perturbed by changing an external parameter; namely x in Cax. Although only two characteristics of the crystal have been measured, cell size and To it is fairly obvious that dramatic changes should take place in other properties o f the system. In particular, transport properties offer an opportunity to measure lifetimes and density of states. Some implications can be drawn from fig. 1. Using fig. 1 for x < 1, the slope of the change in Tc with changing cell height is A T d A c = 1 000 K / A . If this rate of change is generally valid then hydrostatic pressure should be able to convert LaLsBal.sCu3Oy to a superconductor. Perhaps the replacement of La by Ca is important for reasons other than changing the cell size. On the outside change that the cell structure would remain the same with the size decreasing further, Mg was substituted for Ca in the x = 1 compound. Superconductivity was not observed for T > 30 K. The crystal structure was again a triple perovskite with c = 11.71 A and a = 3 . 9 1 A. The cell size and the X-ray pattern corresponded most closely to x = O . The Mg seems to have slipped into the cell without any change in the cell. The same line o f thinking led to the replacement of Ca by Sc in the x = 1 compound. The Sc replace-
455
merit did not superconduct down to T = 5 K. The Xrays showed a multiphase c o m p o u n d which probably contained the triple perovskite structure. The evidence for this is the appearance of the characteristic lines o f the triple perovskite among the others in the spectrum. The turning on of Tc might be due to one of several mechanisms caused by the addition of Ca. Perhaps the most obvious is a change in band structure and enhancement of the density o f states which is important to every model of superconductivity. Secondly the changes in cell size due to the smaller Ca replacing La would change the strength o f the forces whether they be super exchange or electron-phonon. A very interestmg clue to the problem is shown in the saturation of both Tc and the cell size at x = 1. There is a paucity o f other single phase materials which can have their ingredients altered over such a large range, 1 < x < 3 , and still show essentially no variation in To. One obvious conclusion is that the addition of Ca is turning on a mechanism which affects both cell size and T~ in a very closely related way. It should be interesting to see the consequences of this mechanism on other measurements.
Acknowledgements I would like to thank the Department of Chemistry at Mount Holyoke College for their initial and continued support. In particular, Prof. T. Gennett and Prof. E. Weaver have been especially helpful. Many people in the Physics Department at the University of Massachusetts have assisted me in this work. D. Sprague, Professor J. Madsen and Prof. R. Hallock were notable helpful on a number o f different occasions. I would also like to thank Prof. A. Quinton for reading the manuscript and offering useful suggestions.
References [ 1] N. Nguyen, J. Choisnet, M. Hervieu and B. Raveau, J. Solid State Chem. 39 ( 1981 ) 120. [2 ] N. Nguyen, F. Suder and B. Raveau, J. Phys. Chem. Solids 44 (1983) 389. [3] J.G. Bednorz and K.A. Miiller, Z. Phys. B 64 (1986) 189.
456
S. Engelsberg /Superconductivity in the series La3Ba3C'axCu~+.,O,
[4] J.G. Bednorz, M. Takashigi and K.A. Miiller, Mat. Res. bull. 22 (1987) 819. [ 5 ] T. Wada, N. Suzuki, A. Maeda, T. Yabe, K. Uchinokura, S. Uchida and S. Tanaka, Phys. Rev. B 39 ( 1989 ) 9126. [6] R. Cava, B, Batlogg, R. van Dover, D. Murphy, S. Sunshine, T. Siegrist, J. Remeika, E. Rietman, S. Zahurak and G. Espinosa, Phys. Rev. Lett. 58 (1987) 1676. [ 7 ] M. Wu, J. Ashburn, C. Torng, P. Hor, R. Meng, L. Gao, Z. Huang, Y. Wang and C. Chu, Phys. Rev. Left. 58 (1987) 908. [8 ] L. Er Rahko, C. Michel, J. Provost and B. Raveau, J. Solid State Chem. 37 ( 1981 ) 151. [9] C. Torardi, E. McCarron, M. Subramanian, A. Sleight and D. Cox, Mat Res. Bull. 22 (1987) 1563. [10]W. David, W.T. Harrison, R. Ibborson, M. Weller, J. Grasmeder and P. Lanchester, Nature 328 ( 1987 ).
[ 11 ] B. Raveau, C. Michel and M. Hervieu, J. Solid State Chem 88 (1990) 140. [ 12 ] R. Cava, B. Batlogg, R. van Dover. J. Krajeuski, J. Waszczak. R. Fleming, W. Peck Jr., L. Rupp Jr., P. Marsh, A. James and L. Schneemeyer, Nature 345 (1990) 602. [ 13 ] F. Keller-Berest, S. Megtert, G. Collin, P. Monod and M. Ribault, Physica C 161 (1989) 150. [ I4] W. Fu, H. Zandbergen. C.J. Van der Beek and L.J. DeJongh, Physica C 156 (1988) 133. [ 15 ] D.M. De Leuw, G. Mutsaers, H. Van Hal, H. Verwey, A. Carim and H. Smoorenburg, Physica C 156 ( 1988 ) 126. [ 16] J.L. Peng, P. Klavins, R. Shelton, H. Radousky. P. Hahn, L. Bernardez and M. Costantino, Phys. Rev. B 39 (1989) 9074.
Superconductivity in the series La3Ba3CaxCu6+xOy, 0_
The system La3Ba3CaxCu6+xOr was chosen for a study as a system of high temperature superconductors. The system retains its tetragonal triple layer perovskite structure over the range 0 < x < 3. The transition temperature (To) defined by the onset of the transition in the out of phase component of the AC susceptibility rises with x to 80 K for x = 1. Tc remains fixed at approximately 80 K through x= 3. The unit cell size is also strongly dependent on calcium content for x< 1. For x> 1 it is fairly constant. There is a strong correlation between the size of the unit cell and To. 1. Introduction
The B a - C u - L a - O system has proven to be very fertile in the field o f high t e m p e r a t u r e superconductivity. The c o m p o u n d La2_xBaxCuO4_6 has the K2NiF4 oxygen deficient double perovskite structure. The related c o m p o u n d La2_xSrxCuO4_6, studied by Nguyen et al. [ 1,2 ] has this same structure. Bednorz et al. [3,4] showed both c o m p o u n d s to be superconducting at T~~40 K. LaBa2Cu307_6 was found [ 5 ] to be superconducting with T~ = 92 K. The structure o f LaBaaCu3OT_6 is isomorphic to that o f YBa2Cu307_ 6 [6 ]; the latter was the first c o m p o u n d found [7] to have T~ above the boiling point o f nitrogen. The oxygen defect triple perovskite La3Ba3Cu6Ol4+6 was studied by Er Rahko et al. [ 8] for its transport and structural properties. It is very similar in its crystal structure to YBa2Cu307+6. One layer o f La replaces the Y and the rest o f the La mixes together with the Ba to fill the Ba sites. There is no particular Ba, La ordering on the Ba sites. Although La3Ba3Cu6Ol4+6 has a low r o o m t e m p e r a t u r e resistance, it was found to have the t e m p e r a t u r e dependent resistance o f a semiconductor. Furthermore, no superconductivity was found [ 9,10 ] down to liquid helium temperatures. The present study began with La3Ba3fu6Ol4+~.
Recent work by Raveau et al. [ 1 1 ] and Cava et al. [12] have emphasized the role played by intergrowths o f multiple oxygen deficient perovskite layers and "rock-salt" type layers i n the crystal structure o f high-To superconductors. Initially an a t t e m p t was m a d e to a d d a "rock-salt" layer by adding CaCuO2 to the starting compound. A single phase triple perovskite structure was clearly seen in the Xray powder pattern. In a d d i t i o n a sharp (half-width 1 K ) superconducting transition was seen in the AC susceptibility. The onset o f the transition is seen in the out o f phase c o m p o n e n t at 80.2 K. The minim u m in the out of phase voltage is more easily picked out and is 74.4 K. The size o f the unit cell and the X-ray pattern were p r o o f that the CaCuO2 was not adding a "rock-salt" layer to the unit cell but going into the oxygen deficient c o m p o u n d La3Ba3Cu6Ol4+6~ substitutionally. The above findings p r o m p t e d the a d d i t i o n o f m o r e CaCuO2 to form the c o m p o u n d s Ba3La3Ca2Cu8Oy and Ba3La3Ca3Cu9Oy. Both compounds had an onset Tc at approximately 80 K. Since the X-rays indicate a triple perovskite unit cell, the chemical formula should more a p p r o p r i a t e l y be written La(3_z)/2Ba(3_z)/2CazCu3Oy.This notation indicates the three Cu a t o m s in the unit cell. The notation used throughout the text indicates the direction taken by the line o f research. The relation between x and z is z = 3 x / ( 6 + x ) .
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
452
s. Engelsberg / Superconductivity in the serws La3Ba~Ca,Cu~,+ ,0~
No observable i m p u r i t y lines were found in the Xray spectra of these compounds. However, upon increasing the calcium content to La3Ba3Ca6Cu~20,. a large n u m b e r o f i m p u r i t y lines appeared. At this stage a review of the literature revealed that several groups [ 13-16 ] had studied the c o m p o u n d LaBaCaCu30,, which is equivalent to the case x = 3 in this work. They chose this c o m p o u n d by analogy with YBa2Cu3OT_a using the idea o f substituting Ca for Y and replacing half the Ba by La. The present study indicates the advantages o f considering the system La3Ba3Ca,Cu6+,O,.. This system retains its crystal structure as x is varied from 0 to 3, without the formation o f any i m p u r i t y phases. The unit cell d i m e n s i o n s and the transition t e m p e r a t u r e are observed to be strongly correlated, as they vary with x going from zero to one. This correlation is illustrated in fig. 1 which shows the transition t e m p e r a t u r e 7~ as a function o f t , the height o f the unit cell. The system has a very b r o a d region o f x ( 1 < x < 3) in which Tc and the cell size r e m a i n almost constant. This is the type o f system in which the correlations observed might help in clarifying the m e c h a n i s m s playing a role in the onset o f superconductivity.
2. E x p e r i m e n t a l
The starting c o m p o u n d s were high purity CaCO3, CuO, BaCO3 and La203. The first firing was done on Q
o
g
O
11 6 0
i
i
11.62
11.64
J
11 6 6 c
I
11 6 8
I
11 7 0
11
72
(I)
Fig. 1. Transition temperature, To, as a function of unit cell height, c.
a finely mixed powder at 900°C for 12 h under air. The cooling rate was set to 5 ¢ C / m i n down to 600°C. Furnace cooling then took the c o m p o u n d to about 200°C in several hours. The resulting black powder was r e m o v e d from the furnace, reground, and pressed into a disc shaped pellet under a pressure of approximately 1.7 × 108 N / m 2. The resulting pellets had a d i a m e t e r of about 12 m m and a thickness o f a p p r o x i m a t e l y 1.3 mm. The pellets were placed in an a l u m i n a boat which was placed inside a quartz firing tube. Oxygen was set to flow at a slow rate as the c o m p o u n d s were sintered for 24 h at 1000 :C. The cooling rate was the same as described earlier. The X-ray diffraction patterns were obtained after the pellets were tested for superconductivity and reground to a fine powder. The Cu Kct line was utilized in a Phillips X-ray Diffraction Analyser System 1840 to obtain the diffraction spectrum. All c o m p o u n d s synthesized showed the triple perovskite structure. For x small in La3Ba3CaxCu6+ ~O~ only the most intense X-ray lines, 0 0 1 , 0 0 5 , 2 0 5 stood out to distinguish this structure from the cubic perovskite as seen for example with SrTiO3. With increasing Ca content the 001 line d i s a p p e a r e d and a large n u m b e r o f other lines of the triple pcrovskite structure appeared. To within the accuracy o f the measurements, all lines retained their relative intensities for .r < 1. The x = 6 c o m p o u n d which had about the same cell size and T~ as the x = 3 c o m p o u n d showed several i m p u r i t y lines not seen in any o f the other compounds and some o f its lines had intensities greater than those c o m p o u n d s with x < 3. Therefore the data for x = 6 will not be included. Several reductions were m a d e on the x = 0 comp o u n d in an a t t e m p t to transform it to a superconductor. None were successful. Figure 2 shows diffractograms for three o f the c o m p o u n d s synthesized, x = 0 , 1 and 3. In addition, table 1 lists the relative intensities o f the lines appearing between 5 ~ < 2 0 < 6 5 ° for all the values o f x measured. The scattering angles, 20, given in table 1 are a p p r o x i m a t e values which vary with the cell size. They are p r o v i d e d as an aid in identifying the lines o f the pattern. The intensities observed for x = 0 were, to within the experimental uncertainty, equal to those observed by Er Rahko et al. [8].
S. Engelsberg / Superconductivity in the series La3Ba3CaxCu6+xOy
7" tad
I
I
I
t
I
1
60
50
40
50
20
I0
-~
20
Fig. 2. X-ray diffraction patterns for x = O , x = 1 and x = 3 , x=O corresponds to the pattern at the bottom and x = 3 is the pattern at the top of the figure.
The peak positions of the lines in the X-ray patterns were used to determine the average height (c) and width (a) of the unit cell as a function ofx. These results are shown in figs. 3 and 4. It is notable that
453
the major change in cell size takes place for x < 1. Thereafter the cell size remains fairly constant. The identity ( c ) = 3 ( a ) common to tetragonal triple perovskites held in almost all cases. Only x = 0 . 9 and x = 1.0 had slight deviations. Superconductivity of the compounds was detected from measurements of a mutual inductance with the sintered pellet inserted in the primary coil. The mutual inductance was measured using an EGG PAR model 5208 two phase lockin amplifier. The lockin frequency used was 10 kHz. To check for possible skin depth effects the frequency was lowered for one of the compounds to 100 Hz. The superconducting transition was essentially unchanged in both position and width. The signal characteristic of a superconducting transition is seen in both the in phase and out of phase voliages as the temperature was lowered through T¢. A typical example of the in and out of phase components is shown in figs. 5 and 6 for x = 1. The out of phase component acts somewhat like the temperature derivative of the in phase component and so is more sensitive to the onset of the transition. The out of phase component also provides a well defined position of the minimum and half width which characterize the transition. For x < 1, the half width is fairly constant at about 1 K. As x decreased from one towards zero, the width
Table 1 Relative intensities for X-ray lines of La3Ba3CaxCu6+xOy 20
indices
7.6 15.3
001 002
22.9
100 003
x=0
x=0.25
x=0.5
x=0.75
x=0.9
x= 1
x=2
x=3
1.6 1.2 t
11.3
9.8
1.8
1.8
2.6
9.5
7.8
8.3
7.9
8.6
7.9
2.0 100
2.7 100
3.7 100
2.9 100
3.9 100
J
27.7 32.7 33.75 36.4
102 103 111 112
38.6
014 005
2.0
2.0
4.1 100 4.1 1.9
4.3
4.9
6.3
7.0
6.3
8.5
7.8
7.3
13.2
11.8
11.8
11.7
12.1
12.2
11.7
9.8
27.4
27.9
30.5
30.3
30.1
26.9
31.3
35.7
2.1 5.8 2.2 39.9 4.2
2.3 5.6 1.9 38.7 4.2
2.3 4.8 2.4 39.3 4.8
2.9 4.3 2.3 37.0 4.5
2.6 5:8 1.7 39.4 4.4
3.8 5.1 2.4 37.5 3.8
100
100
1.8 t J
40.4
1t3
46.5
200 006
51.6 52.7 55.5 58.4 62.3
115 203 212 213 205
1 )
7.0
6.2
39.5 2.8
38.6 2.5
4 54
S. Engelsberg / Superconductivity in the series La3Ba3CaxCU~ +,0,.
.r O'a e,D
~g cO
J
g,
co to
I
j
o.o
I
06
I
12
18
i I
i b
I
_ _ J
I
24
65
75 8O 85 Temperature (K) In phaselockin voltage as a function oftemperature mr
3.0
x
Fig.
Fig. 3. Unit cell height, c, as a function ofx in La3Ba3CaxCu6+,O,,.
x=
5.
70
1.
C~
F
I 0 2>
CO
I
I
3 g 5--
v
I o cx/
CO
0
I
O0
o ~D I 0 0 o~ I
CO
0.0
06
I2
18
24
30
i
65
70
75 80 Temperature (K)
85
x
Fig. 4. Unit cell width, a, as a function ofx in La3Ba3Ca~Cu6+~O ,. o f the t r a n s i t i o n b r o a d e n s c o n t i n u o u s l y . T h e s e characteristics can be seen in fig. 7 which shows the o n s e t ( • ) o f the t r a n s i t i o n for each v a l u e o f x. T h e l o w e r s y m b o l ( o ) at each x i n d i c a t e s the t e m p e r a t u r e o f the m i n i m u m seen in the out o f phase voltage. At the lowest x, x = 0 . 2 5 , for w h i c h a t r a n s i t i o n was obs e r v e d the o n s e t t e m p e r a t u r e was T ~ = 5 . 7 K. T h e m i n i m u m in the out o f phase voltage d i d not a p p e a r at the lowest t e m p e r a t u r e s accessible in this experi m e n t , n a m e l y T ~ 4 . 2 K. T h e r e t r a c k i n g o f the signal seen at a p p r o x i m a t e l y 72 K in b o t h fig. 4 a n d fig. 5 is a result o f a slight w a r m i n g a n d t h e n a r e c o o l i n g o f the c o m p o u n d to much lower temperatures. This happens occasion-
Fig. 6. Out of phase lockin voltage as a function of temperature lbrx= 1. ally due to a slight, short d u r a t i o n p r e s s u r i z a t i o n o f the H e d e w a r f r o m the H e gas line. T h e small closed l o o p o b s e r v e d in the figures is a g o o d i n d i c a t i o n that the m e a s u r e m e n t s are taken close to t h e r m a l equilibrium. T h e change in b e h a v i o r f r o m x less than o n e to x greater t h a n one is rather striking. T h e r e does not s e e m to be an o b v i o u s reason f o r x = l to be the onset o f saturation. T h u s the reason for the c h a n g e o v e r f r o m a d y n a m i c region to a static one is not clear.
S. Engelsberg / Superconductivity in the series La3Ba~CaxCu6+xOy 0
.• o @
0
0
v~"J
0
O
0.0
J
L
i
i
06
12
IB
2.4
3.0
x
Fig. 7. Transition temperatures defined by the onset (•) and minimum ( o ) in the out of phase lockin voltage as a function of x in La3Ba3CaxCu6+xOy.
3. Discussion The La3Ba3CaxCu6+xO.v system clearly merits study. It provides a single crystallographic array which can be perturbed by changing an external parameter; namely x in Cax. Although only two characteristics of the crystal have been measured, cell size and To it is fairly obvious that dramatic changes should take place in other properties o f the system. In particular, transport properties offer an opportunity to measure lifetimes and density of states. Some implications can be drawn from fig. 1. Using fig. 1 for x < 1, the slope of the change in Tc with changing cell height is A T d A c = 1 000 K / A . If this rate of change is generally valid then hydrostatic pressure should be able to convert LaLsBal.sCu3Oy to a superconductor. Perhaps the replacement of La by Ca is important for reasons other than changing the cell size. On the outside change that the cell structure would remain the same with the size decreasing further, Mg was substituted for Ca in the x = 1 compound. Superconductivity was not observed for T > 30 K. The crystal structure was again a triple perovskite with c = 11.71 A and a = 3 . 9 1 A. The cell size and the X-ray pattern corresponded most closely to x = O . The Mg seems to have slipped into the cell without any change in the cell. The same line o f thinking led to the replacement of Ca by Sc in the x = 1 compound. The Sc replace-
455
merit did not superconduct down to T = 5 K. The Xrays showed a multiphase c o m p o u n d which probably contained the triple perovskite structure. The evidence for this is the appearance of the characteristic lines o f the triple perovskite among the others in the spectrum. The turning on of Tc might be due to one of several mechanisms caused by the addition of Ca. Perhaps the most obvious is a change in band structure and enhancement of the density o f states which is important to every model of superconductivity. Secondly the changes in cell size due to the smaller Ca replacing La would change the strength o f the forces whether they be super exchange or electron-phonon. A very interestmg clue to the problem is shown in the saturation of both Tc and the cell size at x = 1. There is a paucity o f other single phase materials which can have their ingredients altered over such a large range, 1 < x < 3 , and still show essentially no variation in To. One obvious conclusion is that the addition of Ca is turning on a mechanism which affects both cell size and T~ in a very closely related way. It should be interesting to see the consequences of this mechanism on other measurements.
Acknowledgements I would like to thank the Department of Chemistry at Mount Holyoke College for their initial and continued support. In particular, Prof. T. Gennett and Prof. E. Weaver have been especially helpful. Many people in the Physics Department at the University of Massachusetts have assisted me in this work. D. Sprague, Professor J. Madsen and Prof. R. Hallock were notable helpful on a number o f different occasions. I would also like to thank Prof. A. Quinton for reading the manuscript and offering useful suggestions.
References [ 1] N. Nguyen, J. Choisnet, M. Hervieu and B. Raveau, J. Solid State Chem. 39 ( 1981 ) 120. [2 ] N. Nguyen, F. Suder and B. Raveau, J. Phys. Chem. Solids 44 (1983) 389. [3] J.G. Bednorz and K.A. Miiller, Z. Phys. B 64 (1986) 189.
456
S. Engelsberg /Superconductivity in the series La3Ba3C'axCu~+.,O,
[4] J.G. Bednorz, M. Takashigi and K.A. Miiller, Mat. Res. bull. 22 (1987) 819. [ 5 ] T. Wada, N. Suzuki, A. Maeda, T. Yabe, K. Uchinokura, S. Uchida and S. Tanaka, Phys. Rev. B 39 ( 1989 ) 9126. [6] R. Cava, B, Batlogg, R. van Dover, D. Murphy, S. Sunshine, T. Siegrist, J. Remeika, E. Rietman, S. Zahurak and G. Espinosa, Phys. Rev. Lett. 58 (1987) 1676. [ 7 ] M. Wu, J. Ashburn, C. Torng, P. Hor, R. Meng, L. Gao, Z. Huang, Y. Wang and C. Chu, Phys. Rev. Left. 58 (1987) 908. [8 ] L. Er Rahko, C. Michel, J. Provost and B. Raveau, J. Solid State Chem. 37 ( 1981 ) 151. [9] C. Torardi, E. McCarron, M. Subramanian, A. Sleight and D. Cox, Mat Res. Bull. 22 (1987) 1563. [10]W. David, W.T. Harrison, R. Ibborson, M. Weller, J. Grasmeder and P. Lanchester, Nature 328 ( 1987 ).
[ 11 ] B. Raveau, C. Michel and M. Hervieu, J. Solid State Chem 88 (1990) 140. [ 12 ] R. Cava, B. Batlogg, R. van Dover. J. Krajeuski, J. Waszczak. R. Fleming, W. Peck Jr., L. Rupp Jr., P. Marsh, A. James and L. Schneemeyer, Nature 345 (1990) 602. [ 13 ] F. Keller-Berest, S. Megtert, G. Collin, P. Monod and M. Ribault, Physica C 161 (1989) 150. [ I4] W. Fu, H. Zandbergen. C.J. Van der Beek and L.J. DeJongh, Physica C 156 (1988) 133. [ 15 ] D.M. De Leuw, G. Mutsaers, H. Van Hal, H. Verwey, A. Carim and H. Smoorenburg, Physica C 156 ( 1988 ) 126. [ 16] J.L. Peng, P. Klavins, R. Shelton, H. Radousky. P. Hahn, L. Bernardez and M. Costantino, Phys. Rev. B 39 (1989) 9074.