- Email: [email protected]
Synthesis and superconducting properties of the infinite-layer compounds Sr1−xLnxCuO2(Ln=La, Nd, Sm, Gd)
PHYSICA
Physica C 210 (1993) 367-372 North-Holland
Synthesis and superconducting properties of the infinite-layer compounds Srl _xLnxCuO2(Ln = La, Nd, Sm, Gd) N. I k e d a , Z. H i r o i 1, M. A z u m a , M. T a k a n o a n d Y. B a n d o Institutefor Chemical Research, Kyoto University, Uji, Kyoto-fu 611, Japan Y. T a k e d a Department of Chemistry, Faculty of Engineering, Mie University, Kamihamacho, Tsu, Mie-ken 514, Japan Received 15 March 1993
Infinite-layer-type superconductors Sr~_xLnxCuO2a r e synthesized under high pressure of 3 GPa for Ln = Sm, Gd as well as for Ln = La, Nd. Their chemical and superconducting properties are systematically studied as functions of doping concentration and the kind oflanthanide ion. As a result, it is demonstrated that the variation of these properties with doping concentration is similar for all the examined Ln3+ions. The solubility limit lies at x ~ 0.10. CuO2 sheets are expanded with increasing x, while their spacing decreases. The T¢onset determined by magnetic measurements remains constant for any doping concentration; only the Meissner fraction increases with increasing x.
1. Introduction
All the known cupric oxide superconductors have 2D CuO2 sheets d o p e d with carriers as a result o f various chemical modifications o f the parent materials. It has been well established that the carriers in the CuO2 sheets, if the concentration is appropriate, Bose-condense at relatively high critical temperatures (To). The so-called infinite-layer ( I L ) (or alllayer) structure consists o f only CuO2 sheets and alkaline-earth a t o m ( A ) sheets stacked alternately along the c-axis, and this structure has been considered as the parent structure o f all the cupric oxide superconductors [ 1 ]. At a m b i e n t pressure this structure can be stabilized only for a narrow composition range a r o u n d A=Cao.gSro.~ at temperatures just below the melting point, if a s t a n d a r d solid-state reaction m e t h o d is applied [2]. On the other hand, Takano et al. found that application o f high pressure widened the c o m p o s i t i o n range towards large A ions up tO A=Sr2/3Bal/3 [3]. They also reported that A~_xCuO2_z including both alkaline-earth and oxygen deficiencies b e c a m e a p-type superconductor
with 110 K Tc [4]. It has been considered that hole carriers arise from the A-deficiency if the oxygen vacancies, introduced for the sake o f charge balance with the A-deficiency, are filled by application of high oxygen pressure treatment. A n o t h e r type o f IL c o m p o u n d , Srl_xLnxCuO2 (Ln = La [ 5 ], N d [ 6 ] ) has also been prepared under high pressure in a strongly reducing atmosphere. They are considered to become n-type superconductors, because the substitution o f trivalent lanthanide ions for divalent strontium ions reduces the CuO2 sheets. In this study, we tried to extend the composition range toward small Ln ions in an effort to understand the solid state chemistry o f the IL system and to find how the superconducting properties vary with the c o m p o s i t i o n and the resulting structural change. C o m p o u n d s with L n = S m and G d have been prepared in a d d i t i o n to those with Ln = La and Nd, and their chemical and superconducting properties have been studied systematically as functions o f doping concentration and the kind o f lanthanide ion.
To whom correspondence should be addressed. 0921-4534/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
368
N. lkeda et al. / lnfinite-layer compounds,Sr~ ~Ln~('u():
2. Experimental High-pressure synthesis was performed with a classical cubic-anvil-type apparatus with a cell assemblage illustrated in fig. 1. The starting material was an intimate mixture of Ln203, CuO and SrCuO2. SrCuO2 (low-pressure phase) had been prepared by an ordinary solid-state reaction at 1073 K for 100 h with several intermittent grindings. It was put into the high-pressure cell in two ways, directly into the boron nitride sleeve or into a gold capsule (3 mm ~ X 3mm) with Ti powder added in the bottom of the capsule which worked as an oxygen getter. All the previous studies on the Sr~_xLnxCuO2 ( L n = L a , Nd) systems had adopted the former method. These high-pressure cells were compressed almost isostatically up to 3 GPa and then heat-treated with a graphite-sleeve heater for half an hour at 1173 K. The sample temperature was monitored with a P t / P t - I 3% Rh thermocouple placed inside the boron nitride cell. The product was finally quenched to room temperature before releasing the pressure. Thus obtained polycrystalline samples weighed about 50 mg and were characterized by powder X-ray diffraction ( X R D ) measurements using Cu Kct radia-
Upper Bucket
~
tion, electron diffraction (ED) and high-resolution electron microscopy (HREM). The Meissner signal was measured with a SQUID magnetometer (Quantum Design, MPMS2) on cooling in an external field of 10 Oe. Electrical resistivity was measured by a standard four-terminal method. The copper valence was determined by an iodometric titration method.
3. Results and discussion
3.1. Srl_ ~Nd,:CuO: We have examined first the two preparation processes for the Nd-substituted system. Figure 2 compares two typical powder XRD patterns of Sro.gNdo.lCuO2 prepared without a gold capsule (a) and with a gold capsule (b). Without the gold capsule, the pellet was covered with a reddish skin mainly composed of Cu20. This implies that at the surface layer CuO in the starting material is reduced to become Cu20 and, at the same time, Sr- and Nd-components are lost, probably into the boron nitride sleeve. There seems to exist a composition gradient even inside the pellet, as found from the XRD pattern of fig. 2 (a) recorded using a fragment selectively obtained from the core. The IL-phase is major, but another new phase (the "3c"-phase as we shall
101
Anvil
i
Lower Bucket
Pyrophvllite Block
Thermocouple
(a)
. 3c phase 110
* Cu20
g n
200
loo ~'i[~ 11~I 002 112 220 k___ _J/~U_zt~_t_x_Sr[ _ ~ _ ~
e
•
>"
(b)
12.5mm ell
er
Starting Powder Fig. 1. High-pressure cell assembly.
10
20
30
40
50
60
70
2o (deg.) Fig. 2. X R D patterns of Sro.gNdo.iCuO2 prepared under highpressure without (a) and with ( b ) a gold capsule.
N. Ikeda et al. / Infinite-layer compounds Sr ~_~Ln~CuOe
call it), which has a tetragonal unit cell of a = 3.9 ~, and c = 10.8 A, and a small amount of Cu20 coexist. The "3c"-phase crystallizes in a triple IL structure whose intersheet distance has such a sequence as 3.7 A/3.4 A/3.7 ,~ and has a copper-rich composition A / C u < 1. Details will be reported elsewhere. The coexistence of the "3c"-phase makes the final Nd content of the IL phase ambiguous. On the other hand, the use of a gold capsule with metal Ti powder as an oxygen getter proved to yield more homogeneous samples. As seen from the XRD pattern of fig. 2 (b), the "3c"-phase did not appear, because the reaction with BN was suppressed. All the following results have been obtained from encapsulated samples. Figure 3 shows the variation of lattice parameters with x for Sr~_~Nd~CuO2. As x increases, a increases, while c decreases. Both a and c vary almost linearly up to x = 0 . 1 0 and remain constant for x > 0.10. In accordance with this an impurity phase of Srz_xNd~ +~Cu2Oy [ 7 ] appears in XRD patterns. Thus we conclude x = 0 . 1 0 is the solubility limit of Nd in the IL-phase. ,-"
3.950.
.,,," •
369
Microscopic observations by means of ED and HREM have revealed that the sample has the ideal IL-type structure without any inherent lattice defects. Shown in fig. 4 are typical ED patterns taken with the incident electron beam along the a-axis (a) and the c-axis (b). Neither superlattice diffraction spots nor streaks are seen. This presents a striking contrast to the case of the p-type superconductor A~_xCuO2_z containing specific defect-layers [8]. Sr~_xLnxCuO2 is in the strict sense an infinite-layertype superconductor. In addition, no evidence indicating a phase separation has been detected for 0 < x < 0 . I0 in the electron microscopic order. The temperature dependence of magnetization as a function of doping level measured on cooling in an external field of 10 Oe is shown in fig. 5. Surprisingly, the T¢o,~ remains almost constant at ~ 44 K against the variation of Nd content. Only the magnitude of the diamagnetic signal increases up to x=0.10, the decrease above which is due to the impurity segregation. A linear variation of the nominal copper valence
a
• I"1
.~.
3.940-
3.930-
a
o
3.920:
3'420,] ~
O0
• Cu
3.410 3.400 3.390-
• this work
. 3.380-
r~ Smith e t a l .
0.00
~,,
",, •
•
'% ,, o
0.05
0.10
[3
0.15
x in Sq.xNdxCuO2 Fig. 3. Variation of lattice parameters with dopant concentration x for Sr~ _xNdxCuOz. The previous data by Smith et al. are also plotted with open squares.
Fig. 4. ED patterns of the [100] (a) and the [001] (b) zoneaxes. Neither superlattice diffractions nor streaks are seen.
370
N. lkeda et al. /Infinite-layer compounds Sr ~_ ,.Ln,CuO:
valence to ( 2 - . v ) , suggesting that the present compound is a n-type superconductor.
(10 .3)
0
..... I
:
I
3
X = 0.05 0"1
-1.0 m
2.0
0.08
~,
~
~
~ *o° °
(10 6)
m ~ o***~*o°°° ~ $ o~ oo
2.0
o ° oo°
.~e.~; ~
mmmm ®® ~ 1 . 0
oo° ° oo 0.12 oo ooOOo°° 0.10 []
-3.0
3.2. Sr¢_ ,Sm,CuOz and ?;r~ ,Gd,.CuO:
%~o ~°o
m
E I
B
®
o m
-4.0
,
0
10
20
[]
I 0.0
40
30
50
T(K) Fig. 5. Temperature dependences of magnetic susceptibilitb M / H , measured in an applied field of 10 Oe on cooling for SQ_xNdxCuO2 with varying x. The T~ onset is always 44 K, independent ofx.
We have found that smaller lanthanide elements, Sm and Gd, substitute SrCuO2 crystallizing in the Ik structure. Figure 7 shows the powder XRD patterns for L n = S m (a) and Gd (b) synthesized using gold capsules. Their composition dependences of the lattice constants are similar to that of the Nd-substituted system. The magnetic and electrical behaviors of these systems also resemble those of the Nd-substituted system. As shown in fig. 8 for Ln = Sm, the 7",...... , remains at 44 K independently of .r, while the diamagnetic signal increases with increasing x. Both the resistivity curves of fig. 9 for L n = S m and Gd show a semiconducting temperature dependence and begin to drop at ~ 40 K, becoming nearly zero at 35 K (Sm) and 20 K (Gd).
Srl_xNdxCUOy 2.00,
L
1.95
3.3. Srl_xLn.,.Cu02 (Ln=La, Nd, Sm, Gd)
The evolution of structural and physical properties with the kind of doping element will be briefly discussed below. Since the lanthanide ions show a systematic variation in both ionic size and the mag-
',,... • "'-.,
c
I(D >
$ &
8
1.90
"-,,,. "..... 1.85
y=2
101
(a)
Sro92Sin0o80uO2
1.80
0.00
0,05
0.10
0.15
X
Fig. 6. Variation of the formal copper valence with x for Srl xNd~CuO2 . The dotted line assumes oxygen content of two.
%-
110 ~
'~ >"
100
II! L ~
200 111
211
~
~, I I
210°°2 J/ 112 220
# (b)
with Nd content has been confirmed by iodometric titration, as shown in fig. 6. The dotted line in the figure was drawn assuming the formal valences of N d 3+ and 0 2- and also oxygen content of two per chemical formula. The data points fall near this line up to x = 0 . 1 0 , which implies that oxygen atoms within the CuO2 sheets were not lost even in the reducing condition during preparation. Therefore, the Nd substitution works effectively to reduce the Cu
Sro 92Gdo.08CuO 2
iE
20
310
20
I
50
I
60
i
70
28 (deg.) Fig. 7. XRD patterns of Sro.9~Lno.osCuO2 for L n = S m (a) and Gd (b) prepared under high pressure in a gold capsule.
N. lkeda et al. I Infinite-layer compounds SQ_ ~LnxCuOe
Sr0.92Ln0.08CuO2
Sq_xSmxCu02 eeooooooo~oOO
o
+++
•
÷++
x = 0.05
371
•
**+*
:
+ ÷~+me
-1.0
E
I1) +++++
O
,~ "r
o o
+t3mo
++~+m •
E
~.
-1.0 "
+++++
-2.0
~lilO
0.08
..
v oom
212
.
,
..-'"
•
,
10
°°
ee
•
++
...
o
•
° o°
o
oe °
° oo°
.
,
20
30
.
,
40
. . . .
T(K)
La
Nd
+ Sm
.
50
e •
o
. . . . . . ~,~o*o*o eoeeeoe"
...... .,.'"
-4.0
o°.... ~++ +.+
++++ Oo
-2.0 ,•
el
oO
o
-3.0 0.10
~0~1
•
• Gd -3.0
'
'
0
'
,
10
'
'
,
,
•
,
,
20
r
30
•
,
,
r
40
,
,
,
50
T (K)
Fig. 8. Temperature dependences of magnetic susceptibility for Sr| _~Sm~CuO2 measured in an external field of 10 Oe on cooling.
Fig. 10. Comparison of Meissner curves for Sro.92Lno.08CuO2 with Ln= La, Nd, Sm and Gd.
XX
3_
\
\ \
co 2 t-,..
.
(b) Sro.gGdo.lCuOa
rr t-v rr"
1
Gd, roughly proportionally to the magnitude of the magnetic moments of these free lanthanide ions. The slight upturn at low temperature seen in the Gd-curve may be due to the relatively large Curie component. The results indicate that the pair-breaking effect due to a magnetic impurity situated very near the CuO2 sheets is negligible, as has already been observed in LnBa2CusO7_y.
(a) Sro.9Smo.lCuO2
0
J 0
50
1~00
1+50
200 ~
250
T(K) Fig. 9. Temperature dependences of normalized electrical resistivity for Sro.gSmo.lCuO 2 (a) and Sro.9Gdo.|CuO 2 (b).
nitude of magnetic moment, the Ln-substituted system is suitable to study how the lattice size and the impurity magnetic moment affect superconductivity. The structural simplicity is another important factor to be considered. Figure 10 compares the Meissner curves of Sro.9eLno.osCuO2 with Ln = La, Nd, Sm and Gd. Both their diamagnetic signals at 5 K and T¢on~t's are similar to each other, while susceptibility in the normal state increases in the order of L n = La, Sm, Nd and
Figure 11 plots the lattice parameters of Sro.92Lno.oaCuO2 against the ionic size of the dopants. Both the a- and c-axes increase as the ionic radius of Ln 3+ increases. The variation of the c-axis is much larger than that of the a-axis, reflecting the two-dimensionality of the IL structure. In spite of these changes, Tc remains almost constant at ~ 44 K. Such an independence between Tc and the lattice size has also been reported for Al_xCuO2_z [8]. Therefore, we conclude that neither the intra-sheet nor the inter-sheet Cu-Cu distance is essential in determining Tc within the range studied. In contrast, it has been reported that, in the case of n-type superconductors Ln2_xMxCuO4 [9] (Ln=Pr, Nd, Sm, Eu; M = C e , Th), Tc varies with Ln as 24 K ( L n = N d ) , 19 K ( L n = S m ) and 13 K ( L n = E u ) for M = C e . Besides, T~ also varies with M as 19 K ( L n = N d ) and 27 K ( L n = S m ) , for M = T h . This variation in Tc has generally been discussed in relation to the variation of lattice parameters with
372
N. lkeda et al. /Infinite-layer compounds Sr ~_ ~Ln ~CuO:
v
3.950 1 3.945
among n-type superconductors, but is much lower than the highest value of p-type superconductors ( ~ 130 K). It is generally known that the m a x i m u m J[, of a given system is attained at an o p t i m u m carrier density around 0,15 per CuOz sheet [ 10 ]. If so. Sq _ ,Ln,CuO2 with the m a x i m u m x of 0.10 still lies in an under-doped region. This implies that, if the solubility range could be widened, T~. should be higher. We are now trying to do this by optimizing the preparation.
• !
..............
B .......
3.940
" "....
'
3.415 ~ J
," •
3.410 .'"
3.405 ,/
3.400
Q,
/
Acknowledgements
/ /
3.395 ,J
3.390
Gd3~ Srr~* I
Nd3.
I
1.06
Ld'
I
1 0
I
1,14
1.18
rLn3+(A) Fig. 11. Lattice parameters of Sro92Lno.08CuO2plotted against the ionic radius of Ln3+. the ionic radii of the host and the substituting ions. However, the present results on the IL c o m p o u n d s strongly suggest that the relation seen for the Ln2_xMxCuO4 c o m p o u n d s is not essential but may be attributed to the existence of the fluorite-block layers (Ln202). One i m p o r t a n t problem to be clarified is whether the well-known relationship between Tc and carrier concentration is realized or not in the present IL compounds. Though the carrier density seems to increase continuously with k n concentration without any sign of a separation into dopant-rich and dopant-poor phases, Tc remains almost constant. We believe that this results from a microscopic (on a scale of the superconducting coherence length of a few n m ) inhomogeneity of the distribution of Ln ions which is inevitable in a r a n d o m l y substituted system. It would result in an inhomogeneous carrier distribution in the CuO2 sheet owing to a strong impurity potential and, thus, a distribution in To. The m a x i m u m Tc value of the lanthanide-substituted IL c o m p o u n d s is 44 K at this stage. It is highest
The authors express their hearty thanks to O. Ohtaka, S. Kume and T. Yamanaka of Osaka University for permitting us to use their high pressure apparatus. This work was partly supported by a G r a n t - i n - A i d for Scientific Research on Priority Areas, "'Science of High-To Super conductivity" given by the Ministry of Education, Science and Culture, Japan.
References [1 ] T. Siegrist, S.M. Zahurac, D.W. Murphy and R.S. Roth, Nature 334 (1988) 231. [2] H. Yamane, Y. Miyazaki and T. Hirai, J. Ceram. Soc. Jpn. 97 (1989) 143. [3] M. Takano, Y. Takada, H. Okada, M. Miyamoto and K. Kusaka, Physica C 159 (1989) 375. [4] M. Azuma, Z. Hiroi, M. Takano, Y. Bando and Y. Takeda, Nature 356 (1992) 775. [ 5 ] G. Er, Y. Miyamoto, F. Kanamaru and S. Kikkawa, Physica C 181 (1991) 206. [6] M.G. Smith, A. Manthiran, J. Zhou, J.B. Goodenough and J.T. Markert, Nature 351 ( 1991 ) 549. [7] K.N. Ynag, Y. Dalichaouch, J.M. Ferreira, R.W. Lee, J.J. Neumeier, M.S. Torikachvilli,H. Zhou, M,B. Maple and R. Hake, Solid State Commun. 63 ( 1987 ) 515. [8] Z. Hiroi, M. Azuma, M. Takano and Y. Takeda, Physica C 208 (1993) 286. [9] C.C. Almasan and M.B. Maple, in: Chemistry of High Temperature Superconductors, ed. C.N.R. Rao (World Scientific,Singapole, 1991 ). [ 10] M.R. Presland, J.L. Tallon, R.G. Bucky, R.S. Liu and N.E. Flower, Physica C 176 ( 1991 ) 95.
Physica C 210 (1993) 367-372 North-Holland
Synthesis and superconducting properties of the infinite-layer compounds Srl _xLnxCuO2(Ln = La, Nd, Sm, Gd) N. I k e d a , Z. H i r o i 1, M. A z u m a , M. T a k a n o a n d Y. B a n d o Institutefor Chemical Research, Kyoto University, Uji, Kyoto-fu 611, Japan Y. T a k e d a Department of Chemistry, Faculty of Engineering, Mie University, Kamihamacho, Tsu, Mie-ken 514, Japan Received 15 March 1993
Infinite-layer-type superconductors Sr~_xLnxCuO2a r e synthesized under high pressure of 3 GPa for Ln = Sm, Gd as well as for Ln = La, Nd. Their chemical and superconducting properties are systematically studied as functions of doping concentration and the kind oflanthanide ion. As a result, it is demonstrated that the variation of these properties with doping concentration is similar for all the examined Ln3+ions. The solubility limit lies at x ~ 0.10. CuO2 sheets are expanded with increasing x, while their spacing decreases. The T¢onset determined by magnetic measurements remains constant for any doping concentration; only the Meissner fraction increases with increasing x.
1. Introduction
All the known cupric oxide superconductors have 2D CuO2 sheets d o p e d with carriers as a result o f various chemical modifications o f the parent materials. It has been well established that the carriers in the CuO2 sheets, if the concentration is appropriate, Bose-condense at relatively high critical temperatures (To). The so-called infinite-layer ( I L ) (or alllayer) structure consists o f only CuO2 sheets and alkaline-earth a t o m ( A ) sheets stacked alternately along the c-axis, and this structure has been considered as the parent structure o f all the cupric oxide superconductors [ 1 ]. At a m b i e n t pressure this structure can be stabilized only for a narrow composition range a r o u n d A=Cao.gSro.~ at temperatures just below the melting point, if a s t a n d a r d solid-state reaction m e t h o d is applied [2]. On the other hand, Takano et al. found that application o f high pressure widened the c o m p o s i t i o n range towards large A ions up tO A=Sr2/3Bal/3 [3]. They also reported that A~_xCuO2_z including both alkaline-earth and oxygen deficiencies b e c a m e a p-type superconductor
with 110 K Tc [4]. It has been considered that hole carriers arise from the A-deficiency if the oxygen vacancies, introduced for the sake o f charge balance with the A-deficiency, are filled by application of high oxygen pressure treatment. A n o t h e r type o f IL c o m p o u n d , Srl_xLnxCuO2 (Ln = La [ 5 ], N d [ 6 ] ) has also been prepared under high pressure in a strongly reducing atmosphere. They are considered to become n-type superconductors, because the substitution o f trivalent lanthanide ions for divalent strontium ions reduces the CuO2 sheets. In this study, we tried to extend the composition range toward small Ln ions in an effort to understand the solid state chemistry o f the IL system and to find how the superconducting properties vary with the c o m p o s i t i o n and the resulting structural change. C o m p o u n d s with L n = S m and G d have been prepared in a d d i t i o n to those with Ln = La and Nd, and their chemical and superconducting properties have been studied systematically as functions o f doping concentration and the kind o f lanthanide ion.
To whom correspondence should be addressed. 0921-4534/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
368
N. lkeda et al. / lnfinite-layer compounds,Sr~ ~Ln~('u():
2. Experimental High-pressure synthesis was performed with a classical cubic-anvil-type apparatus with a cell assemblage illustrated in fig. 1. The starting material was an intimate mixture of Ln203, CuO and SrCuO2. SrCuO2 (low-pressure phase) had been prepared by an ordinary solid-state reaction at 1073 K for 100 h with several intermittent grindings. It was put into the high-pressure cell in two ways, directly into the boron nitride sleeve or into a gold capsule (3 mm ~ X 3mm) with Ti powder added in the bottom of the capsule which worked as an oxygen getter. All the previous studies on the Sr~_xLnxCuO2 ( L n = L a , Nd) systems had adopted the former method. These high-pressure cells were compressed almost isostatically up to 3 GPa and then heat-treated with a graphite-sleeve heater for half an hour at 1173 K. The sample temperature was monitored with a P t / P t - I 3% Rh thermocouple placed inside the boron nitride cell. The product was finally quenched to room temperature before releasing the pressure. Thus obtained polycrystalline samples weighed about 50 mg and were characterized by powder X-ray diffraction ( X R D ) measurements using Cu Kct radia-
Upper Bucket
~
tion, electron diffraction (ED) and high-resolution electron microscopy (HREM). The Meissner signal was measured with a SQUID magnetometer (Quantum Design, MPMS2) on cooling in an external field of 10 Oe. Electrical resistivity was measured by a standard four-terminal method. The copper valence was determined by an iodometric titration method.
3. Results and discussion
3.1. Srl_ ~Nd,:CuO: We have examined first the two preparation processes for the Nd-substituted system. Figure 2 compares two typical powder XRD patterns of Sro.gNdo.lCuO2 prepared without a gold capsule (a) and with a gold capsule (b). Without the gold capsule, the pellet was covered with a reddish skin mainly composed of Cu20. This implies that at the surface layer CuO in the starting material is reduced to become Cu20 and, at the same time, Sr- and Nd-components are lost, probably into the boron nitride sleeve. There seems to exist a composition gradient even inside the pellet, as found from the XRD pattern of fig. 2 (a) recorded using a fragment selectively obtained from the core. The IL-phase is major, but another new phase (the "3c"-phase as we shall
101
Anvil
i
Lower Bucket
Pyrophvllite Block
Thermocouple
(a)
. 3c phase 110
* Cu20
g n
200
loo ~'i[~ 11~I 002 112 220 k___ _J/~U_zt~_t_x_Sr[ _ ~ _ ~
e
•
>"
(b)
12.5mm ell
er
Starting Powder Fig. 1. High-pressure cell assembly.
10
20
30
40
50
60
70
2o (deg.) Fig. 2. X R D patterns of Sro.gNdo.iCuO2 prepared under highpressure without (a) and with ( b ) a gold capsule.
N. Ikeda et al. / Infinite-layer compounds Sr ~_~Ln~CuOe
call it), which has a tetragonal unit cell of a = 3.9 ~, and c = 10.8 A, and a small amount of Cu20 coexist. The "3c"-phase crystallizes in a triple IL structure whose intersheet distance has such a sequence as 3.7 A/3.4 A/3.7 ,~ and has a copper-rich composition A / C u < 1. Details will be reported elsewhere. The coexistence of the "3c"-phase makes the final Nd content of the IL phase ambiguous. On the other hand, the use of a gold capsule with metal Ti powder as an oxygen getter proved to yield more homogeneous samples. As seen from the XRD pattern of fig. 2 (b), the "3c"-phase did not appear, because the reaction with BN was suppressed. All the following results have been obtained from encapsulated samples. Figure 3 shows the variation of lattice parameters with x for Sr~_~Nd~CuO2. As x increases, a increases, while c decreases. Both a and c vary almost linearly up to x = 0 . 1 0 and remain constant for x > 0.10. In accordance with this an impurity phase of Srz_xNd~ +~Cu2Oy [ 7 ] appears in XRD patterns. Thus we conclude x = 0 . 1 0 is the solubility limit of Nd in the IL-phase. ,-"
3.950.
.,,," •
369
Microscopic observations by means of ED and HREM have revealed that the sample has the ideal IL-type structure without any inherent lattice defects. Shown in fig. 4 are typical ED patterns taken with the incident electron beam along the a-axis (a) and the c-axis (b). Neither superlattice diffraction spots nor streaks are seen. This presents a striking contrast to the case of the p-type superconductor A~_xCuO2_z containing specific defect-layers [8]. Sr~_xLnxCuO2 is in the strict sense an infinite-layertype superconductor. In addition, no evidence indicating a phase separation has been detected for 0 < x < 0 . I0 in the electron microscopic order. The temperature dependence of magnetization as a function of doping level measured on cooling in an external field of 10 Oe is shown in fig. 5. Surprisingly, the T¢o,~ remains almost constant at ~ 44 K against the variation of Nd content. Only the magnitude of the diamagnetic signal increases up to x=0.10, the decrease above which is due to the impurity segregation. A linear variation of the nominal copper valence
a
• I"1
.~.
3.940-
3.930-
a
o
3.920:
3'420,] ~
O0
• Cu
3.410 3.400 3.390-
• this work
. 3.380-
r~ Smith e t a l .
0.00
~,,
",, •
•
'% ,, o
0.05
0.10
[3
0.15
x in Sq.xNdxCuO2 Fig. 3. Variation of lattice parameters with dopant concentration x for Sr~ _xNdxCuOz. The previous data by Smith et al. are also plotted with open squares.
Fig. 4. ED patterns of the [100] (a) and the [001] (b) zoneaxes. Neither superlattice diffractions nor streaks are seen.
370
N. lkeda et al. /Infinite-layer compounds Sr ~_ ,.Ln,CuO:
valence to ( 2 - . v ) , suggesting that the present compound is a n-type superconductor.
(10 .3)
0
..... I
:
I
3
X = 0.05 0"1
-1.0 m
2.0
0.08
~,
~
~
~ *o° °
(10 6)
m ~ o***~*o°°° ~ $ o~ oo
2.0
o ° oo°
.~e.~; ~
mmmm ®® ~ 1 . 0
oo° ° oo 0.12 oo ooOOo°° 0.10 []
-3.0
3.2. Sr¢_ ,Sm,CuOz and ?;r~ ,Gd,.CuO:
%~o ~°o
m
E I
B
®
o m
-4.0
,
0
10
20
[]
I 0.0
40
30
50
T(K) Fig. 5. Temperature dependences of magnetic susceptibilitb M / H , measured in an applied field of 10 Oe on cooling for SQ_xNdxCuO2 with varying x. The T~ onset is always 44 K, independent ofx.
We have found that smaller lanthanide elements, Sm and Gd, substitute SrCuO2 crystallizing in the Ik structure. Figure 7 shows the powder XRD patterns for L n = S m (a) and Gd (b) synthesized using gold capsules. Their composition dependences of the lattice constants are similar to that of the Nd-substituted system. The magnetic and electrical behaviors of these systems also resemble those of the Nd-substituted system. As shown in fig. 8 for Ln = Sm, the 7",...... , remains at 44 K independently of .r, while the diamagnetic signal increases with increasing x. Both the resistivity curves of fig. 9 for L n = S m and Gd show a semiconducting temperature dependence and begin to drop at ~ 40 K, becoming nearly zero at 35 K (Sm) and 20 K (Gd).
Srl_xNdxCUOy 2.00,
L
1.95
3.3. Srl_xLn.,.Cu02 (Ln=La, Nd, Sm, Gd)
The evolution of structural and physical properties with the kind of doping element will be briefly discussed below. Since the lanthanide ions show a systematic variation in both ionic size and the mag-
',,... • "'-.,
c
I(D >
$ &
8
1.90
"-,,,. "..... 1.85
y=2
101
(a)
Sro92Sin0o80uO2
1.80
0.00
0,05
0.10
0.15
X
Fig. 6. Variation of the formal copper valence with x for Srl xNd~CuO2 . The dotted line assumes oxygen content of two.
%-
110 ~
'~ >"
100
II! L ~
200 111
211
~
~, I I
210°°2 J/ 112 220
# (b)
with Nd content has been confirmed by iodometric titration, as shown in fig. 6. The dotted line in the figure was drawn assuming the formal valences of N d 3+ and 0 2- and also oxygen content of two per chemical formula. The data points fall near this line up to x = 0 . 1 0 , which implies that oxygen atoms within the CuO2 sheets were not lost even in the reducing condition during preparation. Therefore, the Nd substitution works effectively to reduce the Cu
Sro 92Gdo.08CuO 2
iE
20
310
20
I
50
I
60
i
70
28 (deg.) Fig. 7. XRD patterns of Sro.9~Lno.osCuO2 for L n = S m (a) and Gd (b) prepared under high pressure in a gold capsule.
N. lkeda et al. I Infinite-layer compounds SQ_ ~LnxCuOe
Sr0.92Ln0.08CuO2
Sq_xSmxCu02 eeooooooo~oOO
o
+++
•
÷++
x = 0.05
371
•
**+*
:
+ ÷~+me
-1.0
E
I1) +++++
O
,~ "r
o o
+t3mo
++~+m •
E
~.
-1.0 "
+++++
-2.0
~lilO
0.08
..
v oom
212
.
,
..-'"
•
,
10
°°
ee
•
++
...
o
•
° o°
o
oe °
° oo°
.
,
20
30
.
,
40
. . . .
T(K)
La
Nd
+ Sm
.
50
e •
o
. . . . . . ~,~o*o*o eoeeeoe"
...... .,.'"
-4.0
o°.... ~++ +.+
++++ Oo
-2.0 ,•
el
oO
o
-3.0 0.10
~0~1
•
• Gd -3.0
'
'
0
'
,
10
'
'
,
,
•
,
,
20
r
30
•
,
,
r
40
,
,
,
50
T (K)
Fig. 8. Temperature dependences of magnetic susceptibility for Sr| _~Sm~CuO2 measured in an external field of 10 Oe on cooling.
Fig. 10. Comparison of Meissner curves for Sro.92Lno.08CuO2 with Ln= La, Nd, Sm and Gd.
XX
3_
\
\ \
co 2 t-,..
.
(b) Sro.gGdo.lCuOa
rr t-v rr"
1
Gd, roughly proportionally to the magnitude of the magnetic moments of these free lanthanide ions. The slight upturn at low temperature seen in the Gd-curve may be due to the relatively large Curie component. The results indicate that the pair-breaking effect due to a magnetic impurity situated very near the CuO2 sheets is negligible, as has already been observed in LnBa2CusO7_y.
(a) Sro.9Smo.lCuO2
0
J 0
50
1~00
1+50
200 ~
250
T(K) Fig. 9. Temperature dependences of normalized electrical resistivity for Sro.gSmo.lCuO 2 (a) and Sro.9Gdo.|CuO 2 (b).
nitude of magnetic moment, the Ln-substituted system is suitable to study how the lattice size and the impurity magnetic moment affect superconductivity. The structural simplicity is another important factor to be considered. Figure 10 compares the Meissner curves of Sro.9eLno.osCuO2 with Ln = La, Nd, Sm and Gd. Both their diamagnetic signals at 5 K and T¢on~t's are similar to each other, while susceptibility in the normal state increases in the order of L n = La, Sm, Nd and
Figure 11 plots the lattice parameters of Sro.92Lno.oaCuO2 against the ionic size of the dopants. Both the a- and c-axes increase as the ionic radius of Ln 3+ increases. The variation of the c-axis is much larger than that of the a-axis, reflecting the two-dimensionality of the IL structure. In spite of these changes, Tc remains almost constant at ~ 44 K. Such an independence between Tc and the lattice size has also been reported for Al_xCuO2_z [8]. Therefore, we conclude that neither the intra-sheet nor the inter-sheet Cu-Cu distance is essential in determining Tc within the range studied. In contrast, it has been reported that, in the case of n-type superconductors Ln2_xMxCuO4 [9] (Ln=Pr, Nd, Sm, Eu; M = C e , Th), Tc varies with Ln as 24 K ( L n = N d ) , 19 K ( L n = S m ) and 13 K ( L n = E u ) for M = C e . Besides, T~ also varies with M as 19 K ( L n = N d ) and 27 K ( L n = S m ) , for M = T h . This variation in Tc has generally been discussed in relation to the variation of lattice parameters with
372
N. lkeda et al. /Infinite-layer compounds Sr ~_ ~Ln ~CuO:
v
3.950 1 3.945
among n-type superconductors, but is much lower than the highest value of p-type superconductors ( ~ 130 K). It is generally known that the m a x i m u m J[, of a given system is attained at an o p t i m u m carrier density around 0,15 per CuOz sheet [ 10 ]. If so. Sq _ ,Ln,CuO2 with the m a x i m u m x of 0.10 still lies in an under-doped region. This implies that, if the solubility range could be widened, T~. should be higher. We are now trying to do this by optimizing the preparation.
• !
..............
B .......
3.940
" "....
'
3.415 ~ J
," •
3.410 .'"
3.405 ,/
3.400
Q,
/
Acknowledgements
/ /
3.395 ,J
3.390
Gd3~ Srr~* I
Nd3.
I
1.06
Ld'
I
1 0
I
1,14
1.18
rLn3+(A) Fig. 11. Lattice parameters of Sro92Lno.08CuO2plotted against the ionic radius of Ln3+. the ionic radii of the host and the substituting ions. However, the present results on the IL c o m p o u n d s strongly suggest that the relation seen for the Ln2_xMxCuO4 c o m p o u n d s is not essential but may be attributed to the existence of the fluorite-block layers (Ln202). One i m p o r t a n t problem to be clarified is whether the well-known relationship between Tc and carrier concentration is realized or not in the present IL compounds. Though the carrier density seems to increase continuously with k n concentration without any sign of a separation into dopant-rich and dopant-poor phases, Tc remains almost constant. We believe that this results from a microscopic (on a scale of the superconducting coherence length of a few n m ) inhomogeneity of the distribution of Ln ions which is inevitable in a r a n d o m l y substituted system. It would result in an inhomogeneous carrier distribution in the CuO2 sheet owing to a strong impurity potential and, thus, a distribution in To. The m a x i m u m Tc value of the lanthanide-substituted IL c o m p o u n d s is 44 K at this stage. It is highest
The authors express their hearty thanks to O. Ohtaka, S. Kume and T. Yamanaka of Osaka University for permitting us to use their high pressure apparatus. This work was partly supported by a G r a n t - i n - A i d for Scientific Research on Priority Areas, "'Science of High-To Super conductivity" given by the Ministry of Education, Science and Culture, Japan.
References [1 ] T. Siegrist, S.M. Zahurac, D.W. Murphy and R.S. Roth, Nature 334 (1988) 231. [2] H. Yamane, Y. Miyazaki and T. Hirai, J. Ceram. Soc. Jpn. 97 (1989) 143. [3] M. Takano, Y. Takada, H. Okada, M. Miyamoto and K. Kusaka, Physica C 159 (1989) 375. [4] M. Azuma, Z. Hiroi, M. Takano, Y. Bando and Y. Takeda, Nature 356 (1992) 775. [ 5 ] G. Er, Y. Miyamoto, F. Kanamaru and S. Kikkawa, Physica C 181 (1991) 206. [6] M.G. Smith, A. Manthiran, J. Zhou, J.B. Goodenough and J.T. Markert, Nature 351 ( 1991 ) 549. [7] K.N. Ynag, Y. Dalichaouch, J.M. Ferreira, R.W. Lee, J.J. Neumeier, M.S. Torikachvilli,H. Zhou, M,B. Maple and R. Hake, Solid State Commun. 63 ( 1987 ) 515. [8] Z. Hiroi, M. Azuma, M. Takano and Y. Takeda, Physica C 208 (1993) 286. [9] C.C. Almasan and M.B. Maple, in: Chemistry of High Temperature Superconductors, ed. C.N.R. Rao (World Scientific,Singapole, 1991 ). [ 10] M.R. Presland, J.L. Tallon, R.G. Bucky, R.S. Liu and N.E. Flower, Physica C 176 ( 1991 ) 95.