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Structural and superconducting properties of oxygenated La2−χMχCuO4+y (M=Sr, Ba; O≦χ<0.1)
PHYSICA
Physica C 198 (1992) 237-246 North-Holland
Structural and superconducting properties of oxygenated La2_xMxCuO4+y(M=Sr, Ba; 0 < x < 0 . 1 ) Y. Ueda, Y. Fujiwara and A. Hayashi Institute for Solid State Physics, Universityof Tokyo, 7-22-I Roppongi, Minato-ku, Tokyo 106, Japan
K. Shibutani and R. Ogawa Superconducting and Cryogenic Technology Center, Kobe Steel, Ltd. 5-5, 1-chome, Takatsukadai, Nishi-ku, Kobe 651-22, Japan
Received 19 February 1992
Oxygenstoichiometry, superconductivity and structural properties ofLa2_xMxCuO4+y (M = Sr, Ba; 0_-
1. Introduction Since the first discovery o f a high-To superconductor by Bednorz and Miiller [ 1 ], the system La2_xMxCuO4 (M = Sr, Ba) has been extensively investigated. It was well established that the substitution of alkaline earth divalent ions for La in the insulative antiferromagnetic La2CuO4 leads to superconductivity above a critical composition o f the substituents [ 2 ]. In pure La2CuO4, on the other hand, a trace o f superconductivity was often observed in samples slowly cooled in oxygen or air [ 3,4 ] and then the oxygenated La2CuO4+y under high oxygen pressure [5-8 ] was confirmed to be a bulk superconductor. Jorgensen et al. [ 9 ] first showed by neutron powder diffraction that the oxygenated La2CuO4+y is a mixture o f two orthorhombic phases which results from a macroscopic phase separation near 320 K under ambient atmosphere. They also claimed that one phase is the insulative antiferromagnetic La2CuO4 with almost stoichiometric composition o f oxygen and the other the superconducting La2CuO4+y with excess oxygen. A two-phase refinement o f a crystal for the oxygenated LaECuO4+y was also car-
fled out by Chaillout et al. [ 10] and they asserted that the excess oxygen occupies the so-called "interstitial site (¼, ¼, z; z ~ ] )" in the form o f peroxide (O2) 2- ions. Anomalous behaviors of some physical properties have also been observed between 200 and 300 K in the oxygenated La2CuO4+y by some groups [ 1 1 - 1 6 ] , and those behaviors were attributed to phase separation without the direct observation by neutron or X-ray diffraction. One of our interests is how to understand this phase separation from a standpoint of thermodynamics or thermal equilibrium. A material prepared under high pressure~and high temperature often remains stable under ambient condition, when the constituent atoms cannot diffuse thermally under the environment. The phase separation in La2CuO4+y, however, shows that the excess oxygen atoms introduced under high oxygen pressure and high temperature diffuse in the crystal under ambient pressure at such a low temperature without decomposition (release o f the excess oxygen). Furthermore, this phase separation has no relation to the oxygen partial pressure to which the material is exposed; that is, the composition of oxygen in each phase (chemical potential
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All fights reserved.
238
Y. Uedaet al. / Propertiesof oxygenated Lae_xMxCuO4+y
of oxygen in both phases) is independent of the oxygen partial pressure. To the authors' knowledge there existed no such compound in oxides. La2NiOa÷y which is similar to La2CuO4 in structure and includes the excess oxygen in large quantity does not exhibit such a phase separation [ 17]. Another interest is what relation there exists between the superconductors resulting from the substitution of alkaline earth metal and from the oxygen nonstoichiometry. In this respect, there are very few reports. To make these problems clear, it is essential that the structural and physical properties of the oxygenated La2_xMxCuO4 +y (M = Sr, Ba) are investigated on the same samples. We synthesized Laa_xMxCuO4 +y (M = Sr, Ba) under PO2 = 360 atm and have investigated its structural and physical properties. In the present paper we report the phase relation and superconductivity of the oxygenated La2_~MxCuO4+y ( M = S r , Ba; 0 < x < 0 . 1 ) . We also report the detailed analysis of oxygen stoichiometry and the decomposition process of oxygenated sampies. The two phase behavior was clearly observed in the samples with x~< 0.02 by X-ray diffraction and the phase separation trends to be suppressed by the substitution of Sr or Ba. We also present the characteristics of this system in the structural properties.
2. Experimental Starting materials were prepared by solid state reaction of an appropriate mixture of predried La203, SrCO3 (or BaCO3) and CuO. The powders were pressed into a pellet, calcined at 900°C for 24 h, then ground, pelletized, and sintered at 1000-1100 °C for 120 h in air with several intermediate grindings. The powder samples obtained were annealed under PO2= 360 atm at 600°C for 6 h using the Kobe Steel O2-HIP system 30T, and were cooled in the furnace prior to the release of applied pressure. To compare with the oxygenated samples, a part of the starting materials was annealed in flowing pure Ar gas at 900 ° C for 48 h. All samples thus prepared were confirmed by X-ray powder diffraction to include no impurity phases. Thermogravimetric and differential thermal analysis were made using a Mac Science TG-DTA 2000
system. Electrical resistivity was measured by a standard DC four-probe method, supplying a current of 1-5 mA from 4.2 K to 280 K. The magnetization was measured under magnetic fields up to 10 G using a Quantum Design MPMS SQUID magnetometer in the temperature range from 4.2 to 50 K. X-ray powder diffraction measurements were carried out using a Mac Science MXP ~8 system with a rotating anode generator and a monochromator of single crystalline graphite for Cu Ket radiation. A closed-cycle helium refrigerator was used to control the temperature between 10 K and 350 K.
3. Results
3.1. Thermogravimetric and differential thermal analysis (TG-DTA) TG-DTA as carried out for the purpose of examining the oxygen stoichiometry and the thermal decomposition. Before the investigation on the oxygenated La2_xMxCuO4+y, the oxygen stoichiometry of Ar-annealed samples was determined by reducing the samples in hydrogen. Figure 1 shows a typical TG-DTA curve for Ar-annealed LaL92Sro.oaCuOy,, heated at 2 K / m i n in flowing hydrogen (4 mol% H2 in He, 400 m l / m i n ) . The compound decreased in weight stepwise showing exo- and endothermic peaks in the DTA curve, as shown in fig. 1. Assuming that the compound is finally decomposed into a mixture of Cu metal, SrO and La203, the oxygen content for this compound was estimated to be 4.00 + 0.005. The oxygen stoichiometry determined by this method was 4.00+0.005 for all Ar-annealed La2_xMxCuOy, ( M = S r , Ba; 0 < x < 0 . 1 ). A plateau around 600 K in the TG curve was observed in all samples. Chou et al. [ 18 ] reported that the plateau in the T G curve for La2CuO4 indicates the formation of a new Sr2CuO3-type phase, La2CuO3.67. In our measurements the oxygen content for the intermediate compounds was about 3.83 and was independent of Sr (or Ba) content x, which are much higher than the value of 3.67. Since the oxygen stoichiometry of Ar-annealed samples was 4.00 independent of Sr or Ba content, the amount of excess oxygen in the oxygenated samples was determined by TG-DTA in flowing pure Ar
239
Y. Ueda et al. / Properties of oxygenated La2_xMxCuO4+y
i
La1.92Sro.oaCuOy, inH2(4%)/Hegas Y'=3"83
y' =4.00 -i - ~ - -
~
50 40
v
3O ~>
§
tO t-
:
/
2O < v
10
y'= 2.96
400
600 800 Temperature (K)
I
1000
Fig. 1. TG-DTA curves for Ar-annealed Lal.92Sro.oaCuOy, heated in flowinghydrogen (4 mol% H2 in He). The oxygen stoichiometryy' was estimated by assuming that the compound is decomposed into La203, SrO and Cu, that is, the decomposed product has total oxygen content y' = 2.96. i
I
I
La2_xSrxCuO4+y(Po2=360arm) ..--. 0
.
0
0
I
in Ar gas
~ x =0.06
(~
~
002
.~ ~-0.05
~
-~--"-~
--
0.04
-0.10
0'00~---~
I
400
I
600
1
800
I
1000
Temperature (~ Fig. 2. Weight-losscurves for oxygenated La2_~SrxCuO4+y (PO 2= 360 arm) heated in flowingAr. Note that the amount of weight-loss corresponds to the excess oxygeny. gas. Figure 2 shows the weight loss versus temperature profiles for the oxygenated La2_xSrxCuO4+y in flowing Ar ( > 99.9999%, 200 m l / m i n ) at a heating rate of 10 K / r a i n . The excess oxygen was released
a r o u n d 600 K in all samples a n d the weight loss of about 0.1% for La2CuO,+ r decreases with Sr content x. Figure 3 shows the x dependence of the oxygen stoichiometry for the Ar annealed a n d the oxygen-
Y. Ueda et al. /Properties of oxygenated Lae_~M~CuO4+y
240
0
0.030 Sr
"
2
0
/
\
1
, , 2 " 0 (a) La2_xBa,,CuO,.oo
Po~ = 360 atm
0.025 P% = 360 atm
\ 0.020
>.,
0.015
8~
o~O\
\ 0.010
\\ 0.005 o
00001: 0.00
0.02 0.04 0.06 x in La2_xMxCUO4¢y
.~" 0.10 -~;
1.0
0.05
0.5
"~. 0.08 I/ 0.00 II
Fig. 3. Excess oxygen content of Ar-annealed and oxygenated La2_xM~CuOa+y (M=Sr, Ba) determined by TG measurements. Note that the Ar-annealed samples have almost stoichiometric oxygen content 4.00 independent of Sr or Ba content and the oxygenated samples have excess oxygen.
0
[
50
f [
1O0
0.10
i ,~
~
150
200
x=°°"l I /
250
0.0 300
i i i I fo) La2-xBa, CuO4*y --I 0.25
°°8/ I\
7 °'2°
"N
Electrical resistivity and magnetic susceptibility were measured to confirm the superconducting transition. Figure 4(a) shows the resistivity versus temperature curves for La2_xBaxCuO4.oo annealed in flowing Ar gas. La2_xM~CuO4.oo shows no superconducting transition in a lightly substituted region and starts to exhibit superconductivity at x = 0.06 for Sr and x = 0 . 0 5 for Ba, agreeing with the previous results [ 2 ]. All of the oxygenated La2_ xMxCuO4 + y, on the other hand, shows a drop of resistivity suggesting the superconducting transition, as shown in fig. 4 (b). Figure 5 shows the compositional dependence of the temperature, T¢, from where the resistivity abruptly decreases with decreasing temperature. Tes decrease and then increase on increasing the content of the substituents, showing a minimum around x = 0 . 0 5 for Sr and x = 0 . 0 4 for Ba. The magnetic susceptibility versus temperature curves showed a low-field flux exclusion in all oxy-
I
T (K)
ated La2_xMxCuO4+y ( M = S r , Ba; 0 < x < 0 . 1 ) . In the samples oxygenated under PO2=360 atm, the content of excess oxygen (y) is about 0.03 for the sample with x = 0 and decreases with increasing x, having a constant value around x = 0.02-0.04. 3.2. Electrical resistivity and magnetic susceptibility
I
1o.,o
"~ o.o4
I1#1
0.00 ~ 0
50
100
150
200
250
I 0.00
300
T (K)
Fig. 4 (a) Resistivity vs. temperature curves for (a) Ar-annealed and (b) oxygenated Laz_xBaxCuO4+y. Note that all of the oxygenated samples exhibit the superconducting transition.
genated samples. A typical example for the oxygenated La2CuO4.o3 is shown in the inset of fig. 6. Figure 6 shows the compositional dependence of the magnetic susceptibility at 5 K as a measure of the relative flux exclusion, where it was assumed that all samples had almost the same particle size. The magnetic susceptibility at 5 K decreases with increasing Sr or Ba concentration and then steeply increases above
Y. Ueda et al. / Properties of oxygenated La2_xMxCuO~+y 50
'
'
o
'I
[
Po~ = 360 arm
40 0 \
o/:1
0
0
30
o° tO
~/a~//.
20
// ,,o : /
/
10
I 0.02
LI
O0.00
x
I 0.04
in
/
; I 0.06
0.08
La2_xMxCUO4+y
Fig. 5. The compositional (x) dependence of the superconducting transition temperature in Ar-annealed and oxygenated La2_xMxCuO4+y(M = Sr, Ba), Tco,~tshowsthe temperature from where the resistivity abruptly decreases with decreasingtemperature. In Ar-annealedsamples the superconductingtransition was observed above x=0.06 for Sr-substituted samples and above x=0.05 for Ba. 10 0
0.~1
j
, , , O.Oi La2Cu04.o3
j
10 "1
10 .2
E ~'~ '1-
o Sr o Ba
110 20
3LO 40
T (K)
a~
10-3
o ~
10.4
/ o
~
10s
T=5K -61
100.00
I
I
L
0.02 0.04 0.06 X in La2_xMxCUO4+y
0.08
Fig. 6. Compositional (x) dependenceof magnetic susceptibility at 5 K for oxygenatedLa2_xMxCuO4+y(M=Sr, Ba). The magnetic susceptibility vs. temperature curves showed the low-field flux exclusion in all oxygenatedsamples. A typical example for the oxygenatedLa2CuO4.o3is shown in the inset. x = 0 . 0 5 where the Ar-annealed samples show superconductivity. It was confirmed from the resistivity and magnetic susceptibility measurements that the oxygenated La2_xMxCuO4 +y (M = Sr, Ba ) under PO2 = 360 atm
241
is a superconductor even in the lightly substituted region.
3.3. Powder X-ray diffraction The oxygenated La2Cu04.o3 was found by the Xray diffraction at 10 K to be a mixture of two phases with a nearly identical structure but different orthorhombic distortion. The lattice parameters and the relative fraction of the two phases were determined by Rietveld refinements based on a mixture of two orthorhombic Bmab phases (the phase 1 and 2), using the analysis program RIETAN [ 19 ]. The refined lattice parameters of each phase at 10 K are in good agreement with the neutron diffraction data [ 9 ], and the relative fraction of the two phases from this analysis is approximately 66% for the phase 1 and 34% for the phase 2, where the phase 1 is the phase with larger orthorhombic distortion. Jorgensen et al. [ 9] concluded that the phase with larger orthorhombic distortion is the insulative phase with almost stoichiometric oxygen content of 4.00, while the superconducting phase with the smaller orthorhombic distortion has excess oxygen. Also in our investigation the phase 1 can be regarded to have the oxygen stoichiometry of 4.00 because the phase 1 at 10 K has almost the same lattice parameters as Ar-annealed La2CuO4.oo, and the oxygen content of the phase 2 is estimated to be about 4.08 in connection with the results of thermogravimetric analysis. Heating the oxygenated La2CuO4.o3 in flowing Ar gas led to a single phase with the same lattice parameters as those of phase l at l 0 K, which showed no superconductivity. So phase 2 was concluded to be a superconducting phase. Figure 7 illustrates the Sr content x dependence of Bragg peaks 400 and 040 for the oxygenated Laa_xSrxCuO4+y at 10 K, where the contribution of Cu Kct2 is arithmetically subtracted. The coexistence of two orthorhombic phases can be clearly seen in the samples with x = 0 , 0.01 and 0.02. The peaks on the higher angle side of 040 peaks of phase 1 are 040 peaks of phase 2, while 400 peaks of the two phases exactly overlap each other. The sample with x = 0.03 was judged to be a mixture of two phases because the 040 peak is rather broad compared with the 400 peak. When Sr or Ba is substituted, the 040 peak of the phase 1 approaches the 040 peak of the phase 2 with
Y. Ueda et al. / Properties of oxygenated La2_~M~CuO4+y
242
n
r
n
i
i
i
I
i
La2_xSrxCuO4+y (Po= = 360 atm)
T=
5.42 = ~ l
T=IOK
10 K
ptwel
_I R;
540
m ~
~
m
5.38
:o.oo
~.
0.01
5.36 a 5.34
0.03
--o-
.o--
o--o--
o-
0.04 5.32
,OW'{~ phase1 Ba{ : phasel
0.05
phase 2
13,18 -
phase 2
//e/
0.06 I
~.s 89.0 6Es 7Eo 7Es 7;.o 71.s 20 (deg) Fig. 7. Sr content x dependence of Bragg peaks 400 and 040 for oxygenated La2_:,Sr~CuO4+y at 10 K. The contribution of Cu Kct2 are subtracted in these figures. The compounds with x < 0.03 are a mixture of two phases with different orthorhombic strain. The relative fraction of phase 2 seems to be almost the same (34%) independent of Sr or Ba content.
increasing x and the two 040 peaks merge to form the single 040 peak in the samples with x > 0.04. The relative fraction of each phase in the samples with x=0.01 and 0.02 was determined by a multiple-peak separation technique using only the 040 peaks, since the Rietveld refinements for the samples were unsuccessful. The result obtained for the sample with x = 0 by this technique was in good agreement with that from the Rietveld refinements, and the fraction of phase 2 was about 30% independent of x. Such behaviors were also observed for Ba substituted compounds. Figure 8 shows Sr (or Ba) content x dependence of lattice parameters at 10 K for the oxygenated samples. The a- and c-axis are longer in a Ba substituted system than in Sr, reflecting the slightly larger ionic radii of Ba 2÷ than that of Sr 2+. On the other hand the b-axis rather depends on the concentration of the substituents; nevertheless it is almost the same in the two systems over the region from twophase to single-phase.
o
13.14 13.12 13.1
•
•
~}~1 . 0
0.02
0.04
0.06
0.08
x in La2_xMxCUO4+y Fig. 8. Compositional (x) dependence of the lattice parameters at 10 K for oxygenated La2_xMxCuO4+r (M = Sr, Ba). Note that only the b-axisshows the same dependence between Sr- and Basubstituted samples. Next in order to examine the phase separation, Xray diffraction measurements for the oxygenated samples were performed as a function of temperature from l0 to 350 K. Figure 9 shows the temperature dependence of lattice parameters for the oxygenated LaE_xSrxCuO4+y with x = 0 and 0.01, which was completely reversible. The temperature dependence of lattice parameters for the samples with x = 0 is in good agreement with the neutron diffraction data [ 9 ]. As can be seen in fig. 9, the phase separation occurs around 300 K in our sample, La2CuO4.o3. Figure 10 shows the temperatures ( T ~ ) where the phase separation occurs in the oxygenated LaE_xMxCuO4+y (M = Sr, Ba), which were determined by measuring the temperature dependence of the full-width at half maximum ( F W H M ) of the 006 peak (the inset of -~fig. 10). The phase separation seems to be suppressed by the substitution of Sr or Ba. To discuss
243
Y. Ueda et al. / Properties of oxygenated La2_~l~CuO~+y
i
i
"l
i
La2_xSrxCuO4.y (Po2 = 360 atm) 381
38o >~ 379
o 0.00 A 0.01
• phase1 I x = 0 0 o phase2 I ' • phase1 I x = 0 0 1 [] phase2 I "
"~P
o_ ..,a.,.'Z'$".~ ~ , ~ 1 ~1 _
~0.15
.__,__~o:...~o:~~:~
~e~
I-
A
o.1
200
T (K)
LB~o./°
J i
378
J
150
b
5.42
=St ~=
•
01,
o.oo
5.40
~"
o~
o.~6
0.08
x in La2_xMxCUO4.y
Fig. 10. Compositional (x) dependenceof the phase separation temperature in oxygenatedLa2_~MxCuO4+y (M = Sr, Ba). Tp,was determined from the temperature dependence of the full-width at half maximum (FWHM) of the 006 Bragg peak, as shown in
5.38
J~
¢r
o.~2
Ba
5.36
the inset. 5.34 -u --°I~I~L-I
5.32 006 o ..~o-aO.5°~
° 13.13L I;
13.10/~e~ 0
mA
La2CuO4"03
~=,1
040
.
115
~
400
A
p~,,1
A
.-./"~'¢:~. i 100
i 200
i 300
Temperature (K)
Fig. 9. Temperature dependence of lattice parameters for oxygenated La2_~Sr~CuO4+ywith x=0 and 0.01. Note that both samples show the phase separation into phase 1 and 2 around 300 K.
JLY/ XL~ K
this more precisely one would need to establish the phase diagram both on the oxygen nonstoichiometry and the substitution, because this suppression may be due to the decrease of excess oxygen accompanied by the substitution. An X-ray diffraction study of the samples with various excess oxygen contents is in progress. Figure 11 shows the temperature dependence of Bragg peaks 006 and 040 in the oxygenated La2CuO4.o3. As can be seen in fig. 11, the phase 2 is a minor phase at low temperature and increases in fraction with increasing temperature. The change in fraction was also reversible and was observed above 200 K, which suggests that the lower limit of temperature for the phase separation is about 200 K. Jorgensen et al. [ 9 ] reported that the relative fraction of the two phase in the oxygenated La2CuO4+y
40.5
41.0
41.5
42.0
42.5 69.0 26 (deg)
69.5
70.0
70.5
71.0
Fig. 11. Temperature dependenceof the portion of X-raydiffraction pattern for oxygenatedLa2CuO4.o3.The relative fraction of the phase 2 seemsto increase with increasingtemperature. is almost independent of temperature, while Chaillout et al. [ lO(b) ] reported that the phase 2 is a major phase at low temperature and increases in fraction with increasing temperature. Such discrepancy among three groups may be due to the difference of the excess oxygen content in the samples.
244
Y. Ueda et al. I Properties of oxygenated La2_xMxCtlO4+y
4. Discussion Present experiments revealed that the oxygenated LaE_xMxCuO4+y ( M = S r , Ba; 0_
0.00 4.1(
0.02
0.04
I
i
(a)
0.06
0.06
I
0.10
I
o Sr Po~ = 360 ,~rn o Ba J
\o 4.0.~
~
~\I\
>, +
Q-~._
/
4.00
Ar
/
Q" " ~ Q
o--o--o
""- .-T--Y=-2 lx
(Cu 2*)
3.95
"- - -
i s:}o ._s 2 5T\° 8
.
-..
}
r\
2.05 . . . . . . . . . .
t
-
~
..........
eQ---
< 2.00
~~/Ar
Ba
I
O.OI
0.02 X in
I
I
I
0.04
0.06
0.06
0.10
ka2_xMxCUO4+y
Fig. 12. (a) Compositional (x) dependence of the oxygencontent for the phase with excessoxygenin L a 2 _ xMxCU04+y (M----St, Ba). The solid line represents the oxygencontent (4.00) for Arannealed samples and the broken line shows the compositional dependence of oxygencontent in the compoundswith constant average valence, Cu2+ (La~+xM2+Cu2+O24+y).(b) Compositional (x) dependence of the average valence of Cu ions for the phase with excess oxygenin La2_xMxCuO4+y (M= St, Ba). Open and closed symbols represent the cases that the excess oxygens exist in the form of O2- and (O2)2-, respectively. The solid line represents the average valence of Cu ions for Ar-annealed samples. Broken lines indicate the minimum values of the average valence of Cu ions for the superconductivity (see the text). not be simply described in terms of the average valence of Cu ions. The orthorhombic strain is also affected by both the oxygen nonstoichiometry and the substitution. Figure 13 shows the orthorhombicity defined as 2(b-a)/(a+b) for the oxygenated and At-annealed LaE_xMxCuO4+y ( M = S r , Ba; 0-
Y. Ueda et al. / Properties of oxygenated La2_~M~CuO4+y 2.0,
F
i
I
• phase 1 a phase 2 A Ar annealed
Sr ~,
1.5
5.43
o phase 2 • Ar annealed
5.42 ' ' - - - e ~ ~°'"o
•~.... ",-.o o
5.41 ' ~ "
•,,,~ii\ \l
\
..Q
~ " 0.5
T=10K )0
o'%. 0
"'~'~ o
\-
,~
5.3!
~a~..
%
\m \
I
oXoo o.o1 0.02 0.03
\
\ I
. o • A
• 0.04 a 0.05
~°'-~.o
tO
OoO"
La2_xSrxCuO4+y plnaae 1
• phase 1
Ba
~ '1.0 /
245
5.3~
a\
\
I
0.05 0.10 0.15 x in La2_xMxCUO4.y
0.20
Fig. 13. Compositional (x) dependence of the orthorhombicity at 10 K for oxygenated and Ar-annealed L a 2 _ xMxCuO4+ y ( M = St, Ba).
] 0 K. As can be seen in fig. 13, the superconducting phase 2 has almost the same orthorhombicity, independent of Sr or Ba concentration, which is also nearly equal to that for the At-annealed samples with the critical concentration (x=0.05 for Ba and x = 0 . 0 6 for Sr). This suggests the existence of the critical orthorhombicity for the superconductivity in these systems. The correlation between the orthorhombicity and superconductivity was also observed in YBCO with various Oxygen contents [ 20 ]. The present work also confirmed that the two phases with different oxygen content result from the phase separation on temperature under ambient atmosphere in the oxygenated La2_xMxCuO4.y (M = Sr, Ba). The oxygen content in oxides is generally controlled by the temperature and the partial oxygen pressure in a pseudobinary system. In the case of La2_xMxCuO4 +y the phase separation and the oxygen content of each phase resulting from the phase separation are independent of the oxygen partial pressure to which the compound is exposed. The phase separation in this system appears to be closely related to the structural properties. As mentioned above the excess oxygen as well as the substitution of Sr or Ba affects the distortion of the lattice. Figure 14 shows the temperature dependence of the b-axis for the oxygenated La2_xSrxCuO4+r In the region of two phases the b-axis of phase 2 hardly depends on the substitution in contrast to the b-axis of phase I.
5.3; 0
i 100
i 200
i 300
400
Temperature (K)
Fig. 14. Temperature dependence of the b-axis for oxygenated LaE_xSrxCuO4+y. The samples with x=0, 0.01, 0.02 and 0.03 show the phase separation and the length of the b-axis at the temperatures where the phase separation occurs is almost equal (broken line) in each sample.
Furthermore, the length of the b-axis at Tps for each sample has almost the same value (broken line in fig. 14) within experimental errors, and the samples with x > 0.04 whose b-axes are shorter than this value above 200 K (lower limit for the phase separation) no longer show the phase separation. Additionally the phase 2 has a smaller b-axis and longer c-axis than phase l, and consequently the unit cell volume of the two phases are almost the same, as shown in fig. 9. These are the characteristics of the oxygenated LaE_xMxCuO4+y ( M = S r , Ba), which suggest that the phase separation is controlled by the structural factor. Such features have not been observed in LaENiO4+y which was also a two-phase mixture but showed no phase separation under ambient atmosphere. The oxygen rich phase in LazNiO4.o7 [21 ] has a smaller orthorhombic strain as well as in LaECuO4.y but has a shorter c-axis and a smaller volume than the oxygen poor phase (at l0 K, a = 5.484 fi, b= 5.498 fi, c= 12.555 fi, V=385.56/~3 for the oxygen rich phase and a=5.452 fit, b=5.489 fi, c= 12.586 fi, V=376.64 fi3 for the oxygen poor phase). These suggest that the formation of the two phases with different oxygen content differs between the two systems in origin. The microscopic study on the phase separation in the oxygenated La2CuO4+y
246
Y. Ueda et al. / Properties of oxygenated La2_xMxCuO~+ r
by means of N Q R , electron microscope a n d DSC is now in progress.
5. Conclusion ( 1 ) The content of excess oxygen of oxygenated ( M = S r , Ba; 0 < x < 0 . 1 ) u n d e r P O 2 = 3 6 0 a t m was about 0.03 for x = 0 a n d decreased with increasing x. (2) The oxygenated samples showed superconductivity even in a lightly substituted region. (3) A phase separation into two phases with different orthorhombic strain was observed in the oxygenated samples with x-_<0.03 a n d the phase with smaller orthorhombic strain was the superconducting phase. (4) The superconductivity in this system is qualitatively explained by the average valence of Cu ions which is affected both by the substitution of divalent ions for La 3+ a n d by the oxygen nonstoichiometry. (5) Some characteristic behaviors of the lattice parameters were found, which suggest that the phase separation is controlled by the structural factor. La2_xMxCuO4+y
Acknowledgements The authors t h a n k K. Motoya (Saitama U n i v e r sity) for offering the opportunity to use a S Q U I D magnetometer. This work was supported by a " G r a n t in-Aid for Scientific Research on Chemistry of New Superconductors" from the Ministry of Education, Science a n d Culture.
References [ 1] J.G. Bednorz and K.A. Miiller, Z. Phys. B 64 (1986) 189. [2] See for example, T. Fujita, Y. Aoki, Y. Maeno, J. Sakurai, H. Fukuba and H. Fujii, Jpn. J. Appl.Phys. 26 (1987) L368; H. Takagi, T. Ido, S. Ishibashi, M. Uota, S. Uchida and Y. Tokura, Phys. Rev. B 40 (1989) 2254.
[3] K. Sekizawa, Y. Takano, H. Takigami, S. Tasaki and T. Inaba, Jpn. J. Appl. Phys. 26 (1987) L840. [4] P.M. Grant, S.S.P. Parkin, V.Y. Lee, E.M. Engler, M.L. Ramirez, J.F. Vazquez, G. Lim, R.D. Jacowitz and R.L. Greene, Phys. Rev. Lett. 58 (1987) 2482. [5]J. Beille, B. Chevalier, G. Demazeau, F. Deslandes, J. Etourneau, O. Laborde, C. Michel, P. Lejay, J. Provost, B. Raveau, A. Sulpice,J.L. Tholenceand R. Tournier, Physica B 146 (1987) 307. [6 ] J.L. Tholence, Physica B 148 (1987) 353. [7] J.E. Schirber, B. Morosin, R.M. Merrill, P.F. Hlava, E.L. Venturini, J.F. Kwak, P.J. Nigrey, R.J. Baughmanand D.S. Ginley, Physica C 152 (1988) 121. [8] G. Demazeau, F. Tresse, Th. Plante, B. Chevalier, J. Etourneau, C. Michel, H. Hervieu, B. Raveau, P. Lejay, A. Sulpice and R. Tournier, Physica C 153-155 (1988) 824. [9] J.D. Jorgensen, B. Dabrowski, Shiyou Pei, D.G. Hinks, L. Soderholm, B. Morosin, J.E. Schirber, E.L. Venturini and D.S. Ginley, Phys. Rev. B 38 (1988) 11337. [ 10] (a) C. Chaillout, S.W. Cheong, Z. Fisk, M.S. Lehmann, M. Marezio, B. Morosin and J.E. Schirber, Physica C 158 (1989) 183; (b) C. Chaillout, J. Chenavas, S.W. Cheong, Z. Fisk, M. Marezio, B. Morosin and J.E. Schirber, Physica C 170 (1990) 87. [ 11 ] M.F. Hundley, J.D. Thompson, S.W. Cheong, Z. Fisk and J.E. Schirber, Phys. Rev. B 41 (1990) 4062. [ 12] Y. Oda, M. Yamada, H. Ochiai, K. Asayama,T. Kohara, Y. Yamada, K. Koga, S. Kashiwai and M. Motayama, Solid State Commun. 73 (1990) 725. [ 13 ] J. Ryder, P.A. Midgley,R. Exley, R.J. Beynon, D.L. Yates, L. Afalfizand J.A. Wilson, Physica C 173 ( 1991 ) 9. [ 14] K.F. MaCarty, J.E. Schirber, S.W.Cheongand Z. Fisk, Phys. Rev. B 43 (1991) 7883. [ 15 ] Y. Oda, A. Sumiyama,T. Kohara, M. Yamada, K. Asayama, S. Kashiwaiand M. Motoyama, Physica C 185-189 ( 1991 ) 917. [ 16] T. Kohara, K. Ueda, Y. Kohori and Y. Oda, Physica C 185189 (1991) 1189. [17]B. Dabrowski, J.D. Jorgensen, D.G. Hinks, S. Pei, D.R. Richards, H.B. Vanfleet and D.L. Decker, Physica C 162164 (1989) 99. [ 18] F.C. Chou, J.H. Cho, L.L. Miller and D.C. Johnston, Phys. Rev. B42 (1990) 6172. [19]F. Izumi, J. Crystallogr. Soc. Jpn. 27 (1985) 23 (in Japanese). [20] A. Hayashi, Y. Fujiwara, Y. Ohya and Y. Ueda, Proc. 2nd ISSP Int. Symp. Physics and Chemistry of Oxide Superconductors, Tokyo, 1991 (Springer, Berlin, Heidelberg, 1992) p.197. [21 ] J.D. Jorgensen, B. Dabrowski, Shiyou Pei, D.R. Richards and D.G. Hinks, Phys. Rev. B 40 (1989) 2187.
Physica C 198 (1992) 237-246 North-Holland
Structural and superconducting properties of oxygenated La2_xMxCuO4+y(M=Sr, Ba; 0 < x < 0 . 1 ) Y. Ueda, Y. Fujiwara and A. Hayashi Institute for Solid State Physics, Universityof Tokyo, 7-22-I Roppongi, Minato-ku, Tokyo 106, Japan
K. Shibutani and R. Ogawa Superconducting and Cryogenic Technology Center, Kobe Steel, Ltd. 5-5, 1-chome, Takatsukadai, Nishi-ku, Kobe 651-22, Japan
Received 19 February 1992
Oxygenstoichiometry, superconductivity and structural properties ofLa2_xMxCuO4+y (M = Sr, Ba; 0_-
1. Introduction Since the first discovery o f a high-To superconductor by Bednorz and Miiller [ 1 ], the system La2_xMxCuO4 (M = Sr, Ba) has been extensively investigated. It was well established that the substitution of alkaline earth divalent ions for La in the insulative antiferromagnetic La2CuO4 leads to superconductivity above a critical composition o f the substituents [ 2 ]. In pure La2CuO4, on the other hand, a trace o f superconductivity was often observed in samples slowly cooled in oxygen or air [ 3,4 ] and then the oxygenated La2CuO4+y under high oxygen pressure [5-8 ] was confirmed to be a bulk superconductor. Jorgensen et al. [ 9 ] first showed by neutron powder diffraction that the oxygenated La2CuO4+y is a mixture o f two orthorhombic phases which results from a macroscopic phase separation near 320 K under ambient atmosphere. They also claimed that one phase is the insulative antiferromagnetic La2CuO4 with almost stoichiometric composition o f oxygen and the other the superconducting La2CuO4+y with excess oxygen. A two-phase refinement o f a crystal for the oxygenated LaECuO4+y was also car-
fled out by Chaillout et al. [ 10] and they asserted that the excess oxygen occupies the so-called "interstitial site (¼, ¼, z; z ~ ] )" in the form o f peroxide (O2) 2- ions. Anomalous behaviors of some physical properties have also been observed between 200 and 300 K in the oxygenated La2CuO4+y by some groups [ 1 1 - 1 6 ] , and those behaviors were attributed to phase separation without the direct observation by neutron or X-ray diffraction. One of our interests is how to understand this phase separation from a standpoint of thermodynamics or thermal equilibrium. A material prepared under high pressure~and high temperature often remains stable under ambient condition, when the constituent atoms cannot diffuse thermally under the environment. The phase separation in La2CuO4+y, however, shows that the excess oxygen atoms introduced under high oxygen pressure and high temperature diffuse in the crystal under ambient pressure at such a low temperature without decomposition (release o f the excess oxygen). Furthermore, this phase separation has no relation to the oxygen partial pressure to which the material is exposed; that is, the composition of oxygen in each phase (chemical potential
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All fights reserved.
238
Y. Uedaet al. / Propertiesof oxygenated Lae_xMxCuO4+y
of oxygen in both phases) is independent of the oxygen partial pressure. To the authors' knowledge there existed no such compound in oxides. La2NiOa÷y which is similar to La2CuO4 in structure and includes the excess oxygen in large quantity does not exhibit such a phase separation [ 17]. Another interest is what relation there exists between the superconductors resulting from the substitution of alkaline earth metal and from the oxygen nonstoichiometry. In this respect, there are very few reports. To make these problems clear, it is essential that the structural and physical properties of the oxygenated La2_xMxCuO4 +y (M = Sr, Ba) are investigated on the same samples. We synthesized Laa_xMxCuO4 +y (M = Sr, Ba) under PO2 = 360 atm and have investigated its structural and physical properties. In the present paper we report the phase relation and superconductivity of the oxygenated La2_~MxCuO4+y ( M = S r , Ba; 0 < x < 0 . 1 ) . We also report the detailed analysis of oxygen stoichiometry and the decomposition process of oxygenated sampies. The two phase behavior was clearly observed in the samples with x~< 0.02 by X-ray diffraction and the phase separation trends to be suppressed by the substitution of Sr or Ba. We also present the characteristics of this system in the structural properties.
2. Experimental Starting materials were prepared by solid state reaction of an appropriate mixture of predried La203, SrCO3 (or BaCO3) and CuO. The powders were pressed into a pellet, calcined at 900°C for 24 h, then ground, pelletized, and sintered at 1000-1100 °C for 120 h in air with several intermediate grindings. The powder samples obtained were annealed under PO2= 360 atm at 600°C for 6 h using the Kobe Steel O2-HIP system 30T, and were cooled in the furnace prior to the release of applied pressure. To compare with the oxygenated samples, a part of the starting materials was annealed in flowing pure Ar gas at 900 ° C for 48 h. All samples thus prepared were confirmed by X-ray powder diffraction to include no impurity phases. Thermogravimetric and differential thermal analysis were made using a Mac Science TG-DTA 2000
system. Electrical resistivity was measured by a standard DC four-probe method, supplying a current of 1-5 mA from 4.2 K to 280 K. The magnetization was measured under magnetic fields up to 10 G using a Quantum Design MPMS SQUID magnetometer in the temperature range from 4.2 to 50 K. X-ray powder diffraction measurements were carried out using a Mac Science MXP ~8 system with a rotating anode generator and a monochromator of single crystalline graphite for Cu Ket radiation. A closed-cycle helium refrigerator was used to control the temperature between 10 K and 350 K.
3. Results
3.1. Thermogravimetric and differential thermal analysis (TG-DTA) TG-DTA as carried out for the purpose of examining the oxygen stoichiometry and the thermal decomposition. Before the investigation on the oxygenated La2_xMxCuO4+y, the oxygen stoichiometry of Ar-annealed samples was determined by reducing the samples in hydrogen. Figure 1 shows a typical TG-DTA curve for Ar-annealed LaL92Sro.oaCuOy,, heated at 2 K / m i n in flowing hydrogen (4 mol% H2 in He, 400 m l / m i n ) . The compound decreased in weight stepwise showing exo- and endothermic peaks in the DTA curve, as shown in fig. 1. Assuming that the compound is finally decomposed into a mixture of Cu metal, SrO and La203, the oxygen content for this compound was estimated to be 4.00 + 0.005. The oxygen stoichiometry determined by this method was 4.00+0.005 for all Ar-annealed La2_xMxCuOy, ( M = S r , Ba; 0 < x < 0 . 1 ). A plateau around 600 K in the TG curve was observed in all samples. Chou et al. [ 18 ] reported that the plateau in the T G curve for La2CuO4 indicates the formation of a new Sr2CuO3-type phase, La2CuO3.67. In our measurements the oxygen content for the intermediate compounds was about 3.83 and was independent of Sr (or Ba) content x, which are much higher than the value of 3.67. Since the oxygen stoichiometry of Ar-annealed samples was 4.00 independent of Sr or Ba content, the amount of excess oxygen in the oxygenated samples was determined by TG-DTA in flowing pure Ar
239
Y. Ueda et al. / Properties of oxygenated La2_xMxCuO4+y
i
La1.92Sro.oaCuOy, inH2(4%)/Hegas Y'=3"83
y' =4.00 -i - ~ - -
~
50 40
v
3O ~>
§
tO t-
:
/
2O < v
10
y'= 2.96
400
600 800 Temperature (K)
I
1000
Fig. 1. TG-DTA curves for Ar-annealed Lal.92Sro.oaCuOy, heated in flowinghydrogen (4 mol% H2 in He). The oxygen stoichiometryy' was estimated by assuming that the compound is decomposed into La203, SrO and Cu, that is, the decomposed product has total oxygen content y' = 2.96. i
I
I
La2_xSrxCuO4+y(Po2=360arm) ..--. 0
.
0
0
I
in Ar gas
~ x =0.06
(~
~
002
.~ ~-0.05
~
-~--"-~
--
0.04
-0.10
0'00~---~
I
400
I
600
1
800
I
1000
Temperature (~ Fig. 2. Weight-losscurves for oxygenated La2_~SrxCuO4+y (PO 2= 360 arm) heated in flowingAr. Note that the amount of weight-loss corresponds to the excess oxygeny. gas. Figure 2 shows the weight loss versus temperature profiles for the oxygenated La2_xSrxCuO4+y in flowing Ar ( > 99.9999%, 200 m l / m i n ) at a heating rate of 10 K / r a i n . The excess oxygen was released
a r o u n d 600 K in all samples a n d the weight loss of about 0.1% for La2CuO,+ r decreases with Sr content x. Figure 3 shows the x dependence of the oxygen stoichiometry for the Ar annealed a n d the oxygen-
Y. Ueda et al. /Properties of oxygenated Lae_~M~CuO4+y
240
0
0.030 Sr
"
2
0
/
\
1
, , 2 " 0 (a) La2_xBa,,CuO,.oo
Po~ = 360 atm
0.025 P% = 360 atm
\ 0.020
>.,
0.015
8~
o~O\
\ 0.010
\\ 0.005 o
00001: 0.00
0.02 0.04 0.06 x in La2_xMxCUO4¢y
.~" 0.10 -~;
1.0
0.05
0.5
"~. 0.08 I/ 0.00 II
Fig. 3. Excess oxygen content of Ar-annealed and oxygenated La2_xM~CuOa+y (M=Sr, Ba) determined by TG measurements. Note that the Ar-annealed samples have almost stoichiometric oxygen content 4.00 independent of Sr or Ba content and the oxygenated samples have excess oxygen.
0
[
50
f [
1O0
0.10
i ,~
~
150
200
x=°°"l I /
250
0.0 300
i i i I fo) La2-xBa, CuO4*y --I 0.25
°°8/ I\
7 °'2°
"N
Electrical resistivity and magnetic susceptibility were measured to confirm the superconducting transition. Figure 4(a) shows the resistivity versus temperature curves for La2_xBaxCuO4.oo annealed in flowing Ar gas. La2_xM~CuO4.oo shows no superconducting transition in a lightly substituted region and starts to exhibit superconductivity at x = 0.06 for Sr and x = 0 . 0 5 for Ba, agreeing with the previous results [ 2 ]. All of the oxygenated La2_ xMxCuO4 + y, on the other hand, shows a drop of resistivity suggesting the superconducting transition, as shown in fig. 4 (b). Figure 5 shows the compositional dependence of the temperature, T¢, from where the resistivity abruptly decreases with decreasing temperature. Tes decrease and then increase on increasing the content of the substituents, showing a minimum around x = 0 . 0 5 for Sr and x = 0 . 0 4 for Ba. The magnetic susceptibility versus temperature curves showed a low-field flux exclusion in all oxy-
I
T (K)
ated La2_xMxCuO4+y ( M = S r , Ba; 0 < x < 0 . 1 ) . In the samples oxygenated under PO2=360 atm, the content of excess oxygen (y) is about 0.03 for the sample with x = 0 and decreases with increasing x, having a constant value around x = 0.02-0.04. 3.2. Electrical resistivity and magnetic susceptibility
I
1o.,o
"~ o.o4
I1#1
0.00 ~ 0
50
100
150
200
250
I 0.00
300
T (K)
Fig. 4 (a) Resistivity vs. temperature curves for (a) Ar-annealed and (b) oxygenated Laz_xBaxCuO4+y. Note that all of the oxygenated samples exhibit the superconducting transition.
genated samples. A typical example for the oxygenated La2CuO4.o3 is shown in the inset of fig. 6. Figure 6 shows the compositional dependence of the magnetic susceptibility at 5 K as a measure of the relative flux exclusion, where it was assumed that all samples had almost the same particle size. The magnetic susceptibility at 5 K decreases with increasing Sr or Ba concentration and then steeply increases above
Y. Ueda et al. / Properties of oxygenated La2_xMxCuO~+y 50
'
'
o
'I
[
Po~ = 360 arm
40 0 \
o/:1
0
0
30
o° tO
~/a~//.
20
// ,,o : /
/
10
I 0.02
LI
O0.00
x
I 0.04
in
/
; I 0.06
0.08
La2_xMxCUO4+y
Fig. 5. The compositional (x) dependence of the superconducting transition temperature in Ar-annealed and oxygenated La2_xMxCuO4+y(M = Sr, Ba), Tco,~tshowsthe temperature from where the resistivity abruptly decreases with decreasingtemperature. In Ar-annealedsamples the superconductingtransition was observed above x=0.06 for Sr-substituted samples and above x=0.05 for Ba. 10 0
0.~1
j
, , , O.Oi La2Cu04.o3
j
10 "1
10 .2
E ~'~ '1-
o Sr o Ba
110 20
3LO 40
T (K)
a~
10-3
o ~
10.4
/ o
~
10s
T=5K -61
100.00
I
I
L
0.02 0.04 0.06 X in La2_xMxCUO4+y
0.08
Fig. 6. Compositional (x) dependenceof magnetic susceptibility at 5 K for oxygenatedLa2_xMxCuO4+y(M=Sr, Ba). The magnetic susceptibility vs. temperature curves showed the low-field flux exclusion in all oxygenatedsamples. A typical example for the oxygenatedLa2CuO4.o3is shown in the inset. x = 0 . 0 5 where the Ar-annealed samples show superconductivity. It was confirmed from the resistivity and magnetic susceptibility measurements that the oxygenated La2_xMxCuO4 +y (M = Sr, Ba ) under PO2 = 360 atm
241
is a superconductor even in the lightly substituted region.
3.3. Powder X-ray diffraction The oxygenated La2Cu04.o3 was found by the Xray diffraction at 10 K to be a mixture of two phases with a nearly identical structure but different orthorhombic distortion. The lattice parameters and the relative fraction of the two phases were determined by Rietveld refinements based on a mixture of two orthorhombic Bmab phases (the phase 1 and 2), using the analysis program RIETAN [ 19 ]. The refined lattice parameters of each phase at 10 K are in good agreement with the neutron diffraction data [ 9 ], and the relative fraction of the two phases from this analysis is approximately 66% for the phase 1 and 34% for the phase 2, where the phase 1 is the phase with larger orthorhombic distortion. Jorgensen et al. [ 9] concluded that the phase with larger orthorhombic distortion is the insulative phase with almost stoichiometric oxygen content of 4.00, while the superconducting phase with the smaller orthorhombic distortion has excess oxygen. Also in our investigation the phase 1 can be regarded to have the oxygen stoichiometry of 4.00 because the phase 1 at 10 K has almost the same lattice parameters as Ar-annealed La2CuO4.oo, and the oxygen content of the phase 2 is estimated to be about 4.08 in connection with the results of thermogravimetric analysis. Heating the oxygenated La2CuO4.o3 in flowing Ar gas led to a single phase with the same lattice parameters as those of phase l at l 0 K, which showed no superconductivity. So phase 2 was concluded to be a superconducting phase. Figure 7 illustrates the Sr content x dependence of Bragg peaks 400 and 040 for the oxygenated Laa_xSrxCuO4+y at 10 K, where the contribution of Cu Kct2 is arithmetically subtracted. The coexistence of two orthorhombic phases can be clearly seen in the samples with x = 0 , 0.01 and 0.02. The peaks on the higher angle side of 040 peaks of phase 1 are 040 peaks of phase 2, while 400 peaks of the two phases exactly overlap each other. The sample with x = 0.03 was judged to be a mixture of two phases because the 040 peak is rather broad compared with the 400 peak. When Sr or Ba is substituted, the 040 peak of the phase 1 approaches the 040 peak of the phase 2 with
Y. Ueda et al. / Properties of oxygenated La2_~M~CuO4+y
242
n
r
n
i
i
i
I
i
La2_xSrxCuO4+y (Po= = 360 atm)
T=
5.42 = ~ l
T=IOK
10 K
ptwel
_I R;
540
m ~
~
m
5.38
:o.oo
~.
0.01
5.36 a 5.34
0.03
--o-
.o--
o--o--
o-
0.04 5.32
,OW'{~ phase1 Ba{ : phasel
0.05
phase 2
13,18 -
phase 2
//e/
0.06 I
~.s 89.0 6Es 7Eo 7Es 7;.o 71.s 20 (deg) Fig. 7. Sr content x dependence of Bragg peaks 400 and 040 for oxygenated La2_:,Sr~CuO4+y at 10 K. The contribution of Cu Kct2 are subtracted in these figures. The compounds with x < 0.03 are a mixture of two phases with different orthorhombic strain. The relative fraction of phase 2 seems to be almost the same (34%) independent of Sr or Ba content.
increasing x and the two 040 peaks merge to form the single 040 peak in the samples with x > 0.04. The relative fraction of each phase in the samples with x=0.01 and 0.02 was determined by a multiple-peak separation technique using only the 040 peaks, since the Rietveld refinements for the samples were unsuccessful. The result obtained for the sample with x = 0 by this technique was in good agreement with that from the Rietveld refinements, and the fraction of phase 2 was about 30% independent of x. Such behaviors were also observed for Ba substituted compounds. Figure 8 shows Sr (or Ba) content x dependence of lattice parameters at 10 K for the oxygenated samples. The a- and c-axis are longer in a Ba substituted system than in Sr, reflecting the slightly larger ionic radii of Ba 2÷ than that of Sr 2+. On the other hand the b-axis rather depends on the concentration of the substituents; nevertheless it is almost the same in the two systems over the region from twophase to single-phase.
o
13.14 13.12 13.1
•
•
~}~1 . 0
0.02
0.04
0.06
0.08
x in La2_xMxCUO4+y Fig. 8. Compositional (x) dependence of the lattice parameters at 10 K for oxygenated La2_xMxCuO4+r (M = Sr, Ba). Note that only the b-axisshows the same dependence between Sr- and Basubstituted samples. Next in order to examine the phase separation, Xray diffraction measurements for the oxygenated samples were performed as a function of temperature from l0 to 350 K. Figure 9 shows the temperature dependence of lattice parameters for the oxygenated LaE_xSrxCuO4+y with x = 0 and 0.01, which was completely reversible. The temperature dependence of lattice parameters for the samples with x = 0 is in good agreement with the neutron diffraction data [ 9 ]. As can be seen in fig. 9, the phase separation occurs around 300 K in our sample, La2CuO4.o3. Figure 10 shows the temperatures ( T ~ ) where the phase separation occurs in the oxygenated LaE_xMxCuO4+y (M = Sr, Ba), which were determined by measuring the temperature dependence of the full-width at half maximum ( F W H M ) of the 006 peak (the inset of -~fig. 10). The phase separation seems to be suppressed by the substitution of Sr or Ba. To discuss
243
Y. Ueda et al. / Properties of oxygenated La2_~l~CuO~+y
i
i
"l
i
La2_xSrxCuO4.y (Po2 = 360 atm) 381
38o >~ 379
o 0.00 A 0.01
• phase1 I x = 0 0 o phase2 I ' • phase1 I x = 0 0 1 [] phase2 I "
"~P
o_ ..,a.,.'Z'$".~ ~ , ~ 1 ~1 _
~0.15
.__,__~o:...~o:~~:~
~e~
I-
A
o.1
200
T (K)
LB~o./°
J i
378
J
150
b
5.42
=St ~=
•
01,
o.oo
5.40
~"
o~
o.~6
0.08
x in La2_xMxCUO4.y
Fig. 10. Compositional (x) dependenceof the phase separation temperature in oxygenatedLa2_~MxCuO4+y (M = Sr, Ba). Tp,was determined from the temperature dependence of the full-width at half maximum (FWHM) of the 006 Bragg peak, as shown in
5.38
J~
¢r
o.~2
Ba
5.36
the inset. 5.34 -u --°I~I~L-I
5.32 006 o ..~o-aO.5°~
° 13.13L I;
13.10/~e~ 0
mA
La2CuO4"03
~=,1
040
.
115
~
400
A
p~,,1
A
.-./"~'¢:~. i 100
i 200
i 300
Temperature (K)
Fig. 9. Temperature dependence of lattice parameters for oxygenated La2_~Sr~CuO4+ywith x=0 and 0.01. Note that both samples show the phase separation into phase 1 and 2 around 300 K.
JLY/ XL~ K
this more precisely one would need to establish the phase diagram both on the oxygen nonstoichiometry and the substitution, because this suppression may be due to the decrease of excess oxygen accompanied by the substitution. An X-ray diffraction study of the samples with various excess oxygen contents is in progress. Figure 11 shows the temperature dependence of Bragg peaks 006 and 040 in the oxygenated La2CuO4.o3. As can be seen in fig. 11, the phase 2 is a minor phase at low temperature and increases in fraction with increasing temperature. The change in fraction was also reversible and was observed above 200 K, which suggests that the lower limit of temperature for the phase separation is about 200 K. Jorgensen et al. [ 9 ] reported that the relative fraction of the two phase in the oxygenated La2CuO4+y
40.5
41.0
41.5
42.0
42.5 69.0 26 (deg)
69.5
70.0
70.5
71.0
Fig. 11. Temperature dependenceof the portion of X-raydiffraction pattern for oxygenatedLa2CuO4.o3.The relative fraction of the phase 2 seemsto increase with increasingtemperature. is almost independent of temperature, while Chaillout et al. [ lO(b) ] reported that the phase 2 is a major phase at low temperature and increases in fraction with increasing temperature. Such discrepancy among three groups may be due to the difference of the excess oxygen content in the samples.
244
Y. Ueda et al. I Properties of oxygenated La2_xMxCtlO4+y
4. Discussion Present experiments revealed that the oxygenated LaE_xMxCuO4+y ( M = S r , Ba; 0_
0.00 4.1(
0.02
0.04
I
i
(a)
0.06
0.06
I
0.10
I
o Sr Po~ = 360 ,~rn o Ba J
\o 4.0.~
~
~\I\
>, +
Q-~._
/
4.00
Ar
/
Q" " ~ Q
o--o--o
""- .-T--Y=-2 lx
(Cu 2*)
3.95
"- - -
i s:}o ._s 2 5T\° 8
.
-..
}
r\
2.05 . . . . . . . . . .
t
-
~
..........
eQ---
< 2.00
~~/Ar
Ba
I
O.OI
0.02 X in
I
I
I
0.04
0.06
0.06
0.10
ka2_xMxCUO4+y
Fig. 12. (a) Compositional (x) dependence of the oxygencontent for the phase with excessoxygenin L a 2 _ xMxCU04+y (M----St, Ba). The solid line represents the oxygencontent (4.00) for Arannealed samples and the broken line shows the compositional dependence of oxygencontent in the compoundswith constant average valence, Cu2+ (La~+xM2+Cu2+O24+y).(b) Compositional (x) dependence of the average valence of Cu ions for the phase with excess oxygenin La2_xMxCuO4+y (M= St, Ba). Open and closed symbols represent the cases that the excess oxygens exist in the form of O2- and (O2)2-, respectively. The solid line represents the average valence of Cu ions for Ar-annealed samples. Broken lines indicate the minimum values of the average valence of Cu ions for the superconductivity (see the text). not be simply described in terms of the average valence of Cu ions. The orthorhombic strain is also affected by both the oxygen nonstoichiometry and the substitution. Figure 13 shows the orthorhombicity defined as 2(b-a)/(a+b) for the oxygenated and At-annealed LaE_xMxCuO4+y ( M = S r , Ba; 0-
Y. Ueda et al. / Properties of oxygenated La2_~M~CuO4+y 2.0,
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] 0 K. As can be seen in fig. 13, the superconducting phase 2 has almost the same orthorhombicity, independent of Sr or Ba concentration, which is also nearly equal to that for the At-annealed samples with the critical concentration (x=0.05 for Ba and x = 0 . 0 6 for Sr). This suggests the existence of the critical orthorhombicity for the superconductivity in these systems. The correlation between the orthorhombicity and superconductivity was also observed in YBCO with various Oxygen contents [ 20 ]. The present work also confirmed that the two phases with different oxygen content result from the phase separation on temperature under ambient atmosphere in the oxygenated La2_xMxCuO4.y (M = Sr, Ba). The oxygen content in oxides is generally controlled by the temperature and the partial oxygen pressure in a pseudobinary system. In the case of La2_xMxCuO4 +y the phase separation and the oxygen content of each phase resulting from the phase separation are independent of the oxygen partial pressure to which the compound is exposed. The phase separation in this system appears to be closely related to the structural properties. As mentioned above the excess oxygen as well as the substitution of Sr or Ba affects the distortion of the lattice. Figure 14 shows the temperature dependence of the b-axis for the oxygenated La2_xSrxCuO4+r In the region of two phases the b-axis of phase 2 hardly depends on the substitution in contrast to the b-axis of phase I.
5.3; 0
i 100
i 200
i 300
400
Temperature (K)
Fig. 14. Temperature dependence of the b-axis for oxygenated LaE_xSrxCuO4+y. The samples with x=0, 0.01, 0.02 and 0.03 show the phase separation and the length of the b-axis at the temperatures where the phase separation occurs is almost equal (broken line) in each sample.
Furthermore, the length of the b-axis at Tps for each sample has almost the same value (broken line in fig. 14) within experimental errors, and the samples with x > 0.04 whose b-axes are shorter than this value above 200 K (lower limit for the phase separation) no longer show the phase separation. Additionally the phase 2 has a smaller b-axis and longer c-axis than phase l, and consequently the unit cell volume of the two phases are almost the same, as shown in fig. 9. These are the characteristics of the oxygenated LaE_xMxCuO4+y ( M = S r , Ba), which suggest that the phase separation is controlled by the structural factor. Such features have not been observed in LaENiO4+y which was also a two-phase mixture but showed no phase separation under ambient atmosphere. The oxygen rich phase in LazNiO4.o7 [21 ] has a smaller orthorhombic strain as well as in LaECuO4.y but has a shorter c-axis and a smaller volume than the oxygen poor phase (at l0 K, a = 5.484 fi, b= 5.498 fi, c= 12.555 fi, V=385.56/~3 for the oxygen rich phase and a=5.452 fit, b=5.489 fi, c= 12.586 fi, V=376.64 fi3 for the oxygen poor phase). These suggest that the formation of the two phases with different oxygen content differs between the two systems in origin. The microscopic study on the phase separation in the oxygenated La2CuO4+y
246
Y. Ueda et al. / Properties of oxygenated La2_xMxCuO~+ r
by means of N Q R , electron microscope a n d DSC is now in progress.
5. Conclusion ( 1 ) The content of excess oxygen of oxygenated ( M = S r , Ba; 0 < x < 0 . 1 ) u n d e r P O 2 = 3 6 0 a t m was about 0.03 for x = 0 a n d decreased with increasing x. (2) The oxygenated samples showed superconductivity even in a lightly substituted region. (3) A phase separation into two phases with different orthorhombic strain was observed in the oxygenated samples with x-_<0.03 a n d the phase with smaller orthorhombic strain was the superconducting phase. (4) The superconductivity in this system is qualitatively explained by the average valence of Cu ions which is affected both by the substitution of divalent ions for La 3+ a n d by the oxygen nonstoichiometry. (5) Some characteristic behaviors of the lattice parameters were found, which suggest that the phase separation is controlled by the structural factor. La2_xMxCuO4+y
Acknowledgements The authors t h a n k K. Motoya (Saitama U n i v e r sity) for offering the opportunity to use a S Q U I D magnetometer. This work was supported by a " G r a n t in-Aid for Scientific Research on Chemistry of New Superconductors" from the Ministry of Education, Science a n d Culture.
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