Complex susceptibilities of Co-substituted YBa2Cu3O7−d synthesized by the polymerized complex method

PHYSICA@ ELSEVIER

Physica C 246 (1995) 37-45

Complex susceptibilities of Co-substituted YBa 2Cu307-d synthesized by the polymerized complex method Hiromasa Mazaki a, *, Hiroshi Yasuoka a, Masato Kakihana b, Hirotaka Fujimori Masatomo Yashima b, Masahiro Yoshimura b

b 9

a Department of Mathematics and Physics, The National Defense Academy, Yokosuka 239, Japan b Research Laboratory of Engineering Materials, TokyoInstitute of Technology, Yokohama227, Japan

Received 4 August 1994; revised manuscript received 25 January 1995

Abstract

Practically impurity-free ceramic samples of YBa2Cu3_xCOxOT_ d with x = 0-0.5 and Y0.8Ca0.2Ba2Cu2.8Co0.2OT_d were synthesized by the polymerized complex method through two different routes, starting either from metal nitrates (route P) or from metal carbonates (route Q). Better compositional homogeneity was achieved in the latter route, where neither colloids nor precipitates were formed during the polymerization procedure. The magnetic response of the samples was measured in terms of AC magnetic susceptibilities and variation of the onset temperature T~(onset) of the superconducting transition was precisely determined as a function of Co content. The T~(onset) were almost unaffected for 0 ~ 0.4. The depression of Tc was interpreted in terms of the localization of holes in the Cu(1)Oy planes. The replacement of 20% of Y with Ca in YBazCu2.sCoo.207_ a enhanced Tc from the initial value of 57.1 K up to 88.6 K. It was suggested that the rise in T~ induced by Ca doping was connected with the delocalization effect of holes in which the hole charge transfer from the Cu(1)Oy planes to the Cu(2)O 2 conduction sheets played a crucial role.

1. Introduction

As one of the useful probes to examine the superconductivity of the cuprate superconductors, many investigations have been carried out by substituting various impurities for the Cu sites, causing structural and compositional modification of the materials [1]. In the case of YBa2Cu307_ d (YBCO), it is reported that the stoichiometry and oxygen ordering are crucial for the critical temperature Tc and that Tc is depressed by the increase of substituted impurities.

* Corresponding author.

This effect occurs for substitutions either in the Cu(1)O chains or in the Cu(2)O2 sheets, suggesting that both of these play an important role for the superconductivity [1,2]. The superconductivity of YBCO is generally believed to occur in the Cu(2)O z sheets through hole charge carriers and the oxygen amount of the Cu(1)O chains governs the hole carrier concentration in the Cu(2)O 2 sheets. From this correlation, the Cu(1)O chains are considered as charge reservoirs [3,4]. In the case where the Cu(1)O chains can contain any disordering of oxygens, we use the notation 'Cu(1)Oy planes' instead of 'Cu(1)O chains' throughout this paper.

0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0921-4534(95)00121-2

38

H. Mazaki et al./ Physica C 246 (1995) 37-45

The effect of partial substitution of Co for Cu in YBa2Cu3-xC°xO7-d on the superconducting critical temperature T~ has been extensively studied with a special interest [5-15], as the dopant Co almost predominantly occupies the Cu(1) chain site with a wide range of Co concentration (0 ~
used for testing the validity of the charge transfer picture previously proposed in YBa2Cu3_xCoxO7_ d where an increasing number of holes becomes localized within the Cu(1)Oy planes with increasing Co concentration. The polymerized complex method is suitable in particular for such a substitution study in which homogeneous distribution of dopants in the lattice is crucial. The method is known to be capable of giving very homogeneous and practically impurity-free superconducting samples as have been confirmed in the bulk synthesis of outstandingly pure YBa2Cu307_ d [20], YBa2Cu408 [21], Y2BaaCu7 O15_ d [22], Bi2Sr2Cal_xYxCu208+ d [23] and Bil. 6Pb0.4Sr2Ca2Cu3Oy [24]. Examinations of the quality of the samples are made by measuring their magnetic response in terms of complex susceptibility X = X' -ix". It should be emphasized that X reflects sensitively the nature of microstructures of superconducting ceramics and importantly their superconducting onset temperatures can be very precisely determined from off-balance temperatures where null-signals of X are broken down. We present experimental details of the polymerized complex synthesis of Co-doped YBCO with an emphasis of the principal advantage of using metal carbonates instead of using metal nitrates as starting chemicals. Of particular importance is the achievement of direct gelation without any precipitates, which means that segregation of metal ions can be avoided and thereby an improvement of the compositional homogeneity of the final product can be expected. The depression of Tc with increasing Co concentration and the restoration of Tc brought about by extra substitution of Ca for Y are discussed in terms of variations of the hole population between the Cu(2)O 2 sheets and the Cu(1)O r planes.

2. Experimental and results 2.1. Sample preparation

Two synthetic procedures are used to prepare the Co-substituted YBCO samples. The first synthetic procedure basically follows the method previously described by the authors [20] (route P; the conventional polymerized complex route starting from metal nitrates dissolved into a mixed solvent of water and

39

H. Mazaki et al. /Physica C 246 (1995) 37-45

ethylene glycol with an approximately equivalent amount of citric acid). In the second synthetic procedure, metal carbonates are used as starting chemicals which are dissolved into ethylene glycol with large excess of citric acid (route Q; an improved polymerized complex route). A marked difference develops when the starting solution is condensed to produce a polymeric gel: a colloid is formed before gelation in the route P, while direct gelation occurs without passing through formation of colloid or sol in the route Q. A detailed synthetic procedure is described below. In the route P, metal-nitrate salts Y ( N O 3 ) 3 • nH20, Ba(NO3)2, Cu(NO3) 2 • n H 2 0 and Co(NO3) 2 • n H 2 0 were used as starting chemicals. The metal content in each nitrate salt was precisely determined by EDTA (ethylenediamine tetraacetic acid) titrations. The required amount of each nitrate salt was mixed together with de-ionized water. Citric acid and ethylene glycol were added in the proportions of 0.7 and 24 mol, respectively, for 1 mol of total metals. The solution was heated and condensed at 110°C. As the solution became concentrated, a brown colloid was formed with evolution of NO x gas resulting from decomposition of the nitrate ions. After completion of this reaction the temperature of the colloid solution was increased up to 190°C to promote polyesterification between ethylene glycol and citrate complexes. Viscous polymeric products thus obtained were decomposed to a fine powder on a hot plate at about 350°C. The powder (referred as 'precursor') was pyrolyzed at 940°C for 30 h under an atmosphere of flowing oxygen. The resulting black product was crushed, pressed into pellets (13 mm diameter and 4 mm thickness), sintered at 947°C for 20 h for annealing, and allowed to cool down to room temperature. This procedure was repeated twice with a change in sintering temperature from 947°C to 952°C. All the heating and cooling procedures took place in an atmosphere of flowing oxygen. Eight samples with x = 0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4 and 0.5 were prepared through the route P. In order to make a possible improvement in the homogeneity with regard to distributions of metal ions in a polymeric gel, a direct gelation route without passing through formation of colloid was attempted. This procedure (route Q) was achieved by using the large excess of cirtic acid relative to metal

Y2(CO3)3"3H20 + 4BaCO3 + xCoCO3 + I (3-x) CuCO3"Cu(OH)2 in c i t r i c acid / e t h y l e n e g l y c o l

I

I evolution of CO2 gas I

Metal-Citrate

Complexes

[ C o n d e n s e at 1 1 0 - 1 3 0 *C I [Polyesterification

1

[gelation]

¢

Polymerized Complex I'Pyrolysis a t 350 "C[

¢

Powder Precursor C a l c i n a t i o n at 940 °C f o r 6 h ( l s t time) and for 12 h ( 2 n d t i m e ) in 02 [Pulverization Pelletization

l

]Sintering [Cooling

1

u n d e r a2Pressure of 3000kg/cm

a t 950 *C f o r 36 h in O21

d o w n to 400 °C w i t h - l ' C / m i n i

[Annealing

at 400 °C f o r 12 h in O2[,

YBa2Cu3.xCoxO7.d Fig. 1. Flow chart for the polymerized complex procedure used to synthesize YBa2Cu3_xCoxO7_a, where starting chemicals are metal carbonates.

ions. The sample preparation procedure is summarized in Fig. 1. The basic procedure in the route Q is similar to that in the route P. It includes, however, six major modifications: (1) Metal carbonates, Y2(CO3)3.3H20 , BaCO3, CuCO 3 • Cu(OH) 2 and COCO3, are used as starting chemicals instead of metal nitrates. (2) Citric acid is used in the proportion of 5 mol for 1 mol of total metals. The quantity of citric acid required in the route Q is 7 times larger than that in the route P. (3) No additional water is used except for crystalline waters in the yttrium carbonate. Only ethylene glycol is used as a solvent. (4) The metal carbonates are directly dissolved in ethylene glycol with citric acid. In contrast to the route P where decomposition of nitrate ions with evolution of NO x gas occurs during the condensa-

40

H. Mazaki et al. / Physica C 246 (1995) 37-45

,//

A_

. . . .

L ~ .

20

40

30

20

(deg)

Fig. 2. PowderX-ray diffractionpatterns of polycrystallineYBa2Cu3_~CoxO7_ d synthesizedthrough the route P. tion, the route Q involves no such gas evolution during the condensation as carbonate ions are decomposed to evolve CO 2 gas already at the early stage of the procedure (Fig. 1). (5) Noteworthy is that neither colloid nor sol has been formed during the polyesterification. (6) Powder precursors were calcined twice at 940°C for 6 h (lst time) and for 12 h (2nd time). The pelletized samples were sintered at 950°C for 36 h and annealed at 400°C for 12 h. Ten samples with x = 0, 0.03, 0.05, 0.07, 0.1, 0.13, 0.15, 0.17, 0.2 and 0.3 were prepared through the route Q. One more sample with a composition Y0.sCa0.2Ba2Cu28Co0.2OT_ a was prepared using CaCO 3 as a source of Ca in a way similar to the route Q. 2.2. X-ray diffraction analysis Powder X-ray diffraction measurements were carried out with a conventional X-ray diffractometer utilizing Cu K o~ radiation. In Fig. 2, we show typical X-ray diffraction patterns for the YBa2Cu3_xCOxO7_d series prepared through the route P from x = 0 to 0.5. The lattice parameters were calculated by a standard leastsquares method fitting to the Bragg peak positions. The results are shown in Fig. 3. The replacement of Cu by Co ions induced an abrupt change of a and b

parameters in the range of 0 < x < 0.05. It has been reported that the orthorhombic to tetragonal ( O - T ) transition for YBa2Cu3_xCOx07_ a takes place at a Co content close to x c = 0.075 [7]. However, the discrepancy of the critical concentration x c for the O - T transition between the two may not be due to an intrinsic difference of the microstructure in the individual samples. In our x = 0.05 sample, a potentially small orthorhombic distortion might be present, which cannot be resolved due to our instrumental resoltion a n d / o r peak broadening resulting from the

3.90

'

C/3

3.88

"~ E

~

'

'

'

178

,

b

177

3.86

~ E

.m

m Q-

,_1

.,

3.84

175

3.82

3.80

a

~

174

,

0

~

'

0.2 X

,

'

0.4

Fig. 3. Latticeparametersand cell volumeof Y n a 2 C u synthesized through the route P.

,

173

3 _ xCOxO7 _

d

H. Mazaki et al. / Physica C 246 (1995) 37-45

41

Si •

X .3

20

30

40

50 20

60

70

(deg)

Fig. 4. Powder X-ray diffraction patterns of polycrystalline YBa2Cu 3_ xCoxO7 a synthesized throuj~h the route Q.

characteristics of our polymer gel-derived sample with relatively smaller grains. For this reason, peak positions of the samples with compositions x >I 0.05 were fit to tetragonal parameters. In order to gain further insight into the O - T transition, the lattice parameters of the samples prepared through the route Q were determined by the Rietveld analysis [25] of the X-ray powder diffraction data (Fig. 4) collected at a step rate of 0.02 ° and a sampling time of 5 s with an internal Si standard, where the crystal structures for samples with x ~< 0.07 and with x >t 0.1 are assumed to be orthorhombic (Pmmm) and tetragonal (P4/mmm), respectively [7]. The substitution-induced O - T transition occurring in YBa2Cu3_xCOx07_ a is indicated by the behavior of the lattice parameters as a function of Co concentration x in Fig. 5. The observation of the O - T transition of YBa2Cu3_xCOx07_ d between x = 0.07 and 0.1 and the general behavior of the lattice parameters on the Co concentration (i.e. (i) in the tetragonal regime the length of the a axis increases monotonically up to the maximum concentration investigated, (ii) the length of the c axis at

higher Co content (x > 0.1) decreases very slightly and (iii) the overall volume of the unit-cell lattice increases throughout the investigated concentration range), are in good agreement with the results reported by other groups [7,8]. 3.90

,

,

, , -- ~

,

C/3

,

~ 178 -4

e--

=~,,,~ 3.88

I

~

177

t.

•=

o<~

3.88

1T8

E ¢g

E

Q. 3.84

175

.,,., 3.82 ,_1

174 t~ o~

3.80 0

q

~

0.1

,

i

0.2

a

i

0.3

173

X Fig. 5. Lattice parameters and cell volume of YBa~Cu 3 ~CoxO7_ d synthesized through the route Q.

H. Mazaki et al. / Physica C 246 (1995) 37-45

42

X=0.3 '...

t"

0.2 !

10.15 .0.1

!0.05

0.5

I

0.4

0

0

r

k

2~0

~

40F

~

6~0

~

8, 0

,

100

T(K)

Fig. 6. Real components of the complex susceptibility X versus temperature T for YBa2Cu3_xCOx07_ d synthesized through the route P, where /%HAc = 0.01 mT and f = 132 Hz. Data are normalized by mass.

The lattice parameters a and c of the Y08Ca02 Ba2Cu2.sCo0.207_ d sample were 3.8636_ 0.0001 and 11.7066 _ 0.0003 A, respectively.

2.3. Measurements of magnetic susceptibilities The magnetic response of the samples was examined in terms of complex susceptibilities X = X ' ix". Measurements of X constitute a well established method which gives us structural information

0.4

0.3 X = 0.3 -I

0.2 0.15 0.1

0.2

0.05

.

:.

.•

0,1

..'

-

=~

__--J

.

:.

/ .- .

i"

.

.jiJi

!.

o14 o

'

2'o

i

4'o

i

do

i

do

i

lOO

T(K)

Fig. 7. Imaginary components of the complex susceptibility X versus temperature T for YBa2Cu3_~.CoxO7_ d synthesized through the route P, where ~0HAc = 0.01 mT and f = 132 Hz. Data are normalized by mass.

of the materials. The measuring system consisted of the Hartshorn bridge and the temperature control system. To avoid the bridge imbalance during one run of the experiment, the sample coil was directly immersed in liquid He bath. A null adjustment of the bridge was made at 105 K and the sample temperature was decreased down to 5 K at a rate of about 0.3 K/min. Phase setting of the lock-in amplifier was carefully made so as to give variation only to the in-phase signals, but not to the out-of-phase signals, against the change in the bridge inductance. The AC magnetic field HAC COS(Wt) was applied to the sample using a function generator, where HAC is the field amplitude (/X0HAc = 0.001-0.1 mT, /% = 4xr X 10 - 7 H / m ) and the frequency f = to/2rr is typically 132 Hz. The whole system was computer controlled. More details of the measuring system were previously reported [26,27].

3. Discussion The real component of X for sintered ceramic superconductors usually exhibits a two-step growth with decreasing temperature [28]. The first step corresponds to exclusion of magnetic fields from the bulk superconducting grains appeared below To(onset), and the second step comes from magnetic shieldings by the supercurrent flowing between these bulk grains, the so-called coupling phase. However, for a well synthesized specimen with good structural and compositional homogeneity, we empirically know that the growth of X' appears more or less one-step like, being similar to that of conventional bulk superconductors. The generation of this one-step like profile of X' for ceramic superconductors is presumably understood as that the superconducting grains with Tc in close vicinity are uniformly dispersed and consequently the coupling phase preferably takes place immediately after the production of the bulk grains. This eventually results in magnetic shieldings of the whole specimen due to the weak-link couplings within a relatively narrow temperature range. In Figs. 6 and 7, we show the real and imaginary components versus temperature for YBa2Cu3_xCo xO7_ d synthesized through the route P (x = 0.050.4), where /.%HAc = 0.01 mT and .f= 132 Hz. As

H. Mazaki et al. / Physica C 246 (1995) 37-45

seen in Fig. 6, the growth of X' for small values of x does not show an explicit two-step nature, but seems to be almost one-step like. Besides, the transition width AT(10-90%) is much smaller than that of sintered samples prepared by the ordinary method, being typically more than several degrees [9,10]. In each x"-T curve demonstrated in Fig. 7, we observe only a very smooth single peak, confirming the compositional homogeneity of each sample. From these facts we may consider that by the polymerized complex method the structural and compositional homogeneity of the sample is much improved. As expected, the onset temperature of X' decreases as Co content x increases. To(onset) for x = 0.05, 0.1, 0.15, 0.2 and 0.3 are respectively 92.0, 82.0, 71.2, 64.3 and 44.9 K, and they are plotted against x in Fig. 8. For larger values of x than 0.4, no superconducting transition was observed in the experimental temperature region (5 K < T < 100 K). In Fig. 9, we show the x'-T curves for the samples synthesized through the route Q. The imaginary components are essentially similar to those of the route P (not shown in the figure). To(onset) for x = 0.03, 0.05, 0.07, 0.1, 0.13, 0.15, 0.17, 0.2 and 0.3 are respectively 92.4, 92.0, 87.5, 82.7, 74.9, 72.8, 66.4, 57.1 and 38.9 K, and they are also plotted against x in Fig. 8. We find between the two series there is no large difference in the variation of To(onset) in the low Co concentration region below x = 0 . 1 5 . Somewhat (6-7 K) lower values of T~(onset) have been obtained for the x = 0.2 and 0.3

80 t

;Ool

i

5O 4C "6 3O 20 10

[]

o

,

, = , , =

013

o.o ' o11 ' o:2

0.4

0.5

X Fig. 8. Onset temperature To(onset) of the superconducting transition versus Co content x in YBa2Cu3_xCoxO7_ d. Solid circles are the data for the samples prepared through the route P and open squares are obtained for the samples prepared through the route Q.

i

43 i

i

0.07 0.05

1.0 ..

'~,4 t,.

X = 0.3"

-

5

.

"'

0 2" 0.17: :'0.13 : •

.

0.5

~:0.0:

..

'inK" o.1. .

0

~

__

0

{

L

2kO

.

• ~i

:

'~

'~

I

410

~

I

6~0

"

'~

I

8~0

1O0

T(K) Fig. 9. Real components of the complex susceptibility X versus temperature T for YBa2Cu3_xCOx07_ d synthesized through the route Q, where i~oHAc = 0.01 mT and f = 132 Hz. Data are normalized by mass.

samples prepared through the route Q. The reason for this discrepancy remains unclear, however, it is most likely that subtle differences in the two synthetic procedures and subsequent heat treatments of the samples may influence both oxygen content and ordering in t h e C u ( 1 ) O y planes of the structure, which in turn affects the superconducting transition temperature. Contrarily to the depression of Tc(onset), the transition width AT(10-90%) gradually broadens with the increase of Co content, probably caused by substitutional disorder. For example, AT(10-90%) for the x = 0.05 sample in Fig. 6 is as small as 1.9 K, while for the x = 0.3 sample in the same figure, it broadens by more than 5 times, 10.3 K. Note that the profile of X' becomes two-step like for larger values of x. Comparison of the x'-T curves of the samples synthesized through the two different routes indicates that AT(10-90%) of the route Q is somehow smaller than that of the route P for all the values of x. For example, in the case of YBa2Cu2.95Co0.0507-d, we find AT(10-90%) = 1.9 K from the route P and 1.3 K from the route Q, suggesting that the improved polymerized complex method is an effective mean for achieving better homogeneity. The observed depression of T~(onset) with increasing Co concentration can be explained by a reduced number of mobile holes on the Cu(2)O 2

44

H. Mazaki et al. /Physica C 246 (1995) 37-45

sheets, which is consistent with reported Hall effect measurements [18] where the number of mobile holes on the Cu(2)O 2 conduction sheets decreases with increasing Co concentration. It should, however, be noted from recent coulometric titration measurements [17] that the formal valence of Cu (or O) is essentially unchanged which would mean that the total hole concentration remains unchanged upon Co doping. Thus it is believed that Co substitution for Cu(1) tends to transfer the holes from the Cu(2)O 2 sheets to the Cu(1)Oy planes where the holes are localized [9-12]. The holes localized within the Cu(1)Oy planes do not contribute to the superconductivity, and the essential variable governing the superconducting properties in YBCO is the number of mobile holes on the Cu(2)O 2 sheets rather than the total hole concentration [29]. It is known that the substitution of Ca 2+ for y3+ tends to increase the hole concentration in YBCO samples when they are well oxidized [30]. The observed rise in T~ on going from YBa2Cu2.sCo0.2OT_ d (Tc = 57.1 K) to Y0.8Ca0.2Ba2Cu28Co02OT_d (T¢= 88.6 K) may therefore be the consequence of reintroduction of holes on the Cu(2)O 2 sheets from the Cu(1)Oy planes, i.e. the so-called delocalization effect of holes [31]. The driving force of the delocalization of holes would come from a reduced positive charge in the Cu(2)O2-(y3+/Ca2+)-Cu(2)O2 layer upon Ca 2+ doping for y3+, which tends to transfer the holes localized in the Cu(1)Oy planes to the Cu(2)O 2 conduction sheets to reestablish the charge balance of the whole system. In some recent reports [4,32] it is proposed that the oxygen ordering in the Cu(1)Oy planes is essential for the ability of these planes to transfer holes to the Cu(2)O 2 conduction sheets. In this context the effect of co-substitution of Ca for Y and Co for Cu(1) in YBCO is not trivial, as the preferential substitution of Co for Cu(1) causes substantial disordering of oxygens in the Cu(1)O r planes [10-14]. The extent of the oxygen disordering in the Cu(1)Oy plane is rapidly increased with increasing Co concentration [9-14], but it appears to be strongly affected by heat treatment procedures employed in the sample systhesis [31]. Further experiments involving different amounts of both Co and Ca doping in Yl_yCayBa2Cu3_xCOxO7_dwould clarify important issues how holes are distributed between the Cu(1)O v planes and the Cu(2)O 2 conduction sheets

and how strongly holes are trapped within the Cu(1)Oy planes with different degrees of oxygen ordering.

4. Summary We attempted to synthesize ceramic samples of YBa2Cu3_xCOx07_ d (x=0-0.5) and Yo.sCao 2Ba2Cu2.sCoo.2OT_ d by the polymerized complex method, starting either from metal nitrates or from metal carbonates. Structural and compositional characterization of the samples was made by means of powder X-ray diffraction and of AC magnetic response. The results show that synthesis of practically impurity-free Co-doped YBCO was successfully achieved by the polymerized complex method. From the measurement of complex susceptibilities, the depression of To(onset) was neatly determined as a function of Co content. The depression of To(onset) can be attributed to a reduced number of mobile holes on the Cu(2)O 2 conduction sheets due to a hole charge transfer to the Cu(1)Oy planes induced by Co doping. The partial Ca 2÷ substitution for y3+ by 20% in YBa2Cu2.sCo0207_ d enhanced T~ from the initial value of 57.1 K up to 88.6 K. This phenomenon can be explained by the delocalization effect of holes, where the introduction of Ca 2÷ in Y planes induces a charge transfer of holes from the Cu(1)Oy planes to the Cu(2)O 2 conduction sheets.

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H. Mazaki et al. IPhysica C 246 (1995) 37-45

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