Effect of La substitution on Tc and electronic structure of Bi 2201 phase

Physica C 231 (1994) 37--49

ELSEVIER

Effect of La substitution on Tc and electronic structure of Bi 2201 phase Yasushi Idemoto *, Hiroki Tokunaga, Kazuo Fueki Department of Industrial Chemistry, Faculty of Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda-shi, Chiba 278, Japan Received 16 September 1993

Abstract

The critical temperature T¢ of Biz(Sr~_xLa~)2CuOy increased with increasing x, reached a maximum of 30 K at x=0.19 and decreased. Determination of the excess oxygen content Ay by chemical analysis revealed that Ay decreased with increasing x. The oxygen content and conductivity 0 of Bit.96(Sro.atLao.,9)2CuLo2Oywere measured as a function of T and P(O2) at high temperatures and a at constant y was determined as a function of T. It was found that 0 increases with the increase in excess oxygen Ay. Assuming that one excess oxygen atom creates two holes, the carrier density n and the mobility were calculated. The extension of high-temperature conductivity linked smoothly with the low-temperature one with the same oxygen content when y is high. From the results, an electronic-structure model whereby the conduction behavior can be interpreted was proposed.

1. Introduction The critical t e m p e r a t u r e o f high-T¢ superconductors is strongly affected by the substitution o f allovalent cations and the oxygen content. It has been rep o r t e d that the critical t e m p e r a t u r e o f Bi 2201 phase is increased by the substitution o f lead for b i s m u t h and l a n t h a n u m for s t r o n t i u m [1 ], and by the replacement o f b i s m u t h and calcium by lead and yttrium [ 2 ], respectively. T h e first p u r p o s e o f the present p a p e r is to interpret the e n h a n c e m e n t o f superconductivity by the replacement o f cations [ 3 6 ] in terms o f the excess oxygen content. In the previous p a p e r [7], the authors and their co-investigators have measured the oxygen nonstoichiometry and the conductivity of (Ndo.67Ceo.33) 2 (Ndo.a3Bao.67) 2Cu3.01Oy superconductor as a function o f oxygen partial pressure and * Corresponding author.

temperature. C o m b i n i n g these two data, they have d e t e r m i n e d the high-temperature conductivity, the carrier density and the mobility as a function o f oxygen content a n d temperature. Also, the t e m p e r a t u r e d e p e n d e n c e o f conductivity was measured on the samples with constant oxygen contents below room t e m p e r a t u r e and the carrier density was calculated using the low-temperature conductivity and the mobility extrapolated from the high-temperature range. C o m p a r i s o n o f the high-temperature c a r d e r density with the low-temperature one revealed that both carrier densities well agreed with each other at high oxygen contents but at low oxygen contents the lowt e m p e r a t u r e one became smaller than the high-temperature one with decreasing oxygen content. These results were well interpreted as the f o r m a t i o n o f acceptor levels originating from the excess oxygen near the t o p o f or in the O 2p band, d e p e n d i n g on the excess oxygen content. The second purpose o f this study is to d e t e r m i n e the oxygen nonstoichiometry and the

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Y. Idemoto et al. / Physica C 231 (1994.) 37-49

38 high-temperature

conductivity of Bil.96(Sro.$1Lao.19)2Cul.o2Ov and propose an electronic-structure model for the interpretation of conducting properties in a similar way as of ( Ndo.67Ceo.33) 2( Ndo. 33Ba0.67) 2Cu3.o iO1,.

2. Experimental 2.1. Preparation o f sample One mol/l solutions of Sr(NO3)2, La(NO3)3 and Cu(NO3)2, and 0.5 mol/l Bi(NO3)3 solutions were prepared, the concentrations being determined with a EDTA (ethylene diamine tetra acetic acid) standard solution, these solutions being mixed in a desired ratio. Oxalic acid 1.5 times more than cations in equivalent was dissolved in ethanol three times larger than mixed aqueous solution in volume, and the oxalic acid-ethanol solution was added to the mixed aqueous solution. The resulting solution was stirred for 4 ~ 5 h, keeping the pH at 3 with ammonia solution. After aging for 12 h, the precipitate was separated by filtration, dried at 100*C, and decomposed into oxides by heating at 400°C for 3 h. Following the heating for 12 h at 800"C, the oxide was heated at 840"C in air for 24 h to get a sample oxide, which was examined to be single phase by means of an X-ray diffractometer.

2.2. Chemical analysis The metal composition of the sample was determined by means of an ICP emission spectrometer, and the total amount of bismuth was determined by the chelate titration method with a EDTA standard solution. About 0.1 g of sample was dissolved in a 20 ml solution of 0.1N MnSO4 in a mixture of acids ( H 2 S O 4 : H N O 3 : H 2 0 = 5 0 : 2 1 : 5 0 0 ) , to react Mn 2÷ with Bi s÷ in the oxide by 5Bi s+ + 2 M n 2+ + 8 H 2 0 --,2MnO~- +5Bi3+ + 16H + .

(1)

Then, a 5 ml solution of 0.1N Mohr salt in IN H2SO4 was added and heated at 67°C under stirring to reduce MnO~- to Mn 2+ by

MnO~- + 5Fe 2+ + 8H + ~ M n 2+ + 5Fe 3+ -I-4 H 2 0 .

(2) The amount of remaining Fe 2+ was back-titrated by a 0.1N standard permanganate solution and the amount of Bi 5÷ was calculated. From the amounts of Bi 5+ and total Bi, the average valence of bismuth was calculated. The iodometric method was employed for the determination of the copper valence [8 ]. First, the sample was dissolved in a hydrochloric acid solution containing potassium iodide to determine the total equivalent of B i s + + C u 3 + + C u 2+ and the amount of Bi s + was subtracted. Next, the sample was dissolved in a hydrochloric solution to convert Bi 5+ to Bi 3+ by Bi 5÷ + 2C1- -~ Bi 3+ + C12

(3)

and Cu 3+ to Cu 2+ by 4Cu 3+ + 2 H z O ~ 4 C u 2+ + 0 2 + 4 H + •

(4)

After purging the evolved C12 by argon bubbling, potassium iodide was added to determine the amount of copper. Combining these results the average copper valence [9] was determined. To prevent iodide ions from oxidation, the dissolution of sample was carried out in flowing argon in a glass apparatus.

2. 3. Determination o f lattice constant Silicon powder was mixed with the sample oxide in about the one-to-two ratio and the mixture was ground. The lattice constant was determined by the internal standard method using silicon as a reference.

2.4. Measurement of resistivity Sample powder was pressed into a pellet at 400 kg/ cm 2 and the pellet was sintered at 840°C in air for 12 h. After annealing under various conditions of temperature and oxygen partial pressure, the pellet was quenched in liquid nitrogen, cut into a rectangular shape and used for the resistivity measurement by the four-probe method.

2.5. Measurement o f oxygen nonstoichiometry About 0.5 g of the sample powder of known oxygen content was weighed and put in a platinum basket,

39

Y. Idemoto et al. / Physica C 231 (1994) 37-49

which was suspended from an automatic recording microbalance with a fine quartz fiber. The initial weight was taken as the reference and the equilibrium weight was determined as a function of temperature and oxygen partial pressure.

50 4O A 30 ~

2.6. Measurement o f high-temperature conductivity The sample powder was pressed at 400 kg/cm 2 into a pellet, which was pressed again in a CIP apparatus at 4 t / c m 2. Then, it was sintered at 800"C for 48 h. The sintered pellet was ground in an agate mortar. The operations of grinding, pressing and sintering were repeated five times to effect complete homogeneization. The final pellet was examined by means of a SEM and the density was measured by the Archimedes method. The pellet was cut into a rectangular shape and employed for the measurement of the high-temperature conductivity. The conductivity was measured as a function of temperature and oxygen partial pressure by the four-probe method.

Bi2 (Srl'-xLax )2'cuOy o :Tc(on) 840Oc,0.2atm o :Tc(zero)

2O 10' 0

0.10 0.20 0.30 0.40 0.50 X Fig. I. RelationbetweenTcand x of Bi2(Sr,_xLax)2CuOr 25.0 6.0 1

BI2(Srl-xLax)2CUOy

24.5

3. Results and discussion

3.1. Bi 2(Sr l _ xLax) 2CuOy 3. I. I. Relation between Tc and x Bi2(Sr~_xLax)2CuOyprepared by the substitution of La for Sr in the Bi 2201 phase was annealed at 840"C in 0.2 atm of oxygen and furnace-cooled to room temperature. Fig. 1 shows the plot of Tc vs. x. As x increases, Tc increases, reaches a maximum at x = 0 . 1 9 and decreases. As shown in Fig. 2, with increasing La content, the lattice parameter a increases slightly whereas c decreases linearly. This result indicates that Bi2(Sr~ _xLax)2CuOy forms single phase in a range of x = 0 to 0.40. The decreases in c with increasing x is considered to be due to the smaller ionic radius o f L a ( 1.16 A) than Sr ( 1.20 A). 3.1.2. Average valences o f Bi and Cu and excess oxygen content The average valences of Bi and Cu are plotted against x in Fig. 3. Both valences decrease with x, which is interpreted as the charge compensation for the increase of positive charge due to the substitution of La for Sr in Bi2Sr2CuO r The decrease in Cu va-

n

5.5

24.0

5.0 i

!

i

i

0.10 0.20 0.30 0.40 .50 X Fig. 2. Relation between lattice parameter and x of Bi2(Srl_xLax)2CuOr lence is remarkable, while the decrease in Bi valence is small. Namely, the charge compensation is effected mainly by the decrease in copper valence. It is noteworthy that copper takes a valence of even less than 2.00 in a high La content region where superconduction appears. Such a low copper valence less than 2.00 has been found in the Bi 2223 phase [8] and YBCO [ l0 ]. In the case of YBa2Cu3Oy, the conductivity increases with increasing oxygen content over the whole oxygen content except for the temperature range above 7500C and low oxygen partial pressures below

40

Y. Idemoto et al. / Physica C 231 (1994) 37-49

6.60 3.10~ u O y

BI2(Sr 1.xLax)2CuOy 840oc, 0.21din

2.10 6.50

Y I

6.40 6.3O

O 0 orJ rQ

3.00~

o e

~

2.00

¢: 0

_o

I: @ r~

(I

&

e

e:D

q[

6.20

1.90

6.00T'" 5.90 A 0 0.10

D :Cu I

0

0.10

I

I

020

i

0 . 3 0 0.40

,, "rl

Ay

6.10~.'"'~z'"

I) >

o:Bi

/

o

4(

2.90

Yo

I

I

0.20

.50

X Fig. 3. Relation between valences of bismuth and copper and x 0 f B i2 (St1 _ xLax) 2CuOr

Fig. 4. Relation between Bi2 (Sri _ xLax ) 2CuOr

.

l0 -3 atm. Namely, YBCO is p-type even when the average copper valence is less than 2.00. According to the result of the thermodynamic calculation of equilibrium concentrations of Cu 3+, Cu 2+ and Cu + ions using the oxygen-nonstoichiometry data, the Cu 3+ concentration increases whereas the Cu + concentration decreases as the oxygen content increases. Moreover, the Cu 3+ concentration equals the Cu + concentration at y = 6 . 5 [ 11 ]. Cu 3+ and Cu ÷ ions correspond to electrons and holes in the physical picture, respectively. If the concentrations of holes and electrons are denoted by p and n, respectively, and the mobilities are represented by pp and g,, the conduction is p-type when p#p> ng,. This condition is fulfilled even when n>p if (/h,//~) > (n/p) holds. This is the reason why YBCO is p-type over the whole oxygen content range. It is considered that Bi2(Sr~_xLax)2CuOy is p-type and exhibits superconductivity in a region where the copper valence is less than 2.00 for the same reason as YBCO. Since the direct correlation between Tc and Cu valence or Bi valence was not found from Figs. l and 3, the relation between Tc and oxygen content was studied. The oxygen content y as a function of lanthanum content x is given by the solid line in Fig. 4. The reference oxygen content Yo defined as the oxygen con-

.

.

.

,

,

,

,

o :Tc(on)

40 ~" 30

contents

,

,

,

and

,

Bi2 (Sr 1.xLax

n :Tc(zero)

x

,

of

,

,

)2CuOy

840Oc,0.2atm

x-O.2S

x-0.20 x,,O.l 5

I-

x

2O

xmO.40

10

~ i

0

0 . 4 0 0.50

X

oxygen

,

I

0.30

a

O

.

~

xeOAO

x.o ~

,

0.05

,

,

i

0.10

0.16

t

Ay Fig. 5. Relation between Tc and excess oxygen content Ay of Bi2 (Sr]_~Lax)2CuOr

tent where Cu and Bi take the valences of + 2 and + 3, respectively, is represented by open squares in this figure. Since the substitution of La for Sr increases the positive charge, Yo increases linearly with increasing x. However, the excess oxygen content Ay= (Y-Yo) decreases with increasing lanthanum content x. This is due to decrease in the average valences of Cu and Bi with the substitution of La. Fig. 5 shows the Tc-Ay curves, on which the lanthanum content x is given. Evidently, with increasing x, Ay decreases, and Tc has a maximum at x = 0.19.

41

Y. ldemoto et al. I Physica C 231 (1994) 37-49

As is easily seen from the figure, the Bi 2201 phase without La has too much Ay to realize a high To; in other words, it is in an oxygen over-doped state. The substitution of La for Sr decreases Ay and enhances Tc. It is concluded that the substitution of allovalent cations changes the excess oxygen content Ay and results in a change of T~.

3.2. Bit.96(SrostLao.tg)2Cul.o20y

Ym~--Yo is nearly the same for both kinds of oxides. The y-log P ( 02 ) diagrams represented in Figs. 6 (a) and (b) show that the whole domain consists of two subphases denoted by a and [3. In subphase ¢zy depends strongly on the temperature and oxygen partial pressure, while in subphase [3the dependence o f y on temperature and oxygen partial pressure is slight. It is seen that the substitution of La makes the ct subphase narrow and the [3subphase wide.

3.2.1. Oxygen nonstoichiometry As already described, Bi2(Sr,_xLax)2CuOy has a maximum T¢ at x = 0.19. The exact chemical formula for the highest Tc was determined as Bii.96(Sro.snLao.19)2Cui.o2Oy. The oxygen content y was determined as a function of temperature and oxygen partial pressure by means of an automatic recording microbalance. The result is shown in Fig. 6(a) as a plot o f y versus log P(O2) at constant temperatures. For comparison, the y-log P (O2) diagram for the Bi 2201 phase of x = 0 determined by the authors and their coinvestigators [ 12 ], is shown in Fig. 6 (b). yo is 6.06 for x = 0 , and 6.165 for x=0.19. If we represent the highest oxygen content by Ymax,Ymaxis 6.19 and 6.29 for x = 0 and x = 0.19, respectively. Although Ymaxis different for these two oxides Ym~-Yo is 0.11 for x = 0 and 0.12 for x=0.19. Namely,

3.2.2. Valences of Bi and Cu and oxygen content Samples of x = 0.19 were annealed under various conditions and quenched in liquid nitrogen. Then, the valences of Bi and Cu were determined by chemical analysis and the oxygen content was calculated. Fig. 7(a) gives the relation of valences versus excess oxygen content Ay(=Y-Yo) for Bil.96(Sro.slLao.tg)2Cul.o2Oy. The copper valence is enhanced remarkably compared to the bismuth valence with increasing Ay. It seems that Cu ions play a major role in compensating for the change in charge caused by lanthanum substitution and oxygen doping. Also, Fig. 7 (a) shows that Bil.96(Sro.81Lao.19)2CUl.o2Oy has a Large Ay range of 0.01 to 0.14. Comparing Fig. 7 (a) with Fig. 6 ( a ), we can conclude that the wide range of Ay for x = 0.19 is caused by the extension of the 13 subphase domain.

6.30

i

l.g'H ' ~

62S

/

6.20

ms:,

yo,,6.16S BIs+, Cu2* SAg (0) BI1.96(Sro.81Loo.19)2Cul.020y -3

-2

k~(Po2/mmi

-

.................... . . . . . . . . . . . . . . . . .

I

-4

a

I

-4

-3

I

I

-I!

-1

0

kig (Po2/litm)

Fig. 6. Oxygen nonstoichiometry as a function of temperature and oxygen partial pressure, for (a) Bii.96(Sro.slLao.19)2Cul.o20~,(b) Bi2.12Srl.s6Cul.o2Oy[ 12 ].

42

Y. Idemoto et al. /Physica C 231 (1994) 37-49

Oxygen content y $.20 o:81 o:Cu

6.25

(|)

(a)

0.30

It 1.N( 8¢0.11 L|0.11)2C u 1.020y

m o

'".

2.1o ,~ g

3.10

E"

3

~

O°o°°°o°°°°°oo °° o° °° o°

~,~ . . . . . °°o°

!

I'""................................ ,~?~

< t.~

2-90 ,

r

,

i , 0.05

,

,

,

, , o.1 o

. . . . . o.15

i

|

i

i

$o

1oo

too

2o0

~y

i 2$@

Tlnltpqlrlltuto (K)

lo

Oxygen content y 6.10

6.13 (b)

o:BJ

'3

III1.118rl.liCu

1.OlOy

(13)

s

:

v,,M.~7

............... -:

....... ..............

-

[3 :Cu

i

it 6.20

2.10

3.1o

8

~ 3.00

~.,

"6

J

i

. .....

. ........

..................

2.00 ~ <

0

1.90

2.90 .

.

.

.

.

n

I

I

I

O.OS

I

0.10

i

I

I

I

I

so

too

150

2oo

260

3oo

Temperature(K)

I

0.15

~y

Fig. 7. Relation between valences of bismuth and copper and oxygen content for (a) Bi,.96(Sro.siLao.19)2CuL020,, (b) Bi2.n2Srt.a6Cun.020v l 12 ].

On the contrary, the width of the [3 subphase domain of Bi 2201 with x=O is narrow as seen in Figs. 7(b) and 6 (b). From these results, it is concluded that the substitution of La for Sr enlarges the width of the subphase domain and Tc is enhanced. The ~ subphase seems to be related to the superconductivity.

3.2.3. Relation between Tc and oxygen content Samples of x = 0.19 with different oxygen contents were prepared by annealing and quenching, as usual. The p - T curves are given in Figs. 8(a) and (b). p decreases with increasing oxygen content between y = 6.233 and y = 6.273 and tends to increase above y=6.273. Fig. 9(a) shows the relation between the critical temperature Tc and the excess oxygen content Ay. With increasing Ay, Tc increases, reaches a maximum and decreases. For comparison, the Tc-Ay relation for x = 0 is given in Fig. 9(b) [ 12]. The oxygen content range where the superconduction appears

Fig. 8. Plot of resistivity Bil.~ ( Sro.st l-~.lg) 2Cu t.o20.,,.

vs.

temperature

for

is extremely narrow compared to x=0.19. Namely, the substitution of La extends the Ay range of superconduction and results in the enhancement of T¢.

3. 2. 4. Conductivity Samples from the same batch as used for the oxygen nonstoichiometry measurement were sintered until the relative density reached 92%. The high-temperature conductivity determined as a function of temperature and oxygen partial pressure by the fourprobe method is given in Fig. 10(a). For comparison, the o-logP(O2) relation for x=O, reported in the previous paper [ 12 ], is given in Fig. 10 (b). Both o-log P(O2) relations are quite similar to each other. Fig. 11 gives the plot o f o against Ay using the data of Figs. 6(a) and 10(a). At constant excess oxygen Ay, the conductivity increases slightly with decreasing temperature. The tT-Ay plot for the [3subphase seems to pass the origin. From the figure it is concluded that the excess oxygen liberates holes responsible for the conduction.

Y. Idemoto et al. / Physica C 231 (1994) 37-49

Oxygon emblem y 6,20

025

: 0 :T(z(on)

0.30

(It) liJl 1~SJro l l l L ~ I l l ~ u

I (~0

I a :Tc(mro)

J 10

0.06

0.10

0.1~;

6y

43

havior (da/dT>O), while all the samples with y > 6.253 exhibit the metal-like behavior (da/d T< 0 ) and superconductivity. The conductivity decreased with increasing oxygen content in the range 6.273
Oxygen content y 0.10

0 :To(on) 0 :T~ro)

;tO

8.16

0.20

3.2.5. Tcand loga

(b) I~Llltlkl.lleCUl.(40y

lo

i

,

i

. . . . . .

,

0.06

0.1o

,

i

i

i

,

,

0.15

~y

Fig. 9. Relation between T¢ and excess oxygen content for (a) Bil.~s(Sro.slLaoag)2Cul.o2Oy, (b) Bi2.12Srl.s6Cut.o20~ [ ! 2 ].

The solid lines above 473 K in Fig. 12 shows the o-Tcurves at constant oxygen partial pressures. Plotting the oxygen content on these solid lines by the use of data in Fig. 6(a) and connecting the points of the same y value, one obtains the lines represented by dashed lines which should be obtained if the conductivity measurement is carried out using samples with constant oxygen contents. The o - T plot at constant y goes upwards with increasing oxygen content y. The temperature coefficient of a at constant y changes from positive to negative and in the negative-sign region, the absolute value of the slope increases with increasing oxygen content. The low-temperature conductivity was calculated from the resistivity data given in Fig. 8. The solid lines below 250 K in Fig. 12 give the cr-Tcurves calculated in this manner. Below room temperature it is considered that the oxygen content is kept constant because the chemical reaction and the diffusion are extremely low. When y<6.242, the samples exhibits semiconducting be-

The conductivities for 250, 100 K and/'co, calculated from the resistivity data in Fig. 8 are plotted as log a versus Ay in Fig. 14. Comparison of Fig. 14 with Fig. 9 (a) clearly shows a good similarity between them. Namely, with increasing Ay both T¢ and log a increase, reach maxima at the same Ay and decrease. Such a similarity has been found in most of the superconducting oxides and indicates that the detailed study of conduction in the normal state as a function of oxygen content would provide valuable information on the elucidation of the superconducting mechanism. However, in the case of TlzBa2CuOy it has been reported that the increase in y decreases Tc but increases a. This is another type of oxygen-overdoping. The example of overdoping due to cation substitution, (La, _~Srx)2CuOy, is well known. The increase in x increase both Tc and a up to x=0.08 and decrease them above x=0.08. The doping of cations changes not only the metal composition but also the oxygen content. The situation is complicated. The temperature dependence of log tr is positive below Ay= 0.07 whereas it is negative above Ay= 0.075. This result indicates that the conduction changes from semiconductivity to metal-like at around Ay= 0.075 and should reflect the electronic structure in the normal state. Fig. 15 shows the Arrhenius plot of a using the data given in Fig. 12. The conductivity behavior for y < 6.242 is semiconducting, where as that for y 2 6.253 is metal-like. The apparent activation energy Eac t w a s determined from the slope at l / T = 5 × 10 - 3 K - ~and is given in Fig. 16. Eac, diminishes

Y. ldemoto et al. / Physica C 231 (1994) 37-49

44

(a) BI 1.96(8rO.81LoO.19)2Cu1.020y

(b) BI2.12$r1.86Cu1.020y -b I(xYc

200

.-----.o----

z ~ # c z

~

__._._o-.o-o-

2O0

6--

o

------o 200"C

300"C

J

Y

o

150

150

n

r-

.-o----o

t~

IO0

50

0

' -4

* -3

i -2

i -1

i 0

km (Po21~m)

o

4'

-;

:2 log (Po2/~m)

:1

Fig. 10. Relation between high-temperature conductivity and oxygen partial pressure at constant temperatures for (a) Bil.96(Sro.siLao.19)zCuLo20r (b) Bi2j2SrL86Cut.ozOy[12].

to zero at around y = 6.25, above which the conductivity becomes metal-like. 3.2.6. Electronic structure The semiconducting properties of metal oxides has successfully been interpreted on the basis of the band model in the field of defect chemistry. Namely, in the case of oxygen-deficient type oxides, the depletion of oxygen produces cations with lower valence, which act as the donor. A schematic drawing of the electronic structure for the oxygen deficient type oxides is given in Fig. 17(a). In the case of oxygen-excess type oxides, the excess oxygen atoms form acceptor levels and produce holes in the valence band, as shown in Fig. 17 (b). Accordingly, the oxygen-deficient type oxides are n-type, while the oxygen excess type oxides are p-type. Superconductivity of oxide superconductors generally appears in the transient region from semiconducting to metal-like and Tc as well as conductivity is governed by the nonstoichiometric

oxygen, as described above and in the previous papers [ 7]. Accordingly it is reasonable to adopt the electronic-structure model given in Fig. 17 for the interpretation of the semiconducting behavior of high-To superconductors. Most of the high-To superconductors are p-type and are in a range of low excess-oxygen concentration; this case is semiconducting. The electronic structure for the oxide could be represented by Fig. 18(a). With increasing oxygen content the gap A E = E A - E v tends to diminish and finally becomes zero, as shown in Fig. 18 (b), and the conductivity behavior becomes metal-like. The electronic structure could be represented by Fig. 18 (b) for that case. The so-called CT gap E 8 estimated from the optical measurement and the temperature dependence is in the range of 1.2 to 1.8 eV [ 13-20].

Y. ldemoto et al. / Physica C 231 (1994) 37-49

Oxygen content y 6.20

6.3

6.25

200

BI1.96( Sr 0.81 Lao.19)2 CU1.020y

150

0/I.7

U

50

,

0

,

,

,

,

0.05

i

i

,

,

i

,

,

i

0.10

Ay Fig. I 1. Relation between high-temperature conductivity and oxygen content for Bil.~(Sro.stLaoag)2Cul.020~.

3.2. 7. Carrier density and mobility The density and mobility of the carriers of semiconductors are usually determined from the Hall coefficient and conductivity. Since the discovery of high-Tc superconductors, many measurements have been carried out on the Hall coefficient of the superconducting oxide. However, a large discrepancy was found between the observed Hall coefficient R , (obs) and the one, RH(Cal), calculated from the dopant concentration n by R = l / n e . Namely, Rr~(cal)/ Rr~(obs) was found to vary from 3 to 10 or more, as n increases [21,22]. R , ( o b s ) of high-To superconductors is in a range of 10 -I to 10 -3 cm3/C compared to 102 to 105 cm3/C of semiconductors. Such low values of RH(Obs) of high-T¢ superconductors indicate the difficulty of a determination with sufficient accuracy. A novel method whereby the carrier density and mobility can be determined from the conductivity and nonstoichiometry on the basis of Figs. 18 (a) and ( b ) was shown for (Ndo.6,Ceo.33)2 (Ndo.33Bao.67)2Cu3.olOy [ 7 ]. This

45

paper describes the determination of carrier density and mobility of Bii.96(Sro.8xLao.19) 2Cu l.o2Oyalong the same line. When y > 6.25 the conduction behavior is metal-like over the whole temperature range, and it is considered that the electronic structure is represented by Fig. 18 (b). Since the acceptor levels due to the excess oxygen overlap with the O 2p band, the holes are formed in the valence band by the complete ionization of excess oxygen. The complete ionization means the creation of two holes from one excess oxygen atom. The carrier density for y > 6.253 was calculated on this assumption. The activation energy Eact of several meV given in Fig. 16 for the semiconducting region, indicates the small gap AE of the same magnitude. Accordingly, the thermal energy k T at high temperatures above several hundred K is much higher than ,4, and the ionization is considered to be complete. The carrier densities at high temperatures for y = 6.242, 6.233 and 6.228, calculated assuming the complete ionization of acceptors are also plotted in Fig. 19. The mobility bt is calculated from the conductivity e and carrier density n by

a=nell,

(5)

where e is the unit charge. The mobility for 6.228 < y < 6.273 at high temperature and the one for y = 6.253 and 6.273, calculated by Eq. (5) are given in Fig. 20. As seen in Fig. 13, the extrapolation of the log a - T relation to the low-temperature side shows a good link between high- and low-temperature portions with the samey value f o r y = 6.253 and 6.273. However, when y~6.242, loge of the low-temperature region is smaller than that extrapolated from the high-temperature region. Since the conductivity extrapolated from high temperature gives the one in the case where the excess oxygen is fully ionized, the mobility ~t is calculated by Eq. (5). Using the/t value, the carrier concentration can be calculated by the same equation. The mobility and carrier density for y = 6.242, 6.233 and 6.228 is given in Figs. 19 and 20, respectively. The mobility of low temperature is nearly independent on temperature and equal to the high-temperature one, while the carrier density increases with increasing temperature and asymptotically reaches a saturated value. The temperature dependence of the carrier density

Y. Idemoto

46

300

.

.

.

.

,

,

et al. / Physica C 231 (1994) 37-49

.

.

.

.

.

.

.

.

,

,

.1,Sro 200

,

,

,

'.o

. . . .

CUoO

~ 6273

E

6 .233

6242 6 242

6 IS233 ~

1-

0

~

- ~ O ..zJtm ]ltm

O228

~ ,---e--?, - - 7 ~ ,

O~

:"',

o ~0.1|tm Ollltm

. . . . . . . . . . . . . . . . . .

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

Temperature(K) Fig. 12. Relation between conductivityand temperature at constant oxygencontents for BiL96(Sro.s~Lao.]9)2Cut.ozO~-

3.C Bi 1.16(S ro.e1L ao.1,)2C ul .o20y 2.~

~

6.373 11.263 |.167

6.273

. ~ _ . ~ . . ~ . - - ~ 6.2~2

~, 2.1

6.133

6.230

_~ 1.5 1.0

0.5

y

•

0

6.328

•

.

.

.

.

.

.

.

I

I

I I

. . . .

I

I

I I

. . . .

100 200 300 400 500 600 700 800 900 1000110012001300

Temperature(K) Fig. 13. Relation between logaand temperature at constant oxygencontentsfor BiL96(Sro.slLao.19)2Cut.o20~.

can be interpreted on the basis of a simple model shown in Fig. 17 (a) as follows. Since the hole density p in the valence band equals the electron density in the acceptor, the Boltzmann statistics provides

P Nv

N^ e x p ( - A / k T ) Nv+NAexp(-A/kT)

(NAINv) exp(-All
(6)

where Nv and N^ are the state densities of valence

47

Y. ldemoto et al. / Physica C 231 (1994) 37-49 oxygen content y 8.20 3.0

.

.

.

.

6.25

.

,

.

.

.

o:250K

7

6.30

.

.

.

,

,

,

L

•

|

•

w

•

•

.228

.

6

~ ' ~

e:T

5

~, 2.o

a)

4

E ¢.)

3

I.U

~i 1.o

Y=6"~ O ~ y = e . 2 4 2

2 1 e" "(s'°"u~"~,c"' °'°,

o ,

,

,

0 6.20

BI1.96(sr 0.81I-a0.19)2CU1.020y

Z ,

,

,

i

i

i

i

0.05

t

,

,

,

0.10

.

I

6.22

3.0

A 2.0I/

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

log o

.

.

.

.

.

.

.

.

.

and

.

.

.

.

6.24

Y

Ay

for

Fig. 16. Relation between Bit.%( Sro.slLao.t9) 2Cul.o2Oy.

Eaa

Conduction band elec~on Ec -4~- donor F.d

.

~6,s**.,=~____=..=.==,.__..=~.,

6.28

6.26

t

0.15

Ay

Fig. ! 4. Relation between Bit.ge(Sro.s|l-,ao.t9)2CUl.o2Oy.

\

•

and

Conduc~on

y

for

band

Ec

S.242 ~--- scce~(x Ex

Ev

ho Valence baud

Valence band

o.ol. :: 0

5

10

15

:'°. '20. .

(a) .

.

.

.

.

25

.

.

.

.

.

.

30

.

.

.

.

.

35

.

.

Co)

40

1/T~10" =/K" I

Fig. 15. Arrhenius plot of a for Bil.~s(Sro.slLao.19)2CuLo20+.

~

Ev

Fig. 17. Schematic drawing ofelectronic structure of a metal oxide for (a) an oxygen-deficient type oxide, (b) an oxygen-excess type oxide.

Conduction band

~

Conducbonband

AC¢~XOf _

" ~'Valence

(a)

band

level~:

Vag'noeband

(b)

Fig. 18. Schematic drawing of electronic structure for the oxide, with (a) low excess-oxygen concentration, (b) high excess-oxygen concentration.

48

Y. ldemoto et al. / Physica C231 (1994) 37-49

1.5

,

,

,

,

,

,

.

.

.

.

.

.

.

,

,

,

,

,

,

.

.

.

.

.

,

Bi, .s6(S ro.61L ao., o);1Cul .o=Ov 1;,273

1.0

~

6

.

2

7

6.253

3

O

-O

$.2S3

-O

O

E

O'-'-O--O" I . 2 4 2

-O

U

~

6.233

6.242 O C

0.5

~ ~

O

~

y=6.226

~._6.233

y=6.;128 ,

,

0"00 Fig.

i

i

100

=

i

200

*

i

300

i

i

400

i

l

n _

i

i

l

.

.

.

.

.

.

.

,

,

900 1 0 0 0 1 1 0 0 1 2 0 0 1 3 0 0

19. Relation between career density and temperature at constant oxygen contents for BiLg~(Sr0.s~Lao.,9)2CUl.O20r

band and acceptor level, respectively, and,~ is the energy gap between two states. At a constant NA/Nv value, P/Nv increases with increasing kT/d and asymptotically approaches

Nv

i

500 600 700 800 Temperature(K)

NA

Nv + NA

in Fig. 21, gives the dotted lines in the same figure. Open circles are the data taken from Fig. 19. The dotted lines fit the open circles fairly well in the low-temperature range. This result seems to support the model given in Fig. 18. However, the dotted lines, especially for y = 6 . 2 3 3 and 6.228 do not agree with the circles in the high-temperature range. The curved Arrhenius plot given in Fig. 15 indicates that the acceptor level would be not single but multiple. The multiple acceptor levels are the cause of the disagreement.

(7) "

The state satisfying Eq. (7) is the metal-like state and the carrier density is constant irrespective of temperature. Insertion of/l and R (=NA/Nv) values given 3.0

\

b

Bi 1.s6(S ro.e,L ao.1 g)2C ul .o=Ov

2.0 "7,

> E 0 ,...,

1.0

6.242

"°'~o..... 6.273

-

O

-

-

-

O

~

O-

-O~O-~O---O--O-

i

0.0

i

;

I

6.;133

6.263

y-6.228

i

i

i

6.242

i

100 200 300 400

i

i

,

,

,

,

i

,

v

.

i

6.226

6.233

i

i

i

.

.

,

•

500 600 700 800 900 1 0 0 0 1 1 0 0 1 2 0 0 1 3 0 0 Temperature(K)

Fig. 20. Relation between mobility and temperature at constant oxygen contents for Bi].96 (Sro.slLao.j9)~Cu,.o2Or

Y. ldemoto et al. / Physica C231 (1994} 37-49

49

Acknowledgement

1.2 B i ~ ( S r 0,,La0,9)zCu , ozO 1,0

y'6.253 R'I A'OmeV o--0--o--¢--o . . . . . . . . . 6 253

This work has been partly supported by a Grantin-Aid for Scientific Research on Priority Areas, "Science of High-To Superconductivity" given by the Ministry of Education, Science and Culture, Japan.

6 253 8 242

0.8 6 242

~t~

~o

y ' 6 242 R ' 0 55 & ' 2 50meV ~ " ..........

0.e

_

"C

y-e.=~

-0.4

2meV

o 6 233

0.2

O00

°~°~233

References y " 6 228 R-O 45

2 0

4 0

A ' 6 60meV

6 0

800

1000

1

0

Temperature(K)

Fig. 21. Relation between carrier density and temperature at constant oxygen contents for Bi:.~(Sro.slLaoa9)2Cu,.o2Oy (solid line: experimental data, dotted line: calculated values).

4. Conclusion (1) The Bi 2201 phase of x = 0 was in the oxygen over-doping state and the substitution of La for Sr decreased the excess oxygen content and enhanced To. (2) The measurement of oxygen nonstoichiometry of Bil.96(Sro.slLao.19) 2Cul.o2Oy bY means of a vacuum microbalance revealed that the substitution of La for Sr enlarges the domain of [3 subphase and results in a decrease in excess oxygen content. (3) In the oxygen-content range where superconductivity appeared, the extrapolation of high-temperature conductivity linked smoothly with the lowtemperature one of the same oxygen content. (4) In the oxygen-content range where the superconduction was not exhibited, the low-temperature conductivity was smaller than the extrapolation of high-temperature one for the oxygen content. The difference in low-temperature conductivity between observed and extrapolated became larger as the oxygen content decreased. (5) The electronic structure model of cuprate superconductors as proposed by an extension of the band model successfully was used to interpret the semiconducting properties of metal oxides. (6) The carrier density and the mobility were calculated as a function of temperature and oxygen content, from the oxygen nonstoichiometry and conductivity on the basis of the proposed model.

[ 1] S. Kambe, M. Kawai and T. Kawai, Solid State Commun. 75 (1990) 435. [2] S. Kambe, T. Matsuoka, T. Takahasi and M. Kawai, Phys. Rev. B. 171 (1990) 2669. [3] R. Yoshizaki, J. Fujikami, T. Ishigaki and H. Asano, Physica C 171 (1990) 315. [4] S. Kambe, Y. Murakoshi, R. Sekine, M. Kawai, K. Yamada, S. Oshima and K. Okuyarna, Physica C 190 ( 1991 ) 139. [5 ] N. Fukushima, H. Niu and K. Amdo, Jpn. J. Appl. Phys. 27 (1988) 1432. [6] K. Fueki, Y. Idemoto and H. Tokunaga, Physica C 185-189 ( 1991 ) 679. [ 7 ] Y. Idemoto, T. Muroga and K. Fueki, Physica C 210 ( 1993 ) 315. [ 8 ] Y. Idemoto, S. Ichikawa and K. Fueki, Physica C 181 ( 1991 ) 171. [9] Y. Idemoto and K. Fucki, Physica C 168 (1990) 167. [ 10 ] K. Fueki, Y. Idemoto and H. Ishizuka, Physica C 166 (1990) 261. [ 11 ] K. Fueki and Y. Idemoto, MRS Syrup. Proc. 209 (1991) 783. [ 12] Y. Idemoto and K. Pueki, Physica C 190 (1992) 502. [ 13 ] H. lshizuka, Y. ldemoto and K. Fueki, Physica C 195 (1992) 145. [14]J. Humlicek, M. Garriga and M. Cardona, Solid State Commun. 67 (1988) 589. [ 15 ] H. Yasuoka, H. Mazaki, T. Terashima and Y. Bando, Physica C 175 (1991) 192. [ 16 ] H. Ishizuka, Y. Idemoto and K. Fueki, Physica C 209 ( 1993 ) 491. [ 17 ] S. Uchida, Springer Series in Solid-State Science, Physics of High-Temperature SuperconductoPs, cds. S. Maekawa and M. Sato, 106 (1992)259. [ 18] K. Horiuchi, S. Hayashi, H. Adachi, T. Mitsuya, T. Hirano, K. Sctsune and K. Wasa, Physica C 160 (1989) 273. [ 19 ] Y. Tokura, H. Taka~i, T. Ar/.ma, S. Koshihara, T. Ido, S. Ishibashi and S. Uchida, Physica C 162-164 (1989) 1231. [20] S. Uchida, H. Takagi and Y. Tokura, Physica C 162-164 (1989) 1677. [21] H. Takagi, Y. Tokura and S. Uchida, Physica C 1162-164 (1989) 1001. [22] A. Maeda, M. Hase, I. Tsukada, K. Noda, S. Takebayashi and K. Uchinokura, Phys. Rev. B 41 (1990) 6418.