Oxygen nonstoichiometry and high temperature conductivity of the 2201 phase of the BiSrCuO superconducting oxide

PHYSICA

Physica C 190 (1992) 502-510 North-Holland

Oxygen nonstoichiometry and high temperature conductivity of the 2201 phase of the Bi-Sr-Cu-O superconducting oxide Yasushi I d e m o t o a n d K a z u o F u e k i Department ~f lnd.~trial Chemzszo,~ Faculty o..fScience and T~hndogl'~ Science L~mrerstO"qf Te~'o. 2641 Yama:akaLNoda-ShL Chiba 278, Japan Received ! 8 October ] 99 i

Ttte oxygen non~oichiome*rv,and lugh temperature conductiv~t)" of Bi. ~ r , ssCu~.o~O~.(2201 phase} g'er¢ delexmined as a function of temperature ~T) an_d~x)gen im~al press~ ~Po~)-The dependences of conducti~'ity on T and Po.~were similar ~o of oxygen content, and two subphases, a and [~. were found in ~uch cases as 2212 and 2223 phases. From the result a close ~elationship wzs referred between condlict/~ity and oxygen co~tenL and the mobilit~ and carrier density were calculated. The c~rricr density 0f2201 phase ¢.~smnMte~~b~L~~O~eof L~e2212 and 2223 phases.

1. latrodeclion The high-T, superconducting oxides possess the pemvskite-related structure as a skeleton ofhhe crystel..Mso, they ha~e a I~rge oxygen n o n s t o i c h i o ~ e t ~ [ i - 3 I, which plays an imporhant role in transport phenomena such as electronic conduction and oxide ion diff~sion~ F o r example, the conductivity as well as the cridc~i temperature o f YBa~Cu~O~_~ decreases with the increase in ~i [ ! ]. There is a close relationship between oxygen nonstoichiometry, a n d the valences o f multiovalent cations, such as copper, bismullL lead, thallium a n d cerium ions. Therefore, the separate determination o f their valences is important in order to obtain detailled information on the oxygen nonstoichiomeL.~: it is kno~xt that in the Bi ~-stem there are three kinds o f phases: 2201, 2 2 t 2 and 2223 phases. In the p r e v i o ' ~ p a p e ~ [ 2~3 ]; the authors have r e . f r e d the relations among oxygen n o n s t o i c h i o m e t ~ °, the valences o f bismuth and copper, and the critical temperature for the 2212 and 2223 phases. Also, a close relationship between oxygen nonstoichiometry and high temperature conductivity has been described [4,51. The present paper is concerned with a similar study o n the 2201 phase. So far, only a few studies have been m a d e on the 220I phase. Sinclair et al. have

delermined the excess oxygen content of Biz+~Sr2_~CaO: ( x = 0 . 1 8 . 0.30 and 0.40) by iodomet~- ~sing citric acid. It was found that the excess oxygen content is 0 . 1 8 ± 0 . 0 2 , irrespective o f composition. The oxygen content for x = 0 . 3 0 ~ s also determined to be 6.33 by the reduction method [6]. lshida aad Sakuma have measured the critical temperature o f BizSr:CuO_, samples which were annealed in air at ~ r i o u s temperatures and then quenched in liquid nitrogen. 1-hey have found that the midpoint critical temperature T,,,~ increased up to ! 7 K and the highest midpoint temperature was obtained when the sample ~as heabmmted at 800°C. However, a compositional analysis was not made

[7]. There is quite a number o f high-temperature conductivity data o f the 220 ! phase. Fiory and his coinvestigators have determined the resistivity o f the 2201 s i n # e cry.stal below 700 K_ They have found that the resistivity in the a - b plane changes linearly with temperature down to as low as the critical temperature and the resistivity in the c-axis direction becomes higher than that in the a - b plane as the temperature decreases [8]. However. to the authors" knowledge no paper has been reported on the resisti~ity, as function o f oxygen partial pressure and temperature. As to the Hall coefficient, Maeda and his coworkers have m a d e the measurement and calcu-

0921-4534/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All rights ~ e d .

Y. Idemoto, K_ Fueki /Oxyges,. nonstoichiometo, and high temwrature conductixqty o f Bi-Sr-Cu-O

luted the carrier concentration for three phases of the Bi system [9 l- However, there are quite a few determinations of carrier concentration and mobility from the high temperature conductivity and oxygen nonstoichiometry of high-T~ superconductors. The purpose of the present study is, firsL to elucidate mutual relations among oxygen content, valences of bismuth and copper, critical temperature, and high temperature conductivity for the 2201 phase; secondly, to compare the present data with those of 2212 and 2223 phases previously reported by the authors [2-5].

503

and copper were determined by chemical analysis [2,31-

2.3. Measurement of o.xTgen nonstoichiometry The oxide sample annealed at 840=C in 0.2 arm oxygen for a long time was subjected to iodometD, for the determination of oxygen corneal An amount of the oxide was suspended from a microbalance, the sensitivity of which is I pg I2], and the equilibrium weight was determined as function of temperature and oxygen partial pressure, in a temperature range of 290 to 890"~Cand an oxygen partial pressure range of 0_3 to 3 × 10-" arm.

2. Experimental

2.4. Cril~cal !emperature rneasurement 2_ I_ Sample 0.5M Bi(NO3)~. IM Sr(NO~)~. and tM C%1(NO3}2 aqueous solutions were mixed so that a ratio of B i : S r : C u = 2 . t 0 : 1 . 9 0 : 1 . 0 0 was attained. Then, an oxalic acid-ethanol soluL~on three times larger than the aqueotts solution in volume was added under stirring condition and the pH was adjusted to 3 with ammonia solution. After aging for one nighL the precipitate v,~asfiltrated, dried at 100~C for several hours, and decomposed ~n a i r m 4~0:C for several hours..After crushing, ~ e powder was heated in air at 800-'C for 15 h, ground, and heated at 84W'C for 24 h in air_ The resulting oxide was examined by the X-ray diffrataion method to be confirmed to be single phase. For the measurement of critical temperature, pellets, lO mm in diameter, were prepared and annealed in various oxygen partial pressures and temperatures. The samples used for the high l,:mp¢:-a:ure conductivity measurement were prepared by pressing powder into pellets, !0 mm in diameter and 0.4 mm in thickness, under a pressure of 400 kgf/cm 2. and by healing them in airat 840°C for 2 h. The relative densiLv was delermined by the Archimedes method.

The measurement was made by the DC four probe method on simevcd samples annealed u~der predetermined conditions and cooled preside the furnace or ,quenched in liquid nitrogen,

2.5~ Itigh to~peral~rc conducti~q.ty t~asurement The high temperature conducfi~fity measurement was made in a temperature range of IO0~C to melting point and an ,exygen pressure range of 1 to i0 -4 area_ The detailed description is given elsewhere [4.5].

3. Results and disewssion

3. L Composition The 2201 oxide confirmed to be single phase by the X-ray diffraction method was subjected to chemical analysis. The composition was found 1o be Bi: ,~Sr~.~C~.o20,. The chemical formula is given so that the mole total of metal elements can be seen to be five [31. 3. Z O.wgen nonstoichiometry

2. 2. ChemicM anatysis Samples confirmed to be single phase were subjetted to chemical analysis and ICP analysis to determine the composition. The valences of bismuth

Figure t gives the oxygen content y as a function of oxygen paniat pressure and temperature. The dotted line shows the me!ti'_,g poinl and the dashed line gives the oxygen content y* when bismuth and cop-

Y. ldemoto, +If+Fueki ! O.wgen nonsloichiometo: and high temperature conductiriO' of Bi-Sr-Cu-O

504

] ~

2

s

:~:=- --

~ /soo~

.

/

o ~

tial molar enthalpy and entropy o f oxygen are given by

%

w~~a00"e

~,+_-~~ _ ~ ~

~

40O°0 !

e.ts

~o

= [O--Rln'°+'l -

i/T)

J,"

(1)

and

1 ¢_

/

//-//; i

6.10p

/

t

[0min e~l

A~qo, = L

, d'<~

] i#+. Ca2+ t 6.0S ~

-3

4

, i~0~ C 8~0~CI

i "! -2

--1

iog(Po2~~m)

Fig. 1. Oxygen rtol~oichiometr~- ~ a fenction of tempemlu~ and o x ~ t~rliM pres_¢~ (Bi: ~#r~ ~Cu~ ~:O+L

per ions take valences of + 3 and + 2 , i"esl~iivel+; The value o f y* is 6_06 for the sample used in the experiment. If the ox_'~gencontent for samples equilibrated with 0 2 aim oxygen at 200-'C is represented by +l,~,the oxygen excess ~.v( =y~ - - r " ) is O. ! 3 ~ilich is about one half o f those of 2212 and 2223 phases [2.3 ]. The oxygen pressure dependence o f oxygen content is quite small below 600°C. and restively lalge above 600=C. Such a d i f f e ~ c e in oxygen oressure dependence bet~een above and below 600:C have also been observed in the 2212 and 2223 phases [2.31. Sinclair et al. have relxtrte~ that the excess oxygen ~ n t e n t Ii+v o f Bi2+,~Sr~+~UuO: (x=0.18, 0,30 and 0.40) determined by the citric acid-iodometry is 0.1 $ _ 0.02 and that the composition o f oxide heated at 800=C is Bi~soSrL~oCuO+z ~ ofwhich total oxygen content was determined by the hydrogen reduction method [6]. The excess oxygen content in the present experiment is 0.13, a little smaller than Sinclairs+ data. The Bi/Cu ratio is 2.30 for their sample and 2.08 for our sample. Probably, the difference in Bi/ Cu ratio would be a cause of difference in excess oxygen content.

3.3. Atio~ and zt-qo~

0T

(2~

j,."

The plot of R tn Po: versus I / T at constant J' is given in fig. 2. The plot is linear when .i, is below 6.150. Above 6.174, only two data points for each y value were obtained because the oxygen content range is quite small. From the slope of the plot, A//o, was determined. Also, the plot o f RTln Po: against T at a constant y value was linear. From the slope, ~0_~ was determined. The plots o f A/]o~ and - ~ o : against 1, are given in figs. 3 ( a ) and (b)+ respect ively. l n a range below y=GA $, both A/to: and ,&-~o~ are small, whereas they are large in the range above y--6.17. Two different sets o f A/-7o~ and A_qo, show the existence of two kinds of subphases, which has been observed in the 2212 ai~d 2223 phases | 2 3 ] . Let us denote the subphase ~ t h h~_g.~er J &I-/o.,] and t '~o++ t by ~ and that wi;h lower IA/]o~ 1 and 1~ o : 1 by ft. respectively+ Figure 4 gives the plot o f x_k/]~ and ,-X-qo: for the a subpha;es o f 220L 2212 and 2223 phases [ Z 3 ] . All the data points fall on a 0 ~

--

--

--

£ t 6.174

e~

6.1~

"~

i;.ioo;~6 +so

-to!

~' ,

Q.8 0.9

1.0

1.1

,.2

-

£

1.~

.

,.8

12g 21o 2.,

104K/T Fig. 2. Plol of R In Po: s~- ! / T at constant y values.

According to chemical thermodynamics, the par-

Y, ldemoto. K_ Fueki / Oxygen nomtoichiomettT and high Iernper~tu,-":6~iduc~iv~O,o f BPSr-Cu-O

single szraight line passing through the origin, and both A//o~ and A~o= values become larger in the order of 2223, 2212 and 2201 phases. This fact is interpreted as follows: the plots o f A G o : ( = RTlnPo~) against Tare given by straight lines as shown in fi~ 5. The equation for the lines is reprcseined by

-~oo ~300

1 (a)

/

~r

/

-400) L

/

-soo! o

/

-600 !!

/

-7001

505

d

Ado_, = A / / ~ - TA~o~.

(3)

The intersection of the tines-~th zhe abscissa gives

-90o; 6.10

6AS 6,20 Oxygen co~te~t y

To _ Afio~ A~qo: "

Figure 5 means that three straight lines for 220 l, 2212 and 2223 pass nearly the same point T ° on the absciss~ and i! is concluded thai the three a subphases are quile similar from the vice,point o f chemical thermodynamics, At a COP~I.3ZZZo x y g e n partial pres~ sure~ y decreases with a gentle slope ~ l o w 800 =C as the temperature increases, bill il decreases with much steeper slopes in the range from around 800 ~C Io the melting point. The foremenlioned phenomenon may be caused in the a subpha~ region.

)

-3oo~ (b)

C E

Im~ -700; ~ ° ~ _ j e -soo~1

6.10

6.15

(4)

E.2Q

Oxygen content Y Fig. 3. A/to; (al and AS~ ( b ) a s a f u n c t ~ of oxygen con~-m

3.4. T, and vaeer~es o.f bismuzk and copper

V.

Table I summarizes fl~e ~ l e n c e s o f bismuth and copper, o~,gen comen~ and T~ o f 220~ 0

-600

,i

j

,

-20 ,- -600

0

7

E

.40

x

(2223

E

.~ 4 o o

¢~ -60

0 -20(]

-200

-400

-600

=zOO

1

-100 8O0

122t 2 tt)

~oo

~,So2/J-mol-* .K-1

looo

11oo

i2oo

,3oo

TIK

Fig4. PlolofAl~o:vs. ASo2fortheasubphasesof2201,22t2

Fig-5. P|ozofAGo~ ( = R T l n P o ~ ) v s - T f o r t h c ~ b p h a s e s o f

and 2223 phase.

220t, 2212 and 2223 phases.

506

Y. ldemoto, K- Fueki /Oxygen nonstoichiometn" and high tetnperature conducttviO" o f Bi-Sr-Cu-O

Table I Oxygen content and valences of Bi and Co in Bi.~teSrt g,Cu~ o.,O~ Annealing

Cooling

Valence

Oxygen content

Too

process Temp• (°C)

Po., latin)

Time (h)

840 g00 700 700 4130 300 800 400

O-] 0.2 02 1.4× l0 ~3 0_2 t.0 0.2 0-2

24 12 24 24 24 24 12 24

(K)

C C C C C C Q Q

Bi

Cu

y'

~"

3_087 3.088 3.090 3_094 3.093 3.093 3.074 3_080

2_016 2.007 2.011 2_013 2_025

6_155 6.158 6.160 6_161 6_166 6. ! 69 6. ~45 6_151

6_158 6.163 6.i69 6_157 6_182 6.185 6.163 6.182

2.030

2.0 t 4 2_01.el

(K)

7_3 7.5

4.3 4.3 5_8

7r7

8.0 6_3 5.2 6.5 6_9

c n'am~ sample cooled ou~d¢ "dief:,~rnacem'~dQ: sa~#e quenched m tiqaid n~t~og~en_

(Biz~_-Srt s~Cut o~,Or) samples together with the heatt r e a t m e m condition. T h e m a x i m u m e r r o r is ± 0.005 for the b i s m u t h valence, + 0 . 0 0 4 for the c o p p e r valence a n d +_+_0.004for the oxygen c o n t e n t s _t"a n d )'°. ~, is t h e oxygen c o n t e n t d e t e r m i n e d f r o m l-ig~ t a n d the a n n e a l i n g c o n d i t i o n a n d ff is t h a t d e t e r m i n e d by chemical analysis. Figure 6 gives the plot o f t h e valences o f b i s m u t h a n d c o p p e r a g a i n ~ oxygen c o m e m y ' . T h e c o p p e r valence is 2.02 in a low _to range a n d i n ~ slightly a b o v e )-"--6.16. T h e b i s m u t h valence increases vdt~h v ~ a n d b e c o m e s c o n s t a n l a b o v e

r" = 6 . t6. Both bismuth a n d cooper valences are small c o m p a r e d to those o f the 2 2 1 2 a n d 2223 phases [2.3I. Figure 7 shows the refdtionship between oxygen c o n t e n t y ' a n d y. T h e d o t t e d line represent the r e | a t i o n )'=3"'. O n the left side o f t h e d o t t e d fine, 3"' is larger t h a n y_ T h i s m e a n s t h a t t h e oxygen take-up would h a v e occurred d u r i n g the cooling o r quenching process. O n the right side o f t h e d o t t e d line, .I," is smaller t h a n )~ This does n o t m e a n oxygen loss d u r i n g the cooling process b u t the existence o f oxygen species u n a b l e io be d e t e r m i n e d b y iodometry. T h e oxygen species h a v e n o t b e e n identified yet b u t might b e O , o r o t h e r neutral species intercalated in a { B i - O ) , block. T h e existence o f such oxygen spe-

~?..10

3-10

7' 6.19~

a

i

0 o

=o a.0,

~2.os i

o

o e-

.

->, 6-181~ C 0 ¢::

, /

6.17:-

0

."

6.16-

T

3.00

!2.00

~ "c

0 6.15~

./~

E

6.14

6. s

6.q6

Oxygen content y' Fig. 6. Valences of bismuth and copper plotted against oxygen conteni 3"-

6.|4= .-" s.14

s.ls

6216 6.1r 6,ia 6.1~

Oxygen content y Fig. 7. Relatiomhip between y and y'.

Y. Idemoto, K Fueki / Ox)wen nonstoichiometry and h~gh tempemwre conducti~'itv of Bi-Sr-Cu-O

cies has been found for 2212 and 2223 phases [2,3]. Figure 8 gives T¢ o, and T¢ 0 as a function of y'. As y' increases, T¢ o~ increases slightly, reaches a maximum at around y" =616 and then deereases. Above r ' =6.I6, the oxide seems to be overdoped with ox)gem lshida and Sakuma have found thin the midizoint critical temperature is enhanced from 6 to ! 7 K when BizSr2CuO~ is annealed in air at 800 ~C and quenched [7]. In the present study, a remarkable increa~e in T, has not been found though the T¢ maximum has been found. The reason is not clear, be cause they did not give the composition determined by the chemical analysis [7]. The composition and the annealing time might both be a cause of the dig ference. In the doping region where T~ increases ~ith oxygen content, the bismuth valence increases and the copper valence remains constant, in the overdoping region where T~ decreases with oxygen content, the copper increases bt~t the bismulh valence remains constant.

3.5. Hight temperature conductit40'

507

4 ¢

~

~5~t,

•%

"3

-2

-~

d

F~g. 9. CendaC~Va~ plotted against- l~gP~- al varioL~S

A sintered sample ~Jth a relative density of 88.7% was used for the high temperature conductivity measurement. The resu!! is summarized in fig. 9_ Ii is easily seen that the conductivity decreas~ with increasing temperature and decreasing oxygen partial p~essure. The dependences of conductivity on temperature and oxygen partial pressure resemble those for 2212 and 2223 phases I4,5]. From comparison

i 10

.!

I,-

I

of fig 9 with fig i, it is concluded that there isa dose relationship between oxygen eontenl and conductivily. as is the ease (or ~he 2212 and 2223 phase~ The sample has an oxygen content .of 6.06 when bismuth and copper ions are assumed to take the valences of + 3 and +2. respectively. The excess ox3.'gencomem A_v=y-6.06 provides the holes which serve as the charge carrier. Figure 10 gives the plot of conductivity against Ay. The conductivity increase with increase in A3-'-Assuming that one execs5 oxygen atom 9rov~des two holes, one can estimate the carrier concentration .and mobility in two ways. From the slope de/d(Ay) of the p]ol the mobtity/~ is calculated by P = .8e .d(Ay) "

(5)

1

0i

6.14

~

~

6.15 6.16 Oxygen content y'

8.17

Fig. 8. Rdadonship between critical temperatures (T~ ~ . T~ ,0) and oxygen COntent y'.

where l ~ is the volume o f the unit cell. Since a0= 5.366 A and c~= 24_576 A, V= is eale~ated to be 7.076 × 10 -2~"cm 3. The plot o f ~ versus Ay is nol linear bu~ curved. Accordingly, the slope was determined from the lX;rtion where data are represented by open circles, because in this range the # o t is nearly straight. The mobility thus determined is given by

508

Y_ Idemoto. K. Fueki /O_'o,gen nonstoichiometo, and high ?emperamre condt*ctirity of Bi-Sr-Cu-O

using the data of a and p. The result is given by open circles and triangles in fig. 12. In the second method, the carrier density n was determined by

IBO

2A|'

n= ~

-2 ~D

5~ !

AY Fig i0. ReLxtion~h~pl x l w ~ conductivityand e ~

~x~.ea

c o n t e n t A3\

m 2> o4

=11

x4

(7)

from Ay. The result is shown by open squares in fig. ! 2. All the data are within 0.5-2.0× 10-~1cm-3 in the temperature range studied. For comparison the data by Maeda et at. [9 ] are given by closed circles in the same figure. They determined the carrier density from the Hall coefficient. Their data are higher than the present datz~ The difference is considered to be due to the difference in measuring method, sample, measuring conditions and others. The carrier density determined in the same way for 2212 and 2223 p h a ~ was 2.74>( 10 :~ c m - ~ and 2.42X 10-~t em -~. respecii~'e|y [4.5]. So. the carrier density of 220t phase is smaller than that of the 2212 and 2223 phases. The tow carder concentration migh~ be a cause of the tow T~ value. in the second method, the mobility is calculated from a and n using eq. (6). The result is given by open squares in fig. 1 I. The temperature dependence of the mobility is small and range in 0.3-1.8 cm: V - t s - ~. So it is concluded that the conduction t}pe is not a thermally activated one, The mobflities of 2212 and 2223 phases were 0.1-0.5 cm-~V-~ s -~ and 0.i~).4 c m : V -~ s -~. respectively [4.5]. So. the mobilil)- of the 220! phase is slightly higher than that of the other two phases.

13 £=

~00

50O

,:3

Temperoture('C) 0 ~00

~2

700

.oA_~edo e~ oL

Fig. t t. M o b i l i t y ~t$ a frocK'tiogaoftetrtper~t~me_

% v

open circles in fig. ! I. In the temperature range of 600 to 800"C, the mobili~' is also determined from the slope of the plot represented by solid circles in fig. 10. The result is plotted by triangles in fig. 11. The carrier density n ~as determined by a=nep,

~000

(6)

J

J

%_ rid

A

13~OD o

o

~

0 oN

Temperoture(K) Fig. 12. Carrierdensiffas a functionof temperature.

K Idemoto, K_ Fueki/Ou,gen nonstowhiometry and high temperature conducllrtO' of B~-Sr-Cu-O

509

Temperature(*C) 0

~

"

_

.

.....

500

=-2

.

3

'

/~

'

'

-

Tem4:,eroture ( K F~g t 3. R e s ~ s t w i l y ~"S. temperature

¢u~es a l

From lhe data in fig- 9. the resistivity ~,~s calculated as a function o f lemperature a~ conslant oxygen pressures• The plot o f resistivity agains~ temperature is given in fig_ ! 3. The resistivity temperature curve below room temperature is the one e b t a i n e d for the determination o f the critical temperature. Figure 14 g~ves the oxygen content difference y0-_r plotted against temperature. The value o f Yo ~,as taken so that the extension of~he re~:sti~ity- ~empera~nre plot passes Ihrou~h the origin. Comparison o f figs_ 13 and 14 indicates a close relationship belween resistivity and oxygen COnlent. Abo~e about ] 000 K, both resistivity and Yo-Y increase remarkably ~ i t h increasing temperature. As shown in fig. 1, such a ,change is due ~o the phase change from [~ to a.

'

i

....

)

c o r t s l a n t ox)~gen

partial pressure',.~

4. Conclusion

(I) The o x y g e n nonstoich~omet~' of B~-.,~~S~I.,~Cu~.~,=O~ ( 2201 phase} was de~.'~'rninedas function of temperat.ure and oxygen pamia] pressure. It was found R a t the oxygen excess fly is about O. 13. which is half o f that for 2212 and 2223 phases. Belob- about 500=C, the dependences o f oxygen conlent on both lempera~ure and oxygen partial pressere were s m a l l whereas they ~,ere large above 600=C. The oxygen content decreases wlth decrease in oxygen partial and increase in ~emperamre. F r o m the therrnodynamical a*~atysis. A/-io: and A ~ : , the partial molar entha~py and entropy of oxygen in the 2201 phase, were determined. Two subphases, a and ~, with differen~ sets o f A/lo: and ASo= vat.ues were

~ernperoture (~C} 500 -o-- ~.og Po2 = -0.~ --o:-tOO

01(

-"-.o.-

>~

+

-

=-2.00 ---3:00

~

~

=-340 ~= " ,

0

500

]000

T e m ~ r a t u r e { K)

Fig. 14. (y~-y) vs. temperature curvesat constant oxygen partia| pressureS-

1

510

Y. ldemoto. K Fueki / Owgen nonstoichiometr), and high telplperature conductivit), of Bi-Sr-Cu-O

f o u n d as in the case o f the 22 ! 2 a n d 2223 phases, ( 2 ) T h e critical t e m p e r a t u r e T, o, was f o u n d to be d e p e n d e n t o f ) , ' . In lhe oxygen d o p i n g region, the b i s m u t h valence increased while the c o p p e r valences r e m a i n e d constant. O n the o t h e r h a n d , in the ove r d o p i n g region, the c o p p e r valence increased while t h e b i s m u t h valence r e m a i n e d constant. ( 3 ) It was f o u n d that t h e d e p e n d e n c e s o f conductivi~" o n t e m p e r a t u r e a n d oxygen partial pressure were similar to those o f oxygen content. ( 4 ) T h e mobility a n d carrier c o n c e n t r a t i o n were calculated f r o m the relation between excess oxygen a n d conductivity. It was f o u n d t h a t the mobility o f 2201 p h a s e is slightly higher than those o f 2212 a n d 2223 phases, a n d the carrier e o n c e n l r a t i o n o f 2201 phase is smaller t h a n those n f l h e ) 9 t 9 :~nd 97"~3 nh?g,~ T h e low c a r r i e r c o n c e n l r a t i o n mig.~t be a cause for t h e low T~ value. (5 } A similar relationship ~ a s f o u n d in the resistiv;,-- , . . . . . . . . . . . . . . . . . (ye-y)-temperalure Oit.~÷e. Acimowtedgemeats T h / s work has b e e n p a r d y s u p p o s e d b y a Gran~-

in-Aid for Scientific Research o n C h e m i s t r y o f N e w S u p e r c o n d u c t o r s from the Ministry. o f F_xlucation, Science and Culture o f Japan. T h e a u t h o r s wish to acknowledge S. Fujiwara a n d N. Yamazaki for t h e i r experimental assistance.

References

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