Standard enthalpies of formation of member oxides in the YBaCuO system

PHYSICA

Physcis C 194 (1992) 177-186 North-Holland

Standard enthalpies of formation of member oxides in the Y-Ba-Cu-O system Yasushi Idemoto, J u n T a k a h a s h i a n d K a z u o Fueki Department of lndustrial Chemistry, Faculty of Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda-shi, Chiba 278, Japan Received 20 January 1992

The standard enthalpies of formation for member oxides in the Y-Ba-Cu-O system have been determined by the solution calorimetric method. The standard enthalpies of formation in units of ~ (mol of atoms)- i, Aft/*,were calculated and represented on a ternary phase diagram. It was found that the heat of reaction AHR (kJ moi ) for the complex oxide formation from three component simple oxides, YOLs, BaO and CuO, is negative except for ( 123 ), ( 210) and (202).

1. Introduction

In order to determine the preparation process and the heat treatment condition, the thermodynamic phase diagrams are necessary. The composition diagram of the Y - B a - C u - O system has been constructed at 950°C in air by several groups [ 1-6 ]. For the construction of the chemical potential diagram that provides the chemical stability region of each member oxide as a function of oxygen partial pressure and temperature, the standard enthalpies of formation and standard entropies of member oxides including YBaECU307_6 are necessary, but only a few data have been reported so far. Pankajavalli et al. and Zhanguo et al. have determined the enthalpy and entropy changes of several reactions by the EMF method [ 7- l 0 ]. Parks et al. have determined the enthalpy of oxidation of YBaECuaOx (5.97 < x < 6.94) by high-temperature reaction calorimetry [ l I ]. The solution calorimetric method has been employed by Morss et al. [ 12-14 ] and Pivovarov et ai. [ i 5 ], and the standard enthalpies of formation have been determined [ 7- l 0, 12-14 ] for several oxides, such as YBaECU307_~, BaCuO2+x, YEBaCuO5 and Y203. However, no systematic measurement has been made on complex oxides in the Y - B a - C u - O system. The purpose of the present study is to determine the standard enthalpies of formation for member ox-

ides in the Y - B a - C u - O system by the solution calorimetric method.

2. Experimental

2.1. Sample The YBa2Cu307_6 sample was prepared by the coprecipitation method. One M solutions of Y (NO3) 3. Ba(NO3)3 and Cu(NO3)2 were prepared and the concentrations were de)ermined by the EDTA chelate titration method. These solutions were mixed so that a desired ratio of m:tal constituents could be attained. Oxalic acid, 1.5 times as much as the equivalent amount of cations, was dissolved in 4 times as much ethanol as the volume of mixed aqueous solution, and the oxalic acid-ethanol solution was added to the mixed aqueous solution while stirring. Then, the pH was adjusted at 3.5 to 4 to complete the precipitation. After aging for one night, the preopltate was separated by intrauon,'- U I I ~ U a~ l l0°C, and thermally decomposed at 400-'C in air for 4 h. Then, the powder was calcined at 850:C ip air for 24 h and heated at 950°C in oxygen for 12 h. The resulting YBa2Cu307_6 was employed as the sample. Six kinds of oxides, BaCuO2, Y2Cu2Os, BaY204, Ba4Y2OT, Y2BaCuO5 and YBa3Cu207_~ were pre-

0921-4534/92/$05.00 © 1992 ElsevierScience Publishers B.V. All fights reserved.

178

Y. Idemotoet al. / Standard enthalpiesofformation

pared by the powder mixing method. Y203 was calcined at 850°C in air for 24 h and it was confirmed by T G / D T A that H20 and CO2 were not absorbed. Y203, BaCO3 and CuP were weighed so that the desired ratio of metal elements could be obtained, ground together in an agate mortar with a small amount of ethanol, and dried. The mixed powder was calcined at 850°C in air for 24 h and then heated at a predetermined temperature to form the sample oxide, which was determined to be a single phase by means of an X-ray diffractometer. BaO was annealed at 1200°C for 3 h under vacuum and then cooled to room temperature in vacuum immediately before the measurement of heat dissolution.

2.2. Chemical analysis An amount of sample was dissolved in a HNO3 solution and the solution was diluted with water in a measuring flask. The concentrations of metal ions in the solution were determined by the ICP method and the composition ratio of metal elements was calculated. The oxygen content was determined by iodometry. An amount of sample was dissolved in a HCi solution containing KI. Copper ions, Cu 3+ and Cu 2+, in the oxide react with KI to liberate I2 by the following reactions: Cu s+ + 3I- -,CuI + I2,

( 1)

Cu2+ + 2l----,CuI + :'I2 ,

(2)

and the liberated iodine was determined with a standard sodium thiosulfate solution. Next, an amount of oxide was dissolved in a hydrochloric acid solution. In the absence of KI, Cu 3+ ions in the oxide liberate oxygen to form Cu z+ ions by the reaction 2Cu3+ + H 2 0 ~ 2 C u z+ + 2 H + + -~O2.

(3)

So, all the copper ions become Cu z + ions, which are determined using reaction (2). From these two kinds of iodometric methods, the average valence of copper ions (oxidation state of copper) was determined. The oxygen content was calculated from the compositions and valences of cations with the assumption that Ba, Y and O ions take the valences of + 2, + 3 and - 2 , respectively, and Cu ions take the

average valence determined by iodometry.

2.3. Measurement of heat of dissolution A twin conduction type multi-calorimeter (Tokyo Riko, MMC-5111 ) was employed for the measurement of the heat of dissolution. The calorimeter consisted of two identical compartments and a temperature sensor. One of the compartments was for the sample and the other was the reference. These two compartments were connected differentially, so the fluctuation common to both of them, such as the temperature fluctuation outside the apparatus, was canceled and only the temperature difference due to the heat generated by the dissolution was detected [ 16 ]. The sample oxide was kept at 120°C in vacuum for one hour, placed in an ampule, weighed, and kept at 120°C in vacuum for one hour again. The ampule was sealed with a gas torch, fixed on a holder and immersed in 25 ml of 1.53M HCIO4 solution contained in the sample compartment. An empty ampule was made and set in the reference compartment in the same way as the sample ampule. After the thermal equilibrium was attained at 298 K, these ampules were crushed by breakers and the temperature difference between the two compartments due to the heat evolved during dissolution was followed with time.

3. Results and discussion

3. I. Composition The compositions of metal elements and the oxygen contents are summarized in table 1. The compositions determined by chemical analysis agree with those calculated from the mixing ratio within an error of + 2%.

3.2. Standard enthalpies of formation In the present paper, a member oxide Y~BabCucO:, in the YO~ 5-BaO-CuO s3stem is expressed as a set of numbers, a, b and c, in the order of Y, Ba and Cu. For example, YBa2Cu3OT_6 is denoted by (123).

Y. ldemoto et al. /Standard en,haipies offormation Table ! Composition of metal elements and oxygen contents. The maximum error is + 0.004 Sample (Y, Ba, Cu )

(! 2 3) (0 1 1 ) (2 0 2) (2 1 0) (2 4 0) (2 1 1) (I 3 2)

Composition

179

( 173 ). For each dissolution reaction, three runs were made. The following results were obtained. Errors are given as the standard deviations of the means For C u O + S o l n . 1 =Soln. 2: AH, = -61.9_+ 1.7 kJ mol- ~;

Y

Ba

Cu

0

0.98 1.99 1.99 1.98 2.00 1.00

2.03 1.01 1.01 4.02 1.01 3.02

2.99 0.99 2.01 0.99 1.98

6.54 2.04 5.00 4.00 6.99 5.01 6.83

(4)

for B a O + S o l n . 2=Soln. 3: AH2 = - 226.8 _+0.5 kJ mol-~;

(5)

for Y203 + Soln. 3 = Soln. 4: A H 3 = - 3 8 3 . 6 _ + 4 . 1 kJ tool- ~;

(6)

and for YBazCu306.54+Soln. 1 =Soln. 4+0.0202: AH4 = - 836.7 _+4.0 kJ/tool- '

(7)

1.53M HCI04(Soln. I) 1.S3M HClO4(Soln.l) For the reaction

3CuO

~ aH 1

0.5Y203 + 2 B a O + 3CuO + 0.202

6x10-4m°lI Cu2+'H+'CIO4{Soln.2) I =YBazCu30654

d 25mI

2BaO

4x10-4m° 1 Ba2+'Cu2+ ' CSoln. L O'H+ 43- 1) (

YBa2Cu306.54 2x10-4mol

l

112Y203 Ixl0-4mol

~ all3

AH4

I

A H = 5.6 + 6.9 kJ mol -I

0.5Y203 + 2BaO + 3CuO + 0.0202 = YBa2Cu306.54 AH : (3~H1 + 2~12 + I/2~H3) - AH4 calorimetric

measurement

(10)

If the composition listed in table I is employed, the enthalpy change is calculated by

AH

for

(9)

and we have

y3+,Ba2+,Cu2+ ,H+,CLO4- I (Soln.4)

Fig. 1. How sheet YBa.,Cu306.54 ( 123 ).

the enthalpy change AH is calculated from the heats of dissolution AH,, AHz, AH3 and AH, by A H = (3AH~ + 2A/42 + -~AH3) - AH4,

q25ml

(8)

A H = (2.99AH~ + 2.03AH= + 0.98/2AH3) - A H 4 ;

of

( 11 )

accordingly

3.2.1. YBa2Cu306.54(123)

A H = 3.2-+ 6.8 kJ mol -~

The heats of dissolution of (123) and its component simple oxides YO,.5, BaO and CuO were determined according to the flow sheet shown in fig. 1. For the dissolution of (123) and CuO, 25 ml of 1.53M HCIO4 solution (Soln. 1 ) were used. Barium

Both AH values given in eqs. (10) and ( ! 2) agree well with each other. So, the calculation of the enthalpy change was made by using the composition given in table 1. Since the standard enthalpies of formation for simple oxides, Y203, BaO and CuO are - 1 9 0 5 . 3 i , - 5 5 3 . 5 . - 157.3 kJ tool- ~ [ 17 ], respectively, the standard enthalpy of formation for (123) is calculated as follows:

_ : J ...... OXIUU Wit~

.a.'___l.._..a UlSbUIYUtOl

"

iFI

"~¢ -'-.3

~! 1111

~ Ol

! ~'~AI" I.,J.JlVl

LII'/-~If'~ 11%-,1%.-I 4

,:,r~ o~k,p-

lution containing 6 × 10 -4 mol ofCuO (Soln. 2 ), and Y203 was dissolved in 25 ml of 1.53M HCiO4 solution containing 6 × 10 -4 a:91 of CuO and 4 × 10 -4 mol of BaO (Soln. 3), so that the sequential dissolution shown on the left in the flow sheet could provide the final solution (Soln. 4 ~ with the same composition as that obtained by the direct dissolution of

ArH°( 123 ) = (0.98/2)AfH ° (Y2 03 ) + 2.03AfH ° (BaO) + 2.99AfH ° (CuO) + AH

(12)

i 80

K Idemoto et aL / Standard enthalpies offormation

= -2521.9+_6.9 El t o o l - '

(13)

According to Morss et al. [131, the standard enthalpies of formation for YBa2Cu3OT_6 with 6 = 0.07, 0.31 and 0.5 are -2713+_ 17 kJ tool -~, -2689+_ 17 kJ m o l - ' and - 2675 +_20 kJ m o t - ' , respectively. The absolute value of the present study is a little smaller than those by Morss et al. According to Morss et al. [ 13 ], the enthalpy o f solution in 4M HCIO4 for YBa2CusO6.s4 is - 844.9 +_6.5 kJ tool- ~. The value of the present study, for AH4, agreed with that by Morss et al. In the experiment by Morss et al., the ratio of solvent to solute was not fixed and the compositions of solutions after the dissolution were not taken into consideration. Moreover, the calorimetric measurement was carried out only on YBaECUsOT_,S, Y203 and Ba(CIO4)2 and the calculation of the AH value was made with the aid of data from other investigators. Moreover, Morss et al. have employed a 4M HCIO4 solution [ 12-14 ] whereas this experiment has used a 1.53M solution. The difference in experimental conditions between Morss' and the present ones would be one reason for the difference. 3.2.2. BaCu02 4 (011) Two routes to reach BaCuO2 were chosen for the determination of the enthalpy of formation for (011 ). The flow sheets are given in figs. 2 (a) and (b). In fig. 2(a), 25 ml of 1.53M HC104 solution containing 5X 10 -4 mol of CuO (Soln. 2) was employed to dissolve BaO. The final solution (Soln. 4) was 25 ml of 1.53M HC104 solution containing 5 × 10 -4 tool of CuO and 5 × 10 -4 tool of BaO. The heats of dissolution were as follows. For CuO + Soln. 1 -- Soln. 2: AH, = -62.10+_0.8 kJ m o l - ' ;

(14)

(15)

and for BaCuO2.o4 + Soln. 1 = Soln. 4: AH~ = - 233.1 _+ 1.8 kJ m o l - '

we have

.

_~.AI'I.

5xlO-4mol_, ~ ~( JL, ~'uZ+,H+ ,ClO41

[ BaO

5xlO-4mol

aacuo2.04

,-

(soln~.~

'5xi0-4 )-4~i

rail3

~AH 2

1Ba2+,Cu2÷,H+ ,CIO4- I (Soln.4)

i

(a) 1.53M HClO4(Soln.l)

1.53M HCIO4(Soln. I)

~] 25m.1. =BaO

25ml

mAH

5xlO-4mol. ~/ 1 Fa 2÷ ,H+,ClO4"I L (sotn.3} I 25ml CuO %1/AH2 '/XH3 5x10-4mol IBa + ,Cu2+ H + ,ClO4-

q

~ct~.04

sx10-%ol

!

AH

BaO + CuO + 0.0202 = BaCuO2.04 AH = (6H 1 + AH 2) - AH 3 (b) Fig. 2. How sheet for calorimetric measurement of BaCuO:.o4 (011).

A H = (AHI + AH2) - AH3 = - 6 2 . 9 +_4.4 kJ m o l - '

(18)

The standard enthalpy of formation for (011 ) is AfH°(011 ) = 1.01AfH°(BaO) + 0 . 9 9 A f H ° ( C u O ) + AH ( 19 )

In fig. 2 ( b ) , a 25 ml 1.53M HC104 solution containing 5 × l 0 - 4 mol BaO (Soln. 3 ) was used for the dissolution of CuO. The standard enthalpy of formation for (011 ) is

(~6) AfH°(Ol 1 ) = - 7 7 6 . 0 + _ 2 . 0 kJ mol -I

For the reaction BaO + CuO 4- 0.20: = BaCuO2.o4

CuO.,

= - 777.7 +_4.4 kJ m o l - '

for BaO + Soln. 2 = Soln. 4: ZM-/2=233.9+_4.0 kJ m o l - ' ;

1.53M HClO4(Soln.l) 1.53M HCl04(Soln.l) _lZS~' zsm.].

(17)

(20)

Good agreement is seen between these two AfH ° (011 ) values. According to Morss et al. [ 14], the standard enthalpies of formation for BaCuO2 o5 and

Y. Idemoto et aL / Star 4ard emhalpies of formation

BaCuO2.o95 are - 797 _+6 kJ mol- ' and - 809_+ 7 kJ mol -~, respectively.

3.2.3. Y2Cu205 (202) The measurements were carried out according to the flow sheet given in fig. 3. Y203 was dissolved in 25 ml of 1.64M HCIO4 solution containing 1 × 10 -3 mol of CuO (Soln. 2), and we obtained: for CuO + Soln 1 = Soln. 2: AH~ = - 6 2 . 9 + 0 . 1 kJ m o l - ~;

(21)

for Y203 + Soln. 2 = Soln. 3: AH2 = - 3 9 0 . 2 + 6 . 8 kJ m o l - ' ;

(22)

K) = - 2 2 9 7 kJ m o l - ' using Neumann-Kopp's rule [18]. However, the calculation by authors using Wiesners' AfH°(Y2Cu2Os, 1100 K) value has yielded AfH ° (Y2Cu2Os, 298 K ) = - 2122.1 ld mol- '. Wiesners' method of calculation seems to be erroneous.

3.2.4. Y2Ba04 (210) The flow sheet used for the measurement of heats of dissolution is given in fig. 4. For the dissolution of Y203, 25 ml of 1.53M HC104 solution containing 3 × 10 -4 mol o f B a O (Soln. 2) was used. We found the following. For BaO + Soln. 1 = Soln. 2: AH~ = - 2 3 3 . 2 + 5 . 7 kJ m o l - ' ;

and for Y2Cu2Os + Soln. 1 = Soln. 3: AH3 = - 524.8 + 2.0 kJ m o l - '

181

(23)

for Y203 + Soln. 2 = Soln. 3 zL//2 = - 3 8 8 . 0 + 1 1 . 6

For the reaction Y203 + 2 C u O = Y 2 C y 2 0 5

(24)

(27)

kJ m o l - ~;

( 28 )

and for Y2BaO4+Soln. 1 =Soln. 3: AH3 = - 6 2 1 . 6 + 6 . 4 kJ m o l - t

we have A H = (2AH, + AH2)

-

For the reaction

Z~k/-/3

=8.8_+7.1 kJ t o o l - '

(25)

The standard enthalpy of formation for (202) is AfH°(202) = - 2203.2 _+7.1 kJ tool- '

(29)

( 26 )

Morss et al. [14] estimated AfH°(Y2Cu205) = - 2229 kJ m o l - ' and Wiesner et al. have determined AfH°(Y2Cu2Os, 1100 K ) = - 2 1 1 3 . 8 kJ mol - t by the E M F method, and calculated AfH°(Y2Cu2Os, 298

Y2 03 + BaO = Y2 BaO~

( 30 )

the heat of reaction is A//= (AH, +

AH2) - AH3

= 0 . 4 + 14.4 kJ tool -j

(31)

Accordingly, the standard enthalpy of formation for (210) is ArH°(210) = - 2455.2_+ 14.4 kJ m o l - '

1.53M HCIO4(Soln. I ) 1.53M HCIO4(Soln. I) | 25mi 2CuO ~ AH Ixl0-3mol. ~ 1 2+ ÷ .. Y2Cu205 [ (Soln.2) ,! 5x10-4mol ~25mi Y203 ~ ~H2 /AH3 5x10-4mol ,

(32)

1.53M HClO4(Soln.1) 1.53M HClO4(Soln.1) _[ 25mi -25mi BaO 1. ~ H

3×io-4~oI_

L

Y203 __ ___-4

V

I,

3xl0-4rr~l

lSoln.2l j ~ 25mi ~[t AH) ,~

AH3

-

y3+, a ,tt ,t_xu 4 (Soln. 3 ) AH Y203 + 2CuO = Y2Cu205 AH= (2~H 1 + all2) - AH 3 Fig. 3. Flow sheet for calorimetric measurement o f Y2Cu205 (202).

nH Y203 + BaO = Y2BaO4 AH = (allI + ~H 2) - a H 3 Fig. 4. Flow sheet for calorimetric measurement ofY2BaO4 ( 210

182

Y. Idemoto et aL / Standard enthalpies offormation

3.2.5. YeBa407 (240) The measurement was made according to the flow sheet given in fig. 5. We found the following results. For BaO + Soln. 1 = Soln. 2: AH, = - 2 2 7 . 9 + 4 . 8

kJ m o l - ' ;

sacra'04

+all1

5xlO-4m°] Ba2+ Cu2+..+.ClO4- ] ~ _ _ . _ _ . ~ . ~ ( soXn. 2) 25ml

~

(33) 5x10-4mol

for Y 2 0 3 + S o l n . 2=Soln. 3: AHz=-387.7+9.1

1.53M HCIO 4(Soln.l) 1.53M HCIO 4(Soln.l) .J 25mi 25.11 '

kJ m o l - ~;

5xlO_4mol

¢ all?

y3+ ,Ba2+ ,Cu2+ ,H+ ,CIO4(Soln.4) ]

(34)

and for Y2Ba4OT+SOln. 1 =Soln. 3:

Y203 +

AH3 = - 1036.4+ 15.7 kJ m o l - '

aH 2

[

aH BaCuO2.04 = Y2BaCuO5 + 0.0202

(35)

AH -- (allI + 6H2) - AH 3

(a)

For the reaction 1.53M HCIO4(Soln. I ) 1.58M HCIO4(Soln. I )

(36)

Y203 +4BaO=Y2Ba407

b2_T the heat of reaction is A H = (4AH, +/~r/2)

.I2Sml

25mi

~AH 1

-- z~kn 3

= - 262.9 + 26.4 kJ m o l - ' ,

( 37 )

and the standard enthalpy of formation for (240) is AfH°(240) = - 4374.2 4. 26.4 kJ m o l - '

( 38 )

5x10-4m° 1 y3+,Ba2+(Soln.3) 'g+'clO4- ] ~......-_ Y2BaGIlO~i 5x10-4mol CuO .[ 25mi 6H2 ~/ ~H3 5x10-4mol IY3+ 'Ba2+~Soln. 'Cu2+ ( 4')H+ 'CIO4

~

AH Y2BaO4 + CuO = Y2BaCuO 5

3.2.6. Y,BaCuO5 (211) Two routes to reach Y2BaCuO5 were chosen as shown in figs. 6(a) and (b). In fig. 6 ( a ) , the condition of dissolution of BaCuO2 o4 was the same as that in fig. 2(a): we employed BaCuO_.o4+Soln. 1 =Soln. 2, AH, = - 233.1 4. 1.8 kJ m o l - '

(16)

AH = (gH 1 + ~H2) - AH 3 (b)

Fig. 6. Flow sheet for calorimetric mcasurement of YzBaCuO5 (211). The calorimetric measurement was carried out on the dissolutions of Y203 and YEBaCuOs, and we found for Y203 + Soln. 2 = Soln. 4:

1.53M HCIO4(Soln.2 ) 1.53M HCIO4(Soln. I )

l-'Iill

J 25ml

4BaO 4 4x10- tool.

~ 2+

~H !.

Y203 .

. _-4

IxlU ,~i

(39)

and for Y2BaCuO5 + Soln. 1 = Soln. 4:

+

~______Y.Ba40

l

AH2 = - 3 8 6 . 4 + 0 . 3 kJ m o l - ' ;

(Soln.2)

I

J 25mi

[ aH2 N/ v7 nH 3 y3*,Ba ,H ,CIO4 (Soln. 3 ) ~H Y203 + 4BaO = Y2Ba407

AH3 = - 669.4 4- 1.2 kJ mol- '

(40)

For the reaction

BaCuO204 + Y 2 0 3 =Y2BaCuO5 + 0 . 2 0 2

(41)

the heat of reaction is 3d~'= (AH~ + AH2 ) - AH3

~H : (4AH 1 + ~H2) - AH 3

F~g. 5. Flov~ sheet for calorimetric measurement of "Y,Ba~O: t240).

= 4 9 . 9 ± 2 . 2 kJ t o o l - '

(42)

The standard enlhalpy of formation for (211 ) is

Y. ldemoto et ai. / Standard e,~ "halpies offormation

ArH°(211 ) = -2621.2-+4.9 kJ mol-i

(43)

183

i').5 Y:,O3 + 3BaO + 2CuO + 0.16502

The processes shown in fig. 6 ( b ) yielded ArH°(211 ) = - 2622.3 _+ 15.5 kJ mol-l.

= YBa3Cu206.a3

(44)

The two values of Ark/° (2 i i ) given in eqs. (43) and (44) agree well with each other. Morss et al. [14] obtain ArH°(Y2BaCuOs)= - 2 7 0 7 - + 9 kJ mol -~

The measurement was carried out according to the flow sheet given in fig. 7 and we obtained for CuO + Soln. 1 = Soln. 2: &H, =

-62.4_+0.8 kJ m o l - ' ;

AH= (2AH. +

3AH 2 + ½AH 3 ) - ~L/4

( 50 )

= - 155.6_+ 10.0 kJ tool -~ The standard enthalpy of formation for ( 132 ) is (51)

3.3. Standard enthalpies of formation in units of kJ Onol of atoms)-'

(45)

for BaO + Soln. 2 = Soln. 3: AH2 = - 2 3 0 . 0 + 1.9 kJ m o l - ' ;

(46)

for Y203 + Soln. 3 = Soln. 4: AH3= -384.7_+ 1.6 kJ too!- ~;

(47)

and for YBa3Cu2Or.s3+Soln. 1 =Soln. 4+0. ! 6502: AH,,= - 8 5 1 . 5 + 8 . 0 kJ mol -I

(48)

For the rcaction 1.53M HCIO4(Soln. I ) 1.53M HCIO4(Soln. I) 25mi 25mi 2,CuO +aH i 4 x 10-4re°l[

we have

ArH°(132) = -3091.3_+ 10.0 kJ m o l - '

3.2.7. YBasCueO6ss (132)

(49)

Cu2+ 1Soln. 'H+' ( CIO42)

_] 25mi 3Ba___O_O "~ AH ,I YBa3Cu206.83 6x10-4mol, ~ 2 2x10-4mol IBa + Cu2+ 'H + 'CIO4[ (Soln.3) i/2Y20~ ~ 25mi ixl0_4mol AH~ AH4 y3+ BaZ+ cuZ+ H +,CIO4-I i (soln. 4 ) .__[ £',H

0.5Y203 + 3BaO + 2CuO + 0.16502 = YBa3Cu206.83 AH = (2~H1 + 3AH2 + I/2AH3) - AH 4 Fig. 7. Flow sheet for calorimetric m e a s u r e m e n t ofYBa~Cu20683 (132).

The standard enthalpies of formation for the oxides in the Y - B a - C u - O system a~e summarized in the second column in table 2. However, the standard enthalpy of formation in units of kJ (mol of a t o m s ) - ' , AfH*, is more convenient than AfH ° in the discussion on chemical stability, so AfH* was calculated from AfH °. The AfH* values are given in the third column in table 2. and in a ternary phase diagram of the YOt 5-BaO-CuO ternary system (fig. 8). In this figure, the composition is expressed by the set of three numbers, a, b and c in Y,Ba~,Cu,O,. For example, YBa2Cu3OT_,~ is expressed by (123). Such a figure provides a bird's eye view of how much each complex oxide is stabilized from the enthalpy point of view when the oxide is formed from constituent elements. Let us calculate next how much the member oxides are stabilized in the formation process from Table 2 Standard enthaipies of formation for the oxides in the Y - B a - C u O system Sample

AfH ° k3 m e ! - '

',-%,H'~ k_t (m..o! of aT_ores) - '

YBa_,Cu3Oo 5a BaCuO2.o4 Y2Cu205 YzBaO~ Y2Ba~O7 Y2BaCuO5 YBa3Cu20~ 83

- 2 5 2 1 . 9 +6.9 - 7 7 7 . 7 _*_4.4 - 2203.2 _+7.1 - 2 4 5 5 . 2 + 14.4 - 4 3 7 4 . 2 + 26.4 - 2621.2 ± a 9 - 3 0 9 1 . 3 + 10.0

- 2 0 1 . 1 *_0.:, - 192.5 Z 1.I - 244.8 + 0.8 -350.7+2.0 - 3 3 6 . 7 ± 2.0 - 290.9 ± 0.5 -240.9+0.8

Y. Idemoto et al. ! Standard enthalpies offormation

184 CuO

~

OXl~

(-

'..

,~2o2 -

BaO

(-276.8)

(

....

.

240 (-336.7)

): ~fH

St

210 {-350.7}

.8)

YO1.5 (-381.I)

I kJ.(mol of atoms)-1 }

Fig. 8. Standard enthalpies of formation, AfH*, for the oxides in the Y-Ba-Cu-O system.

simple oxides CuO, BaO and Y203. As an example, we take the case of (123). For the reaction

viewpoint, this fact indicates that the complex oxides, except for (123), (210) and (202), are stabilized in their formation from the component oxides, Y203, BaO and CuO. Also it seems that two kinds of oxides, (011 ) and (240) lying on the sides of ternary phase are highly stabilized. Now, let us plot the AfH* values vertically on the ternary phase diagram. The distances from the base are -381.1, - 2 7 6 . 8 and - 7 8 . 7 kJ (tool of atoms) -t for YO~.s, BaO and CuO, respectively, and these three points form corners of a planar ternary phase. The values with double parentheses in fig. 9 show the distances from the base to the planar ternary phase for respective compositions. The underlined values represent the differences between the experimental values and the calculated ones lying on the planar ternary phase. Such a calculation can be extended to a subsystem consisting of a complex oxide and several oxides surrounding it. As an example, let us take (123) and surrounding oxides, (011 ), (211 ) and CuO. A part of the diagram including these oxides is shown in fig. 10. For a hypothetical reaction

YO,.5 + 2BaO + 3CuO + 0.0202 = YBa2fu306.sa

CuO

(52)

(-78.7)

we calculate YArH°, the sum of ArH° multiplied by the number of moles for substances on the left side, and divide ~AfH ° by u, the total number of mol of atoms. The calculation gives us:

YArH°lu= -201.6

Ol

kJ (mol of atoms) -~

(53)

((-175.2)) JL-----d~,_gn1\~ _-i _ ~ +

~

02

((-245.8)) 00___.I

Since AHp(123) is -201.1 kJ (mol of atoms) -~, AHR = AfH*(123) - Z ArH°/12.54 =0.5 kJ (mol of a t o m s ) - '

/

(54)

AHR is the enthalpy change in units of mol of atoms -/" %T~_ f",.. UI I D~2K.~U3V6.54.

The values of YArH° divided by u, the total number of tool of atoms, for member oxides are given by the values with double parentheses in fig. 9, and AHR, the enthalpy changes in the formation of one mol of atoms of member oxide, are expressed by the underlined values. The underlined values are negative except for (123), (210) and (202). From the enthalpy

l

~l - " ~ ' ~ , . ~

BaO (-276.8)

240 ((-316.5) -~:v.2

): £XfH

((-290.8))

'-.4,

210 ((-350.7)) 0

YOI. 5 (-381.17

{ kJ'(mol of atoms) -I 1 o

}):E~fH /w[kJ'(mol of atoms} -I } : ~H R

{kJ.(mol of atoms} -I }

Fig. 9. Enthalpy changes, ~,SJ/R,in the formation of member oxides.

Y. Idemolo et aL /Standard onthalpies offormation

-

.7)

123~

0,/1 )

23

b20244

',

(-2oi.i).

(-192.5

185

l

/ ~x

/

• -,,

.B)

•

(-210.1)} 210

1-35e.71

1: AfH

(

t

{ kJ-(mol

of atoms)-I }

)):Z&EH / u l k J . ( m o l

of atoms) -1 )

) : ZXfH

t

I kJ.lmol

o f a t o m s ) -1 )

11 : Z A f H ' / u { k J - ( m o l

o f a t o m s ) -1 )

: AH R

(kJ . ( m o l of a t o m s ) -1 }

o

((

: AH R

IkJ-(mol

of atoms)-I

Fig. I 1. Enthalpy change, , ~ k n R , in the formation of (211 ).

I

Fig. I 0. Enthalpy change, AHR, in the formation of ( 123 ).

-3 BaCuO2.042 + ~Y2BaCuO5 + C u P =YBa2Cu306.s4 +0. IO2

(55)

we obtain ~Arn °

-----210.1 P

4. Conclusions

( 1 ) The heats of dissolution for the member oxides of the Y - B a - C u - O system were determined and the standard enthalpies of formation were calculated as follows: AfH°( 123 ) = - 2521.9 + 6.9 kJ m o l - i

kJ (mol of a t o m s ) - '

(56)

Subtracting ~ArH°/vfrom AfH*( 123 ), we have + 9.0 kJ (mol of a t o m s ) - ! . The difference in enthalpy between both sides of reaction (55) is positive. This result indicates that ( 123 ) is not stabilized in its formation from the surrounding oxides, (011 ), (211 ) and CuP. As a second example, let us take (211) and surrounding oxides (123), (202) and (210), and consider a hypothetical reaction ~YBa2Cu306.54 + ~Y2Cu205 + ]Y2BaO4

=Y2BaCuOs+0.00302.

(57)

ZArH°/v is - 2 8 9 . 4 kJ (mol of a t o m s ) - ' and the enthalpy change for reaction (57) is only 1.5 kJ *-'.A t t , ) l' of a t o m s ) - ' . The small difference indicates that AfH*(211 ) lies on the same plane of a triangle having three corners, ArH*(123), ArH*(202) and AfH*(210) in fig. 11. - -

~

AfH°(011 ) = --777.4+4.4 kJ mol-~, AfH°(202 ) = -2203.2 + 7.1 kJ m o l - t , AfH°(210) = - 2 4 5 5 . 2 + 14.4 kJ mol -!, AfH°(240) = -4374.2 + 26.4 kJ m o i - ' , ArH°(211 ) = - 2 6 2 1 . 2 + 4 . c kJ moi -!,

AfH°(132) =

- 3 0 9 1 . 3 + 10.0 kJ m o l - '

(2) The standard enthalpies of formation in units of kJ (mol of atoms) -~, AfH*, were calculated and represented on a ternary phase diagram. It was shown that AHR was negative for the reactions, except for (123), (210) and (202), by which the complex oxides were formed from three component simple oxides YOts, BaO and CuP. A similar calculation was made for the reactions by which a complex oxide was formed from several oxides surrounding it.

i 86

Y. idemoto et al. / Standard enthalpies of formation

Acknowledgements This work has been partly supported by a Grantin-Aid of Scientific Research on Chemistry of New Superconductors from the Ministry of Education, Science and Culture of Japan. The authors wish to acknowledge Y. Yasuda for her experimental assistance.

References [ ! ] R.S. Roth, K.K. Davis and J.R. Dennis, Adv. Ceram. Mater. 2 (1987) 303. [2] R.S. Roth, C.J. Rawn, F. Beech, J.D. Whitler and J.O. Anderson, in: Ceramic Superconductors II, ed. Man F. Yah (Am. Ceram. Soc., Ohio, 1988) p. 13. [ 3] K.G. Frase, E.G. Liniger and D.R. Clarke, J. Am. Ceram. Soc. 70 ( ! 987) C204. [4] D.R. Clarke, Int. J. Mdn. Phys. B I (1987) 169. [5] D.M. De Leeuw, C.A.H.A. Mutsaers, C. Langereis, H.C.A. Smoorenburg and P.J. Rommers, Physica C 152 ( 1988 ) 39. [6] B.J. Lee and D.N. Lee, J. Am. Ceram. Soc. 72 (1989) 314.

[7] R. Pankajavalli and P.M. Sreedharan, J. Mater. Sci. Lett. 7 (1988) 714. [8] A.M. Azad and P.M. Sreedharan, J. Mater. Sci. Lett. 8 ( 1989 ) 67. it.] R. Pankajavalli and P.M. Sreedharan, J. Mater. Sci. Lett. 8 (1989) 225. [ 10 ] F. Zhanguo, J. Chunlin and Z. Zhongxian, J. Less-Common Met. 161 (1990) 49. [11] M.E. Parks, A. Navrotsky, K. Mocala, E. TakayamaMuromachi, A. Jacobson and P.K. Davies, J. Solid State Chem. 79 (1989) 53. [ 12] L.R. Morss, D.C. Sonnenberger and R.J. Thorn, Mat. Res. Soc. Symp. Proc. 99 (1988) 571. [13] L.R. Morss, D.C. Sonnenberger and RoJ. Thorn, Inorg. Chem. 27 (1988) 2106. [ 14] L.R. Morss, S.E. Dorris, T.B. Lindemer and N. Naito, Eur. J. Solid State Inorg. Chem. 27 (1990) 327. [ 15 ] M.M. Pivovarov, M.V. Razumeenko and M.M. Shul'ts, in: XXII Vsesoyuznaya Konferentsiya po ~daimicheskoi Termodinamike i Kalorimetrii. Tezisy Dokladov, I (Izd. Gor'k. Gos. Univ., Gor'kii, 1988) p. 4. [ 16] S. Hagiwara, Funtai to Kougyou, 12 (1973) 1. [ 17 ] D.D. Wagman, W.H. Evans, V.B. Parker, R.H. Schumm, I. Halow, S.M. Bailey, K.L. Churney and R.L. Nuttall, J. Phys. Chem. Ref. Data 11 (1982) Suppl. No. 2. [ 18 ] U. Wiesner, G. Krabbes and M. Ritschel, Mat. Res. Bull. 24 (1989) 1261 l.