Poole Ch.P., Jr. Handbook of Superconductivity

636 Chapter 14: Phase Diagrams

exhibit an orthothomic-tetragonal phase transition depending on the oxygen

content (Wong-Ng

et al.,

1988; Jorgenson

et al.,

1987).

The high-temperature reaction of CO 2 with Ba2YCu306+ x is known to lead,

upon completion, to BaCO 3 and copper and yttrium oxides (Gao

et al.,

1990);

however, oxycarbonates may form prior to complete carbonatization (Karen and

Kjekshus, 1991). Phase diagrams of the Ba(O/CO3)-Y(O/CO3)-Cu(O/CO3)

pseudoternary system, as determined by powder X-ray diffraction after repeated

firings of samples at (a) 800~ and (b) 900~ in 1 atm oxygen containing

.~40ppm CO2, are shown in Figs. 14.2 and 14.3. In these two diagrams, the

envelopes of oxycarbonate stability are shown bounded by dotted curves. The

carbonate stability regions are near the BaO(BaCO3) region and are bounded by

the broken curves. The Ba-rich oxides in the Ba(O)-Y(O)-Cu(O) system have a

high affinity for CO 2, leading to the formation of oxycarbonates. Three oxycar-

bonates were identified: (1) The first phase is near the "other perovskite phase"

composition (8:1:4; Roth

et al.,

1988), with a homogeneity region extending

toward Y, and has a formula of

BagYl+xCu4+z(CO3),O11+w ,

with u~2,

Fig. 14.2.

BoO (EIoCOz)

80ooc

105 Po

02

40 ppm

C02

BAY204

-....-

f

/. ".,JR.

BaCu02

///" V. .... V ~, _V _ V ~'~

0 20 '40 Y2Cu205 60 80 100

1/2(Y203 (Y2(C03)3)) Mol ~ CuO (CuC03)

Phase diagram of the system

BaO(BaCO3)-~Y203(Y2(CO3)3)-CuO at 800~

105 Pa

02,

and

40 ppm CO2 (Karen and Kjekshus, 1991).

D. Representative Phase Diagrams

637

Fig. 14.3.

BaO (I

900~

105

P a

02

40 ppm C02

Bo3Y2(CO~)05

BasYCu4(C03)2012

BAY204

~__ .. V . V ~, V _V

0 20 40 V2c 205 60 80 .... -1-00

1/2(Y203 (Y2(C03)3)) Moi % CuO (CuC03)

Phase diagram of the system BaO(BaCO3)-~21Y203(Y2(CO3)3)-CuO at 900~ 105 Pa

02,

and

40 ppm CO 2 (Karen and Kjekshus, 1991).

0x < 0.03, 0 < z < 0.04. There is a pressure-temperature equilibrium between

oxygen gas and vacancies in the solid; for 0.05 < w < 0.08, x- z = 0. (2)

The tetragonal Ba3Yz(CO3)uO6_u, u~ 1, phase is stable up to 960~ in

purified oxygen. (3) The third carbonized phase is the important 2:1:3

phase. It appears that accommodation of the rather small "size" of cation at

the the Ba site is promoted by the carbonization. An extended solid solution

(Bal_yYy)zYCu3(CO3)0.zO6.y+y , 0 <y < 0.2, was obtained. For u -~ 0.2, v ~ 0.1,

the

BazYCu3(CO3)uO7_u_ v

phase is tetragonal, with unit cell a - 3.877(2) A and

c = 11.573(3)A. The structure of BazYCu3(CO3)uO7_u_ v is basically derived

from BazYCu307 by replacing some of the oxygens by carbonate groups (Karen

et al.,

1999).

2. Prepared in Air in the Absence of CO 2

The diagram of the CO2-free system prepared at ~950~ is shown in Fig. 14.4

[42]. BaO 2 or Ba(NO3) 2 was used as one of starting reagents. Instead of three

638 Chapter 14: Phase Diagrams

temary oxide phases as found in Fig. 14.2, five were found. The Ba2YCu306+ x

(2:1:3) and BaY2CuO 5 (1:2:1) phases are the same as those prepared in pure

air, but the perovskite solid solution discussed previously (Ritter, 1988) has

become two point compounds with compostion Ba4YCu308. 5 (4:1:3) and

Ba8Y3CusO17.5 (8:3:5). Another phase with composition BasY1Cu20 x

(5:1:2) was also found. The Ba4Y207.CO 2 and Ba2Y2Os.2CO2 compounds

are absent in this diagram. A comparison of this diagram to Fig. 14.2 (prepared

with BaCO3) reveals that the tie-lines connecting the 2:1:3, 1:2:1, BaCuO2,

and perovskite phases are different. There are still uncertainties involved these tie-

lines due to the hampered kinetics at 950~

3. Prepared in Oxygen

The phase relations of the BYC system at lower temperatures of 800~

(P(O2)- 1, 105 Pa

< Ptotal -< 5 • 105

Pa, p(CO2)~ 0.000), and at 700~ 105

Pa 02 are schematically shown in Figs. 14.5 and 14.6 (Roth, 1990). In these two

diagrams, the 2" 1 9 4 and 4" 2" 7 phases were indicated as stable at 105 Pa 02 at

Fig. 14.4.

BaO

Cu03

Ba3Y409

8:3:5~ BaCuO 2

1"2:1

I V V X ~ V 9 I

0 20 40

Y2Cu205

60 80 100

1/2(Y203) Mol ~ CuO

Phase diagram of the system

BaO-~Y203-CuO at

~ 950~ in (Deleeuw

et al.,

1988).

D. Representative Phase Diagrams 639

Fig. 14.5.

BoO

IPo~_ =

~.0 .........

I

iPco~ = o.ooo

B2C IPt = I ~, .,x __,..5 at m

Be

BAY204

11

t I

i

IfI '''~ I I I

B2C3

:2:1

Y2o3

9

/ 4:2:7

,/

v v v .... v., v ,~__v, ,v

-

Y2Cu205

Mol

%

CuO

Phase diagram of the system BaO~Y203-CuO. Schematic phase relations in

0 2 at

~800~

(pO 2 = 1.0 and 1 atm < PYotal --< 5 atm, Pco2 -< 0.000) (Roth, 1990).

800~ The 2" 1" 4 phase decomposes at >850~ However, at lower tempera-

ture, such as ~700~ neither the 2" 1 9 3 nor the 4"2" 7 phase can formed, and a

tie-line exists between Y203-Ba2Cu3Os+x. The Ba2Cu3Os+x phase only forms in

the absence of CO2. This phase is only stable at pO 2 ~ 0.2 if the CO 2 content is

well below that of the ambient atmosphere. It decomposes after formation if

heated in air.

4. Effect of Oxygen Partial Pressure

Figure 14.7 shows for the 213, 214, and 427 phase regions of the BYC system vs.

the oxygen partial pressure plotted vs inverse temperature over the range between

500 and 1000~ and 10 -2 Pa <p(O2) <107 Pa (Morris, 1990). The approximate

boundaries between the phases are indicated by the dotted curves. Included in this

plot for the 213 system, the decomposition line (thick solid line [Bormann and

Noelting: 1989), and the orthorhombic-tetragonal transition line (bold dashed

line) (Lindermer and Sutter, 1988; Specht

et al.,

1989), lines of constant oxygen

640

Chapter 14: Phase

Diagrams

Fig. 14.6.

BaO

i,-,7o0' I

Y203

BAY204

f

J

f

J

i

f

/ Ba~

f"

- j

..-

f ._..._.._..._- --- ---" --" --"

......

No 2:1:3

----~

f

J

i

11 t

t

I

v v v V V, v

Mol ~,

Po2 = 0.2 ~ x _~ 1.0

Pco2 - 0.000

Pt = 1.0

Ba2Cu30x

"4

..=(,2:1

\

\\

v v V

\\\

OuO

Phase diagram of the system BaO1Y203-CuO. Schematic phase relations in

0 2

at ~700~

and 1 atm O 2 (Roth, 1990).

content (full lines) in the 213 stability region, and dashed lines where 123 is

metastable. The 2CuO ~ Cu20 + 102 phase boundary above 900~ (alternat-

ing dots and dashes) lies close to the 123 decomposition boundary. The stability

boundary between 213 and 214 phases continues to decrease to lower oxygen

partial pressure as temperature decreases. This 213/214 phase boundary occurs at

considerably higher temperatures than that of the 213 tetragonal/orthorhombic

transition, indicating that the fully oxidized orthorhombic 213 superconductor is

not thermodynamically stable at any temperature or pressure. The slope of the

213/214 phase boundary is so large that at lower temperatures (<600~ the

213/214 boundary will intersect the 213 decomposition line. The oxygen-

depleted tetragonal 213 phase is unstable at all oxygen pressures below 600~

5. Liquidus Diagrams

Melt processing investigations of this high-T c material for viable commercial

applications constitute a major activity within the high-T c superconductor

D. Representative

Phase Diagrams

641

Fig. 14.7.

1000 900 800 700 T, ~ 600 500

i:.. "q..'-.. -'q~ r<.. I

,

.... I ~>

_

".~ ~ ~ ~, ~. z

6 ~

"...-,,, ~'~_ "~-~ "~,

x

-.~ "'-L. " "-. 214 ""~ ~'-~ ~'... /

-~ ---...~..-..

~.. "~.-~_ -~..

"-..%~

_

"-.4 '0 ~ ."

"-_ "~_

"-- ~ ~"'~ "~ 6=

"--

9

9 ~. ~.. i,-. -,~ ~ .Q

4 -'~ ~ '~--. _ "-._ "-_ "-.~__ ~'--. .~ "-..

9 , ,~ ~ ~. o/ "~

~_ ~ " .~

~ .~. ".4~_ ~-... -~

,

9 ~ -~ ~ ~. .,~

13-- -- -'~_ ~ 9 .~ "~ "--_ -~. O.R '~'~_ "~_ -~

o ~

"'--L.

"~ "~

"~"~'~ "~'~'~ "~. "4

c~ 2~

". "~ "-.~ "-~ "~--7 ~!

~. "~ ~.~ ~,. --,,

unstable ~

"~.~6_,.,. "~--.

~"

T

~ud "-...

-2 I i I 9 I ....... , I ~ .I ..... J ...... ,-.-~

8.0 9.0 10.0 11.0 12.0 13.0

104/T, K-1

Oxygen pressure

pO 2 vs

1 / T for oxygen contents x in the range 6.01 to 6.99 near 2 : 1 : 3 (--)

and 2:1:4 (- - -) compositions in the BaO--~21YzO3-CuO system (Morris, 1990). The

decomposition (----) and orthorhombic-to-tetragonal transition (---) lines are shown as

well as the phase transition boundaries ( .... ). The orthorhombic phase exists above the (---)

line. Lines of constant oxygen content are shown by solid lines in the 2 : 1:3 stability region,

and dashed lines in the 2:1:4 region where 1:2:3 is metastable.

research community. The liquidus information for the Ba-Y-Cu-O system is

critical for crystal growth and melt processing. The primary phase field for

Ba2YCu306+x, and univariant reactions in the phase diagram near the CuO-rich

comer have been reported (Aselege and Keefer, 1988; Krabbes

et al.,

1993; 1994;

Zhang and Osamura, 1991). The most referenced schematic liquidus based on a

reaction melting sequence (Table 14.1) is shown in Fig. 14.8. (Aselege and

Keefer; 1988). According to Table 14.1, the eutectic melting of the system took

place at 890~ (el). Other eutectics (e), melting (m), and peritectic (p) events are

also listed.

The liquidus diagram of Fig. 14.8 is in general a good approximate model

for bulk material processing. More detailed studies, however, provide evidence

that the liquid field of the BYC phase has a miscibility gap, as shown in Fig. 14.9

(Wong-Ng and Cook, 1998). This diagram is presented by "stretching" the

customary temary composition triangle in order to magnify the yttrium oxide

contents of the liquids, all of which were below 4 mole fraction (%). The phase

642 Chapter 14: Phase Diagrams

Table 14.1.

The melting reactions of the BYC system (Aselege and Keefer, 1988) that are

identified as eutectic (e), peritectic (p), or peritectic melting (m) (Ullman

et al.,

1989) (see Fig. 14.8). The symbol L is used to represent liquid.

Type Temp (~ Phases involved

el 890

e2 920

pl 940

p2 975

e3 1000

p3 1000

ml 1015

m2 1015

1026

e4 1110

m3 1122

m4 1270

Ba2YCu306+ x -t- BaCuO 2 + CuO --+ L

BaCuO 2 + CuO --+ L

Ba2YCu306+ x + CuO -+ BaY2CuO 5 + L

BaY2CuO 5 + CuO --+ Y2Cu205 + L

BaCuO 2 + BaY2CuO 5 --+ L

BaCuO 2 + Ba2YCu306+ x ~ BaY2CuO 5 + L

Ba2YCu306+ x ~ BaY2CuO 5 + L

BaCuO2 ~ L

CuO ~ Cu20

YzCu205 + Cu20 --+ L

YzCu205 --+ Y203 -k- L

BaY2CuO 5 --+ Y203 -k- L

Fig. 14.8.

BoO

e3~BoCuO2

p3

1:2:1

~/2(Y2O3)

..... .V __

40 Y2Cu205 60 80 e4 1 O0

Mol ~

CuO

Liquidus surface and subsolidus relationships in the BaO-Y203-CuO system (see Fig. 14.4) in

air (Aselege and Keefer, 1998). Table 14.1 provides the temperatures and involved phases for

the eutectic (e), melting (m), and peritectic (p) points, and Table 14.2 provides corresponding

DTA onset temperatures.

D.

Representative Phase Diagrams

643

Fig. 14.9.

Bo4u

"BoCu02"

Y203

A

3.0-

2.5-

2.0-

973 o

I

!1015 o

BaY2Cu05

Y2O3

Y2Cu205

)710 / ~ I

~/

/ ~ I N,~1048 o

Bo2YCu,O,+x / / ~ / ~x

I ~ 947 o

I Two Liquids / "~

! (Liq.l + Uq.2) / ~~

I / Bo2YCu

! / ~J23O

/

I

/

"BoCuO

I.~1972~ ./ ~ r q?R

967 o

CuO

40

-~---BoO

45 50

/#-55 60 65

70 75

80 85

"BaCu02" Mol ~ CuOx---,.-

Liquidus surface in the BaO-Y203-CuO system in air, showing the liquid immiscibility field

of the Ba2YCu306+ x phase (Wong-Ng and Cook, 1998).

fields of BaY204, Y203, BaY2CuOs, Ba4YCu3Ox, Y2Cu2Os,

Cu20 ,

nominal

BaCuO2, and CuO are also shown. The Ba2YCu306+ x superconductor, which

contains 16.7 mol fraction (%) 89 would plot off the top of the diagram at a

height more than four times that of the diagram. The crystallization field of the

Ba2YCu306+ x phase occurs in two segments. Both segments of the field are

entirely below the 2.0 mol fraction (%)

1Y203 level.

The fight-hand segment of

the Ba2YCu306+ x field is bounded by the crystallization fields of BaY2CuOs,

"BaCuO2," Y2Cu205, and CuO, and the left-hand segment was bounded by the

Ba4YCu3Ox, BaY2CuOs, and "BaCuO2" primary phase fields.

A sequence of isothermal reactions that describe the melting sequence are

found in Fig. 14.10, (a) to (i). In these isothermal sections, a split scale is used as

indicated by the break in the side axes in order to give a more comprehensive

view of the solids participating in the equilibria, and simultaneously present the

schematic details of the participating liquids near the base in expanded form. L1

and L2 represent two immiscible liquids. The dominant reaction at each

temperature is as follows:

644 Chapter 14:

Phase

Diagrams

Fig. 14.10.

1/2(Y203)

BaY2CuO 5 ~,, ^

-'~Y2uu205

(a)

eo4VCu;=O=

ill

..

I .... .v. .................. ;:...L.!q;.~.. v.

~40 "Ba(~u02" 60 Mol % 80 100

9 -~---BaO CuOx

~ 2(u

30,

40 "BaCTu02'; 6"0

uoi %

Liq., 80 100

BaO CuOx

Topological sequence of the melting events in the BaO--~2]Y203-CuO system (Wong-Ng and

Cook, 1998), from 923 to 1274~ The main overall reaction for each temperature is given in

the text.

(continued)

D. Representative Phase Diagrams 645

Fig. 14.10.

(continued)

~/2(Y2O3)

BaY2Cu05 ~. Y2Cu205

67oci

[(c)

\\

\\\\

BazYCu306+ •

B x

40 "BaC'u02" 60 Mol ~.

Llq.1 80 100

9 ~-'- BaO CuOx

~/2(Y2O3)

(d ~

~o~o~

40 "BaCu02" 60 Mol g \Liq.1 80 100

-~--BaO CuOx

(continued)

Poole Ch.P., Jr. Handbook of Superconductivity

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