Poole Ch.P., Jr. Handbook of Superconductivity

646

Chapter 14: Phase Diagrams

Fig. 14.10.

(continued)

~/:'(u

BoY2Cuo5 \

(

Bo4YCu30x

t

40

Mol ~. l.Jq I 80

3 ~iq.2 "BaEu02" 60 100

"~---BaO CuOx

~/2(Y2O3)

BaY2Cu05 ~, ,., ..

ci

Ba4YC

t

40

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

..=-- BaO CuOx

(continued)

D. Representative Phase Diagrams 647

Fig. 14.10.

(continued)

1/2(Y203)

Bou

(g)

...... \,,,,co,o,

9 ,

_

. .

///

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

Ba4YCu;~)x .""

Liq_.,'2

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

BaO CuOx

Y203

~ 1/2(Y203)

BeY2CuO 5

(h) --~- -

B C

V

40 60 Mol % 80

4--BaO

11o48oo. i

100

CuOx

(continued)

648 Chapter 14: Phase Diagrams

Fig. 14.10.

(continued)

(i)

Y20~I~

BAY20"4 .... ".. "... i; aY2Cu05 1/2(Y2031

"

Os

\ 1i 274~

\

\ u

V V V

40 60 Mol % 80 100

9 ,.--- BaO

CuOx

(a) 923~ "BaCuO2" + CuO + Ba2YCu306+ x ---, L 1 +

0 2

(b) 947~ BazYCu306+ x + CuO --+ BaYzCuO 5 + L 1 + O 2

(c) 967~ BaYzCuO 5 + CuO

--+ YzCu205

-'t- L 1 + O 2

(d) 971~ Ba4YCu30 x + "BaCuO2" ---, BazYCu306+ x + L 2 +

O 2

(e) 972~ "BaCuO2" + BazYCu306+ x --+ L 1 + L 2 --t- O 2

(f) 973~ Ba4YCu30 x + BazYCu306+x --+ BaYzCuO 5 + L2 + O2

(g) 1015~ BazYCu306+x --+ BaYzCuO5 + L1 + L2 + O2

(h) 1048~ BaYzCuO 5 + Y2Cu205 --+ Y203 --t- L1 + O2

(i) 1274~ BaYzCuO 5 --+ BaY204 + Y203 + L + O 2

b. Ba-R-Cu-O Systems (R = Lanthanide)

The discovery that the substitution of the most of lanthanide (3+) ions, R, for Y

also produced a superconductor with a transition temperature of ~90 K has

provided numerous altemative materials for investigations of possible desirable

properties (Le Page

et al.,

1987). Proceeding from the La system, which has the

largest ionic size of R, toward the Er system with a smaller ionic size, a general

D. Representative Phase Diagrams 649

trend of phase formation, solid solution formation, and phase relationship was

found to be correlated with the ionic size of R. The ternary phase compatibility

diagrams of the systems BaO(BaCO3)-89 and BaO(BaCO3)-89

CuO in the vicinity of the CuO corners (most relevant to the processing of the

high-T c materials), where R- La, Nd, Sm, Eu, Gd, Er, are shown schematically in

Fig. 14.11 (a) to (f) (Wong-Ng

et al.,

1990). Features of the progressive changes

in the appearance of these temary diagrams near the CuO comer include the

following: (1) The La system has the largest number of ternary compounds and

solid-solution series; this number decreases as the size of R decreases. (2) The

superconductor phase, Ba2RCu306+x, for the first half of the lanthanide family,

that is, R- La, Nd, Sm, Eu, and Gd, which are relatively larger in size, exhibit a

solid solution of

Ba2_zRl+zCu306+ x

with a range of formation that decreases as

the size of R decreases; this solid-solution region terminates at Dy and beyond,

where the superconductor phase assumes a point stoichiometry. The size

compatibility between Ba 2+ and R 3+ is a predominant factor governing the

formation of this solid solution. As the mismatch between R 3+ and Ba 2+

increases, the range of substitution decreases.The approximate upper limit of

the solid solution range ofz

of Ba2_zRl+zCu306+x

are La" 0.7, Nd" 0.7, Sm" 0.7,

Eu" 0.5, and Gd" 0.2, (3) A trend is observed regarding the tie-line connections

between BaR2CuO 5, CuO, the superconductor phases

Ba2_zRl+zCu306+x,

and

the binary phase R2CuO 4, or R2Cu2Os; note that the binary phase R2CuO 4 is

replaced by the binary phase R2Cu205 after the tie-line connection changes.

More complete diagrams of the systems with R- La, Nd are shown in Figs.

14.12 and 14.13, respectively. It is within the Ba-La-Cu-O system that the first

30 K high-T c phase in polycrystalline form,

BaxLas_xCusOs(3_y),

was discovered

by Bednorz and Mfiller (1986). The isothermal section of the Ba-La-Cu-O

system (Klibanow

et al.,

1988) shows a total of five solid solutions:

Ba2+xLa4_2xCu2_xOlo_2x

(242), BaLa4CusO13+x (145),

BaxLa2_xCuOa_(x/2)+ ~

(021), and Ba l+xLaz_xCuzo6_(x/2

) (122),

and a solid solution

Ba3+xLa3_xCu6014ix that spans from the 213 composition to the 336 composi-

tion. The limits of most of these solid solutions have not been quantified. The

solubility limits for

Baz+xLa4_zxCuz_xOlo_zx

were reported to be 0.15 _< x _< 0.25

[54]. The tie-line connectivity of the figure is schematic.

The ternary diagram of the Ba-Nd-Cu-O system at 890~ in air [56] is

reported in Fig. 14.13. In the barium-rich region, samples were annealed in air

with CO2 < 3 ppm. A total of three phases were found in this system. In addition

to the solid solution of the superconductor (213), Baz_xNdl_xCu307_ ~

(0.04 _< x_< 0.6) and

Baz+xNd4_zxCuz_xOlo_2x

(242) (x is negligible), a 6"1"3

phase (orthorhombic: a-3.886(2), b-3.984(2) and c-13.001(5)A) is also

found. The existence of the

Baz_xNdl+xCu3Oz-Baz+xNd4_zxCu2_xOlo_zx two-

phase field enables one to select a starting composition that leads to composite

superconductors of these two phases that are completely devoid of the minor

second phases that segregate at Baz_xNdl+xCu30~ grain boundaries after a solid-

state sintering.

650

Chapter

14:

Phase Diagrams

Fig. 14.11.

BaO

(a) ~k

//

1:2::

_~ . .. ~-~ ...- : . .'------~ ~ -~- ,.

0 20 La2CuO 4 40 60

11z(La203) Mol

2:4:2

Bao (E~aC03)

BaCu02

80

100

CuO

(b) /

///

1:2:1

0 20 Nd2CuO 4 40

~/2(Nd203)

2:1:3

_V v

60 80 1 O0

Mol % CuO

Subsolidus phase equilibira of the BaOIR203-CuO system near the CuO comer. (a) R = La,

(b) R = Nd (Wong-Ng

et al.,

1990).

(continued)

D. Representative Phase Diagrams 651

Fig. 14.11.

(continued)

Boo (E}oC03)

(c) /

v V

0

20

Sm2Cu04.

40

I/2(Sm203)

Mol %

2:1:3

v

6O

BoO (IE)aCO~)

V

80

~oo

CuO

(d)

/

1:2:1

v v

0 20 EuzCuO 4 40

I/2(Eu203)

2:1:3

v v

60 80 100

Mol %

CuO

Subsolidus phase equilibira of the BaO4R203-CuO system near the CuO comer. (c) R - Sm,

(d) R- Eu (Wong-Ng

et al.,

1990).

(continued)

652

Chapter 14: Phase Diagrams

Fig. 14.11. (continued)

BoO (I~IoC03)

(e) /

1:2:1

0 20 Gd2Cu04 40

1/2(Gd203)

Moi

%

2:1:3

V ..... V '~%~.

....

60 80 1

O0

CuO

BaO (IBaC03)

(0

/

///:

1:2:1 "

/ M ...... V

0 20

I12(Er203)

BaCu02

2:1:3

\ M _v

40 Er2CuzO 5 60 80 100

Mol % CuO

Subsolidus phase equilibria of the BaO1R203-CuO system near the CuO comer. (e) R = Gd,

and (f) R = Er (Wong-Ng

et al.,

1990).

D. Representative Phase Diagrams 653

Fig. 14.12.

BaO

BaLa204

0 20 La2Cu04 40 60 80 100

1/2(La203) Mol ~ CuO

Phase diagram for the system BaO-~La203-CuO at 950~ in air (Klibanow

et al.,

1988).

A melting study of the

Ba2_xNd1+xCu30 z

solid solution has been

conducted by Goodilin

et al.,

(1997). A sequence of schematic quasiternary

sections of the Cu-rich comer of the BaO-89 system between 970

and 1060~ in air is shown in Fig. 14.14, (a), (g). Figure 14.14a shows the phase

relationships below liquid formation, and at the bottom Fig. 14.14b shows the

onset of liquid formation in the system. From Fig. 14.14c to 14f, the stability

region

of Ba2_xNdl+xCu30:

is extensive and its maximum range was found in air

at 995~ (0<x< 1). At 995-1045~ the

Bae_xNdl+xCu30 z

phase with the

maximum x value co-exists in the copper-rich comer with the Ba-free Nd2CuO 4

phase and Cu-rich liquid. At 1060~ (Fig. 14.14g); the simultaneous presence of

the

Ba2+xNdn_2xCu2_xOlo_2x

and Nd2CuO 4 phases was found, and only the

Ba2+xNda_2xCu2_xOlo_2x

phase was detected in equilibrium with Nd-poor

Ba2_xNdl+xCu30z

solid solution.

Examples of the effect of atmosphere and different lanthanide ions on the

various invariant points on the liquidus surface for the BYC and BRC (R = Er,

Gd, and Nd) systems in oxygen, air, and argon are given in Table 14.2. The

reactions of the Y system were used as a model (Table 14.1) (Le Page

et al.,

654 Chapter 14: Phase Diagrams

Fig. 14.13.

BaO

Nd2Bo04

I I ~

l~ l''-ooc'

/ \

I \ Ba2CuO 3

! 61

/

I

/

I/

= __ ~ ~

V V V l l

0 20

Nd2Cu04 40

60 80 100

I/2(Nd203) Mol 7, CuO

The subsolidus phase diagram of the BaO1Nd2

O3-CuO

system around the

Ba2_xNdl+xCu30z

compound in air at 890~ (Yoo and McCallum; 1993).

1957). In this table, while the temperatures of the low-melting reactions are

almost independent of the rare earth species, they are strongly dependent on

oxgyen partial pressure. The peritectic decomposition temperature of the

Ba2RCu307 phase (ml) was found to be a function of rare earth with a

significantly higher value for the Nd compound than for the other rare earths.

c. (La, Sr)-Cu-O Systems

In this system, the composition,

La2_xSrxCuO4_y

in

polycrystalline form was

found to exhibit onset T c temperatures in the 20 K range. Figure 14.15 shows the

dependence of the tetragonal-to-orthorhombic phase transition temperature on the

hole concentration of oxygen-deficient

La2_xSrxfuOa_y

(Birgeneau and Shirane,

1989). Figure 14.16 shows a schematic phase diagram of the SrO-La203-CuO

system at 980~ in air (Hahn

et al.,

1989). The precise solid solution limit has not

been determined. Extensive solid solutions were observed for quite a few phases.

D.

Representative Phase Diagrams

Fig.

14.14.

655

NdO~.5

021

242

@

21

3/~-..~

BaO~ .~ v v .....

1 01 60 80 1 o0

Mol ~ CuO

BaC

242

@

(b)

213 s

213

/

\

101 60 80 1 O0

Mol

~ CuOx

Schematic quasiternary sections of the Cu-rich corner of the BaO-~Nd203-CuO system phase

diagram in air at different temperatures: (a) phase relations below melting, (b) beginning of

liquid formation in the system.

(continued)

Poole Ch.P., Jr. Handbook of Superconductivity

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