Poole Ch.P., Jr. Handbook of Superconductivity

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656

Chapter 14: Phase Diagrams

Fig. 14.14.

(continued)

Ndl)l.5

1995ocl

242 I (C)

2.00

171 /

1.sy(X

1.29// k k

,,'\\\\

60 80 1 O0

1 01 Mol ~. CuOx

BaO~

242

Nd~01.5

!1020ocl ~21

(d)

1.96

1.87/(

1.1

BOO-:- -- ~ v , ....... --. .....

::~,-',

I 01 60 80 100

Mol ~. CUOx

Schematic quasitemary sections of the Cu-rich comer of the BaO--~Nd203--CuO system phase

diagram in air at different temperatures: (c) 995~ (d) 1020~

(continued)

Representative Phase Diagrams

657

Fig.

14.14.

(continued)

1!o3oo01

24-2

1.86

1.67~

1.55 X' '

1.34

/ \

1.13 /' \ '

1.07/,\ 'X

1.00/1\\ '

BaO~ =

101 60

~t021

(e)

".._ ... '\\

_._ .. \

k --.. \

--.".... \\

\ ".....-._ '... \\

', __-.

\.N~" Uq [.? ....... ..

~"~- " i "'" .... "":""

.......... .::

Mol %

100

CuOx

11045~

242

1.14

1.09

1.00/

Nd01.5

\o,1

1.70 /

(f)

9 . . . 9

~ -.....

9 ,

Ba 0--- -- v v

101 60

Mol ~,

9 9 -j

9

"""

~9N d2C u 04-

9 9 "

eut.

Liq. 1.. "-

I...'.'.'.'.'.':

100

CuOx

Schematic quasiternary sections of the Cu-rich corner of the 1

BaO-~Nd203-CuO system phase

diagram in air at different temperatures: (e) 1030~ (f) 1045~

(continued)

658 Chapter 14: Phase Diagrams

Fig. 14.14.

(continued)

i1060ocl

242

~021

(g)

1.0{

1.00

BoO ~ = v

101 60

i : i ii iiiiii

~iq I'/"-i''" - "'- "%".

80 1 O0

Mol ~ CuOx

Schematic quasitemary sections of the Cu-rich comer of the BaOINd203-CuO system phase

diagram in air at different temperatures: (g) 1060~ (Goodilin

et al.,

1997). The numbers on

(e), (f), and (g) correspond to the x value in the solid solution Ba2_xNdl+xCu3Oz.

The superconducting solid solution series

La2_xSrxCuO 4

that crystallized with

the K2NiF 4 structure was confirmed to exist with complete substitution of

0 < x < 1. This solid solution is in equilibrium with all the three end members

of this ternary oxide system, the 1"4"5 phase and another solid solution series,

La2_xSrl+xCu206_x/2+ 6

(1"2"2). The 1 "2"2 phase is an end member of the

La2_xSr l+xCu206_x/2+

series, which has the

Sr 3Ti

structure, and has a

smaller solid solution range of 0 < x < 0.14.

d. {Bi, Pb}-Sr-Ca-Cu-O Systems

In 1987, the ability of bismuth to form lamellar oxides was recognized, and the

single-layer Bi-Sr-Cu-O compound having a critical superconducting tempera-

ture ranging from 9 to 22 K was discovered (Michel

et al.,

1987). Superconduct-

ing bismuth cuprates were later reported to form a family of layered-structure

phases with ideal formulas Bi2Sr2Can_ 1CunOa+2n with n = 1, 2 and 3, depending

on the number of (CuO2)n layers. These phases exhibit variations of cation ratio.

D. Representative Phase Diagrams

659

Table 14.2.

DTA onset temperatures (~ for various reactions identified as eutectic (e),

peritectic (p), or peritectic melting (m) (Ullman

et al.,

1989) (see Fig. 14.8).

Approximate uncertainties are indicated in parentheses next to the selected

temperature values.

Reaction Argon Air Oxygen

Yttrium

Erbium

Gadolinium

Neodymium

el ... 898(5) 925(3)

e2 816(17) 902(2) 938(2)

pl 862(7) 933(2) 962(3)

p2 897(5) 956(5) 990(5)

p3 ...... 1010(5)

e3 ...... 1015(1)

ml ... 999(3) 1028(3)

el ... 898(5) 923(3)

e2 ... 902(2) 943(1)

p 1 860(5) 928(5) 957(2)

p2 ...... 999(1)

p3 ...... 1000(5)

e3 ...... 1018(5)

ml ...... 1012(3)

el ...... 923(2)

e2 815(5) 907(5) 936(3)

pl 870(10) 949(5) 927(5)

p2 ...... 1007(1)

p3 ...... 1020(5)

e3 ...... 1017(5)

ml ...... 1063(5)

el ...... 923(2)

e2 ...... 945(5)

pl ...... 963(3)

p2 ...... 1014(1)

ml ...... 1108(5)

e3 and p3:~1005 to ~1020~

To date, the three well known superconductor phases in the BSCCO system are

commonly referred to as the single-layered 2201 (Bi : Sr: Ca : Cu) phase, the two-

layered 2212 (Bi : Sr: Ca : Cu) phase, and the three-layered 2223 (Bi : Sr : Ca : Cu)

phase. Among them, the two most widely investigated are the 80 K Pb-free two-

layered 2212 phase (MacManus-Driscoll and Bravman, 1994; Majewski

et al.,

1993; Sung and Hellstrom, 1995) and the 110 K Pb-doped 3-layered 2223 ((Bi,

Pb) : Sr: Ca : Cu) phase (Merchant

et al.,

1995; Hu

et al.,

1995; Jiang and Abell,

1996). Because of the small temperature range and sluggish kinetics of formation

of the Pb-free phase, little has been reported on the phase equilibria or processing

of this phase. Pb substitution was found to increase the processing window and to

stabilize the 2223 structure.

660 Chapter 14: Phase Diagrams

Fig. 14.15.

-100

-200

300-

200

~xT,(x

100 _ -- ~~

0.0

I ' !

let

~- Ortho-P

_~ \

--i TN(x)

- ~

AF I

-- I SG

type phase

0.1 0.2 0.3

x in Laz-xSrxCu04

Temperature diagram of the system SrO-La203-CuO , section of La2_xSrxCuO 4 (Birgeneau

and Shirane; 1989). T c refers to the superconducting transition temperatures of the ortho-

rhombic paramagnetic phase and the tetragonal phase, and "IN represents the Nrel temperature

between the orthorhombic antiferromagnetic phase and the paramagnetic phase; Ortho-

P =orthorhombic paramagnetic phase; AF =orthorhombic antiferromagnetic phase; Tet =

tetragonal phase; SG = spin glass type phase; Su = superconductor phase; T s -- temperature of

the orthorhombic-P and the tetragonal phases.

The interpretations of phase relations in a quaternary system is the same as

those in the binary and ternary systems. In the quaternary system, however, it is

more difficult to present the relations in a simple form because of the many

variables present. When detailed knowledge of data is available, it is common

practice to build a tetrahedral model of the system to portray the phase relation-

ships. Figure 14.17 shows the tetrahedral system used to illustrate the position

of the three superconductors in the Pb-free BSCCO system. (Hettrich

et al.,

1991).

1. Phase Diagrams for One Cu02 Layer

Figure 14.18 shows the phase equilibria of the Bi-Sr-Cu-O system reported by

Roth

et al.,

at 875-925~ Four ternary oxide compounds were found. Note the

D. Representative Phase Diagrams

661

Fig. 14.16.

SrO

Sr2Cu03

SrCuO 2

"2:,3"

0 20 ka2Cu0440 60 80 I00

I/2(La203) Mol % CuO

Phase diagram of the system SrO-La203-CuO at 980~ in (Hohn

et al.,

1989).

coexistence of the solid solution

Bi2.2_xSrl.8+xCuO z

(commonly referred to as the

Raveau 11905 phase) and the Bi2Sr2CuO 6 (2201) phase. It is this 11905 that is

the single-layered 9-20 K superconductor, whereas the 2201 phase does not

superconduct. The Raveau phase and the 2201 phase are in equilibrium with each

other. The Raveau solid solution was found for the approximate range

0<x_<0.15 for

Srl.8_xBi2.2+xCUl:Lx/20 z.

This phase is structurally similar to

the n- 1 member of

SrzBizCan_]CunO2n+4.

The 2201 phase was found to be

monoclinic and CuO-deficient (<1 mol%) and only has a small homogeneity

region. The observed X-ray diffraction of this phase does not match that of the

Raveau phase. Thoughout the literature, the 2201 symbol is commonly used in

place of the Raveau phase and may be interpreted as a part of the extended single-

phase region of the Raveau phase (Fig. 14.17). The Raveau phase was found to

melt at 870~ (Nevfiva

et al.,

1993). The other two ternary compounds

(BiaSr8CusO x and Bi2Sr3Cu2Ox) are not superconductors.

662 Chapter 14:

Phase

Diagrams

Fig. 14.17.

SrO

Sr3ai=o'~l I

jl, I

/I !i I I i

"l,,Sr~4cu=4o,,~-,

/I Ii I 2223

CoO

Mol g

CuO

Tetrahedral phase diagram of the

Bi203-SrO-CaO-CuO

system at 850~ showing the

presence of various phases and the locations of the 2201, 2212, and 2223 phases (Hettrich

et al.,

1991).

2. Phase Diagrams for Two CuO 2 Layers

Solid solution region.

Various investigations of the solid solution bound-

aries of the 2212 phase have been published. Figure 14.19 summarizes some of

the literature data and indicates the range in size and shape reported for the solid

solution field up to 1993 (Krizek

et al.,

1993; Holesinger

et al.,

1993; Majewski,

1994; Hong and Mason, 1991; Golden

et al.,

1991; Muller

et al.,

1992).

Disagreements among the reported solid solution regions indicate the compli-

cated nature of the equilibria in this system. Many factors influence the

experimental results, including the sensitivity of the phase assemblages to

processing conditions, the sluggish kinetics of phase formation, and the very

closely spaced phase stability fields of the high-T c phases.

Figure 14.20 shows the temperature stoichiometry dependence of the

Bi2.18Sr3_yCayCu208+ x composition (Majewski

et al.,

1995b). In this projection,

the 2212 phase exhibits an extended single-phase region with variable Sr, Ca, Bi,

and oxygen content. This single-phase region approximates a half-moon shape,

with the greatest width of y at about 800~ With increasing temperature the

extension of the single-phase region shrinks and is shifted to Sr-richer composi-

tions. The four-phase regions outside the single-phase area vary depending on the

D. Representative Phase Diagrams

663

Fig. 14.18.

SrO

Sr6Bi209

Sr3Bi20s

orth-Sr2Bi205

t et~,-" Sr.9 Bi ~. 102.5s'~

mon-"SrBi204" ~ o

0 0

Sr2Cu03

SrCu02

"Sr14Cu24041"

0 20 Bi2CuO 4 40 60 80 100

1/2(Bi203) Moi ~ CuO

Phase diagram of the system Bi203-SrO-CuO at 875-925~ in air (Roth

et al.,

1990).

y value of the formula. The high-temperature annealing of the Ca-rich 2212 phase

leads to precipitation of CazCuO 3 and a liquid, whereas annealing of the Sr-rich

2212 phase leads to the formation of Bi2Sr3Cu2Os8, cuprates, and liquid. At

temperatures above 870~ the Sr-rich 2212 phase decomposes. The ratio Sr:Ca

of the critical composition of the 2212 phase was determined to be about 2:1

(Biz.18Sr2CaCuzO8+d). At the maximum melting temperature, the 2212 phase

melts to 2201 4- cuprates 4- L.

Subsolidus four-phase compatibilities.

The 2212 solid solution was found

to be in equilibrium with 10 phases at 830~ (Wong-Ng

et al.,

1998). The

equilibrium phases were 0x21 {[(Ca, Sr)zCuO3], x is used to represent the solid

solution concentration of the lesser component}, l19x 5 [(Bi,Pb)z.zSrl.8_ x

CaxCuOz], 2110 [Bi16(Sr, Ca)14Oz], 014x24 [(Sr,

Ca)14Cu24041],

2310

[Biz(Sr, Ca)4Oz], 4805 [Bi4SrgCusOz), 2201 [(Bi,

Pb)zSrz_xCaxCuOz],

(Ca, Sr)O,

CuO, and 0xl 1 [(Srl_xCax)CuO2, Ca-rich]. Because of the presence of extensive

ternary and quaternary solid solutions, the 2212 phase compatibilities include a

number of relatively "flat," or shallow, four-phase equilibrium volumes. The

implication is that a small variation of composition, or temperature, can lead to a

664 Chapter 14: Phase Diagrams

Fig. 14.19.

0.5

1.0

2.5

1.5

/ .,~_~-__-.~-t- ~,... b '"

2...... >

,, I ,,,

! i

1.5 2.0 2.5

Summary of various determinations of the single-phase solid solution region of the 2212 phase

in the Bi203-SrO-CaO-CuO system (Knizek

et al.,

1993). The cross-section shown

corresponds to the formula

Bi2+ySr3_y_xCaxCu208+,~.

Knizek

et al.

(1993), (e 9 9 850~

air; Holesinger

et al.

(1993), ( .... ) 865~ oxygen; Majewski (1994), (----) 850~ air;

Hong and Mason (1991) (------) 860~ air; Golden

et al.

(1991), (---) varying

temperature; Muller

et al.

(1992), (--) 830~ air.

dramatic change in the phase equilibrium assemblage. Table 14.3 lists a series of

16 four-phase equilibrium volumes arranged according to the initial melting

temperatures obtained from the DTA events (Wong-Ng

et al.,

1998). They are

mutually stable in a topologically consistent manner (i.e., no compositional

overlaps with each other), and they form a close-packed arrangement. The

eutectic temperature (~825~ or the lowest temperature at which the 2212

phase is in equilibrium with liquid, was found to involve a liquid near the

strontium-poor part of the Bi-Sr-Ca-Cu oxide system: 2110 + 119x5 +

2212 + CuO--+ L.

Primary crystallization field of the 2212 phase.

Table 14.3 also shows the

initial melt compositions for the 16 four-phase volumes involving the 2212 phase.

The Bi concentration was found to cover a range from 24 to 42%, Sr from 7 to

33%, Ca from 2 to 27%, and Cu from 19 to 43%. The three-dimensional

pictorialization of the 2212 crystallization volume is shown via the two views in

Representative Phase Diagrams 665

Fig. 14.20.

900

850

800

750

700

- [

- I //////

650~~

0.4 0.8 1.2

y in Bi2.18Sr3_yC~u201~

----r I r 1 T I r A

:__..._

2201 + Cuprotee

4- Liquid . .

- t~-- 2212+

t \ *,~.~._~.. ~ e.~0~ /

"-/ \:

~ -.t

2CuO3 +

1 ,.---"l

2212 + 2212

Sr14Cu24041--= /

+ s,'~o /

- + 2201

/ -

- ,/ -

2212 +

Co~uO~

sr~i~o=

+ CuO

1.6 2.0

Temperature-(Sr, Ca) stoichiometry phase relations for the 2212 phase with composition

Bi2.18Sr3_yCayCu208+ 6. The designation 2201 is used to represent the Raveau superconduct-

ing phase (Majewski

et al.,

1995b) Table 14.3 gives initial melts of four-phase equilibrium

volumes containing the 2212 phase.

Figs. 14.21 and 14.22, where approximate temperatures of the four-phase initial

melting equilibria are also indicated. In these diagrams the volumes are expressed

in Cartesian coordinates. The polygonal areas on the surface of the crystallization

volume have been labeled according to the presence of a second solid in

equilibrium with 2212. The maximum melting temperature in Fig. 14.21 is

889~ corresponding to the initial melting equilibrium for the four-phase volume

2212-2310-0x21-CaO. The overall wedgelike shape of the volume is apparent in

Fig. 14.22. In the lower-melting region (low SrO), the volume is very thin and

terminates along a sharp edge. The higher SrO end of the volume is much wider

and is terminated by the 2201 crystallization surface. Inside the volume, the 2212

phase is in equilibrium with liquid. At the comers, it is in equilibrium with three

phases and with L. At the edges, 2212 is in equilibrium with two phases plus L,

and on each face 2212 is in equilibrium with L and the labeled phase.

Effect of Ag addition.

The use of silver as an additive is widespread in

BSCCO tape and wire processing. Except at very high temperaures, the presence

of Ag does not affect the stability of the 2212 phase (Driscoll

et al.,

1993). Figure

14.23 shows a schematic temperature-composition phase relations of the