Chemically substituted high temperature superconducting oxides studied by electrical resistivity

CHEMICALLY OXIDES

SUBSTITUTED

STUDIED

HIGH TEMPERATURE

BY ELECTRICAL

A.M. STEWART*, J.S. ANDERSON, I.D. FITZGERALD, Institute

J.G. THOMPSON,

M.S. PATERSON

B.G. HYDE, R.L. WITHERS

AND J. BITMEAD

of Advanced Studies, The Australian

Box 4? Canberra,

SUPERCONDUCTING

RESISTIVITY

National

University,

G.P.O.

ACT, 2601 (Australi:i)

Received April 14, 1988; accepted May 26, 1988

ABSTRACT The electricaf properties based compounds

substituted

(where R = rare earth) have been investigated.

of the superconducting been observed.

of a number of chemically

transition

temperature

It is found generally

No increase

above the value of 92K has

that metallic behaviour

is accompanied

by superconductivity

suggesting that they are both a consequence

correlated

state in these metallic oxides.

electronic

RBa,C!u,O,

of the same

INTRODUCTION The discovery of ceramic oxide materials temperatures expectation

of over 9OK, in particular that many applications

that superconduct

at

YBa,Cu,O, [1,2,3] has lead to the

will arise from them.

Consequently

large amount of work is being carried out in an attempt to understand mechanisms materials

of superconductivity

further.

superconductivity chemical

and to improve the properties

In particular

higher than N 90K associated with different

and physical treatments

paper we report results of resistivity substituted

ceramics including

of these

there have been a number of reports of

at temperatures

substitutions

a the

of the ceramic.

measurements

In this

on chemically

some of those in which high T, anomalies

been reported by others.

*Also at: Centre for Superconductivity Research, Department University of Maryland, College Park, MD. U.S.A.

of Physics,

have

EXPERIMENTAL The samples were prepared by standard ceramics methods, typically by mixing and grinding firing in air. cold pressing

powders of R,O, (R = rare earth), BaCO, and CuO and

The process was repeated several times and was followed by into pellets of Icm diameter and 1 or 2mm thickness,

at high temperature

and then annealing

of times and temperatures

in oxygen at 4WC

are given with the discussion

The samples were characterized

sintering

- 500°C.

Details

of each material.

by X-ray powder diffraction

and optical

microscopy. The resistivity currents

measurements

of 0.03 to 10 mA.

were made by the four terminal

spring blades, silver paint or both. uniformity

of temperature

method using

Contacts were made to the sample by means of Great care was taken to ensure

during the measurement,

on a copper block (but insulated

the sample being mounted

from it by Mylar tape) and surrounded

by

two copper or brass shields between it and the inside of the nitrogen Two rigs were used for measurement,

cryostat.

a computer controlled

d.c.

system using a silicondiode thennometry[41 and an a.c. systemusing a The gaseous environment of the samples thermocouple.

copper-constantan

during all measurements

was nitrogen

samples were cut into prismatic resistivity

was flO%

form of half-pellets

at a pressure of one atmosphere.

bars in which case the uncertainty

Some

in

due to sample shape, and others were measured in the in which case the uncertainty

was f30%.

RESULTS

In order to verify that the methods used in this work produced samples of quality comparable and HoBa.#u,O, annealing

to those made by other workers samples of

(the oxygen content is nominal,

treatment

would have produced close to seven oxygen atoms per

formula unit) were prepared. at 92SC material

The HoBa&u,O,

pattern

(cobalt radiation)

un~~~nties

of 91.7K.

temperature

The resistivity

(in

(zb 10% due to

falling to Z.OpQm

The width of the transition

of the resistivity

at the transition

is 2K.

The downward

from linearity just above the transition

is due to ephemeral

this effect in HoBa.$u,O,

lattice.

at room temperature

of sample dimension)

temperature

The powder

shown in Fig. l(a) shows that the

is single phase with an orthorhombic

Fig. 2) has a value of 7.0&m

deviation

sample was fired and sintered

then treated at 5OO‘C in flowing oxygen for 48 hours.

diffraction

YBa$u,O,

but in most cases the oxygen

Su~r~nduGting

is comparable

fluctuations.

The size of

to or smaller than that in YBa&u,O,

399

(a) Fig. 1. HoBa&u&.

X-ray powder diffraction

pattern

(cobalt radiation)

The lines are indexed to an orthorhombic

lattice.

(b) X-ray diffraction pattern of Ea.&&o@,, sample E-Can 3% The splitting characteristic of the orthorhombic lattice is missing.

This sample is tetragonal.

of

400 4

.:::::::,::::::::::::::::, . ..J .. ...-f .. ..- _...-*.

3.2 . . . . ..-. .. ... . . cE

._....

..

. . . ...

. . . ...

_..... . . . . . .

:2.4 r-

./

.9

0

.:..:::::t::::::l::::::::* 70 90

110

130

TEMPERRTURE

Fig. 2.

Resistivity

against temperature

suggesting

that the Landau-Ginsberg

containing

compound is comparable

non-magnetic

compound,

critical current

170

of HoBa,Cu,O,.

coherence length in the localized moment or even longer than that in the

a further confirmation

decoupled from the electrons

150

IKI

responsible

that the rare earth site is

for superconductivity.

The

density in this sample was found to be 70Alcmz, limited by

contact heating.

Mixed Rare Earths A sample of composition

RBaaCu307, where R consists of equal molar parts

of Y, Ho, Gd, Eu, Sm was prepared. The room temperature indicating compounds

metallic behaviour containing

The resistivity

p is shown in Fig. 3.

value is 14.4 &%n, and although dpldT is positive the slope is much smaller than those of the

pure Y or Ho.

This may be related to the presence of

Eu in this sample as will be discussed in the next section.

However the

sample shows a broad superconducting transition with zero resistivity attained by 83.5K. This suggests that for practical applications it may be feasible to fabricate

superconducting

oxides from unseparated

rare earths.

401

150

100

Fig. 3.

Resistivity

RBa,Cu&

200 TEMPERATURE (K) against temperature

where R indicates

of a sample of com~sition

mixed rare earths (Y, Ho, Gd, Eu, Sm

in equal molar proportions).

EuBa,CusO, Several samples of composition reproduce

the observation

temperatures reducing

However all the samples prepared,

and oxidizing conditions,

showed semiconducting

For example, the natural

against inverse temperature

in an oxygen atmosphere

were prepared in an attempt to

by Ruang et al. [S] of a resistive

around 25OK.

sign of any superconductivity. resistivity

EuBa&u,O,

of sampIe Fl.B,

is shown in Fig. 3(a).

resistivity

is O.OlS~m and the temperature

negative.

This semi~nducting

transition

behaviour logarithm

with no of the

which was annealed

At room temperature

derivative

at

both in

of the resistivity

the is

behavior is associated with the sample being

tetragonal. The X-ray diffraction pattern of another EuBa&u,O, sample annealed in oxygen is shown in Fig. l(b). The splitting between the (h,k,l) and (k,h,I) lines that is characteristic in HoBa&!u,O,

of the orthorhombic

[Fig. l(a)] is found to be missing.

we have been unable to prepare orthorhombic

distortion

We do not understand

and superconducting

as many others have succeeded in doing so [6].

seen why

EuBa,CusO,

14.5,

:

:

:

:

.

:

:

:

:

:

:

:

:

:

*:..e

13.5.

c

_.-.

E

y-12.

A

z L

c 1z 2 (L 10. 11. ii:::::::::: 9.5 3

5

7

-1

g

13

TEHPERRTIJRE

Fig. 4.

Plot of the natural

temperature

logarithm

of the resistivity

for a sample of composition

line A corresponds

to a semiconducting

EuBa,Cu,O,.

p against inverse The slope of

energy gap of O.l2eV, that

of line B to a gap of 0.033eV.

Assuming

that the semiconducting

behavior is due to the excitation

single species of carrier across an energy gap (an impurity energy or the band gap) the resistivity dependence

of the carrier mobility)

temperature

T as -

exp(EsRkT)

d(lnp)ld(+)

= E,/2k

.

Such a linear relation

p (neglecting

of a

ionization

the temperature

would be expected to depend on and hence:

(1) is not observed over a wide temperature

However the slopes of the lines A and B in Fig. 4 correspond

This suggests that at low temperature

of 0.12eV and 0.033eV respectively. the carriers arise from the ionization but at higher temperature

excitation

range. to energy gaps

of impurities

with low binding energy

across the band gap begins to be

important.

Fluorine and Chlorine Substitution Following reports of high transition fluorinated

compounds

temperatures

[7] several samples of nominal

were prepared by incorporating

observed in some composition

BaF, in the starting materials.

YBa&u,F,O,

All were

403

TEMPERRTURE

Fig. 5. indicated

(a)

X-ray diffraction

by the vertical

(200) diffraction (b) Resistivity

semiconducting diffraction

(Kl

pattern of YBaJ!u3FOr

arrows are identified

sample.

The lines

as the (020) and

lines. of the sample against temperature.

(dp/dT

pattern

< 0) except the sample with x = 1 whose X-ray

and resistivity

are shown in Fig. 5.

prepared by firing and sintering followed by annealing but of a metallic form. of low conductivity

at 61K.

sample is multiphase.

at the relatively

This sample was

low temperature

of 85oOC

in oxygen.

The room temperature resistivity is high This suggests a microstructure consisting of grains

surrounded

broad superconduct~g zero resistivity

200

150

100

tr~sition

by a skin of metallic material.

There is a

centered at 6XK with the sample attaining

The powder diffraction The orthorhombic

pattern

shows that the

phase is present together with

404 35

:

l

t

:

:

;

:

:

:

:

:

:

:

:

:

:

:

:

:

:

:

:

:

*

28 c

E

621

/

z z14 im 2 =?

0

+::::::::::::::::::::::::r 50

100

200

150 TEMPERATURE

Fig. 6.

Resistivity

of sample of nominal

250

300

(Kl

composition

YB~.$u,C!~,~O, plotted

against temperature.

unreacted

BaF,, and other unidentified

splitting of the superconducting HoBa&!u,O,. superconducting

It is therefore transition

phases.

However the orthorhombic

phase is only one third of that of not clear whether the reduction

temperature

is a consequence

fluorine into the lattice of the superconducting

in the

of incorporation

of

phase or merely of the

reduced oxygen content that is indicated by the smaller orthorhombic distortion

(the compounds

Further fluorinated

RBa,Cu306 are tetragonal however retreatment

longer time at high temperature

evaporated

of fluorinated

processing history. A sample of nominal

of these materials

for a

(950°C) resulted in samples of lower and

having a superconducting

suggests that the fluorine the properties

composition

transition

at 90K.

This

leaving behind the YBa.&u,O,

materials

the Y@,, BaCOs and CuO together,

therefore

depend strongly on

YBa&u3(Cl~,0,) grinding

phase;

was made by first firing

and mixing with CuCl then slowly

heating from 400°C to 900% followed by an oxygen anneal at 400°C. sample was multiphase and unidentified

[8]).

samples with x = 0.5 and 1.0 fired and sintered at

9OO’C were semiconducting; metallic resistivity

and semiconducting

phases.

and contained

traces of the orthorhombic

The resistivity

is shown in Fig. 6.

The

phase, BaCuO,

It shows a

semiconducting behaviour with an indication of a partial superconducting transition at 90K. However the resistivity has not decreased to zero by 57K.

405

Compounds

containing

Following temperature

Strontium

the report of [9] apparent in YBaSrCusO,

proved to be multiphase normal metallic Fig. 7.

and had a room temperature

resistivity

The resistivity

superconductivity

and attained

transition

even at 56K.

of 8.0pQm,

A bump is noted around 275K.

cooled to 77K, warmed up to a temperature for several hours. temperature; difference

beginning

at 85K

In the first run the This resulted

In the second run the sample was of 239 f

1K [lo] and held there

It was then cooled to 56K and allowed to warm to room

this run is shown by b.

One possible explanation

between the two runs is that small quantities

phase with a superconducting sample initially

transition

but transformed

between the two runs.

temperature

for the

of a metastable

of 280K existed in the

to a stable non-superconducting

phase

However the result was not reproducible

considered to be unambiguous

evidence for superconductivity

However, we note that indications 240 K in a multiphase

and cannot be

above 90K.

of possible stable superconductivity

sample of nominal

composition

at

Y,Ba,,Sr&u,O,

have been

reported by Bhagat [12].

35

:

:-

28 E

a

.;--+==-

z21

50

100

150 TEHPERRTURE

Fig. 7.

a

is shown in

sample was cooled to 56K and warmed up to room temperature. in curve a.

It

at 75K.

Y,Ba.,Sr.,Cu,O,

There are signs of a superconducting is not attained

was examined.

resistivity

zero resistivity

of a sample of composition

but zero resistance

above room

a sample of this composition

Resistivity

sample of composition the text.

against temperature Y,Ba,,Sr.,Cu,O,.

200

-4 300

250

1Kl

during two warming

runs of a

Details are discussed in

A sample of composition

Y,,Sr,,Ba.#u,O,

was prepared in an attempt to

raise the valence of the copper by substituting yttrium resistivity

site.

divalent

The sample proved to be multiphase

of 11.7pQm

and a superconducting

transition

89.7K, sugesting that little chemical substitution

strontium

onto the

with a room temperature temperature

was achieved.

300 -z ci 200; lF F? 1007) iii

TEMPERATURE 1KI

( b) 100

Fig. 8.

150

Resistivity

pressed at 9W

200 250 TEMPERATURE (K)

of two isostatically

hot pressed samples (a) Hot

and cooled in air (b) Annealed

hours in oxygen after hot pressing.

2

at 500°C for 24

of

407

Hot Pressed

Samples

Two samples of YBa#&O,

were isostatically

hot [ll]

During pressing and subsequent

pressure of 2kbar at 900°C for 2 hours.

cooling they were in contact with the atmosphere. sample cooled in air after hot pressing resistivity

above 77K.

behaviour

In contrast

transition.

higher, the superconducting difficulty

transition

(Fig. 2).

The resistivity

is is

are likely to be due to the

through the dense sample, and

suggests that there may be difficulties properties

and a

were not so good as

is broader and zero resistivity

These deficiencies

that oxygen has of diffusing

with good electrical

there is no

low resistivity

However the properties

those of samples made without hot pressing not attained until 81K.

apparent;

of the

The

a sample annealed in oxygen for 24

hours at 500°C after hot pressing has a relatively superconducting

The resistivity

is shown in Fig. S(a).

is high and semiconductor-like

superconductivity

pressed at a

in forming the dense bulk samples

that will be needed for many applications.

CONCLUSIONS A number of chemical compounds materials

substitutions

(where R = rare earth) and the electrical measured.

Only in one (non-reproducible)

suggestion that the superconducting above the conventional (dpldT

have been made in RBa.$u,O,

92K.

of the

case is there any

temperature

In the samples examined

> 0) is always accompanied

the case as multiphase

transition

resistivities

based

is increased

metallic behaviour

by superconductivity.

The converse is not

samples may exhibit both semiconductivity

and

superconductivity. Superconducting

samples can be made from mixed rare earth oxides

suggesting that it may not be necessary fabricate

to separate the rare earth oxides to

samples for applications.

We have not been able to make electrically

good hot pressed samples

because of the difficulty

of oxygen diffusion

The electrical

properties

of samples that have been fluorinated

incorporation

of BaF, in the starting

processing

through the dense material.

materials

by the

depend strongly on

conditions.

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