Thermodynamic desorption of oxygen in high Tc superconducting oxides

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Thermodynamic desorption of oxygen in high Tc

superconducting oxides

J.P. Burger, L. Lesueur, M. Nicolas, J.N. Daou, L. Dumoulin, P. Vajda

To cite this version:

Thermodynamic

desorption

of oxygen in

high

Tc

superconducting

oxides

J.P.

_Burger,

L.

_Lesueur+,

M.

_Nicolas,

J.N.

_Daou,

L.

Dumoulin+

and P.

_Vajda

Laboratoire

_Hydrogène

et Défauts dans les

_Métaux,

Bât.

350,

91405

_Orsay,

France

+C.S.N.S.M.,

Bât.

_108,

91406

_Orsay,

France

(Reçu

Ie 7

_{juiIIet 1987, accepté}

Ie 9

_juillet

₁₉₈₇₎

Résumé.- Nous mesurons les

_{pressions d’équilibre}

de l’oxygène desorbé par des

oxydes

supracon-ducteurs de formule MBa2 Cu3 O6+x. Nous en désuisons

l’enthalphie

de formation de

l’oxygène x

en excès

_{(-0,7eV/atome)}

ainsi que l’énergie d’interaction

répulsive

entre deux

oxygènes

(0,11eV).

Nous observons _également que l’oxygène désorbé est réabsorbé au-dessus de

_600°C,

avec formation

d’oxydes plus

stables.

Abstract.- We measure as a function of _temperature and

composition

the equilibrium pressure

of desorbed _oxygen from

_{superconducting}

oxides _{MBa2 Cu3 O6+x}

_{(M=Eu, Y).}

_Using

the Lacher model we determine the

enthalphy

of formation of the excess oxygen x

_{(-0.7 eV/atom)}

as well as the

repulsive O-O interaction energy

(0.11eV).

We also observe that there is a _spontaneous

_reabsorption

of the oxygen above about 600° C with formation of more stable oxides.

Classification

Physics

Abstracts

74.30E - 64.70K

1. Introduction.

In this

_work,

we

_present

an

_experimental

inves-tigation

of the

thermodynamic desorption

of oxygen

in Y

Ba2 Cu3

and Eu

Ba2 Cu3

06+x.

After the

dis-covery of the new

family

of

superconductors

[1]

it be-came

immediately

evident that air

annealing

at

_high

temperatures

was an

important

feature for the

for-mation of these materials and that vacuum

anneal-ing destroyed

the

_{superconducting}

state

_[2].

This

clearly

shows the

_importance

of the excess _oxygen

content x involved in these

_absorption

and

_desorption

processes. For this reason it is

_important

to mea-sure

_{quantitatively}

the

_equilibrium

_{oxygen pressure}

p(T, 2:)

as a function of

_temperature

and

composition

which _may

_give

information on the

stability

of this

oxygen and on the

sign

and nature of the 0-0

in-teractions. For this we start with saturated

_samples

(z = 1)

and enclose them at room

_temperature

under

vacuum in a finite volume V :

subsequent heating

permits

us to follow the

_desorption

of oxygen and

to measure the

equilibrium

_{pressure p} as well as the

change

ð.x in

_composition.

We

prefer

in fact to mea-sure the

_desorption

_process rather than the

absorp-tion one because the latter

_phenomenon

_may be

com-plicated

by

the _necessary dissociation of the oxygen

molecule at the surface.

1.1 MODEL OF LACHER

_[3].-

The

_stability

of the cop-per oxides is about the same as that of numerous

hydrides,

like the rare earth

_hybrides

_REHX

_(the

cor-responding enthalpy

of formation is for both

_systems

in the 50-100

_kcal/mole

₀₂

_range).

For this reason

one

_expects

similar

equilibrium

pressures for

02

and

for

H2

and one can then

_apply

to

₀₂

the model

de-veloped by

Lacher

_[3]

for the calculation of the

equi-librium pressure in the

hybrides.

In this model one admits that the _energy of the

oxygen takes the form :

1420

(N

is the. number of oxygen

atoms,

No

is the

num-ber of

_empty

sites available to the _oxygen, Eo is the

binding

energy of an isolated _oxygen

_atom,

ei is the interaction energy between two oxygen atoms and z

is the oxygen coordination

number).

The

_{p(T, x)}

_equilibrium

pressure is then obtained

by equalizing

the chemical

_potential

of the oxygen

inside the material with that of oxygen in the

OZ

gas.

In this way one obtains

(zc

is the maximum

_possible

_oxygen

_{concentration,}

AH =

Eb-EO-X(ZEi)

where Eb is the

binding

energy of oxygen in the

02

molecule,

po is a standard

pressure).

This formula is

_only

valid for a

_repulsive

interaction

enrgy E;

(i.e.

when Ei

0).

For attractive interactions and for T

_{Tc =}

_E;, one

_expects

a

_segregation

of

the _oxygen into two

_phases

_(a

low

_{concentration xl}

phase

and a

_high

concentration _xl,

phase).

Formula

(1)

remains valid for T >

_Tc

but for T

_Tc

and x; xxh, one

_expects

a constant

_plateau

_pressure,

independent

of x and

_{given by :}

Detailed

_{investigation along}

this line on several

metal-hydrogen

system

are

given

in references

[4].

2.

_{Experimental.}

The

_samples

were

prepared

from a mixture of

pure

Y2

03,

Eu2

03

and Ba

C03

(Johnson

Matthey,

Chemicals

_{"Specpure" )}

in atomic ratios

_{of Y(or Eu)/}

Ba/Cu== 1/2/3.

The mixed

_powders

were

pressed

into

pellets

and sintered at 950° C in air

_during

24 h. The

resulting

black

_compound

was then

ground, pressed

and heated at 950°C in

_flowing

oxygen

(1 atm.)

for 20 hours. The oven was allowed to cool down

_slowly

to

_{room-temperature.}

After a new

_grinding,

a

sec-ond

_annealing

was carried out at 950° C

_during

12

hours

_always

in

_flowing

oxygen

(1

atm.).

The

room-temperature

was reached _very

_slowly

from 950° C.

The

_{X-ray powder}

_patterns

show that both

ma-terials have at room

_temperature

the sole

orthorhom-bic structure with

parameters

in

_good

_agreement

with the

_already

_published

results

_[5].

The

_{superconducting}

transition

temperature

ob-tained

_by

resistivity

measurements is

Tc =

93.5 K for the

_{yttrium compound}

_(with

a 10-90

%

width of

0.7

_K).

Similar results are obtained for the Eu

com-pounds.

The _pressure

_{p(z, T)}

is measured for

_samples

with a

weight

between 30 and 600 mg enclosed in a

volume of about 300

cm3.

The _pressure is recorded

by

a _gauge

(baratron) ;

from the increase of _pressure we deduce the _oxygen loss Az

_through

the relation

pV

= nRT where n is the number of desorbed

02

moles. We start the measurements at room

temper-ature with an initial vacuum _presure of

10-7 torr ;

the increase of

_temperature

is done at a rate of 1

de-gree/minute

but the

points

in

_figure

1 are obtained

after

_stabilizing

the

_temperature

_during

a

_period

of

about one hour.

Fig.l.- Variation of the desorbed of oxygen with temperature.

Note the _reabsorption of the _oxygen above 550°C. In the insert

we _plot

Inp

as a function of

_{(1000/ T)}

in the range 570 K T

820 K : the

_slope

is related to the

_enthalpy

of formation OH

(lnp

is expres in units of 10-3

torr).

3. Results and discusion.

’

’

3.1.- INCREASE OF PRESSURE AS A FUNCTION OF TEMPERATURE.- There is no

_{appreciable desorption}

(p

10-3

_torr)

_up to 200°C

_(Fig.l).

The _pressure

in-creases

quickly

between 300° C and 520° C and it is in

this

_temperature

_range that we will

apply

the Lacher

model ;

in fact above about

_550°C,

we observe a

dra-matic reduction of the

₀₂

_pressure

_indicating

that the

_sample

reabsorbs its _oxygen

_completely.

An

X-ray

investigation

of the

samples

heated above 600°C

shows that other

_phases

have

_formed,

_among which

there is the _green

_phase

_M2

Ba Cu

₀₅

_{[4] ;}

this

_clearly

indicates that the initial oxide has a low

_stability

and transfoms into more stable

_phases

for which there is

is nevertheless to be noted that the

_reabsorption

did not occur in all

investigated samples :

if one chooses a small

sample

mass and a

_{large volume,}

then the

equilibrium

pressure does not reach values as

high

as in

figure

1 : the

desorption

continues then up tr 950° C with a marked increase of the

_slope

(dp/dT)

near

820° C ;

at this

_temperature,

there occurs

proba-bly

the

_{orthorhombic-tetragonal}

transformation and

the

_change

of

_slope

indicates that the oxygen is then

less stable in the

_tetragonal

_phase.

The insert in

_figure

1 shows the variation of

_log

p with

1 j T

(in

the range T 820

K)

from which one can deduce the

_enthalpy

of formation AH. The

_slope

is -0.55eV _{per 0} atom if one does not take into

ac-count the

_log

_{(xlxo - x)}

term which is related to

the

_{configurational}

_entropy

of the _oxygen. In the

tetragonal phase

one has _{Xc = 2} and the

correc-tion of the

_entropy

term to OH is then small and

nearly compensated by

the variation of

_{po(T) ;}

but in the orthorhombic

_phase

there are now two

non-equivalent

sites

_(each

with Xc

==l)t

the oxygen

occu-pying

for energy reasons

mainly

one of these

_{sites ;}

with zc =

_1,

_we obtain a

_larger

value for

JAHI ;

a

reasonable

_compromise

between these two extremes

leads to OH =-0.70:f:0.15 eV. Such a

binding

_energy

is about two times smaller than the

_{corresponding}

value in Cu oxides and it may

explain qualitatively

the trend for the formation of more stable oxides. It

is also to be remarked that OH can be modified

_by

the presence of a

desorption

_energy barrier : the real

JAHI

is then somewhat smaller than the measured

one.

3.2.- OXYGEN LOSS 4lz AT CONSTANT TEMPERATU-RE.- In order to follow an isotherm we start at a

_given

equilibrium

pressure and then we _pump out

_quickly

the

_{corresponding}

_oxygen

_(this

takes

_only

a few

sec-onds) :

after this we isolate

_again

the

_sample

in the

closed volume and we wait for a new

_equilibrium.

Fig-ure 2 shows

clearly

that the new

equilibrium

_pressure

which is reached after about one hour is smaller than

the initial one ; further

pumping always

leads to a new

decrease of the

_equilibrium

_pressure. This means that

there is no

_plateau

_pressure or that the 0-0

interac-tions are either

_negligible

or

repulsive.

The insert

of

figure

2 shows the evolution of x after successive

pumpings separated by

equilibrium

times of 75

min-utes. It _appears that the measured

_slope

_(dlnp/dx)

= 24:f:5 _cannot be

explained solely by

the

configura-tional

_entropy

but is also involves the term

_(dAH/dx)

resulting

form 0-0 interactions. A detailed

analy-sis shows that the interaction energy is

negative

(i.e.

repulsive)

with a value zei = - 0.45 :f: 0.15eV

(or

ei

-O.lleV)

in

rough

agreement

with the

previous

data of Monod et al.

_{[21 ;}

the

_{large uncertainty}

Fig.2.- Decrease of the _{equilibrium pressure}

_(at

constant

tem-perature)

with the variation of

_composition

Ax obtained

_by

two successive

_{quick pumpings}

of the _oxygen.

_Equilibrium

is

reached after about one hour. The _slope of the insert is related

to the _repulsive 0-0 interactions

_(Inp

is _expressed in units of 10-3

_torr).

is

_again

related to

_possible

variations of the param-eter

_{x,. ’Lt}

is also to be noted that the absence of a

plateau

pressure is restricted to the domain T600°C

and 6.5x7.

At this level it is

_interesting

also to discuss the

possible origin

of these

_{repulsive interactions ;}

it _may for instance

_correspond

to true interactions related

to

_charge

transfer

_(ionic

_{interactions) ;}

one can then

wonder if the

_{tetragonal-orthorhombic}

transition and the associated order-disorder transition and the

asso-ciated order-disorder transition in the oxygen lattice

are not related to these interactions. But it is also

possible

that the

_quadratic

term in formula

₍₁₎

re-flects

_mainly

non-linearities in the

_binding

energy, for

instance if Eo is a function

of x ;

the

orthorhombic-tetragonal

transformation must then be related to a

different

_{phenomenon :}

an electronic transition

(like

a Jahn-Teller

transition)

_{triggered by}

a lattice

distor-tion due to different

_occupations

of the two available

oxygen sites can be a

good candidate ;

it is clear that

some-1422

how this

_{tetragonal-orthorhombic}

transition in order to be

complete.

, Our main conclusion is that the

energy of the

0 atoms in excess of the formula M

Ba3

Cu3

Oe

is

governed by

two terms : a

relatively

small

binding

energy and an effective

repulsive

0-0

interaction ;

but there remains to connect this

_{phenomenological}

model with the electronic

_properties

of the

_system

i.e. with the metal-insulator transition and the associated lattice distortion and order-disorder transition of the

oxygen atoms. It is

interesting

to note that all these

problems

also occur in the

REH2+..,

_systems

which

are metallic for x=0 and insulator for x=1.

We would like to thank Ph. Monod and A. Rev-colevschi for fruitful discussions.

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