Incommensurability and transfer mechanism in bismuth high Tc superconductors

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Incommensurability and transfer mechanism in bismuth high Tc superconductors

G. Pan, S. Megtert, G. Collin

To cite this version:

G. Pan, S. Megtert, G. Collin. Incommensurability and transfer mechanism in bismuth high Tc su- perconductors. Journal de Physique I, EDP Sciences, 1992, 2 (6), pp.887-898. �10.1051/jp1:1992186�.

�jpa-00246609�

Classification

Physics

Abstracts

74.70V 61.60

Incommensurability and transfer mechanism in bismuth high l~ superconductors

G. Pan

('),

S.

Megte~ (')

and G. Collin (~)

(1) Laboratoire de

Physique

des Solides, Bt. 510, Universit6 Paris-Sud, 91405

Orsay

Cedex, France

(2)

Groupe

C-P-C-M-, L-L-B-, CEN

Saclay,

91191Gif _sun Yvette Cedex, France

(Received 2 March J992, revised and

accepted

2

April

J992)

Abstract. The _structure of bismuth

high

T~

superconductors

is

investigated.

A correlation is found between incommensurate modulations and transport

properties

_:

B12Sr2Cu06

is not _a

good

metal because the transfer of electrons from the

Cu-02 layers

into the Bi-O

bi-layers

induces _an incommensurate and monoclinic distortion,

responsible

for the localization of carriers.

Substitution of Sr2+

by

^La3+ reduces the transfer

degree

and the distortion a

superconductive

transition is therefore observed with the

remaining

carriers after _a monoclinic-orthorhombic

phase

transition. Further substitution

tending

to a

complete compensation

of transfer leads to a

compensated

insulator. It is

suggested

that the c* component ^{of the modulation is lbJked} to the lattice distortion and the b* component to the

charge

transfer

degree.

Similar

changes

_are

observed in

Bi~sr~(Caj

_{_~Y~)CU~O~ except} that the monoclinic distortion is _never present.

I. Introduction.

High

critical

temperature superconductors, (H.T.S.C.),

consist

essentially

in copper-oxygen

layers, responsible

for

superconductivity

with

planar

formula

Cu02.

These

layer8

_are

localized in-between M-O

layers (M

=

alkaline-earth _{or rare} earth

elements),

close to

perovskite _type packing,

which

provide

the

stability

of the

crystalline

framework.

Moreover,

in _some materials additional

layers,

such _as

Cu-CUO, Tl-O, Pb-O, Bi-O,

...,

are present. ^The

prototype ^.H.T.S.C.

compounds

_are

La~_~Sr~Cu04,

without additional

layers

and

CUBa2YCU~O~ (YBa~CU~O~)

with _one additional CUD

layer,

_{« copper} chain _», distinct from the _two

CUO~ layers,

_{« copper}

planes

_». In

addition,

_an e8sential characteristic of

(H.T.S.C.)

materials is the presence of _a

charge

transfer.

Indeed,

copper-oxygen

layers

_{are not}

spontaneously

conductor.

Thus, La~Cu04

is _an

anti-ferromagnetic

insulator

and,

to make it

metallic,

it is necessary to «

dope

_» it

according

to the ionic schema _:

La(

+

Cu(

+

O( charge equilibrated

_~

(La( _~Srj

+

) (Cu(

⁺

O(

_{+ holes}

The carriers created

by

the

Sr~

+

-La~

+ substitution _are transferred into the

hybridized

_copper-

oxygen band, thus

leading

to a metal. This mechanism of

transfer by doping

is

quite general

ⁱⁿ

this type ^{of materials.}

However,

there is another kind of H.T.S.C. materials with spontaneous

transfer.

An

example

is

given by

the thallium

compounds,

with formal

equilibrium

of

charges,

but which _are

spontaneously

metallic and _even

superconducting, through

_an electron transfer from copper-oxygen

layers

into additional Tl-O

layers.

The transfer is stabilized

by

_a valence

fluctuation of thallium,

6s°6p°(Tl~

⁺

)-6s~6p°(Tl~

⁺

according

to the schema

Tl(

+

Ba(

~

Cu(

+

O(

~

(Tl( ~Tl(

⁺

)Ba(

+

(Cu(

+

O(

₊ holes

Neve~heless,

it is sometimes _{not so}

simple

_to

point

out the spontaneous ^{transfer mechanism.}

For

example,

in bismuth materials like

Bi(~Srj+Cu(~O(~, Bi(+Srj+Ca(+Cu(+O(~

_et

Bi(+Srj+Ca(+Cu(+O(I

all of them with formal

charge equilibrium,

the bismuth 3+ ion is

already

in _a

6s26p°

configuration, and,

in

principle,

could _{not attract} additional electrons.

However,

these materials are

superconducting

metals

(T~

_m ^{20 K}

(substituted),

90

K,

110 K

respectively).

But

they

are also materials in which strong ^{structural disto~ions} are observed

resulting

in

« incommensurate _»

superstructures.

^If these distortions are linked to the transfer

mechanism, they

could _even constitute _a direct measurement of the

charge

transfer

degree

as we will

suggest,

by examining single crystals

of the

simplest

of these

materials,

the _«

one-layer

_»

compound, Bi~Sr~CUO~.

2. Structure of the _{« one}

layer

_»

compound B12Sr2Cu06.

All bismuth materials _can be described in _a

tetragonal cell, 14/mmm,

Z

=

2, [1, 2],

with the

same lattice constant,

(a

m b

-

3.8

A),

and lattice _{constant c}

depending

_on the number of

layers.

Its value for

B12Sr2CuO~

is

m 24.6

A (Fig. I).

In this sub-structure there _are three types ^{of cationic sites} :

the strontium

site,

nine-fold coordinated

the bismuth

site,

in octahedral coordination

(strongly distorded)

_; the copper

site,

in _a rather

regular

octahedral coordination.

In

fact,

_an o~horhombic

description

is _most

commonly retained,

with a~~_

mb~~_

_m

~~i_

/.

_It

must also be noticed

that,

whereas

cation-oxygen

distances _are consistent with lattice dimensions for strontium and copper, this is _not the _case for the

bismuth-oxigen distance,

which

ideally

should be much shorter.

Thus,

there is _a mismatch for this

pa~icular

distance.

Under these conditions it is _not

surprising

that

X-ray

diffraction pattems ^{reveal the}

occurrence of superstructures as shown in

figure

2. The actual cell is monoclinic

C2,

with _a

volume _ten times

larger

than that of the

tetragonal

sub-structure

[3]

and with _{a two}

component

modulation _wave vector,

along

the o~horhombic b* and c* directions. In

addition,

careful determination _{on a} four-circle diffractometer indicates that this super-

structure is

only approximately

commensurate.

Indeed,

_{as an}

example,

the

strongest

satellite

reflection,

whose ideal indexes in the monoclinic superstructure

(a~~~,

=

2b~n~_

+c~~~,

b~~~

₌ a~~_, c~~~_ = 5

b~~~,)

^{should be}

-14, 0,

it, exhibits _an incommensurate

indexing

such

as

14.03(1), 0, 10.94(1).

The _same result is obtained for other satellite reflections, with

twice the

incommensurability

_wave vector for second order

satellites, Moreover,

this

material exhibits _a

pa~icular

behavior _: it is

metallic,

but with _an increase of

resistivity

at low temperatures,

10-20K, suggesting

_a

charge

localization.

Only

_some of the substituted materials exhibit _a

superconducting

transition between 5 and 25 K

[4-7].

In section I, we have

pointed

out the difficulties encountered for the

interpretation

of

Bi

'

Bi Cu

it

Sr Sr

o

m Bi

. _cu

Fig.

I. Schematic

tetragonal

structure of

B12Sr~CuO~.

transfer in the bismuth materials.

Especially,

there is _no stable bismuth electronic state to trap the transferred electrons.

However,

in this kind of

layer,

it is

possible

to observe _a

6s-6p

^band

coupling

with the lattice

(the

_« lone

pair »), leading

to

polaronic

states and

resulting

in

altematively

Bi-concentrated and deficient bands _as observed

by

H-R-T-E-M-

[8].

The carriers _are thus localized and will behave _as

charged

«ions» and

modify

electrostatic

equilibrium

within the

layers. But, by

that very

fact,

the number of

charges

has

absolutely

_no

reason to be commensurate with the lattice

periodicity.

Its consequence will be the existence

of incommensurate

modulations,

which _are

actually

observed. The

special

character of the

« one

layer

_»

compound

is that the

magnitude

of the disto~ion induced

by charge

^transfer is

large.

In

addition,

_we have _seen that the

super-structure

^{is monoclinic}

(Fig. 2),

with

appreciable

values of the modulation _wave vector components ^{in both}

pseudo-o~horhombic

b * and c ^* directions. We will define

incommensurability

in this _{case as}

(Fig. 3)

the

reciprocal

distance of the

position

of _a first order satellite reflection with

respect

to the nodes of _an average orthorhombic superstructure :

I-e-

along b*,

to the commensurate _wave vector in _an ideal commensurate

superstruc-

ture with

parameter

a X 4 b _{x c as} observed in

insulating

substituted

compounds [9, 10]

I-e-

along c*,

to the closest commensurate

position

with odd index.

In

Bi~Sr~CUO~,

the

incommensurability

components, as

previously defined,

_are

respecti-

vely

0.24 and 0.66

along

^{b* and c* direction.}

Fig.

2. -Diffraction pattem

(MOK~)

of the 0 kf

layer

of pure

B12Sr2CuO~.

The modulation _wave

vector direction is indicated

by

the Q arrow.

superstructure

^satellites b ^* ~

@ @

~

2(n+I)

@ @

°

@ @~ _@

~~

o @

, ,

@

~

411 average

4(n+I)

lattice

Fig.

3. Definition of the b* and c* _wave vector components.

In _our

hypothesis

of _an incommensurate

super-structure

^linked to

charge transfer,

it is clear that any modification of the transfer

degree

will lead _to structural

modifications,

which will

manifest

by

_a

change

^of the incommensurate modulation. This is

why

_we will examine

substituted materials in the next section.

3. Substitution.

Several kinds of substitution _are

possible

in this

compound.

We will

only

examine here substitution _on the strontium

site,

the

only

cationic site

exhibiting complete charge

transfer.

Two

types

^of

simple

substitution _can be

performed.

3.I «Iso-ELECTRONIC» suBsTiTuTioN.

Bi~(Sr~_~M~)CUO~,

in which M

=Ca~+

or

Ba~+

_is substituted _to

Sr~+,

then without modification of

charge equilibrium.

In this _case,

only slight

structural modifications _are

observed,

_as indicated in the first two columns of table1.

Table I. Lattice constants and

shifts

with respect to

commensurability ofsatellite reflection indexes,

in _a

(a

_X 4 b _X

c) (Pseudo-)

orthorhombic cell and in _a

Bi~Sr~Cu06

monoclinic cell.

~i12~~2~U°6 ~i12(~~l.4~~0.6)~U°6

~i12(Srl.41~~0.6)CU°~~6>,

a

(A) 5.378(1) 5.372(1) 5.419(1)

b

(A) 5.380(1) 5.368(1) 5.449(1)

c

(I) _{24.621(4) 24.537(3) 24.166(5)}

a

(°)

90,ll

(2)

90.15

(1)

90.00

(Ii

fl (°)

90.00

(1)

90.00

(1)

90.00

(2)

y

(°)

90.00 90.00 90.00

q("~'

_0.00

₍₁₎

_0.00

₍₁₎

_0.00

₍₁₎

q("~'

0.24

(1)

0.23

(1)

0.04

(1)

qf~.

_0.66

₍₁₎

_0.58

₍₁₎

_0.00

₍₁₎

q?°".

₊ 0.03

(1)

0.03

(1)

0.47

(1)

qi°"'

0,00

(1)

0.00

(1)

0.00

(1)

qf°".

0.06

(1)

0.03

(1)

₊ 0,18

(1)

This result is in agreement ^with our

previous interpretation;

_{as an} «iso-electronic»

substitution does not

change

the

charge equilibrium,

transfer is

only

modified at a second

order, essentially by

the

geometrical change

in distances due to the difference in ionic radius.

3.2 _« NON-iso-ELECTRONIC _» suBsTiTuTIoN.

Bi~(Sr~_~M~)CUO~,

^{in which M} =

La~+

or

Bi~

+ is substituted to

Sr~+,

_{then with}

a modification of

charge equilibrium.

The substituted 3+ ions will counterbalance the

transfer, playing

^{the role} of _«

anti-dopant

». Under these

conditions, appreciable

structural modifications are observed

(column 3,

Tab.

I), with,

for

lanthanum,

for

example (Fig. 4)

_:

a first

region,

for 0.00

< y~~ <

0.07,

where the lattice remains monoclinic with two

components

^{for the modulation} wave vector.

However,

_a

rapid

^{decrease of the} incommen-

surability along

c* is observed _;

a second

region,

for 0.07

< y~~ <

0.20,

characterized

by

_an orthorhombic

lattice,

due to a zero component

along c*,

where the

incommensurability along

^b*

keeps

a finite value,

which decreases with

increasing

_{y~~ ;}

0.7

0.6

~ O-S

0.4 qJ)

0.3 ~"*~~~l~l

0.2 o-i o

-o.1

0 O-1 0.2 0.3 0.4 O-S

Fig.

4.

Change

of b* and c* _wave vector components ⁱⁿ

Bi~(Sr~_~La~)CUO~

versus y~.

a last

region,

_{for y~~ ~}

0.20,

in which the b*

incommensurability

is around 0.03-0. 04 and does not

change

further.

Moreover, and for any substitution

Sr~+-M~+ _(M

=

La, Pr, Bi, ),

the _same type ^of

change

in lattice _{constants, as}

reported

in

figure

5 for the La _case, is

systematically

observed

at the monoclinic-orthorhombic

transition,

with _a non-linear behavior for the b and

c =

f~y~)

_curves, the two axes which define the modulation

plane.

Thus,

and contrary to the _« iso-electronic _»

substitution,

_a modification of

charge

transfer

by

the introduction of _excess electrons from the La3+ substitution induces strong ^structural

changes

_: monoclinic-orthorhombic

transition, large change

in lattice constants,

increasing

for

a and b _{on one}

hand,

and

decreasing

for _{c on} the other

hand, incommensurability

lowered

along

b* and cancelled

along

c*

5.45 24.65

5.44 , +

~~

24.60

5.43

/~'

24.55

I

~ 4~

~

- KA)

l + «Al

5.41

)

~

24.45

',

5.40

/

24.40

I

5.39 24.35

',

'~

5.38 24.30 , _-,,,

5.37 24.25

0 O-1 0.2 0.3 0.4 0.5 0 O-1 0.2 0.3 0.4 0.5

a) b)

Fig.

5.

-Change

of lattice constants versus

(La3+)

substitution level, I) _a and b, 2) _c.

3.3 THE SUPERCONDUCTING TRANSITION. Results

conceming

these substituted

materials,

and the reference

Bi~Sr~CUO~ compound itself,

_are rather controversial. Different authors

[4-7, 11, 12],

have

published contradictory results,

most often obtained with ceramics _;

they

are described sometimes _as

superconductors

and sometimes without any

superconducting

transition,

for the _same materials with the _same nominal

composition.

We will _come back later _{on an} additional _source of

problem

due to the

preparation

conditions and

specially

those

conceming

the role of _oxygen

present during

the

synthesis. However,

with _our

results,

obtained

exclusively

_on

single crystals,

it is

possible

to propose ^the

following interpretation

_:

a

comparison

between _«

one-layer

» materials _on the one hand and _« two- and three-

layer

_» materials _on the other hand is rather

lightening. Indeed,

the

« two- and

three-layers

_»

compounds

_are

spontaneously superconductors,

which suggests ^that a spontaneous

charge

transfer between copper-oxygen and

bismuth-oxygen layers

_occurs. There is

absolutely

no

reason that such _a transfer does not exist in the _«

one-layer

_»

material, only

the transfer

degree

could

change

from _one material to the other _;

however,

Bi~Sr~CUO~,

is _not

superconducting (we

have verified this

point

on

single crystals),

but exhibits _a

quite specific

structural distortion.

Indeed, only

this

particular

material exhibits _a monoclinic

disto~ion,

whereas _{« two-} and

three-layer

_»

compounds

remain o~horhombic _;

only

_«

one-layer

_» materials substituted

by

3+

ions,

exhibit _a

superconducting

transition

[4, 5],

but for _a substitution level

larger

than y~ m

0,10,

that is to say with the o~horhombic

structure, identical for the _«

one-layer

_»

compound

to that

spontaneously

formed in the other members of the

family.

It is therefore

possible

to

assign

the

following specific

kinds of behavior _to each _one of the

homogeneity

_range

regions

_:

Region I)

the transfer

degree

is maximum for the reference

material,

but the

charges trapped

in the Bi-O

bi-layers

induce _a

strong disto~ion,

which is

coupled

from Bi-O

bi-layer

to

bi-layer, leading

to the monoclinic structure. This

disto~ion,

because of the c* component of the modulation _wave vector, is not limited _to the

bismuth-oxygen layers

but also _concems the whole structure, and

specially

the copper-oxygen

layers [13, 15].

The carders

(holes)

created within this

plane

_are thus

trapped by

localization due _to the incommensurate modulation.

Therefore,

the material is _{an «} insulator

by

distortion _or

by

localization

». In this

first

region

_{we are}

dealing

with _a

two-component (q~, q~)

^modulation wave vector. The

incommensurability along

b* could be associated with the transfer

degree, producing

_an in-

plane

disto~ion due _to the _excess

charges,

and the

incommensurability along

c* could reflect the

degree

of

interplanar coupling

of the lattice distortions. A substitution

by

^M3+ ions counter-balances the

spontaneous charge

transfer

leading

to a decrease of

incommensurability along b*,

and _a reduction of the lattice disto~ion with _a

rapid

decrease of c* component ^of

incommensurability (Fig. 4).

Region 2)

for

larger

^M3+

concentration,

_{we are now} within the

« metallic

region

_», the transfer and

consequently

^the

disto~ion, being reduced,

the monoclinic

coupling

^between

successive Bi-O

bi-layers vanishes,

and the o~horhombic structure appears, with

only

_a strong

in-place incommensurability,

but without

interplanar component

as in the 2-2-1-2

compound

with the _same o~horhombic structure, ^{thus with} a reduced disto~ion of the

Cu-O~

planes [15].

As

charge

transfer is _not

completely compensated by M3+

counter-transfer

ions, superconductivity

_{can occur.}

However,

the increase of the substitution _amount

progressively

reduces this

charge

transfer and results in _a decrease of

T~,

up to the critical concentration y~ m 0.3.

Region 3)

for

higher concentrations,

the electrons

injected by

the substituant

compensate

the

charge transfer,

and the

incommensurability along

b* is stabilized _{at a} low value and does

not

change

fu~her. We _are in the presence of _{a «}

compensated

insulator

». In

fact,

in this context, a residual transfer remains

(non-zero incommensurability along b*),

but it is not

large enough

_to

produce

a metallic behavior. Zero transfer leads to a

perfect

commensurabi-

lity

_as observed in the _case of substitution _on the copper site

[10].

4. The _«

two-layers

» material.

The _«

two-layer

_»

material, B12Sr2CaCu~Os,

exhibits _{a very} similar behavior

[14, 15], except

that the monoclinic disto~ion is _never observed. The transfer

degree

is very

high

with copper-

oxygen

coupled bi-layers

instead of

single layers,

but the Bi-O

bi-layers

are much _more shifted

(and screened)

from _one another and the «transverse»

coupling, responsible

for the

monoclinic disto~ion observed in the _«

one-layer

_»

compound,

does not occur. The material

adopts

an o~horhombic structure, with

only

_{one component} of the modulation _wave vector,

along

b

*,

similar _to that observed for the _«

one-layer

_» material within the so-called metallic

region.

A

completely parallel study

_can be carried out

by performing

an

yttrium

3+ substitution _on the calcium 2+ site

(lanthanum

substitutes _on both strontium and calcium sites and leads to

similar results but _more difficult to

interpretate).

This

corresponds

to the solid solution

J~13⁺~~2 +

(~~2

+

y3

+

)~~2

+

~2

2 2 _{z z} 2 8

between metallic

(Bi(

^~ ₊

electrons)Sr

⁺

Cal

+

(Cu(

+

O(

₊ holes and

insulating

Bi(

+

Sr~

⁺

Y(

⁺

Cu(

+ O Its

phase diagram

coincides with that of the

previously

mentioned

2)

and

3) regions

of the _{« one}

layer» compound

_: the

regions

with _a

strictly

o~horhombic

structure and with

only

_one incommensurate

component, along

b*

Region 2')

for 0 _{< z <}

0.4,

the _« metallic

region

_», the b ^*

incommensurability

decreases between 0.16 and

0.10,

which

corresponds

ⁱⁿ our

interpretation

to a decrease of the transfer

degree

which _can be associated with _a

lowering

of the

superconducting

transition temperature.

Region 3')

for

z~0.4,

the material is _a

«compensated

insulator» and the residual b*

incommensurability

does not

change

further.

5. Structural distortion and

charge

transfer.

From these results _{we can} extract _some

trends, associating

structural

prope~ies

and

superconductivity,

as

schematically presented

in

figure

6.

Superconductivity

is linked _to orthorhombic _structure in which copper-oxygen

plane(s)

are

only slightly

disto~ed. In this

framework,

the pure «

one-layer

_» material is _not

superconducting

because these

planes

_are

strongly

distorted

[15].

In this _case, the incommen-

surability along

c* and the monoclinic disto~ion result from _a strong

coupling

between

substantially charged

Bi-O

bi-layers.

As _a consequence the in-between Cu-O

layers

_are

sizeably

disto~ed. When the substitution level is

increased,

this disto~ion cancels and the material becomes _a

superconductor. However,

this cancellation of distortion is caused

by

_a reduction of the

charge

transfer

and, therefore,

the critical

temperature

^{observed is rather low}

(m

20

K). Figure

6a

suggests that,

if pure

B12Sr~CuO~,

with _a

high

transfer

degree,

could be obtained without _a monoclinic

disto~ion,

its critical

temperature

^{could be}

compared

to that of the thallium

equivalent (undistorted), Tl~Ba2Cu06

» 90 K. On the

contrary,

for the _« two-

layer

_»

compound,

and in

spite

of _a

high

transfer

degree,

^the structure does not exhibit any monoclinic

distortion,

and

consequently

_a

high

_T~ is obtained.

the

incommensurability along

b* _measures the

in-plane

distortion in the Bi-O

planes

and is linked _to the transfer

degree, responsible

for the

charge injection coming

from the Cu-

q

~

Temperature ^transfer

~l b

^~~~~~

m-knk ^'*+*

wh«Mmw

?

~~ _distcdbn

cc~tcr

~~ ~sulatcrby_

~

ccmpcnsuion

c

~c

^25K

~

~ ~

0

~~~~~~

o y@I3+)

o +---.onelayer ---,I ^Y@'~~l

0 w--- one1ayer ,1

o

w----two-layers

_.---, ^I

o w---- two"layers _----~ l

a)

b)

Fig.

6.

Superconducting

_versus structural

properties

in bismuth

compounds.

O~ planes producing

this distortion. A decrease of the

charge

transfer results in _a

lowering

of this

pa~icular

incommensurate

component.

^{This is} illustrated in

figure 6b,

which

points

out the strict

parallel

between _«

one-layer

_» and _«

two-layer

_» materials.

Moreover, _some

generalization

could be

attempted (Fig. 7).

Indeed,

when,

in

(Bi~_~Pb~)Sr~CUO~,

bismuth

(3+)

is

replaced by

lead

(2+),

the

charge

transfer should be enhanced

by

this substitution. This

experimentally

results in the fact

that,

in the concentration range

0<xp~< 0.07,

the incommensurate distortion is also

increased,

in both b* and c* components ^{of the} modulation _{wave vector,} in agreement ^with our

interpretation.

However,

this

regime rapidly

becomes unstable

and,

for xp~ ~ 0.07 the modulation _wave

vector

splits

into two distinct vectors, as shown _on the diffraction pattem ^of

figure 8,

which

stresses the fact the

composition

of pure

Bi~Sr~CUO~

is not a

particular (locking) point

in the

phase diagram.

[Bi(~~Pb~~]Sr(~CufO~ Bi(~[Srj$jL~]Cu(~O~

-x(Pb2+) y(La3+)

_-

PUre

4-

tr~er

+~ distortion

'

~ ~~~~~~~ Ivector lvector

o 2

components

I

component

Fig.

7.

Proposed generalized phase diagrarr

for substitution in

B12Sr2Cu06.

.~ ~.

~i~ ~

Fig.

8.- Diffraction pattem

(MOK~)

of the 0ki

layer

of

(Bij~-Pbo_i)Sr~CUO~.

The _two different satellite types ^{associated with each} wave vector are indicated

by

_arrows.

6. The role of oxygen.

All the

previous

results _are obtained when the

crystal growth

process is carried _out in

nitrogen (10-3 02).

When

crystals

_are

prepared

in air _or in pure oxygen, another

phase diagram

is

obtained characterized

by

_a much _more dramatic

change

in lattice constants and incommen-

surability. Everything

_{occurs as} if additional oxygen ^atoms could be

trapped

within the structure and if

they played

the role of very efficient counter-transfer elements. Even if the exact nature of this

phenomenon

is not

completely solved,

_we

suggest

that the final formulas of substituted materials _are not

equivalent.

In _case

I) below,

substituted materials

retaining

_a

formula close to

O~

_are out of

equilibrium

from the

point

of view of formal

charges.

On the contrary, ⁱⁿ case

it) below,

if additional oxygen atoms,

corresponding

to the Sr2+-La3+

substitution,

_are absorbed

by

the lattice _as

suggested by Zandbergen

et al.

[16],

_a spontaneous

equilibrium

_can be reached _:

I)

ⁱⁿ

nitrogen,

out of

equilibrium

Bi(~ [54 ~Y~~~~~~~~~ ~~~~

it)

in oxygen,

equilibrated

Bi(

~

iS4 _~Y~~~

~

~~~~

^~

~~

~ ~~

This is manifest in _case

it) by

an increased range of

homogeneity

for La

substitution,

at least

up ^to y~~ = I, and strong

changes

in lattice constants and

incommensurability.

In

particular,

the orthorhombic modification is _never stable and the lattice remains monoclinic _as shown

by

the

comparison

of the lattice

parameters

for

single crystals

of pure

Bi~Sr~CUO~

and

Bi~ [SrLa ]CUO

~_~ ~

a

(h)

_b

(h)

c

(h)

a qb qc

Bi~Sr2CuO~

5.38 5.38 24.62 90,10° 0.24 0.65

Bi2iSrLa]Cu06

+

5.45 5.51 23.82 90.78° 0,14 0.36

7. Conclusion.

In this paper we propose ^a correlation between

physical

^{and structural}

properties

of

high

T~

superconductors.

Bismuth materials appear to be

especially interesting

because

they

exhibit _a

charge

transfer process different from that of

La~ _~Sr~Cu04,

^{where the} transferred electrons _are

trapped

in _a chemical

bond,

_on the

Sr(2+) ions,

and also different from that of

T12Ba~CuO~,

where _«

nearly free

electrons _»,

weakly coupled

to the

lattice,

_are created in the empty ^{Tl 6s band. In} our

interpretation,

bismuth

compounds

^could represent ^the case of

charge

stabilized

by

_a

coupling

to the

lattice,

which makes them very sensitive to any

change

introduced which modifies the

charge equilibrium,

such _as substitution

by

trivalent ions.

A

general

and

complete analysis

^of

displacive

mode symmetry ^{associated with} the various observed

phases

is under progress ^and will be _soon

published

elsewhere.

Acknowledgments.

We _are

especially grateful

to P. Severin and Y. Dumont for

sample preparation

and also _to J.- P.

Pouget

for

helpful

discussions.

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