Copper-deficiency in Ln2-xCexCuO4 (Ln=Nd, Gd) crystals and oxygen disorder in Gd2CuO4 crystals

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Copper-deficiency in Ln2-xCexCuO4 (Ln=Nd, Gd)

crystals and oxygen disorder in Gd2CuO4 crystals

Ph. Galez, G. Collin

To cite this version:

579

LE

JOURNAL

DE

_PHYSIQUE

Short Communication

Copper-deficiency

in

_Ln2-xCexCuO4

_{(Ln=Nd, Gd) crystals}

and

oxygen

disorder

in

Gd2CuO4

crystals

Ph. Galez and G. Collin

C.P.C.M., DPhG,

C.E.N.

_Sachay,

91191 Gif sur

_Yvette,

France

(Requ

le 24 novembre 1989,

_accepté

sous

_{forme définitive}

le 16

_{janvier 1990)}

Résumé. 2014 La structure de _sept monocristaux de

_{Nd2-xCexCuO4 (x=0 et x=0.18)}

et

_Gd2CuO4

est

déterminée par diffraction des rayons X. Tous les cristaux ont la structure

_tétragonale

T’ du

_Nd2CuO4

(groupe

d’espace

I4/mmm,

n°

_139).

Un défaut de cuivre de l’ordre de 3 à 6% est trouvé dans six cristaux alors _que les deux sites de

_{l’oxygène (dans}

les

_plans

_CuO2

_O(1)

et entre les

_plans

de terre rare

O(2))

sont

_{complètement}

_occupés

dans la limite d’erreur. Les

_ellipsoïdes

de vibration des sites

_Nd,

Cu et

_{O(2) sont}

_plus

_{allongés parallèlement}

à l’axe c dans les cristaux recuit sous

_atmosphère

inerte que dans les cristaux bruts de croissance. L’atome

d’oxygène

des

_plans

_CuO2

_(O(1))

dans

_Gd2CuO4

est trouvé décentré dans la direction de l’axe a de 0.18

₍₂₎

Å.

Abstract. 2014 The structure of _seven

Nd2-xCexCuO4

(x=0 and x=0.18)

and

_{Gd2CuO4 single crystals}

is determined

_by

means of

_X-ray

diffraction. All

_crystals

have the

_{Nd2CuO4 tetragonal}

T’ structure, space group

I4/mmm (139).

The _copper site is found deficient

_{(3 -}

_6%)

in six

_crystals

whereas both

oxygen sites

_(within

the

_CO2

_planes

_O(1)

and between rare earth

_layers

₀₍₂₎₎

remain

_{fully occupied}

within standard deviation. The vibration

_{ellipsoids of Nd,}

Cu and

_O(2)

_appear to be more

_ellongated

in the c direction in Nd-based

_crystals

reannealed in an inert

_atmosphere

than in _as-grown ones. The oxygen of the

CuO2

basal

_plane

₍₀₍₁₎₎

in

_Gd2CuO4

is found off-centered in the a direction

_by

0.18

(2)

Å.

1

_Phys.

France 51

₍₁₉₉₀₎

579-586 ler AVRIL _1990,

Classification

Physics

Abstracts 61.10-61.70B-74.70

1. Introduction.

Superconductivity

was

_recently

discovered in the

_{Ln2_~Ce~.CuO~}

_(Ln

=

_Nd,

_Pr,

_Sm,

Eu)

and

Ln2-x ThxCu04

(Ln

=

Nd,

Pr) copper-oxide

materials with a critical

_temperature

_up to 24 K four x=O. 16

_{[1 - 3].}

These

_compounds

are not

_{spontaneously}

_{superconducting}

but need to be annealed

580

(900 °C)

in an inert

_{atmosphere (N2, Ar)}

to achieve

_{superconductivity. X-ray}

_absorption

near-edge

structure measurements of the Ce

_L3

_edge

_{[4, 5],}

Ce ionic-radius

_analysis

_{[6, 7]}

and Ce

_X-ray

photoemission

measurements

_[8]

indicate that the valence of Ce is

_greater

than + 3. These

com-pounds

can therefore be considered as

_{electron-doped superconductors}

in

_agreement

with Hall

coefhcient measurements

_{[1, 9]}

which show

_that,

in the normal _state, the

_transport

_properties

are

essentially

due to electrons. At least one

_parent

_compound

of these

_materials,

_namely

_Nd2Cu04,

exhibits

_magnetic

order of the Cu moments at 245 K with an

_{antiferromagnetic}

_arrangement

of

spins

in the

_Cu02

_{planes [10, 11].}

At room

_temperature,

all

_{electron-doped superconductors}

have the

_Nd2Cu04

_tetragonal

T’

structure

_(space

_group

_{I4/mmm, Z=2) [12]}

which differs from the related

_high

_temperature

tetra-gonal

T structure of

_{La2Cu04_y}

in that the two

_apical

_oxygen atoms are removed from the

_(0,0,

z)

position

to the

_{(0, 1/2, 1/4)}

_position

_(Fig.

_1).

Hence the coordination numbers of Cu and Ln

(Ln

=

Nd, Pr, Sm,

Eu)

become

respectively

4

(square plane)

and 8 instead of 6

_(octahedral)

and

9.

Fig.

1. -

Body

centered

_tetragonal

unit cells of the

_high

_temperature T-structure of

_La2CU04

_(left)

and the

T’-structure

of

_{Nd2 CU 04}

_{right).

Several neutron and

_{X-ray powder}

diffraction structural studies have been

_performed

on

these

_compounds.

The main results for the

_prototype

material

_{Nd2-~Ce~.Cu04}

can be

summa-rized as follow :

(i)

the c

_parameter

decreases from 12.17

A

to 12.06

A

when the Ce content is increased from

x = 0 tox =

_0.20,

upper limit of the

solubility

range, while the a

_parameter

remains

quasi-constant

at 3.945

A

_{[8, 13],}

(ii) superlattice

reflexions have been

_reported

_by

Izumi et al. in

_agreement

with electron diffraction results

_[14]

while other _groups did not mention such extra

_{peaks [8,12,13,15]}

and

(iii) oxygen-deficiency

on both sites

_{0{1) (within}

the

_Cu02

_planes

_{(0, 1/2, 0))}

and

_{0(2) (o,1/2,}

1/4)

have been encountered

_by

Izumi et al.

_[14]

and

_{’~akayama-Muromashi et}

al.

_[16]

on

supercon-ducting NdissCeo isCuO4

and on Ce free

_sample.

For non-reannealed

_{(non-superconducting)}

Ndl.85CeO.15CU04,

only

the

₀₍₂₎

site was found deficient. In the

_light

of

_{thermogravimetry}

underestimated

and,

eventually,

that _oxygen defects are introduced at least into the

₀₍₁₎

site

_by

reduction.

We have

_performed

a detailed

_{Ndz_x Cex Cu04}

_{single crystal analysis, using}

_X-ray

_diffraction,

in order to obtain valuable structural information and to

_{investigate possible}

structural

_changes

between _as-grown and reannealed

_samples.

One

Gd2Cu04

single crystal

is also included in the

present

study.

2.

_{Experimental.}

Several Nd-based

_crystals

were

_prepared

_using

two different

_methods,

_growth

from a flux

(CuO)

between 1150 ° C and 1000 ° C for the Ce free

_{samples (N1,}

_{N2, N3,}

_N4)

and

_spontaneous

crystallisation

from the

_powders

at 1200 ° C for the

_{Ce-containing}

_{samples (N5, N6).}

N3 was

extracted from the

_shiny

free surface of the

_preparation,

N1 and N4 from the innermost

_layers

of this surface and N2 from the flux at the bottom of the

_preparation.

N4 and N6 were further annealed at 900 ° C under

_{flowing nitrogen}

for 12 hours. The

_Gd2Cu04

_crystal

_(Gd)

was obtained

by

spontaneous

crystallisation

at 1200 ° C.

All the

_crystals

were found

_tetragonal

with

_systematic

extinction conditions

_{corresponding}

to

a

_body

centered unit cell in

agreement

with

_unpublished

_X-ray

_powder

diffraction results. 8 - 2B and scans

_performed

on the

_{(200), (020)}

and

₍₀₀₆₎

reflexions of each

_sample

revealed a

_high

crystalline quality

and no trace of

_twinning.

The Ce content of N5 and N6 was estimated from the

c

_parameter

to lie between 0.17 and 0.19

_{(Thb. I).}

The data collection was

_performed

on a half

sphere

of the

_reciprocal

_space

_up to 65°

_{(Mo Ka).}

After

_absorption

_correction,

the

_independent

reflexions were obtained as an _average of 8

_equivalent

reflexions. The whole set of

_independent

reflexions were

_kept

in the refinement

_including

those with zero

_intensity.

%ble 1. - Lattice

parameters

(A)

and volume

_of

unit cell

_(A3).

The lattice constants are obtained

from a

reflexion

centering

routine

_{(25 reflexions).}

3. Results and discussion.

The first refinement was _{carried out,}

_using

atomic structure

_factors,

with the structural results

of Rietveld

_analysis

as

_starting

_parameters

_[14,16J,

_isotropic

thermal

_parameters

and

_occupations

factors constrained to their ideal values. The

_temperature

factors for all sites were then allowed

to be

_anisotropic

with

_symmetry

constraints

Ull

=

_U22

for

Nd(Ce),

Cu and

0(2)

sites and

_{Uij -}

0

582

3.1 COPPER DEFICIENCY. - For six

crystals (Nl,

N2, N3, N5,

N6 and

_Gd),

a

_significant

copper-deficiency

was found which led to a decrease of the R-factor. In table III are

_reported

the results

of the

_Rl/R2

ratio

_{(respectively}

without and with a variation of the Cu

_{occupancy) compared}

with the minimum value it should exhibit to be

_{statistically significant}

_{1?b,n-m,O.OOS [17].}

In _{all cases,} the

_{Rt/ R2}

ratio is

_higher

than R. Furthermore it is to be noted that :

(i)

the standard deviations on the

_occupation

factors of Cu are at least four times lower than the deviation from the ideal value and

(ii)

the decrease of the R-factor is all the more

_important

than the

_deficiency

is

_significant.

This

_{copper-deficiency}

could result from a

_preparation

_temperature

of

_{crystals higher}

than the maximum

_stability

_temperature

of CuO

_{(~ 1050 ° C).}

Above this

_{temperature CU20}

rather than CuO is the stable form of

_copper

oxide which leads to a

_fugacity

of

_copper.

Indeed a reddish

hue,

caracteristic of

Cu20,

is observed in the flux after

_crystal

_growth.

In addition

_{Nl, N2,}

N3 and N4

_prepared

at lower

_temperature

than

_N5,

N6 and Gd and with CuO in excess exhibit a

smaller

_deficiency.

Such a defect has

_already

been encountered in

_{YBa2Cu30~_y crystals [18]}

but affects

_only

the

_Cu(l)

site

_(chain

_site)

because of lower

_preparation

_temperature.

In the

_present

case,

however,

the

_Cu02

_planes

are concerned and this should affect both

_{superconducting}

and

magnetic properties

of

_Nd2-xCexCu04

_crystals.

In fact Thrascon et al.

_reported

that

_they

could

not obtain zero resistance down to 2.5 K in the basal

_plane

of

_{superconducting}

Nd~_~CexCu04

(x

= 0.14 and x =

0.16)

crystals

while it is

currently

achieved at 20 K in

polycrystalline

ceramics

[19].

However we

_point

out that this

_deficiency

is not

_systematic

since it is not observed in N4. It

seems to be

_closely

related to the

_preparation

method and _{may vary} from

_crystal

to

_crystal

as a

function of local

_{surrounding during}

_growth.

We now come to the

_problem

of _oxygen content.

_Keeping

in mind the

_copper

defect

_reported

above,

it is reasonable to think that it should induce a related _oxygen

_deficiency

at least on the

0(1)

site. However no

_significant

deviation from the ideal value was found

_except

for the

₀₍₁₎

site of N2

_{(Thb. II).}

Our results do not allow to _{draw any definite conclusion.}

’1’able II. - Structural

parameters

for

N2, N5,

N6 and Gd

_(space

_group

_I4~mmm;

Z =

2).

The

_possibility

of interstitial oxygen at the _apex

_position

03

_(0,

0,

z)

was also

_{investigated.}

Indeed some electronic

_density

was found at this site

_resulting

in a

_slight

decrease of the R factor and in

_large

standard deviations over the

_multiplicity

g, the

_{position z}

and the

_temperature

factor B. However the additional _oxygen content

_(about

the same for all

_{crystals :}

_doped

or

_undoped,

reannealed or

_not)

was found to be

_{approximately}

0.25

₍₁₂₎

which is unrealistic. Moreover the refined 03

_position

_(z

_{N 0.2)}

would lead to much too short Nd-03 and 02-03 distances. This

result,

although

highly

unprobable,

may indicate

complex

structural _{features associated with}

oxy-gen disorder which can

_hardly

be solved with

_X-ray

diffraction.

_Single

_crystal

neutron diffraction is needed to answer this

_question.

Thble III. -

Refinement

results and Cu _occupancy.

3.2 EFFECTIVE VALENCE OF CERIUM IN Nd-BASED CRYSTALS.

_{- Knowing}

the _average distances between atoms in one

_compound,

it is

_possible

to

_compute

the ionic radius of each element and therefore its valence state for a certain coordination number. From our structural results we

may evaluate the mean valence state of cerium in

_{Nd2_ x Ce~ CuO4}

_{(x =}

0.18 f

_1).

In table IV is

reported

the _{average distance} between the rare earth ion and its

_eight

_surrounding

_oxygens for each

_crystal.

For Ce-free

_sample

this distance is 2.501

₍₁₎

A

in

_good

_agreement

with the theoretical

value,

2.499

_A,

_computed

from the

_following

ionic radii :

In

_{Ce-containing}

_crystals

_(N5

and

_N6),

we find

_2.4945(10)

to

which leads to a mean ionic

_radius,

1.1045(10)

5i,

and

_eventually

to a mean valence state of

cerium,

_{3.61(7), computed}

from :

The error bar takes into account both the standard deviation on distances and the

_uncertainty

on Ce content

_(x

= 0.18 ~

1).

This result is similar to that found

by

Hor et

al., 3.59,

from the

variation of the volume of the unit cell

_[8]

but much lower than that

_{computed by Huang et aL}

from the decrease of the c

_parameter,

3.84 and would lead to a total extra-electron concentration of

_1.17(13)

x

10+21

CM-3

in the absence of _any other chemical difference between

Nd2Cu04

and

Ndl.82CCO.18CU04-584

Stable IV - Ln-Ox distances

_(A).

3.3 ANISOTROPY. - In all Nd-based

_crystals

the

₀₍₁₎

site exhibit

_{strongly anisotropic}

tempe-rature factors

_(U22

_U11,

_U~3)

due to Cu attraction

_along

the b direction

_{(Thb. V).}

For other

sites,

we must

_distinguish

between _as-grown

_(Nl,

_N2,

N3 and

_N5)

and reannealed

_crystals

_(N4

and

N6).

In the first _{case, the} Cu and

₀₍₂₎

thermal

_parameters

are found

_isotropic

within standard deviation while the Nd site exhibit smaller

_temperature

factor in the c direction. For the latter

crystals

however,

the

Nd,

Cu and

_Q(2)

sites have

_larger

thermal

_parameter

in the c direction than

within the basal

_plane.

Another difference between _as-grown and reannealed

_crystals

is

_readily

seen from table I : N4 and N6 have a lower c

_parameter

than

N1,

N2 or N3 and N5

_{respectively.}

Table

_{V -Anisotropic}

_temperature

_{factors for}

N2,N5

and N6

_(A2

x

104) .

3.4 OFF-CENTERING OF

₀₍₁₎

IN

_{Gd2Cu44. -}

The refinement of Gd was carried out in the same manner as for the other

_crystals.

_Isotropic

thermal

_parameters

for the

_Nd,

Cu and

₀₍₂₎

sites

were found very similar to these of the _as-grown Nd-based

_samples.

In contrast, the

_temperature

factor of

₀₍₁₎

was much

_higher

_{(Beq. =}

_{1.7(4)) indicating}

a

_possible

_{off-centering}

of the atom

10-4

_A2,

U22

=

70(23)

x

10-4A2

and

U33

= 145(40)

x

10-4t~2) .

The x

_parameter

of the atom was

therefore allowed to _vary and reached

_rapidly

its _convergence

_value,

_0.046(5),

that is an absolute

off-centering, 0.18(2)

A.

A

_significant

decrease of the R factor was obtained

_giving

a

_{Ri /R2}

_ratio,

1.093,

higher

than the minimum statistical

value,

1.033. In this

_{configuration,}

the

_temperature

factor of the off-centered atom is

_nearly

divided

_by

two

_(j9eq

=

0.9(2)).

Another feature in

Gd2Cu04

arises from the

_experimental

value of the mean distance

bet-ween the Gd cation and its

_surrounding

_oxygens,

_2.461(1)

_A,

which is

_{significantly}

lower than the theoretical one, 2.443

_A,

_computed

from :

R(Gd3+, VIII)

= 1.053

A

4.

_Summary.

The main feature

_reported

in this work is the occurence of

_copper

defects within the

Cu02

plane resulting

from a

_preparation

_temperature

_higher

than the

_stability

_temperature

of CuO. This result obtained on

_{single crystals}

is in

_agreement

with the

_difficulty

in

_obtaining

zero resistance in the basal

_plane

of

_{Nd2-.pCe.pCu04}

_(x

=

0.14,

0.16) [19].

In _{contrast, no}

significant

_oxygen

deficiency

could be detected

_except

in one

_crystal

extracted from the inner

_part

of the

_preparation.

Further

_{single crystal}

neutron diffraction

_analysis

is

_required

to

_investigate

this

_problem

more

accuratly.

The mean valence state of cerium in

Ndl.82Ceo.18CU04,

evaluated from the mean rare

earth-oxygen

distance,

is found

significantly

higher

than +

3,

3.61(7),

thus

_confirming

the results ob-tained

_by

means of

_X-ray

_absorption

and

_X-ray

_{photoemission}

measurements

_[4,5,8].

Detailed

investigations

of the

_magnetic

and

_transport

_properties

of

_{Nd2_~CexCu04}

are in _progress as well

as

_{single crystal}

neutron diffraction studies.

Eventually

the

₀₍₁₎

site in

_Gd2Cu04

_appeared

to be off-centered in the a direction

_(0.18(2)

5i)

which can

_perhaps

be related to the lack of

_{superconductivity}

in

Gd2_ x Cex Cu04.

Acknowledgements.

The authors thank J. Godard for his

_help

in

_growing

the

_crystals

and for

_making

available a

high

temperature

furnace.

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