Structural effect of heavy ion irradiation on GdBaCuO ceramics

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Structural effect of heavy ion irradiation on GdBaCuO

ceramics

M.-O. Ruault, H. Bernas, M. Gasgnier, J.-C. Levet, H. Noel, P. Gougeon, M.

Potel

To cite this version:

Structural effect of

_heavy

ion irradiation

on

GdBaCuO ceramics

M.-O. Ruault

_(1),

H. Bernas

_(1),

M.

_Gasgnier

_(2),

J.-C. Levet

_(3),

H. Noel

_(3),

P.

_Gougeon

₍₃₎

and M. Potel

₍₃₎

(1)

Centre de

_{Spectrométrie}

Nucléaire et

_{Spectrométrie}

de Masse,

IN2P3,

91405

_{Campus Orsay,}

France

(2)

UPR210 CNRS Bellevue, 1

_place

A. _Briand, 92195 _Meudon, France

(3)

Laboratoire de Chimie Minérale B, LA254, Université de Rennes 1,

Campus

de Beaulieu, 35042 Rennes,

France

(Reçu

le 28 mai 1989,

accepté

le 24 août

₁₉₈₉₎

Résumé. - Nous _avons irradiés des cristaux GdBaCuO _avec des ions Kr de 480 keV à 40 et 300 K. L’évolution

d’un cristal

_présentant

une structure

_monoclinique

voisine de la structure

_{orthorhombique}

a montré

qu’une

petite

déformation initiale du réseau n’a _{pas d’influence} sur les _{défauts étendus induits par irradiation. Nous}

avons mis en évidence le rôle de

_puits

des

_joints

de macles sur les défauts créés. Un film vidéo montre en effet

l’interaction

_dynamique

des dislocations avec les

joints

de macles. Une

_{amorphisation progressive}

est ensuite observée pour des doses > 4 - 5 x

1012 Kr/cm2.

Dans tous les cas une occasionnelle transition vers la structure

tétragonale n’apparaît qu’après

le début de

_{l’amorphisation.}

Abstract.

- The influence of twin boundaries

as sinks on defects induced

_by

480 keV Kr ion irradiation in GdBaCuO

crystals

was observed in situ at 40 and 300 K. The interaction of the dislocations with the twin boundaries followed on a video

_recording.

A

_crystalline

to

_amorphous

transition was observed above a total fluence of ~ 4 - 5 x

1012 Kr/cm2.

A

_comparison

between orthorhombic

_(Os)

_crystals

and a monoclinic structure

(Ms) (close

to Os and whose _parameters were

_calculated)

shows that the behaviour of irradiation-induced

extended defects does not

_depend

on a small initial deformation of the orthorhombic cell. In both case, an

occasional orthorhombic

_{(or monoclinic)}

to

_{tetragonal phase}

transition

_only

occurs when the

_{amorphization}

process has

begun.

Classification

Physics

Abstracts 61.80J -

61.16D - 74.70V

Ion irradiation studies of the

_{high Tc}

superconduc-tors are

_{providing increasingly interesting}

infor-mation on their structural

_stability

_{(e.g. [1-8].}

The controlled introduction _{of defects may also lead} to

important applications

[9-11].

Understanding

the

nature of these

defects,

their evolution and their relation to structural and electronic

_property

_changes

Structural studies related to electrical

investi-gations

with various radiation fields

_(e.g.

120 keV to

1 MeV electrons

_{[3, 6, 12, 13],}

50 to 300 keV He ions

[2, 14],

or 400 to 2 MeV 0 ions

_[1,

_15,

_16])

have demonstrated that with the

_possible

_exception

of GeV-ion irradiations

_[17],

irradiation

_by

different ions at

_{varying energies, produced essentially}

ident-ical effects on the

_{superconducting}

critical

tempera-ture

_T,,

the critical current Jc or on the normal

conductivity

as

_long

as the

_displacement

_per atom

(dpa)

ratio was the same. An orthorhombic to

tetragonal

(OTT)

transition occurs due to a local

oxygen

disordering

and

_then,

at a level 5 to 10 times

higher, amorphization

occurs due to cation

disorder-ing

[2, 7].

This was true, at

_least,

as

_long

as the

deposited

energy

density

(i.e.

the instantaneous concentration of

_displaced

atoms

_along

the incident ion

_trajectory)

remained

_{comparatively}

low such as

for MeV Ar+ or keV He+ 18 .

In _{this paper} we show

_dynamical

transmission electron

_microscopy

_(TEM)

observations of the irradiation-induced defect evolution in GdBaCuO

crystals

under

_heavy

ion irradiation

_(Kr

ions at

480

_keV).

Such

_heavy

ions

_produce

_{high deposited}

energy densities in an individual cascade and hence

large

localized defect concentrations. This leads to

dislocation

_(or

dislocation

_loop)

formation as well as

visible clusters creation

_{(yield *}

_0.1,

mean size

- 10 to 20

nm).

As the cluster

density

increases,

amorphization progressively

occurs

(threshold

50

ence = 9 x

1012

Kr/cm2).

Occasionally

we observed in

_parallel

an orthorhombic to

_tetragonal

_(OTT)

transition

_(no

_phase

transition was observed below

the

_{amorphization}

threshold

_dose).

A second

import-ant

_point

is the

_{high mobility}

of the irradiation induced defects which

_gives

rise to a

_spectacular

interaction with the twin boundaries at 40 K as well as at room

_temperature.

This

effect,

observed for the first time to our

_knowledge,

leads to both the dislocation motion and twin

_boundary

deformation. All these observations are still valid in the case of

a monoclinic structure

_{(Ms) corresponding}

to a

deformation of the basic orthorhombic

_(Os)

lattice. The lattice

_parameters

are calculated and some

information relative to _{the space group of the}

monoclinic structure are obtained.

Experimental.

1. SAMPLE CHARACTERISATION. - Small

GdBaCuO

_single

_{superconductor crystals}

_{[19, 20]}

were crushed and

_deposited

on carbon-coated

_grids

for

_{microscopy investigations.}

Some of the

_deposited

small

crystals

exhibit a monoclinic structure

_(Ms)

close to the Os. The determination of this structure

is

_reported

below. In this work we

_mainly

followed

the structural evolution at 40 and 300 K of Os

crystals

with the c axis both

_{perpendicular}

and

parallel

to the free surfaces which exhibit twin boundaries. At 40 K a monoclinic

_crystal

was also

studied.

2. IRRADIATION CONDITIONS. - The 480 keV Kr

ion irradiations were

_{performed in}

situ in a

_Philips

EM400 electron

_microscope

on line with the ion

implantor

[21].

We have used a standard double tilt

sample

holder for the 300 K irradiation and a

custom-built

_single-tilt

helium-cooled

_sample

holder

[22]

for the 40 K

_{investigations.}

To avoid a

_sample

temperature

increase

_during

irradiation,

the dose

rate was

_kept

down to - 2 - 4 x

109

Kr

cm- 2

s-1.

The maximum fluence reached was 2.5 x

1013

Krlcm2,

so that even the maximum fluence

_only

leads to a Kr

doping

level of a _{few ppm in the matrix}

and the induced

_damage

is

_essentially

due to ir-radiation effects.

Results.

1. MONOCLINIC STRUCTURE DETERMINATION.

-Electron diffraction

_patterns

_(EDP)

taken

_prior

to

irradiation show a deformed

[111]

zone axis of the

Os structure

_(i.e.

the

_angle

between the

₍₁₀₁₎

and

(011)

directions is not 90° but 86.5°

_(Fig.1)).

From such

_pattern

we deduced a monoclinic structure with the

_following

_parameters

am = 0.403 ± 0.02 _nm,

bm

= 0.384 _± 0.002 _nm,

cm = 2.350 ± 0.05 nm and

f3

= 75° _± 1°.

Comparing

with the orthorhombic

Fig.

1. - EDP from _a GdBaCuO

crystal showing

a

monoclic structure

_{([1 l 1]}

_{zone axis).}

parameters

the

_{corresponding}

ratios are

amlbos

=

1.055,

bm -

aos,

cm - 2

cos. The

interplanar

distances between Os and Ms structures are close :

During

irradiation an

_apparent

defor-mation/rotation of this structure leads to the obser-vation of three successive zone axes

(Fig. 2)

[521 ],

[121

]

and

_[411]

_]

_{(respectively}

above - 3 x 1011,

1.2 x

1013

and 1.7 x

1013

Kr/cmz).

These zone axes were indexed

_according

to the Ms structure

par-ameters found above. We

_specifically

checked that

Fig.

2. - Same

crystal

as in

figure

1

_during

480 keV Kr ion irradiation. EDP

_showing

different zone axes

_{(see text).}

As

_crystal

thickness varies

_through

the area

studied,

little information can be obtained from the

_spot

intensities. In addition no

_ring

_pattern

from a

polycrystalline

area is

_available,

so that the

_missing

planes

cannot be determined. This does not allow an

rules can be deduced

_(e.g.

reflexions are allowed for

(hhf )

with f odd,

(mi)

with f

even,

(h0f )

and

(hk0 )

with h odd and

_{(OkQ )}

with k

_odd).

2. DEFFECT EVOLUTION. - At both 40 and 300

K,

the sequence of observation as a function of

increas-ing

fluence is as follows.

i)

Both in orthorhombic and monoclinic

_crystals,

defect clusters are observed above fluences - 3 x

1011

Kr/cm2.

These clusters

_{(or dislocations)}

interact

immediately

with the twin boundaries : the

_moving

and

_pinning

of dislocations on the twin boundaries are observed on a video

_recording

[23].

Above a

fluence of - 5 x

1011 Kr/cm2

the

_trapping

of defect

clusters on the twin boundaries is observed. At

higher

fluences we observe

_{inhomogeneous}

contrast

and

_{(at -}

1012

_Kr/cm2)

the formation of

_separate

clusters inside the twin boundaries. Defect cluster

trapping

and dislocation

_pinning

on the twin bound-aries thus lead to the deformation of the twin boundaries

_(see

_Figs.

3,

4).

Detailed

_analyses

of this process will be

reported

elsewhere

[24].

These observations show that the defects created in a

_high

damage

cascade

_(i.e.

with

_heavy

_ions)

are _very

mobile

_(even

at low

_temperature)

when the ir-radiation level reaches the cascade

_overlapping

re-gime.

At this

_stage

of

irradiation,

no evidence of an

orthorhombic to

_tetragonal

transition was found in

the Os

_phase

and in Ms

_only

an

_apparent

defor-mation/rotation is observed.

ii)

Above 4 - 5 x

1012 Kr/cm2

the

_heavily

dam-aged

twin boundaries

_{progressively disappear}

as

diffuse

_rings

characteristic of an

_amorphous

structure

appear and are

_{progressively}

enhanced.

-Fig.

3. -

Bright

fields of the same

_crystal

as in

figure

1 : defect evolution

_during

480 keV Kr ion irradiation at

40 K. Doses

_(Krlcm2)

are

_reported

on the

_pictures.

At this

stage

a new

_apparent

deformation/rotation

occurred in the one Ms structure observed and

finally

we observed a Ms to

_tetragonal

transition

(above -

1.7 x

1013

Kr/cm2).

Occasionally

the OTT

transition also occurs in Os

_crystals.

Discussion and conclusion.

i)

From the above results we conclude that a

defor-mation in the Cu-0 basal

_plane

of the orthorhombic

structure

_leading

to a monoclinic structure has no

drastic influence on the structural evolution of the

crystal during heavy

ion irradiation. Whether or not

this deformation _{has any influence} on the

supercon-ducting properties

is still an _open

_question.

ii)

Previous He ion irradiation studies led us to

propose that at least two

types

of defects were

involved in the structural

_changes

observed

during

light

ion irradiation.

First,

very mobile anionic defects _{associated with oxygen} are

_responsible

for the cluster formation at low

temperature

and for the OTT transition.

_Secondly

less mobile cationic defects

are related to

_{amorphization}

_[8].

For

_heavy

ion irradiation OTT occurs at the same level of disorder

as for

light

ion irradiation

( -

0.1

displacement

per

atom ratio :

_dpa).

For reasons discussed in reference

[8],

it is

_univocally

related to sublattice

_disordering.

The results obtained here

_strongly

indicate that there is no difference in the process

leading

to the OTT between

_light

and

_heavy

ion irradiation. The

amorphization

process is

simply sufficiently

efficient in the

_heavy

ion irradiation case to

_partially

mask the OTT.

iii)

In the case of

_heavy

ion

irradiation,

the

amorphization

occurs at fluences which are more

than 3 orders of

_magnitude

lower than for He irradiation

_(above

4 - 5 x

1012 Kr/cm2

instead of

10 16

_He/CM2).

For

_heavy

ion

irradiation,

the

aver-age energy transferred per collision is

significantly

larger

so that cation

_{displacements}

are

_definitely

enhanced. Also the

_deposited

energy

density

is about two orders of

_{magnitude larger}

than in the

light

ion irradiation

_(according

to TRIM calculation

[25]

the average numbers of

displaced

cations for He and Kr ion irradiation are

_{respectively -}

60 and

1 600).

iv)

In order to

_analyze

the

_{amorphization}

_process,

e o _{owmg ques} ion anses. or r irra ia

_ion,

could

_amorphous

zones be created

directly by the

individual cascades so that

_{amorphization}

would

proceed by simple

cascade

_{overlap ?}

From our

results,

we obtain the

following

information

regard-ing

this

_question.

1)

The threshold fluence of

_{amorphization}

is at

least an order of

magnitude higher

than the

threshold fluence for the cascade

_overlap

( - 1010 Kr/cm2).

This value does not take into

52

Fig.

4. - Defect evolution of

an orthorhombic GdBaCuO

crystal

(with

c axis

parallel

to the zone

_axis)

irradiated at

300 K with 480 keV Kr ions.

cascades as we consider the

_overlap

of the whole

extension of the cascade

_(i.e.

both

_light

and

_high

damage density

zones)

instead of

_only

subcascade

overlap

(high

damage density

zones).

This result indicates that

_{amorphization}

does not

_{proceed by}

the

_overlap

of

_{light damage}

_density

zones. However

amorphous

zones could be created

_directly

inside subcascades

_(i.e.

_{large damage}

_{density zones).}

2)

The clusters observed

_{( >}

1011

_Kr/cm2)

below the threshold fluence for subcascade

_overlap

5 x

1011

_Krlcm2)

are

_directly

formed inside subcascades.

However cluster interaction which leads to the formation of dislocations and their interaction with twin boundaries

_imply

that the clusters involved in this interaction _{process do} not have an

_amorphous

structure.

3)

Although

most of the visible clusters are

cer-tainly

not

_{amorphous, they}

_represent

less than 10 % of the

_high

_{density damaged}

zones created

_directly

in

partially amorphous

is still an unresolved

_question

[23, 26],

but in any case the

_{amorphization}

is not

induced

_by

the

_overlap

of visible

_amorphous

clusters. This leads us to _{propose that}

_amorphized

zones are created inside an individual cascade but are

either relaxed or _{very small in size}

₍

2

_nm)

and do

not lead to visible strained

_amorphous

clusters. The

overlap

of these

_amorphous

zones induces

amorphi-zation. Therefore two

_types

of extended defects are

probably

formed

_{during heavy}

ion irradiation :

i)

unresolved dislocation

_loops

which leads to dislo-cation formation and interaction with twin

bound-aries ;

ii)

amorphous

zones,

presumably

invisible

by

TEM,

which induce

_{amorphization.}

Acknowledgments.

We thank 0. Kaïtazov for

_help

with the irradiations. This work was

_{partially supported by}

the PIRMAT

(ARC

«

Microstructure »)

CNRS,

France.

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