Experimental evidence of a new crystallographic structure of samarium deposited by cluster beam

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Experimental evidence of a new crystallographic structure of samarium deposited by cluster beam

P. Mélinon, G. Fuchs, M. Treilleux

To cite this version:

P. Mélinon, G. Fuchs, M. Treilleux. Experimental evidence of a new crystallographic structure of

samarium deposited by cluster beam. Journal de Physique I, EDP Sciences, 1992, 2 (7), pp.1263-

1269. �10.1051/jp1:1992208�. �jpa-00246619�

Classification Physics Abstracts

81.15 61,16D 68.55

Short Communication

Experimental evidence of

_{a new}

crystallographic structure of samarium deposited by cluster beam

P.

M41inon,

G. Fuchs and M. lieilleux

D4partement de Physique des Mat4riaux, ^Universit4 Claude Bernard-Lyon 1, 43 Boulevard du II Novembre 1918, 69622 Villeurbanne Cedex, France

(RecHved

21 April 1992, accepted 5

May1992)

ILdsumd Une nouvelle structure du samarium obtenue _par ddp6t d'agr4gats _a 4t4 observ4e.

Cette _{structure est} du type cubique faces centr4es. Dans le _cas d'agr6gats, les formes particu-

libres de la structure cfc

(particules

multimacl4es, icosabdre,

...)

_permettent d'avoir

une phase commensurable qui accomode l'importante difl4rence de parambtre de motile entre les _atomes de surface divalents et les _atomes de _coeur trivalents des agr4gats d4pos4s.

Abstract. A _new fcc phase of Sm has been obtained by deposition of Sm clusters with the low-energy cluster beam deposition technique. The formation of this _new phase corresponds to a commensurable phase which accomodates the large lattice parameter of divalent surface atoms and trivalent _core atoms of the supported Sm clusters.

One _reason of the intensive

study

of _rare earth elements and their

compounds

is the dis-

covery of _{a new} type ^of

ground

state named intermediate

(or mixed)

state.

Among

rare-earth

elements,

samarium is the most known

example exhibiting

an intermediate valence _state. This

state

corresponds

to the coexistence of divalent and trivalent states of Sm

leading

to very

interesting properties especially

in the first stage ^{of thin film}

growth (when

Sm

deposited particles

_are still

isolated).

In this _case, theoretical calculations

[I]

^and

experimental

results [2-4] ^{show that the surface of Sm}

particles

is

composed

of atoms in _a divalent

configuration (atomic

state

[Xe]4f~6s~)

^while atoms of the

particle

_core have the trivalent

configuration (bulk

state

[Xe]4f~5d~(6sp)~).

As both

configurations

are

nearly

_energy

degenerated,

the valence is

strongly

influenced

by

the chemical

surrounding

_or coordination number [5, 6]. ^{Valence state}

strongly depends

_on the

degree

of order in the

layer,

its thickness and the _nature of the sub-

strate. Some interfaces exhibit different

properties

_{as a} function of samarium coverage that

means a

change

in

bonding

versus thickness. In these

studies,

the valence _state is

usually

de-

termined

by X-ray photoemission

spectroscopy

(XPS),

ultra violet

photoelectron

spectroscopy

(UPS),

_or

X-ray-absorption

_near

edge

structure

(XANES).

For very low coverages samarium

1264 JOURNAL DE PHYSIQUE I N°7

is

expected

to be

weakly

bound _on the substrate. In this

regime,

divalent-trivalent transition is observed with

increasing

_coverage. For _an intermediate coverage,

disruption

and chemical

mixing

involve modifications of the

overlayer/substrate

bonds. For

higher

_coverages trivalent metallic samarium is observed

corresponding

to the formation of Sm

overlayer

_on the reacted

overlayer.

For small matrix-isolated Sm

clusters,

for which chemical interactions between the Sm de-

posits

and the supports are

minimized,

a mixed-valence behavior is known to be _very sensitive to the cluster size

[7-9].

The divalent-trivalent transition with the

increasing

_coverage has also been observed

by

Mason et al. [10] ^{for isolated samarium}

particles deposited

on

amorphous

carbon substrate. Some difference between the results of _{rare gas} matrix and solid supports

experiments

have to be outlined: the cluster size threshold

corresponding

to valence transition is lower for small matrix-isolated Sm clusters

(13

atoms [9]) ^{than for Sm clusters} on C supports

(about

400 atoms

[10]).

The _most

popular interpretation

^{of the valence} transition of Sm clusters

gives

_a trivalent state for "bulk"

(core)

atoms

(corresponding

to _a

high coordinance)

and _a divalent _state for the outermost surface _atoms

(corresponding

to a low

coordinance).

In the _{same way,} ther-

modynamic

arguments [I] ^{show that} a

compression (increase

of

coordinance)

will favor the trivalent state.

According

to these

considerations,

the Sm

aggregate

structure is

expected

to have _a compact

layer (trivalent state)

for the bulk atoms and _a strong ^surface reconstruction _to obtain the divalent f~ character for the outermost surface atoms. In this _{paper a new}

crystal- lographic

structure for small samarium clusters is

experimentally

exhibited and supports the classical model of valence transition. In

addition,

the valence state is measured _{as a} function

on the

experimental

cluster size.

Small samarium clusters _are

generated by

the gas

aggregation technique.

^{Our cluster} source

has been described in

previous

_paper

[iii.

Free neutral clusters _are ionized

by

electron beam and

analyzed

in _a time

offlight

_mass spectrometer. ^{The observed} size distribution shows

large peaks (magic numbers)

at _{n =}

13,

19,

23,

26, 29,

32,

34... [12]. ^The

beginning

of this _sequence has been observed in _argon clusters and in barium clusters

[13].

^{This is}

interpreted

in terms of

icosahedral structures

corresponding

to the addition of _{caps on a core} icosahedron of13 atoms

(Mackay sequence) [13].

^{Several authors}

[14-16]

^{have observed the} coexistence of abnormal structures such

multiply-twinned,

cuboctahedron and icosahedron structures in free clusters

as a function of cluster size. As the size cluster

increases,

fcc structure is

always

observed and

corresponds

to the bulk structure.

So,

free samarium clusters _are

expected

to be in the fcc structure type. The present _paper reports ^{results obtained with} a Sm cluster size distribution of 400 _{atoms mean} size

(neutral clusters) deposited

at room temperature on

amorphous

carbon films

(5

nm

thickness) supported

_{on copper}

microscopy grids.

The rate of

deposition

_was

checked

by

a

crystal

quartz ^{monitor located} near the substrate. Neutral free clusters _are

deposited

in _{a vacuum}

of10~~

_Pa. Chemical

analysis

and valence state _were

performed by Auger

electron spectroscopy

(AES)

and XPS in _a UIIV Nanoscan-Cameca system. ^{The AES} and XPS spectra were recorded _{on a} MAC2 spectrometer

respectively

with _a 2 kev incident

electron energy and _a

Mg-K

_a radiation. The _mean size

ofsupported particles

_was measured

by

transmission electron

microscopy (TEM)

_{on a} 200-CX Jeol electron

microscope operating

at 200 kV

accelerating voltage.

The

crystallographic

structure was determined

by optical

micro- difsraction _on IIRTEM

(high

resolution transmission electron

microscopy) images

of isolated

particles.

Figure

I presents a

typical

TEM

micrograph

of _a I _nm

equivalent

thickness Sm

deposit.

The coverage (@) is about 20 iii and the size distribution of the

supported

samarium

particles

is centered _on 4 _nm which

corresponds

to 1200 atoms

(see

inset of

Fig. I).

This diameter

roughly

corresponds

to the _mean size of the 400 atoms incident clusters

(3 nm)

that

suggests

a low

coalescence

regime.

This behavior has been

already

observed for

antimony low-energy

cluster beam

deposition (LECBD) _{[17, 18]:}

the low nucleation rate allows _a

paving

of the substrate

by

the incident free clusters.

IIRTEM observations show that all

supported

clusters _are

crystallized.

The Sm

particles presented

in

figure

2 _are two

examples

of the

typical

HRTEM

images

recorded. The

particle

of

figure

2a presents a fivefold symmetry ^similar to

multiply-twinned particles (MTP) previously

observed in _some fcc metals

[19, 20].

^The

shape

of this

particle

is _a decahedron _{seen on} the five-fold symmetry axes

according

to IIRTEM

image

simulations [21] ^{of fcc structure material.}

This

particle

is faceted in the

(l10)

_type orientation: the symmetry ^{of the lattice}

image

_on

each face

corresponds exactly

to

( iii) planes

of _a fcc

crystallographic

structure. In the _same way, the

crystallographic

structure of the

particle presented

in

figure

2b has been determined

by

lattice

image

and

optical

micro-diffraction

pattern analysis.

The structure of this

particle

coincides with

a fcc

particle imaged along

the

[l10]

axis. The lattice parameter calculated from the micro-difsraction pattern is 0.55 _nm. This value

gives

Sm-Sm distance of 0.389 _nm

compared

with the Sm diameter

(0.36 nm)

in the rhomboedric

phase.

The external facets _are

(

ii

ii

and

(100) planes.

From this

image,

the determination of the

particle

3

dimension-shape

is not trivial.

However,

_we mention that the

shape,

the contrast and the orientation of such

particles

_are similar with

previously published

IIRTEM

micrographs

of fcc cuboctaedron [22].

This fcc structure, also observed in _our

experiments

for smaller

particle

^diameter

(down

to 3

nm),

is correlated _to the

predicted

free cluster fcc structure. This _structure is

undoubtly strongly

^related to the intrinsic

properties

of free Sm clusters since the structure of the

deposited

clusters remains

unchanged

_even after interaction with the support.

The structure of these

particles

does not

correspond

to the well-known rhomboedric _structure of bulk Sm [23]. ^{It is well-known that thin samarium}

deposits

_{are very} sensitive to oxygen.

So,

a

particular

attention _was

paid

to check that _no other oxide structures

(SmO [20], cubic-Sm203

[24] ^{and monoclinic}

Sm203 [25])

^{fit the}

experimental

lattice

images

and difsraction patterns.

In

addition,

from AES and XPS

results,

_we have checked

that,

in _our

experimental

_vacuum

background

_{pressure range, no} oxidation _occurs

during

LECBD Sm

deposition.

The

depth profile Auger

electron

analysis gives

a Sm metallic _core surrounded

by

_a carbon contamination

layer occurring during

the transfer between the

deposition

and the

analysis

system

[26].

To compare our results with those of the literature _an XPS

study

of the transition

3d~'~

_{has been}

made. This

study

of LECBD Sm

deposits

_[26] and TEM results allow _us to

give

the correlation between the _mean valence

(V),

the

crystallographic

structure and the cluster size

(Tab. I).

The relation between the obtained _mean valence and the _coverage rate is in

good

agreement ^{with the}

predictions

of Mason et al. [10] ^{and the} theoretical model of _a cluster

composed

with trivalent

core atoms and divalent surface atoms fits _our

experimental

^results

(see

Tab.

I).

The _mean valence calculated here

corresponds

to the _mean size of the

deposited

cluster size distribution and _{so we} cannot

distinguish

the real valence of surface and _core atoms and their variation

versus cluster size. At this

point,

_a size-selected cluster

study

could

bring

_some

quantitative

information if coalescence of free clusters

deposited

_on the surface is inhibited.

The lattice

images presented

here _prove that samarium clusters obtained

by

LECBD

crystal-

lize in the fcc structure

though

the usual Sm bulk structure is rhomboedric. To _our

knowledge,

these observations present ^{for the first time} an

experimental

evidence of the fcc structure of Sm clusters.

According

to theoretical

arguments,

^{the fcc} structure

expected

in the trivalent

lanthanides

corresponds

to the structure that allows the maximum d-band _occupancy [27]

(the

4f-electrons _are treated _as valence

electrons).

This increase of the d-band _occupancy from the Sm bulk

configuration ([Xe]4f~5d'(6sp)~)

to

([Xe]4f~5d~(6sp)') requires

too much _energy to be

possible

_[28].

Still,

this _{energy can} be balanced

by

the

no-promotion

^{of 4f~ electron} on the

1266 JOURNAL DE PHYSIQUE I N°7

ii

~~°

.l

~

(~ ~s

~o

la

lo

s

°1 ~ a 4 a e v e o

mm

2 O _nm

Fig. I. TEM micrographs of1 _nm thick samarium deposits obtained with low-energy cluster beam deposition

(LECBD).

The incident _mean cluster size is 3 _nm diameter

(corresponding

to 400

atoms)

which approximatively corresponds to the _mean size of the supported clusters (4

nm).

The size distribution of supported aggregates is given in inset.

a 2nm b ^2nm

Fig. 2. HRTEM micrographs and the corresponding optical diffraction patterns of two shapes of samarium clusters obtained by LECBD. The lattice images correspond in both _cases to fcc particles in the [110] ^{direction. The} particle

(a)

is _a fivefold symmetry multiply-twinned particle while the particle

(b)

has _a cuboctaedral shape.

Table I. Valence of samarium clusters _{as a} function of the size. The results of this work

(a)

are

compared

to XPS values

(b)

obtained

by

Mason et al.

[10].

^{We also} report ^the theoretical valence value

(c)

calculated with _a model of _a Sm cluster formed with divalent surface atoms

surrounding

trivalent _{core atoms.}

number of

atoms/per

1200 bulk

cluster

valence 2 2.50

(a)

3

2.35

(b)

2.63

(c)

structure c.f.c.

surface.

Rosengren

et al.

[ii give

a value of 0.72 eV for the surface core-level shift

corresponding

from _a

completely

trivalent Sm metal

(trivalent

_core surrounded

by

trivalent

surface)

to _a

1268 JOURNAL DE PHYSIQUE I N°7

trivalent-divalent Sm cluster

(trivalent

_core surrounded

by

divalent

surface).

The

large

lattice parameter in surface is essential to avoid the 4f orbital

overlap corresponding

to its

promotion (the

ratio between the

density

of

hypothetical

_pure divalent

phase

and bulk

phase

is about

2/3).

This lattice

expansion

has been

already

observed _on samarium

epitaxially

_{grown on} Mo

(l10) _[29].

For

non-epitaxially deposited

^Sm clusters the stabilization of the lattice

expansion

can be

explained by

the formation of MTP'S _or truncated cuboctaedron _structures observed in the fcc elements [30]. ^{The basic} structure of MTP

particles

is described

by

the collection of

primitive

tetrahedra twin related _on their

adjoining

faces. This

configuration

does not fill the _space and it is _necessary to introduce

homogeneous

_or

inhomogeneous

internal strains

[31, 32].

The formation of MTP is favored

by

extensive

faceting,

small twin

boundary energies

and small surface stresses. The surface relaxation _can be modelled

by

_a surface shell of abnormal lattice parameter: Marks [33] ^{has observed in small}

gold

surface _a 2 _x I reconstruction with _a 20 iii _{outer atom}

expansion

that could _occur in MTP

particles.

As

Stenborg

et al. have

suggested

_a low

melting

surface temperature

(below

_room tem~

perature)

in the 5 _x 5 Sm surface reconstruction

[29],

it _can be

surprising

to observe in _our

experiments

such _a lattice

expansion

at room temperature. The _presence of carbon contami- nation

(shown

_on IIRTEM

micrographs

of

Fig. 2) embedding

Sm clusters _may stabilize the fcc

structure. At this

point,

_we have to recall to the reader that the fcc _structure is

directly

linked to the free cluster structure and the carbon does not initiate the fcc

phase

formation.

Roughly,

_{we can} define _a

qualitative

criterion to observe fcc

particles

^of N atoms. If

Ns

is the number of surface atoms per cluster and Ed the _energy of the _new d electron

promoted,

we have:

Ed _<

[0.65

x

(Ns IN)]

eV

From this criterion _a critical size of cluster is

expected.

The

Ed

value _cannot be

easily

estimated but _a maximum value _can be

given. Assuming

that the

bigger

fcc cluster observed is about 4 _nm

diameter,

_{we can}

predict

that:

Ed

< 0.24 eV.

The present work establishes the existence of _a fcc structure for Sm clusters. The direct relation between the valence and the cluster size agrees with the well-known model of the intermediate valence Sm clusters. This fcc Sm _structure is

compatible

with _an electron _pro- motion from

(sp)

to

(d)

state. To _our

knowledge,

the size efsects

already

evidenced _on other metallic cluster structure are limited to lattice parameter

change, MTP'S,

cuboctaedron

(fcc

type

structure)

_or icosaedron

(fcc

structure

precursor)

structure formation. In these _cases, the

structure ofthe metallic clusters

basically

remains the _{same as} the bulk metal. So

we present in this work the first

experimental

evidence of _a real structure

change

between metallic clusters

(fcc structure)

and bulk

(rhomboedric).

Among

the

properties

of this _new structure, ^the

magnetic properties

are now in _progress.

Magnetism

^{arises when} the orbitals _{are so} localized that

overlap

with of

neighbors

is _very small and there is _very little

bonding. So,

the _new d-electron is

expected

to

play

_a

major

role in the

magnetic properties

of the fcc Sm structure. In the _{same way,} the electrical

properties

of

LECBD thin films would

change

with this _new electronic

configuration.

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