Raman scattering and local order in GexSe 1 -x glasses for 1/3 ≤ x ≤ 1/2

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Raman scattering and local order in GexSe 1 -x glasses for 1/3 ≤ x ≤ 1/2

P. Tronc, M. Bensoussan, A. Brenac, G. Errandonea, C. Sebenne

To cite this version:

P. Tronc, M. Bensoussan, A. Brenac, G. Errandonea, C. Sebenne. Raman scattering and local or- der in GexSe 1 -x glasses for 1/3

≤

x

≤

1/2. Journal de Physique, 1977, 38 (12), pp.1493-1498.

�10.1051/jphys:0197700380120149300�. �jpa-00208724�

RAMAN SCATTERING AND LOCAL ORDER IN GexSe 1-x GLASSES FOR 1/3 ~ ^{x ~} 1/2

P.

TRONC,

^M.

BENSOUSSAN,

^A.

BRENAC,

G. ERRANDONEA

Centre National d’Etudes des

Telecommunications, 196,

^{rue de}

Paris,

⁹²²²⁰

Bagneux,

^France

and C. SEBENNE

Laboratoire de

Physique

^{des Solides}

(*),

Université

Pierre-et-Marie-Curie,

⁷⁵²³⁰ Paris Cedex

05,

^France

(Reçu

^{le 2 mai}

1977, accepté

^{le 19 août}

1977)

Résumé. ²⁰¹⁴ Après avoir étudié auparavant l’ordre local des verres GexSe1-x ^entre le sélénium pur et x ⁼ 1/3, nous avons

préparé

des échantillons pour 1/3 ~ x ~ 1/2. ^Des matériaux vitreux ont été obtenus jusqu’à ^{x =} 0,44. Pour les valeurs de x supérieures les échantillons sont constitués de cris- tallites de GeSe contenus dans une matrice vitreuse

GexSe1-x,

^{avec x =} 0,41 ± 0,01, ^le pourcentage de verre tendant vers 0

quand x

^{tend vers} 1/2. ^{Des mesures de diffusion Raman ont} été effectuées sur

les échantillons à la température ambiante. Les résultats conduisent au modèle de structure locale décrit ci-après. ^Pour 1/3 ~ x ~ 0,42, ^{les verres} présentent ^un ordre local du type GeSe2 : ^{les coor-}

dinances des atomes de Ge et de Se sont égales respectivement ^{à 4 et à} 2, les liaisons Se2014Se ainsi que deux liaisons Ge2014Ge par ^atome de Ge étant statistiquement exclues. Pour x ~ 0,43, ^il apparait

de fortes indications d’une structure du type ^GeSe (coordinances ^{des atomes de Ge et de Se toutes}

deux

égales

^à 3) : ^{un tel} type de coordination n’avait

jamais

^été signalé

jusqu’à

présent ^{pour des}

verres.

Abstract. ²⁰¹⁴ Having

previously

^{studied the} local order of GexSe1-x glasses between pure selenium and x = 1/3, ^{we have now}

prepared

samples ^for 1/3 ~ x ~ 1/2.

Glassy

materials have been obtained up ^{to x =} 0.44 and, ^at

higher

^x values, ^the

samples

^are made of GeSe

crystallites

embedded in a

glassy ^{matrix of}

GexSe1-x,

^{with x = 0.41 ±} 0.01, ^the percentage ^of glass going ^{to zero when x} goes to 1/2. ^Raman scattering measurements have been

performed

^{on a set of such}

samples,

^{at room} temperature. The results lead to the

following

^model of local order : for 1/3 ~ x ~ 0.42, ^the

glasses

have a GeSe2-like ^local order with four-coordinated Ge atoms and two-coordinated Se atoms, Se2014Se bonds and two Ge2014Ge bonds per Ge ^atom

being statistically

forbidden; ^for x ~ 0.43 strong indications appear for ^a GeSe-like local order, ^{with both} three-coordinated Ge and Se atoms : such a type ^of coordination has never been

previously

reported ^for glasses.

Classification

Physics ^Abstracts

61.40 - 78.30

1. Introduction. ^- The

importance

^of

optical

methods to the

study

^{of local structure in}

binary chalcogenide glasses

^{has been} demonstrated

recently

both for the

GexSel-x [1]

^and

GexS 1 _ x [2] systems.

In reference

[1],

which will be called 1

hereafter,

Raman

scattering

measurements allowed us to draw

some conclusions about the local structure and its

changes

^{with x for 0 x}

1/3

^{in the}

GexSel _ x glasses.

The structural model could account for the

changes

^{in the}

optical-absorption edge

^{as a function}

of x.

In the

present

paper, the results of a similar

study

(*) ^{Associé au} Centre National de la Recherche Scientifique.

for

GexSe 1 _ x glasses

^with

1/3 ,

^x

1/2

^are

given :

this is of

particular

interest for reasons which are

developed

^below.

In

I,

^{the current views on the}

phase diagram

^{of the}

Ge-Se

system

^were

reviewed, together

^{with the}

crystalline

structures of

GeSe2

and GeSe. In the

particular

^{case of}

^GeSe2,

^a

complete knowledge

^{of the}

structure is not

yet

established.

However,

^{some infor-} mation has been obtained

recently [3]

^{on the}

crystalline

form of interest for _us, that is the form into which our

glassy samples crystallize

^{when cooled at a too slow}

rate.

(The experimental

results which were

reported

in 1 about

crystalline GeSe2

^{both for}

optical-absorp-

tion

edge

^{and Raman}

scattering

^{are relative to this new}

crystalline form.)

^In

short, GeSe2 crystals

^{should be}

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0197700380120149300

1494

considered as a set of more or less distorted tetrahedra with a Ge atom at the centre, ^{bonded to four Se atoms}

at the _corners, each Se atom

being

^{bonded to two}

Ge atoms

(Fig. la).

^GeSe

crystals

^are such that each atom, ^{Ge or}

Se,

^{has three nearest}

neighbours

^{of the}

other

kind,

^{one at 2.56}

A

and two at 2.59

A,

^{the next}

three nearest

neighbours being

^{at 3.32}

^À

^{and 3.7}

^A

(Fig. lb).

^,

FIG. 1. ^- Local structures of crystalline germanium selenides : 0 Ge atom, 0 ^{Se atom ; a :} GeSe2, ^{b : GeSe.}

In

Ge,,Se 1 -,, glasses,

^{as x} varies from 0 to

1/3,

^{it has}

been shown in 1 that the local structure tends towards

a

pseudo-tetrahedral

one, with four coordinated Ge atoms and two coordinated Se atoms, ^Ge-Ge bonds

being statistically

^forbidden

(Fig. 2),

^which

implies

^a

vanishing

number of Se-Se bonds.

FIG. 2. ^- Local structure of GexSe1-x glasses (x 1/3) : ^{0 Ge}

atom ; 1 Se atom.

For x

higher

^than

1/3

^{in the}

GexSe 1 - x system,

^such

a situation is no

longer possible

^{and the}

question

^{is to}

determine its behaviour _up to x ⁼

1/2,

^{where the}

crystalline

form shows both three coordinated Ge and Se atoms.

Three

possibilities

^can be considered :

-

First,

^{if the} system ^{does not} accept ^Ge-Ge

bonds,

the three-coordinated Ge and Se atoms must appear ^{as soon as x} >

1/3.

2013

Second,

^{if the} _system ^admits any number of Ge-Ge

bonds,

^no three-coordinated Ge and Se atom is _necessary.

- Third,

^{if the}

system

^admits

only

^a limited number of Ge-Ge

bonds,

three-coordinated Ge and Se atoms must appear ^{at some} intermediate value

of x ;

^for

example, if only pairs

of bonded Ge atoms surrounded

by

^{Se atoms are}

accepted,

the limit is for the

Ge2Se3 composition

and three-coordinated Ge and Se atoms must appear for x >

2/5.

The local structure of

GexSe 1 - x

thin films at

high x

values has been studied elsewhere

[4-8]

^either

by

electron diffraction or

by

^extended

X-ray absorption

fine structure.

Except

for Mikolaichuk and

Kogut [7]

who claim a GeSe-like local order at x ⁼

0.50,

all the authors

support

^a

GeSe2-like

local order with a bond

length

^{of 2.3 to 2.4}

^A

for x values

ranging

^{from 0.32 to}

0.73. Such results

give

^{another reason to}

study

^bulk

samples

^{where the strain can} be lower than in thin films.

In the present paper, it will be shown that an answer can be obtained to the

question

of local order of

GexSel-x glasses (x

>

1/3),

^{from Raman}

scattering

measurements. In section

2,

the main features concern-

ing

^the

preparation

and the characterization methods

are described. After

giving

^the

experimental results,

section 3 is devoted to the discussion of the results which leads to a model of local structure of

GexSe1 _x glasses

for the different values of x in the studied interval.

2.

Samples préparation

^and characterization. ^-

GexSel-x glasses

^{can be}

prepared

^for

germanium

concentrations x

higher

^than

1/3,

^as

reported by

Baidakov

[9]

^{for x}

0,40,

and later

by

^different

authors

[1,10,11] for x

^{0.42. In the} present case, the

samples

^{with a} concentration _up to 0.42 are

prepared

^and characterized as described in 1 : amor-

phous

^{materials are obtained}

by cooling

^the quartz cell

containing

^the

homogenized liquid

^{of proper}

composition

^{at a suitable rate.}

However,

^{for x} > 0.42 such a method does not lead to an

amorphous

material and it is _necessary to use a

different method which is described in detail else- where

[12].

^In

short,

the mixture is heated _up to about 30 °C above the

liquidus

temperature

[1]

^for

5 minutes in an open silica crucible

encapsulated

with

B203

^{under an 8 bar} argon pressure. Then a

water-cooled _copper

finger

^is

quickly plunged

^{into the}

liquid.

^{It is}

possible

^to

get

^an

amorphous layer

^about

500 _gm thick for x _up to and

including

^{0.44. The}

amorphicity

is verified

by X-ray

diffraction

(the

diffraction

spectra

^are

absolutely flat)

^{and the} average and local chemical

compositions by colorimetry

^and

electron

microprobe.

For ^x

higher

^than

0.44, optical microscopy

^and

electron and

X-ray

diffraction show that ^even the surface

layer

^is

partly crystallized :

^GeSe

crystallites

are embedded in an

amorphous

^{matrix made of}

GexSel _x

^{with x = 0.41 ± 0.01}

(as

^measured

by

electron

microprobe).

GeSe

single crystals

^{have been}

prepared using

^the

vapor

phase transport

^{method as}

proposed by

Wiedemeier et al.

[13].

3.

Expérimental

results and discussion. ^-T The Raman spectra ^of the different

amorphous

^and crys- talline

samples have , been

obtained with the

experi-

mental set up described in I.

Except

^for

crystalline GeSe2,

where the laser source was either Kr+ ^or

Ar+,

the excitation was obtained

through

^{a YAG laser}

at 1.06 _gm CW. The

beam

was focused on the sur-

face of the

sample,

^{with an incidence}

angle

^{of about}

750. The measurements were

performed

^{at room}

temperature

and the anti-Stokes

spectra

^were sys-

tematically

recorded since

they give

better results

taking

^{account of the} variation of the

photomultiplier sensitivity

^versus

frequency,

^{on the one}

^hand,

^{and of}

the effect of Bose-Einstein statistics _upon the line

magnitudes

^on the other hand. The results for the different values of x are

given

^on

figures

^{3 and 4}

(the

dotted lines represent intensities of the main

peaks

which can be

distinguished

^{in the Raman}

spectra).

When x varied from 0.33 to

0.44,

within the accuracy of an

X-ray

diffraction test, ^the

samples

^{were amor-}

phous

and remained so even after

being exposed

^{to the}

focused laser

during

the Raman

scattering experi-

FIG. ’3. ^- Anti-Stokes Raman spectra ^of GexSe1-x glasses (1/3 ^x 0.42).

FIG. 4. ^- Anti-Stokes Raman spectra ^of GexSe1-x compounds (0.42 x 0.50).

ments. Several

recordings

^were

taken,

the location of the laser beam on the

sample

^{or the}

samples

^itself

being changed

^each

time,

^for every x value.

For the same nominal value

of x,

^the

reproducibility

is

generally good

if one considers

only

^the

frequencies,

the relative intensities and the widths of the différent

peaks.

The absolute intensities ^can

change easily by

^a

factor of two

depending

^{on the}

quality

^{of the}

sample

surface and the details of the

optical alignment.

However,

^{in the x = 0.42 + 0.01} range, where

rapid changes

^are observed when x

varies,

^a

good

repro-

ducibility

^{in the} relative intensities of the different

peaks

^{is not}

always obtained,

which indicates that local

.

composition

fluctuations of about 1

%

^can

occur

(in

^{the later} case, spectra ^{which are}

presented

^{are the}

averaged

values of the different

recordings).

From these

considerations,

^some normalization

rule

^{had to be}

adopted

^for

representing

the results.

In

figure 3,

^{for x}

0.42,

^the

spectra

^are

given

^with

the 200 cm -1

peak intensity proportional

^{to x. Such a}

choice will be

justified

^{later. In}

figure 4, for x > 0.42,

the

spectra

^are

given

^{with the}

highest peak

^{at the same}

intensity independent

^{of the}

peak frequency

^and

the x value. This choice is

quite arbitrary

^{and may}

even hide part ^{of the}

experimental information ;

^its

only advantage

^{is to}

give

^a well balanced _represen-

tation, but,

ⁱⁿ

fact,

^{as x} increases from 0.45 to 0.48 and

again

^{to GeSe}

crystal

^an increase of the overall intensities

clearly

appears with a

multiplying

^factor

of 3 to 4. The Raman

spectra

of the GeSe

crystal

^were

obtained on cleaved

samples

^{with the same}

geometrical

arrangement ^{as for}

amorphous samples.

1496

To discuss these results it is _necessary to refer to a

model. One can

rely

^{on the data}

provided by

^the

crystallization

behaviour of

GexSel _x glasses

^to

give

^a

hint to start

interpreting

the informations

provided by

^Raman

scattering.

The

crystallization

behaviour of

GexSe 1 _ x

^com-

pounds depends strongly

^{on the thermal} process :

- If the

cooling

^{rate is}

high enough,

^{as described}

above in part

2, X-ray

diffraction patterns ^{show that}

strictly amorphous samples

^{are obtained for x 0.44}

and GeSe

crystallites

appear for x > ^0.45.

- If the

cooling

^{rate is a little}

lower, partly

crys- tallized

samples

^are obtained : for

1/3 x 0.42, only GeSe2 crystallites

^are

observed,

^{while for}

x >

0.43, only

^GeSe

crystallites

^{are detected.}

-

Upon annealing,

^{which is}

equivalent

^{to a} very low

cooling

rate,

only GeSe2 crystallites

^{are observed}

for _x 0.35 but GeSe

crystallites

^{start to} appear,

together with GeSe2,

^{for x} > 0.35.

Since the increase of the

cooling

^{rate seems to delete}

the GeSe-like local order and to favor the

GeSe2-like

local order for x

0.42, glasses

^{in that}

compositional

range ^can be assumed to have a local structure similar to

GeSe2

with four-coordinated Ge atoms and two- coordinated Se atoms. On the contrary the x ‘> 0.43

glasses

^{show a}

tendency

towards the local order of

crystalline ^GeSe,

with three-coordinated Ge and Se atoms. The

GeSe2 crystal

^has

only

^{Ge-Se bonds.}

One can assume that no

dangling

bonds exist in

chalcogenide glasses [1]. Then,

^{for x} >

1/3,

^{a similar}

local order as

GeSe2 crystal implies

the existence of Ge-Ge bonds. Moreover, ^Se-Se

bonds,

^if any, ^are

certainly

^{in small}

number,

^as

proved by

^{the Raman}

spectra ^of

figure 3,

^{where the}

peak

^{at 250}

cm -1,

^which

characterizes such bonds and is intense in x

1/3 glasses [1],

^{is almost}

negligible.

From these

considerations,

^{we shall} propose the

following

^{model :}

- For

1/3 x 0.42,

^the

samples

^are constituted of true

glassy

material. There is no

dangling bond,

Ge atoms are

four-coordinated,

^{Se atoms are two-}

coordinated,

Se-Se bonds are forbidden and the local structure is of tetrahedral type.

- For 0.43 x

0.44,

^the

samples

^{are still made}

of true

glassy materials,

the rules for x 0.43 still hold but local structures with three-coordinated Ge and Se atoms appear which means a dramatic

change

in the first

neighbours configuration.

- For 0.45 x 0.50 the

samples contain

^GeSe

crystallites

^{in an}

amorphous

matrix made of a

glassy

material x -

’0.41,

^{with a similar structure as a true}

glassy sample

of similar

composition.

One has now to check if this model fits with the

experimental

^results.

a) 1 /3 x

^{0.42. - In the}

high frequency

range of interest _{to us,} the Raman spectra ^of

figure

^{3 show}

the existence of three

peaks

^at

175, 200

^{and 215}

cm-1

respectively

^{and of a} wider band in the 230-330 cm -1 range

containing

^{a wide}

peak

^{at about 290}

^cm-1.

As shown in

I,

^the

peak

^{at 200 cm-1 can} be attributed to the tetrahedral structure where a Ge atom is surrounded

by

^{four Se atoms. We} shall consider that it still holds if one or more Se atom is

replaced by

a Ge atom at one or more corner of the tetrahedron.

As in

I,

^the

peak

^{at 215 cm-’ can} be attributed to

a Se atom bonded to two Ge atoms. Both

peaks

at 175

cm -1

and 290 cm -1 have intensities which are

very low for x ⁼

1/3

and increase with x : we shall consider these two

peaks

^can be attributed the Ge-Ge bond. To support

this,

^{let us} remark that the Raman spectrum ^of

amorphous

^Ge

[14]

^{shows a wide}

band centered at 270 cm -1 with a shoulder around 170 cm-1.

In order to

verify

^the

picture

^we have drawn for

1/3 x 0.42,

^{let us} calculate the

intensity changes

which are

expected

^{as a} function of x for the different Raman

peaks.

^{If a}

good

fit with the measured inten- sities is

obtained,

it will demonstrate at the same time the

consistency

of the model and the

peak

attribution.

Since

only

^the

highest frequency

first order Raman

peaks

^are

considered,

^{we assume that the}

frequency depends only

^{on the local}

configuration,

that is first

neighbour

interactions. The masses of Ge and Se atoms are very close

together

^{and the}

density

^{of the}

glasses

^varies

only

very

slightly

^{with x : we can}

therefore

safely

^{assume that a Ge atom}

occupies

^the

same volume as a Se atom and that this volume does not

depend

^{on x.}

Using

^{all the}

preceding

^conside-

rations,

statistical calculation of the same

type

^{that is} made in 1 shows that the

intensity

^{of the 200 cm-1}

peak

^{must vary as x, the}

intensity

^{of the 215}

^{cm -1} peak

as 1 ^- x and the intensities of the 175

cm -1

and 290 cm -1

peaks

^{as 3 x - 1.}

Practically,

^{a linear}

variation of the intensities is

expected,

^{with a}

positive

or

negative slope,

the absolute values of the different

slopes,

^which

depend

^{on Raman}

scattering

^cross

sections and other uncontrolled

parameters, being insignificant.

^{After a} normalization of the intensities where the 200

cm-1 peak intensity

^is

imposed

^{to be}

proportional

to x, ^as

presented

^on

figure ^3,

^the

experi-

mental intensities of the other

peaks

^are

given

^on

figure 5, together

with the best linear fit

corresponding

to the

proposed

^{model. In}

spite

^{of an} unavoidable

dispersion

^{of the}

experimental

values due to

composi-

tion fluctuations and local order

distortions,

^the

agreement

appears

quite satisfying

^and

justify

^the

proposed

^{model as well as the}

peak labelling.

b)

^{0.43 x 0.50. - As soon as x becomes} greater ^than

0.42,

^{new intense}

peaks

which charac- terize a local structure of the

crystalline

^GeSe

type (Fig. 4),

appear ^at 150

cm -1

and 187

cm -1

while the

peaks

^at

200, 215

^{and 290}

cm-’

which characterize a

local structure of the

crystalline GeSe2 type,

^almost

disappear

^{for x = 0.44.}

Only

^the

peak

^{at 175}

^cm-’

appears all

along

^{and cannot} be considered

by

^itself

FIG. 5. ^- Raman peak intensities for GexSel_x glasses (1/3 x - 0.42) : ^{1 175} cm - ’ ; ib ²¹⁵ cm - ’ ; ^{0 290 cm-1.}

as

characterizing

^one type of local order.

Nevertheless,

let us remark

that,

for x >

0.44,

^the

intensity

^{of the}

peaks

^at

150, 175

^{and 187 cm-’ behave}

similarly

^and

converge towards the

crystalline

^GeSe

spectrum.

^The

175

cm-1 peak

^therefore agrees with both local

configurations

and its behaviour does not

impair

^our

conclusions. The behaviour of the Raman spectra ^{as a} function of x demonstrates the sudden appearance of

a local order of the

crystalline

^GeSe type ^{for x = 0.43} in an

amorphous sample.

^The

question

which arises is to know

why,

^{at the same}

^time,

^the

^peaks

characteriz-

ing

^a local order of the tetrahedral type ^almost

disappear

^{while it seems that there must}

^be,

^{at the}

atomic

scale,

^{a local}

composition corresponding

^to

x ⁼ 0.50

compensated by

^{a local}

composition

^cor-

responding

^{to x =}

0.41,

^{as it is}

proved

^for

partly crystallized samples

^at x > 0.45

(for example,

^{if the}

nominal

composition

^{is x =}

0.44,

^{there must be one} third of the atoms in a GeSe-like

configuration

^with

x ⁼ 0.50 and two third in a tetrahedral like confi-

guration

^{with x =}

0.41).

^{If the}

superposition

^{.of the}

two

corresponding

^Raman spectra ^{is not observed}

experimentally

^{this is due to} the difference in Raman

scattering

^cross sections : as we remarked in the _pre- sentation of

experimental results,

^the

intensity

^{of the}

main

peak

^increases

qualitatively by

^{a factor}

^of

^{3 to 4}

as x goes from 0.42 to 0.50. _{It proves} that the

peaks correspond

^{to the}

crystalline

^GeSe-like

configuration

dominate the spectrum as soon as a notable _propor-

tion of that

configuration

^is present ^{in the}

sample

^and

hide the

peaks corresponding

^to the tetrahedral-like

configuration.

^{One can} wonder whether the GeSe

crystal peaks appearing

^{in Raman}

spectra

of x = 0.43 and x ⁼ 0.44

compounds

^{are not} accountable to GeSe

microcrystals.

Scherrer formula

[15]

^shows

that, provided

the concentration is

high enough (a

^few

at.

0/00

^at

^least),

^rather

large microcrystals (let

^us say

measuring

^{at least one hundred}

A,

^{to fix the}

point)

would

produce sharp peaks

ⁱⁿ

X-ray

diffraction spectra, ^{which is not the case.} Therefore the amount in the

compounds

^{of GeSe}

microcrystals

^{with süch a size}

is _very low

(less

^{than 0.5 at.}

% approximately)

^and

cannot justify

^intense

peaks corresponding

^{to the GeSe}

configuration

^{in Raman}

spectra

^{of x = 0.43 and}

x ⁼ 0.44

compounds :

the intensities of these

peaks

should indeed be at least one hundred times less intense than for _pure

crystalline

GeSe. If the com-

pound

would contain smaller GeSe

microcrystals,

^the

X-ray

diffraction

peaks

would be wider

(up

^{to ten}

degrees approximately

^{for ten}

^A microcrystals).

^All

the theoretical studies

[16-18]

undertaken _up to now

show that the intensities of the

peaks

^{would be} great.

This allows to conclude that the amount of _very little

microcrystals

^{is not} either sufficient to

explain

^the

Raman

peaks.

^{Let us remark at last that}

edge

^effects

probably

^disturb

angles

^and

bondlengths

^{on the}

crystal

boundaries : for this _reason, below 10

À,

^the

concept ^of

crystal

^is

probably meaningless.

Another

question

arises which concerns the number of Ge-Ge bonds in the

glassy

^{material with x 0.42.}

If

only

^{one Ge-Ge bond was}

accepted by

^{each Ge}

atom, the maximum value of x

compatible

^{with the}

tetrahedral-like

configuration

^is

0.40,

and for x ⁼

0.40,

we would have a local structure as

given

ⁱⁿ

figure

^6a.

If two Ge-Ge bonds were

accepted by

^{each Ge} atom,

a tetrahedral-like

configuration

^{would be}

possible

up to x ⁼

0.50,

^{with a local structure of the} _type

given

in

figure

^6b.

However,

^{one can} prepare

glasses

^{with a}

practically

pure tetrahedral-like

configuration only

up ^{to x =} 0.42 while at

higher x

the GeSe-like confi-

guration

appears. It _proves

that, statistically, only one

Ge-Ge bond is

accepted by

^{a Ge atom. Such a result}

shows that the Ge-Se

system

^{is similar to the Ge-S} system where Feltz et al.

[19]

demonstrated from ESCA measurements that all the Ge atoms were

equivalent

in

Geo.40SO.r,,O,

^{which means}

only

^{one Ge-Ge bond}

per Ge atom is

accepted.

FIG. 6. ^- Hypothetic ^local structures of GexSe 1 _ x glasses :

0 Ge atom, 0 Se atom ; ^{a :} Ge2Se3, ^{b : GeSe.}

1498

4. Conclusion. ^- A model of local structure,

supported by

^Raman

scattering

measurements, has been established for

GexSel _x glasses.

^{It is confirmed}

by X-ray

measurements on the

crystallized samples.

In

I,

^we have shown that for 0 x

1/3

the coordi- nation-numbers of

germanium

and selenium atoms

are

respectively equal

^to four and two, the Ge-Ge bonds

being statistically

forbidden. Moreover the

germanium

^{atoms tend to}

part

^{from one another as far}

as

germanium

^content of the mixture allows.

Now,

^we

see

that,

^for

1/3 x 0.42,

^the

glasses

^are constituted of four-coordinated Ge atoms and two-coordinated Se atoms, ^{Se-Se bonds and two} Ge--Ge bonds _per Ge atom

being statistically

forbidden. For x > ^0.43

a GeSe-like local order _appears, with three-coordinat- ed Ge and Se atoms the material

being

^still

completely amorphous.

For x > ^0.45

completely amorphous

material cannot be

prepared

^{and GeSe}

crystallites

appear embedded in a

glassy

^{matrix of}

GexSel _x

^with

x - 0.41. To our

knowledge,

it is the first time that the existence of a three-three coordination is observed in a

chalcogenide glass :

^{it is} obtained in a

quite

^narrow

interval and needs a very fast

quenching

^{rate. This is}

different from the thin film case where a tetrahedric local order has

generally

^been

reported

^for any x value : such a difference suggests ^{that the structure of} the lattice is

probably

^{closer to} that of the

crystal

^of

neighboring composition

ⁱⁿ

glasses

that in thin films.

The results we have

reported

about the three-three coordination are

completely

^new. About this

question

we believe that our present ^{work is to} be considered

only

^{as a}

starting point.

^{It will have to be}

improved

and

developed by

^other

experimental techniques

before the existence of a new type of coordination _may be considered as

definitely

established in

chalcogenides glasses.

Acknowledgments.

^{- We are} very

grateful

^{to the}

CCM

Department

of Centre National d’Etudes des

Télécommunications,

where the

samples

^were pre-

pared

^and

controlled,

^and

particularly

^{to G. Le Roux}

for the

X-ray

measurements, ^{A. M.}

Pougnet

^{for the}

chemical

analysis

^{and C.}

Daguet

for the electron

microprobe

measurements. We are indebted to R.

Beserman,

^{J. F.}

Morhange

^{and M. Massot who}

let us use their Raman set up.

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