Computer-generated models of a-SiSe2: evidence for a glass exhibiting medium-range order

Journal of Non-Crystalline Solids 97&98 (1987) 383-386 North-Holland, Amsterdam

COMPUTER-GENERATED EVIDENCE

L.F.

MODELS

FOR A GLASS

GLADDEN

and

383

OF a - S i S e 2 :

EXHIBITING

S.R.

MEDIUM-RANGE

ORDER

ELLIOTT

D e p a r t m e n t of P h y s i c a l C h e m i s t r y , C a m b r i d g e CB2 IEP, U.K.

University

of C a m b r i d g e ,

A new approach to modelling glass structures is presented, with specific application to a-SiSe 2 and the medium-range order exhibited by this system. A systematic investigation of the parameter-space describing possible amorphous structures has been undertaken, and the effect of different structural features on the total pair correlation function studied. This work suggests that the structure of a-SiSe 2 consists of both randomly oriented chains of edge-sharing Si(Sel/2) 4 tetrahedra and cross-linked chain-cluster units. i. INTRODUCTION Glassy order

SiSe 2 is

(MRO)

polyhedra

connect.

comprises

extend

Si2(Sel/2) 8 units as

Raman

'packets'

to

of

Chain-Clusters

contain

the

studies

It

a high

specific

has

way

provide

tetrahedra

polymorph I.

must

to

from

edge-sharing

crystalline system

expected

resulting

also

the

include

spatial

of medium-range

which

evidence

characteristic

beyond

been

of

of

the that

chains

correlations

tetrahedral

the

structural

that

suggested

existence

edge-sharing

degree in

of

between

chains 2

the

glass

low-pressure MRO

in

chains,

and

this

edge-sharing such

Cross-Linked

(CLCCs) 3.

2. RESULTS AND DISCUSSION The

philosophy

construct allowing

behind

completely full

the

modelling

control

over

the

geometry

dihedral angle)

and the development

random chains).

The models,

from

'seeds'

within

model.

specific

T(r) built

regions to

to

from the a

of the

structural

of the model

'seeds'

two

play

three The

of

(e.g.

roles,

total for

0.0105

Si

is

to

structure, units

(e.g.

'flexibility'

of

were generated

firstly

providing

and secondly to introduce

MRO,

chains

scattering

here

glass

and in a random orientation,

two

constituting

calculated

neutron

density

or

other.

was

a

containing about 1200 atoms,

features of

each

(= J(r)/r)

obtained

of

for random chain development,

structural

and

parallel

The

presented

models

which were placed at random,

the

starting points CLCCs

approach

computer-generated

of

e.g.

microcrystallites,

varying

length

running

pair

correlation

function

model

and

to

that

data of Johnson 4. All models

were

each

atoms

density 5 of 3 . 2 5 g c m -3.

0022-3093/87/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

compared

A-3, corresponding

to

a

glass

L.F Gladden, S.R. Elliott / Computer-generated models of a-SiSe 2

384

4

a

3

2

,,---

0

3

..

1 0 -1

0

2

4

6

_

'

'

'

'

10

(~)

r

FIGURE 1 Typical T(r) distributions for models containing (a) random chains only and (b) a pure CRN. The points (+) are experimental data 4 and the solid lines are the calculated T(r) distributions for each type of model. In addition by taking (which

to

the

contains

oxygen

atoms

random

chain based

coordinates only

with

models,

of the Gaskell

corner-sharing

selenium

atoms,

CRN models

and Tarrant

tetrahedra)

and varying

and

the

were produced

model 6 of

a-SiO 2

replacing

Si-Se-Si

the

bond angle

and Si-Se bond length. Energy-relaxation

of all the models

the Keating 7 expression Lennard-Jones relaxation direction force 8.

12-6 potential

was

achieved

A

approximating

by

moving

all

of the force acting on each,

Tests

performed

study are independent complete

results

constructed

of

conclusions

are

associated structures

with

shewed

that

the

study

of will

presented models

respectively.

the be

below.

Clearly,

non-bonded interactions. atoms

simultaneously

conclusions

drawn

The

in

the

to that

from

this

in the relaxation potential.

parameter-space given Fig.

containing

using

with an additional

a distance proportional

of the parameters

discussion

this

was performed

for the local strain energy,

pure

investigated

elsewhere9; l(a)

and

random

only

(b)

show

chain

the CRN description

and

and

the

major

the

T(r)s

pure

CRN

is inappropriate

for a-SiSe 2 since it does not reproduce the feature occurring at - 2 . 8 ~ and

the

third

correlations

peak

in

the

are characteristic

experimental

T(r).

These

real-space

of chains of edge-sharing tetrahedra.

L.F. Gladden, S.R. Elliott / Computer-generated models of a-SiSe 2

-i 0

7"7 ~

385

--X--7 .--~ --7--7 ,-V-V-q--, 'a ''--V-I-q--'

)

D

CLCC(

)

,

<~~

C

b

Random Chain h,

D

<> Parallel Ch~

I~

) FIGURE

2

M o d e l s h o w i n g the b e s t o v e r a l l s t r u c t u r e in T(r). A p p r o x i m a t e l y 15% of t h e a t o m s a r e p a r t of t h e C L C C units. T h e p a r t i c u l a r f e a t u r e s in T(r) a s s o c i a t e d w i t h s p e c i f i c s t r u c t u r a l u n i t s are shown.

L.F. Gladden, S,R. Elliott / Computer-generated models of a-SiSe e

386

The effects

of microcrystallite,

on the calculated T(r)

~ost

indicated

strongly in Fig.

a significant present)

T(r)

were

affected

by

2. To provide

number

embedded

CLCC and parallel

studied systematically. particular

structural

a good fit throughout

of CLCC units

chain

of the

features

are

the 0-~IOA range

(-15% of the total

number

in a random chain matrix are required.

of microcrystallites

structures

The regions

of atoms

Incorporation

comprising more than 15% of the atoms in the glass

were found to give too much structure at high-r in T(r). This

modelling

structure

and

approach

MRO

in

may

other

readily

be

extended

non-crystalline

to

systems,

the

study

e.g.

of

organic

polymeric materials and inorganic covalent glasses. ACKNOWLEDGEMENTS We

thank

Gaskell

and

Dr.

P.H.

Tarrant

Gaskell

model

of

for

supplying

vitreous

The British Petroleum Company p.l.c,

the

silica.

coordinates

L.F.G.

for financial

wishes

of

the

to

thank

and

R.K.

Phys.

Rev.

support.

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M.

Tenhover,

Grasselli, 3. B30

J.E.

R.S.

Henderson,

Solid State Commun.

Griffiths,

(1984)

M.

Malyj,

4. R.W. Johnson,

J. Non-Cryst. S. Susman,

Johnson,

(1986)

G.P.

Hazle

Espinosa,

J.P.

Remeika,

Solids 88

(1986)

366.

41,

7. P.N. Keating, (1974)

M.A.

455.

J. McMillan and K.J. Volin, Mat. Res. Bull.

6. P.H. Gaskell and I.D. Tarrant, 8. M.G.

(1984)

(1982) 1270.

Lukco,

6978.

5. R.W. 21

51

D.

Duffy,

Phys. Rev. B145 D.S.

Boudreaux,

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and D.E.

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265.

J. Non-Cryst.

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9. L.F. Gladden and S.R. Elliott,

(1980)

637.

to be published.

Solids

15