X-ray absorption near edge structure of quartz. Application to the structure of densified silica

(1)

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X-ray absorption near edge structure of quartz.

Application to the structure of densified silica

P. Lagarde, A. Flank, G. Tourillon, R. Liebermann, J. Itie

To cite this version:

P. Lagarde, A. Flank, G. Tourillon, R. Liebermann, J. Itie. X-ray absorption near edge structure of

quartz. Application to the structure of densified silica. Journal de Physique I, EDP Sciences, 1992, 2

(6), pp.1043-1050. �10.1051/jp1:1992113�. �jpa-00246584�

(2)

Classification Physics Abstracts

61.10D 61.40 62.50

X-ray absorption

_near

edge structure of quartz.

Application to the structure of densified silica

P-

Lagarde ii),

A. M- Flank

ii),

G- Tourillon

ii),

R. C. Liebermann

(2)

and J. P- Itie

(3)

II)

LURE (CNRS, CEA, MENJS) Bit. 209d, Centre Universitaire, 91405

Orsay,

France (2) Center for

High

Pressure Research, State

University

of New York,

Stony

Brook, New York

11794, U-S.A.

(3)

Physique

des Milieux Condens£s, Universit£ P. et M. Curie, 4

place

Jussieu, 75230 Paris Cedex 05, France

(Received 20

January

1992, revised and

accepted19

March 1992)

Abstract.

X-ray absorption

spectra of quartz

(10T0),

_pure _silica _and _densified _silica _under

pressure have been recorded at the silicon and oxygen

K-edges.

The spectra ^of quartz show intense

polarization

effects which have been

interpreted

_on the

single scattering approximation,

using geometrical considerations. The

comparison

between the spectra ^of ^silica ^and ^densified silica, _on the basis of this

interpretation, gives

_some information on the structural effect suffered

by

the silica network under _pressure.

Introduction.

X-ray absorption

spectroscopy

(XAS)

has been, and is

still,

_a

powerful

tool for the

understanding

of the local order in

amorphous systems.

^This comes from the

sensitivity

of the Extended

X-ray Absorption

Fine Structure

(EXAFS)

to the very local order due to the 2 k-

dependence

of the

oscillatory signal [I], despite

the fact that the structural disorder becomes often _a very

strong

^limitation to the extraction of _a reliable radial distribution function

[2].

For

instance EXAFS

analysis

in

amorphous

metallic

alloys

has been able _to show the

splitting

of the first coordination shell into two different subshells

[3]

and this accurate

description

^of the local order could not be achieved

by

_any other method _: in that _case EXAFS appears ^to be

short-range

^limited

compared

to

scattering techniques

but at the _same

time,

much _more sensitive to fine details of the local order. When

applied

to covalent

glasses

or

oxides,

this

technique

does not

yield

useful information _: the first shell is well defined because of the

stability

of the valence bond

angles

and it is very close _to its

crystalline

counterpart ; however the distribution of the dihedral

angle

is wide

enough

to eliminate the

signal

from the medium range order.

Nevertheless,

_some

interesting pieces

of information _can be

gained

from _a _more detailed data

analysis

in _some

special

_cases

[4]

or

by taking

into _account the three

body

correlation function

[5],

at the cost of

increasing

the

complexity

^of the calculations.

(3)

1044 JOURNAL DE PHYSIQUE I N° 6

At the _same time it is also well known that the

X-ray

Near

Edge

Structure

(XANES)

part ^of

the spectrum ^contains ^much more information _on the

medium-range

order

[6]:

its

quantitative analysis

is still _a very difficult task which has

only

been

attempted

ⁱⁿ some

selected _cases

[7, 8], primarily crystalline

materials with _a

simple _symmetry

and

closely

_systems.

Among

the _numerous

glasses,

silica is of

particular

interest because of its natural abundance _as well _as its wide range of industrial

applications

nevertheless its structure is still not

fully

elucidated. Since it is _one of the most abundant materials in the earth's

interior,

the behavior of the structure of this

glass

when submitted to

high temperature

^and/or

high

pressure is of a fundamental interest in

geophysics. Finally,

silicon and silicon oxide and suboxides _are present ⁱⁿ all the electronic devices based _on the MOS

technology

_; the _structure of the first oxide

layers

_on the

top

of a silicon substrate

[9],

the structure of silicon suboxides

[10]

and that of different kinds of silicas obtained

through

the

sol-gel

route

[I

I are still the

subject

of fundamental research in materials science

by

XAS-

All of these _reasons make the

understanding

of the

X,ray absorption

of the silicon in an

oxidized environment very valuable. Since the

analysis

of the EXAFS data has _not been able to

give

much _more than the first oxygen shell around the silicon atom, we have focused _our attention to the _near

edge part

^of ^the spectrum with the

goal

of

extracting

_some

signature

_on

the

absorption

spectra ^from ^the

medium-range order,

which could then be used to understand the structure of _an unknown material.

Therefore,

_we started with the

interpretation

of the X- ray

absorption spectra

^of

quartz, keeping

in mind that the

spectra

^of ^the

amorphous

state is sometimes reminiscent of that of the

crystalline

material. As _an

illustration,

the

change

_on the middle range of silica when densified under very

high

_pressure will be

analyzed.

This first

approach

will be

completed by

a full

multiple scattering analysis

which is _now in progress to

quantify

these conclusions.

1.

Experimental techniques.

XANES spectra of quartz, silica and densified silica have been taken at the silicon and the oxygen

K-edges.

In the _case of

quartz, polarized spectra

^have ^been ^recorded ^with ^the ^z-axis ^of the

crystal aligned parallel

_or normal _to the

photon

electric field. The

experiments

^have been

done at

SuperAco

_: silicon spectra ^have ^been ^taken on the SA32 soft

X-ray

beam line

using

the _two

crystal

monochromator

equipped

with

Insb(ill) crystals.

The

experimental

resolution is then of the order of 0-7 eV _; the

absorption signal

is collected

by measuring

the

total electron

yield

with _a channeltron in the

counting

mode while the

incoming

flux is

monitored

by

an ion chamber.

Oxygen spectra

^have ^been ^collected on SA72

using

_a TGM

monochromator in that _case, the

experimental

resolution is of the order of 0-3 eV. Three

samples

have been _run _: _a

quartz (1010)

set normal _to the

photon

^beam ^and rotated

along

_an axis

parallel

to the

photon beam,

_a _pure silica

glass

and _a densified silica. This densified

sample

was

produced

at the

Stony

Brook

High

Pressure

Laboratory compressing

rods of

normal

a-Si02

in _an uniaxial

split-sphere

_apparatus _up to pressures of 16GPa at room

temperature.

^From

density

measurement, ^the densification _can be estimated to about 20 fb- The _two

amorphous samples

_were set at the _same incidence

angle.

2. Results and discussion.

2-1

QUARTZ. Figure

I shows the two silicon _near

edge _spectra

of the

(lfl0) _quartz

_: the

(1010) _plane

is oriented normal _to the

photon

beam and the c-axis has been oriented

parallel

or normal _to the electric field of the

photon.

These _two

spectra

are raw data since the

(4)

1850 1860

E(eV)

Fig. I. Silicon XANES spectra of quartz I do _with _two orientations of the

crystal

_i,ei,ins the

photon

electric field _I

: solid line I

perpendicular

_to the z-axis, _crosses _: I

parallel

to the =-axis.

incoming

flux is monitored _at each

experimental point by

the ion chamber and the

quantity I/Io

^is ^known to be

proportional

to the

absorption

coefficient, While the two white lines

appears ^to ^be identical there _are considerable differences between the two

spectra

ⁱⁿ ^the ^first 40 eV

beyond

the

edge

which

obviously correspond

_to

polarization

effects due _to the electric

dipole

operator

entering

the matrix element

[I]-

Figure

2 shows the _structure of the quartz

(1010) plane

where

only

_one unit cell

along

the direction normal _to this

plane

has been taken into account

depending

on the orientation of

e

Fig.

2. View of the quartz

(ld0) plane.

Silicon atoms are small circles.

Only

one unit cell has been used

along

the x-axis and the

figure

has been rotated

slightly

around the

y-axis

for

clarity.

The z-axis is

horizontal. The labels refer to the text.

(5)

1046 JOURNAL DE

PHYSIQUE

I N° 6

the electric field with

respect

^to ^the ^z-axis we obtain the situations described

by figure

I. The XAS

spectra

^is

govemed by

^Fermi's

golden

rule

p cc

£ )(I)I,r) f))~6(E~-E;-hw)

where

I)

and

f)

_are the initial and final states. Because the matrix element contains the scalar

product

I _r where I is the

photon

electric field and _r the interatomic vector which links the

absorbing

atom and the

neighbors, only

these interatomic _vectors which have _a _non-zero

projection

onto the

photon

electric field will appear on the spectrum. Examination of

figure

2 shows that _:

I)

the first and the second shell must contribute almost with the _same _amount _to the two

polarized

_spectra _: around the silicon _atom A for

instance,

the first shell of four oxygen ^atoms at 1.6

I

_has

a pure tetrahedral symmetry ^and ^the four-second

neighbor

atoms at 3.06

/k

_must

contribute _more _or less

isotropically-

This is in accord with the observation that the

shape

resonance at 865 eV has almost the _same energy and the _same

intensity

for the _two spectra.

From

multiple scattering calculations,

this _resonance has been attributed _to the 9 atoms cluster

composed

^of _one silicon atom, the four oxygen

nearest-neighbors

and the four silicon

next-nearest

neighbors [12]

it)

the main differences in the local environment of _one silicon atom

projected

onto the electric field _comes from _outer shells which _are active in _one orientation and _not in the other

one- The _most obvious effect _comes from the atoms at 4-9

/k

_labelled _B _and _B'

along

the

y-axis

and the atoms A and A' at 5.4

/k along

the z-axis _: _a

large polarization

effect is to be

expected

from this situation since there is _no

intervening

atom between the central and the

scattering

ones. To _a lesser extent the

third-neighbor

_oxygen atoms also

produce

_a

polarization

effect

which appears in

figure

2 _: while the third

neighbors

at

3-6i _(atoms _c)

are distributed

isotropically

around the central _atom

A,

this is not the _case for those _at 3.9

I _{(atoms d)}

or at 4-1

I _{(atoms e)}

; the former will be influential when the electric field is

parallel

_to the z-axis and the latter when the field is

polarized

normal _to the z-axis.

To _a first

approximation,

each group of

scattering

atoms at one

given

distance will

contribute to the

absorption spectrum by

a

shape

resonance. The energy

position

AE of such _a _resonance, when measured from the

edge

_energy, scales with the interatomic distance R _as

[13]

:

AE *R ^~

= constant

Therefore _we _can

interpret

the structures

present

ⁱⁿ ^the ^XANES quartz spectra ⁱⁿ ^the

following

_way :

the _two lines at 1853 and 855 eV

correspond

to the

scattering by

the silicon atoms

along

the _z and the

y-axis respectively,

at 5-4 and 4.9

I,

the two _resonances at 857.4 and 859 eV _are due _to the oxygen third

neighbors,

the

first _one

being

attributed to those atoms at 4.I

I

_while _the _second _feature

comes from the

oxygens ^at 3.9

I- _Using

_this identification for these four _resonances _we obtain the _same value of 843 eV

(within

I eV

accuracy)

for the energy ^of

origin.

This energy is the

low,energy

^limit

of the white

line,

as

previously

indicated the wide

shape

_resonance around 865 eV is due to the Flrst and second

neighbors

of the silicon.

2.2 SILICA AND DENSIFIED SILICA. The two spectra ^of ^silica ^and densified

silica,

in the

same energy range as that of the

quartz,

are shown in

figure

3- These spectra ^have ^been normalized to the _same

edge _step

taken at the maximum of the

shape

rdsonance at 865 eV-

(6)

1850 1860

E(eV) Fig.

3. a) solid line

= silicon XANES spectrum of the pure silica

glass,

b) _crosses

= silicon XANES spectrum ^of ^the densified silica. The two spectra ^have ^been ^normalized at the _same values before the white line and _at 1865 eV-

The _most

significative

differences between them and with the

quartz

spectra are the

following:

I)

in both _cases the white line appears ^at the _same energy as in the

quartz

^but ^{it is}

slightly

wider in silica than in quartz. ^It ^is even wider in the _case of the densified silica

it)

in the range 850-1 860 eV _some _structures _are still present ^which vary from _one silica to the other _one. These

changes

have

already

^been

reported

without any

explanation [12]

iii)

the structure at 1865 eV is still

present,

ⁱⁿ an

equal

_amount, in both

spectra

^and ^it

keeps

_some resemblance with that of the

crystalline

material.

2-3 DiscussioN- The white line

corresponds

to the ls

-

3p

transition. Since quartz ^is an

insulating

material with

4sp3 hybridization,

the first level above the gap is of strong _p character and it is empty,

leading

then _to _a strong

absorption

coefficient. This white line is of the _same

type

as the discrete _resonances due to the

promotion

of _a _core electron into _a low-

lying

empty ^molecular ^orbital ^which ^have been observed in tetrahedral molecules like

SiC14, SiF4

_or

Si(CH~)4 l14]. Therefore,

the short range ^order ^alone

gives

the main character of this final level and the

position

of the white line and its width remain in first

approximation

the

same since the elemental

Si04

unit is considered to remain

unchanged

in

going

from quartz to silica _: full

multiple scattering

calculations of _a 5 atom cluster

[15] composed

of _one central silicon _atom and its four

surrounding

_oxygens show that this white line _can be

explained by

this local environment.

Nevertheless,

the

long

_range order _must determine the exact value of the width of this level and _we expect ^therefore ^the ^width ^of ^the ^silica ^white ^line ^to ^be greater

than that of the

quartz.

^The

change

in the

shape

^but not the energy of this line after densification is therefore _a

signature

of the

broadening

of the

3p

states

following

_a

slight

distorsion of the tetrahedron which _occurs without any

significant change

of the interatomic distances for the Si-O first

neighbors.

This is in line with neutron diffraction results _on the

same

sample

where _no

change

in the Si,O and O-O _mean distances have been detected whereas the width of the distribution of this last distance _was

enlarged [16]-

It could also indicate that the disorder is _even _more

pronounced

in the _case of the densified silica.

That the two

shape

_resonances around 865 eV _are similar in both silica and rather close to that _on quartz appears ^to indicate that the

high-pressure cycle

does _not

affect,

in first

(7)

1048 JOURNAL DE

PHYSIQUE

I N° 6

approximation,

the close environment of the silicon _atoms up ^to the second shell apart ^for ^the

slight changes

which have been discussed above. In

fact,

full

multiple scattering

calculations

[17]

at the silicon

edge

show that this

spectral

feature is almost insensitive _to the Si-O-Si bond

angle

in the

expected

domain of variation of this

angle

_upon

amorphisation

_or densification.

While little

insight

can be obtained from the silicon

K-edge,

_some information _comes from the oxygen

K-edge

shown in

figure

4 _: it is

composed

of _a wide white line at 540 eV followed

by

_a

shape

_resonance at about 560 eV which

slightly

shifts to

high energies

in the _case of the

densified silica. XANES calculations

[17]

have attributed this _resonance _to the Si-Si second

neighbor

distance related to the Si-O-Si bond

angle

_an increase of this

angle

leads _to _an increase of the energy

splitting

between this _resonance and the main white line. Therefore the variation _we observe in the densified silica _as

interpreted

on the basis of this

calculation,

seems not in agreement ^with ^other ^results

[18],

which conclude _to _a decrease of the Si-O-Si

angle,

but definite conclusions from XAS need _more _accurate

experimental

and theoretical results.

a

a Si02

b

ji

_b _densfied _Si02

§

d

z a

o

#

~ o b

~i

535 540 545 560 565 570 575 580

ENERGY (ell~

Fig.

4.

Oxygen K-edge

XANES of silica and densified silica. Notice the shift in energy of the

resonance at 560 eV.

The main modifications between the

crystal

and the

glass,

and between the two

glasses,

appear in the XAS spectra ^between 1850 and 1860 eV and thus _comes from interatomic

distances

beyond

the third shell. In order _to

clarify

this

point,

_we have shown in

figure

5 the difference

spectra

:

figure 5,

_curve

a)

is the difference between the quartz spectrum ⁱⁿ ^the

parallel

^and

perpendicular

orientations

(Fig. I)

while

figure 5,

_curve

b)

shows the difference

between silica and densified silica after

being

normalized to the _same

edge step (Fig. 3)-

In

this latter _case,

only

the

part

^between ¹⁸⁵⁰ ^and ¹⁸⁷⁰ ^eV ^has ^been

plotted

because the

difference between the two white line

yields meaningless

values in the range 840 to

848 eV. Two main features appear in

figure 5,

_curve

a)

at 853 and 859 eV _we attribute them to the structural differences described

previously

_on ^the

(1010) _plane

_{of the} _quartz _: _the

first _one _comes from the silicon atoms at 5,4 and 4.9

I _along

_the

z and

y-axis respectively

while the second feature _comes from oxygen ^third

neighbors.

^Without calculation of the XAS spectrum, ^it ^is ^not

possible

to be _more

specific, Figure 5,

curve

b)

shows the first

spectral

(8)

1858 1866 E(eV)

Fig.

5. Differences between the normalized silicon XANES spectra : curve a) crosses quartz

parallel

to the z-axis minus quartz ^normal to the z-axis. Curve b) solid line _: pure silica minus densified silica.

Only

the energy

region beyond

the white line has been

plotted.

feature and

perhaps

_some hint of the second _one _; this

implies

that the oxygen environment of the silicon at around 4

/k

_is

only slightly changed

_upon densification and that the

principal

modification of the structure at this

mid-range

interatomic distance affects the Si-Si _at about 5

/k-

This suggest ^that ^the ^silicon atoms at about 5-4

I _(which

_exist

only

for 6-members of

larger rings),

_move to _a

position

similar to the _atoms situated _{in the xy}

plane

of

quartz

at 4-9

I la position

which _can

only

be obtained for smaller

rings).

This result could be _an indication for

a

change

in the

rings

statistic upon

densification,

with _an increase of the concentration of low- membered

rings,

in

good agreement

^with the data obtained from neutron

scattering

results _on the _same

sample [16],

Conclusions.

A

comparison

between XAS

spectra

^of quartz ⁱⁿ ^two

polarisations,

of normal silica and densified silica _at both the silicon and the oxygen

K-edges

has led _us to the

following

conclusions _:

the XANES structures of

quartz

have been attributed to

specific

interatomic

distances,

Si-O _at 3.9 and 4, I

I,

_Si-Si _at _4.9 _and _5,4

I.

_These _results _have _to _be _confirmed

_by

a

multiple scattering approach

;

the white line, due _to the

Si04

tetrahedron, is

unchanged

from the

crystalline

to the

amorphous

state but is

slightly

wider upon

densification,

_an indication for _a

slight

distorsion of this

Si04

unit under pressure ;

the oxygen

K-edge

shows _a small

displacement

of the _resonance at 560 eV _to

high

energies

_upon

densification,

difficult to

interpret

in _terms of the Si-O-Si bond

angle

variation _; in the Si

spectrum

of normal

silica,

there exists _a

signature

from Si,Si

resulting

from

large-membered rings.

This interatomic distance of 5.4

I

_decreases _in

compression

to a value close _to that

present

ⁱⁿ ^the

crystal along

the xy

plane (I,e.

4.9

h).

(9)

1050 JOURNAL DE PHYSIQUE I N° 6

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