Abstract. The change of short and intermediate range orders in a model of amorphous silica
at 500 K in the 0-100 GPa pressure range is investigated using molecular dynamics simulation.
The pressure dependence of the bond length, bond angle and coordination distribution is
analysed in detail. The transformation from tetrahedral to octahedral network structure and
corresponding structural unit transition from SiO4 to SiO6 is found under compression. At
pressure greater then 20 GPa, most of the structural units are SiO6 and silica tend to transform
from an amorphous phase to a crystal phase at high pressure. Moreover, the results also show
that the first peak spliting of Si-Si PRDF under compression is caused by the redistribution of
Si-O-Si bond angles in the process of structural unit transition and the main densified
mechanism of silica is a change of intermadiate order.
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170
JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2017-0046
Mathematical and Physical Sci. 2017, Vol. 62, Iss. 8, pp. 170-175
This paper is available online at
THE SHORT AND INTERMEDIAT RANGE ORDERS IN A MODEL
OF AMORPHOUS SILICA UNDER COMPRESSION
Nguyen Thi Thu Ha and Mai Thi Lan
School of Engineering Physics, Hanoi University of Science and Technology
Abstract. The change of short and intermediate range orders in a model of amorphous silica
at 500 K in the 0-100 GPa pressure range is investigated using molecular dynamics simulation.
The pressure dependence of the bond length, bond angle and coordination distribution is
analysed in detail. The transformation from tetrahedral to octahedral network structure and
corresponding structural unit transition from SiO4 to SiO6 is found under compression. At
pressure greater then 20 GPa, most of the structural units are SiO6 and silica tend to transform
from an amorphous phase to a crystal phase at high pressure. Moreover, the results also show
that the first peak spliting of Si-Si PRDF under compression is caused by the redistribution of
Si-O-Si bond angles in the process of structural unit transition and the main densified
mechanism of silica is a change of intermadiate order.
Keywords: Short and intermediate range orders, phase transition, crystalline, simulation, silica.
1. Introduction
Silica is one of the abundant components of the Earth and is an important application
material. It is a material of common interest in past decades. Recently, polymorphism and
structural transformation under high pressure have been of great importance not only in physics
but also in materials science and geophysics [1, 2]. The experimental works [2-5] show that
coordination of Si atoms increases from 4 to 6 with compression. The results in this work [4]
show that a structural transformation occurs in the 8 - 28 GPa pressure range. At ambient pressure,
the first peaks of Si-Si, Si-O and O-O PRDFs are determined at 3.07 Ǻ, 1.59 and 2.61 Ǻ,
respectively. The first peak position of Si-O PRDF increases from 1.59 Ǻ to 1.64 Ǻ under
compresion up to 28 GPa and it is about 1.66 Ǻ at higher pressure. At 42 GPa, the O-Si-O bond
angle in glass silica is found at about 96
o
which is intermediate between the tetrahedral and
octehedral values of 109.5 and 90
o
, respectively. In the works [2, 5], the authors also investigated
pressure dependence of the Si-O-Si bond angle. The Si-O-Si bond angle is around 145
o
at ambient
pressure and decreaces to 130
o
as compression to 10 GPa [2]. The results in this work [6] also
show that the Si-O-Si bond angle decreases from 146.36
o
to 129.49
o
under compression in the
0 - 12.7 GPa pressure range. In addition, by using molecular dynamic simulation, the authors have
shown that a structural transformation occurs strongly in the 15 - 25 GPa pressure range [7-9].
Received February 17, 2017. Accepted July 29, 2017.
Contact Nguyen Thi Thu Ha, email: hahuy197808@gmail.com
The short and intermediat range orders in a model of amorphous silica under compression
171
In a recent work [6], the model of amorphous silica is constructed using an ab initio method
consisting of 1296 atoms. The authors confirmed that the Si-O bond length increases suddenly in
pressure range occuring structural transformation. This result agrees with the previous work [7]
showing that the Si-O bond lengths be long to SiO4, SiO5 and SiO6 units are different. Moreover,
the work [6] also showed that the first peak position for Si–Si pairs shifts to the left under
compression in the 0-23.8 GPa pressure range. At higher pressure, the first peak position of the
Si–Si pair shifts strongly to the right and the distribution become more broad. The changes of O-
Si-O and Si-O-Si bond angles with compression is also describled in the work [6]. The authors
confirmed that the first peaks of O-Si-O and Si-O-Si bond angle distribution are about 108
o
and
146
o
at zero pressure, respectively. The O-Si-O bond angle distribute around 90
o
while the Si-O-Si
bond angle distribution tends to split into two peaks around 126
o
and 96
o
under compression to
79.8 GPa. In the work [9], the authors also showed the change of Si-O-Si bond angle with
pressure and confirmed that the bond angle distribution decreases under compression and this
relates to the change of intermediate range order structure.
In this paper, the structural transformation of amorphous silica under compression is
investigated. The effect of pressure on short and intermediate range orders will be clarified. A
relation between the structure and the Si-O-Si bond angle distribution as well as the first peak
spliting of Si-Si PRDF is also discussed in detail.
2. Content
2.1. Calculation method
Amorphous silica models at different
pressure (in the pressure range from 0 to
100 GPa) consisting of 1666 Si and 3332 O
atoms are contructed by molecular dynamic
simulation with Beest–Kramer–van Santen
(BKS) potential and periodic boundary
conditions [10, 11]. To integrate the
equation of motion, Verlet algorithm is
used with time steps of 0.478 fs. Initially,
all atoms are placed randomly in a
simulation box and heated to 6000 K to
remove initial configuration. After that the
sample is cooled to 5000, 4000, 3000, 2000,
1000 and finally to 500 K. Next, the sample
is relaxed in an isothermal–isobaric (NPT)
condition for a long time (10
7
time steps) to
get equilibrium state. The structural data of
considered models is determined by
averaging more than 1000 configurations
during the last 10
4
time steps.
2.2. Rerult and discussion
Figure 1 displays pair radial
distribution functions (PRDFs) of Si-Si,
Si-O and O-O pairs. It can be seen that at
ambient pressure, the first peaks of PRDFs
are very sharp. This demonstrates that
Figure 1. Pair radial distribution functions gSi-Si (r),
gO-O (r) and gSi-O at different pressure.
0
2
4
6
2 3 4 5 6 7
0
5
10
15
20
g S
i-S
i(r
)
g O
-O
(r
)
0GPa
15GPa
20GPa
100GPa
g S
i-O
(r
)
r(Å)
0
2
4
6
8
10
Nguyen Thi Thu Ha and Mai Thi Lan
172
short range order structure or local structure in amorphous silica is more ordered at low pressure.
The positions of these peaks are 3.14, 1.60 and 2.60 Ǻ corresponding to Si-Si, Si-O and O-O
PRDFs which is in good agreement with the results in the experimental work [4]. At higher
pressure (15 GPa), the height of the first peaks decreases. This shows that the short range order
structure is significant and dependent on pressure. At pressure greater than 20 GPa, the PRDFs
have many peaks which show the formation of crystal structure. This means that under
compression, amorphous silica tends to transform into a crystaline structure (Figure 2). It can be
seen that network structure tends towards crystalline formation.
To clarify short range order structure, we investigated structural unit transformation and the
change of Si-O length under a pressure of 0-100 GPa. Evolution of structural unit transformation
in silica is presented in Figure 3. It can be seen from Figure 3 that at ambient pressure, the number
of SiO4 structural units with tetrahedral network structure is greater than 95%. Under compression,
there is a transition from a tetrahedral to octahedral network structure corresponding to structural
unit transition from SiO4 to SiO6 via SiO5. This structural unit transition is always accompanied
by the transition from OSi2 to OSi3 linkages as shown in Figure 4. At high pressure (greater than
20 GPa), the number of SiO6 structural units is over 94% and that of OSi3 linkages is over 89%.
This indicates that the coordination number of both Si and O atoms increaces under compression.
Investigating the change in Si-O average bond length of SiOx (x = 4, 5, 6) structural units under
the influence of pressure, we found that the bond lengths are almost unchanged and are
approximately 1.6 Ǻ, 1.64 Ǻ and 1.7 Ǻ corresponding to the SiO4, SiO5 and SiO6 structure units in
the 0-20 GPa pressure range (Figure 5). This data agrees well with the results introdued in work [7]
which showed that at 16 GPa and 300 K, Si-O bond lengths corresponding to SiOx (x = 4, 5, 6)
structural units are 1.6 Ǻ, 1.68 Ǻ and 1.75 Ǻ. Figure 5 also shows that Si-O average bond lengths
of all SiOx structural units decrease slightly under pressure of 20-100 GPa. From figure 5, it can
be seen that the Si-O average bond length of SiO4 is the shortest and that of SiO6 is the longest at
any pressure within the 0 - 100 GPa pressure range. Thus, it can be determined that the Si-O
average bond length increases under compression in the 0-20 GPa pressure range and decreases at
higher pressure. This rule is also observed from the moving of the first peak position of the Si-O
PRDF under compression as describled in Figure 6.
The change of intermediate range order in the model of amorphous silica under compression
was also a topic of interest in this paper. As shown in the analyzed results above, at the low
pressure, most of the structure units are SiO4 and the linkages are OSi2. In this case, we found that
the Si-O-Si bond angle is appoximate 145
o
which agrees with the results shown in [6] that the Si-
Figure 2. Network structure of SiO6 at 15, 20, 40 and 100
GPacoressonding from left to right.
The short and intermediat range orders in a model of amorphous silica under compression
173
O-Si bond angle is appoximately 146
o
at zero pressure. The structural unit transition from SiO4 to
SiO6 is accompanied by the change of linkage from OSi2 to OSi3 under compression. It can be
seenin Figure 7 that at ambient pressure, there is a peak at appoximate 120
o
and a shoulder at
around 95
o
in the Si-O-Si bond angle fraction ditribution of OSi3. This shoulder tends to grow
with increased pressure forming a new peak at 20 GPa. Moreover, Figure 7 also shows that the
positions of two peaks shift to two sides with increasing pressure. At about 20 GPa, the positions
of the two peaks are appoximate 135
o
and 90
o
. This result agrees with the previous work [12]
which found the change of peak position of the Si-O-Si bond angle ditribution from 147° to 135°
under compression in 0-20 GPa pressure range in a silica model at 500 K. However, a formation
of two peaks of Si-O-Si bond angle ditributions was not found with this work. We also observed
that the position of the peaks is almost unchanged at pressures exceeding 20 GPa.
0 20 40 60 80 100
0
20
40
60
80
100
F
ra
c
ti
o
n
o
f
S
iO
x
(%
)
P(GPa)
SiO4
SiO5
SiO6
0 20 40 60 80 100
0
20
40
60
80
100
F
ra
c
ti
o
n
o
f
O
S
i y
(
%
)
P (GPa)
OSi2
OSi3
Figure 3. Distribution of fraction of SiOx
as a function of pressure
Figure 4.Distribution of fraction of OSiy
linkage as a function of pressure
0 20 40 60 80 100
1.3
1.4
1.5
1.6
1.7
P(GPa)
SiO
4
SiO
5
SiO
6
r S
i-
O
(Å
))
1.4 1.5 1.6 1.7 1.8 1.9 2.0
0
5
10
15
20
25
g
S
i-
O
(r
)
r(Å)
0GPa
5GPa
15GPa
20GPa
30GPa
50GPa
100GPa
Figure 5.The Si-O average bond length of
SiOx as a function of pressure
Figure 6. The first peak of pair radial
distribution function gSi-O (r)
The formation of new peaks and a shifting of the peaks of the Si-Si PRDF describled in
Figure 8. The positions of Si atoms replaced with the pressure increasing leads the distance
change of Si-Si atom pairs. We found that at ambient pressure, there is only an initial peak of Si-
Si PRDF at appoximate 3.14 Ǻ. Under compression to 5 GPa, this PRDF appears as a shoulder to
the left of this peak. The shoulder grows with pressure and forms a new peak at a pressure of
about 20 GPa when a new peak of Si-Si PRDF is formed. Specially, Figure 7 also shows that at a
Nguyen Thi Thu Ha and Mai Thi Lan
174
pressure range of 0 - 20 GPa, the initial peak shifts to the right and the new peak shifts to the left.
This shows that there are a pair of Si-Si atoms present corponding to the larger or smaller Si-O-Si
bond angles under compression. It can be seen that the influence of pressure on the distance
between the Si-Si pairs observed in Figure 8 agrees with the rule observed in Figure 7. These
results show that the change of distance of the Si atom pairs is caused by the redistribution of Si-
O-Si bond angles in process of structural unit transition under compression. We also confirmed
that the positions of the two peaks of the Si-Si PRDF are at 2.62 Ǻ and 3.16 Ǻ at 20 GPa and
these peaks tend to shift to the left under higher pressure. This indicates that the distance between
the Si-Si pairs is shorter while the Si-O-Si bond angles are almost unchanged in the 20-100 GPa
pressure range, and that most of the structural units are SiO6 units. This is due to the decrease of
Si-O bond length as pressure increases in the 20 - 100 GPa pressure range.
80 100 120 140 160 180
0.00
0.05
0.10
0.15
0.20
0.25
0.30
OSi
3
0GPa
5GPa
15GPa
20GPa
30GPa
50GPa
100GPa
F
ra
c
ti
o
n
Si-O-Si bond angle (deg)
2.2 2.4 2.6 2.8 3.0 3.2 3.4
0
1
2
3
4
5
6
7
8
0GPa
5GPa
15GPa
20GPa
30GPa
50GPa
100GPa
g
S
i-
S
i(
r)
r(Å)
Figure 7. Si-O-Si bond angle fraction
distribution of OSi3
Figure 8. The first peak spliting
of pair radial distribution function gSi-Si (r)
The change of short and intermediate
range orders under compression leads to
increased density and this is the densification
mechanism in silica. Figure 9 describles the
molar volume dependence on pressure. It can
be seen that the molar volume decreases with
increasing pressure. Namely, the molar volume
is appoximate 24 cm
3
/mol at ambient pressure,
14 cm
3
/mol at 20 GPa and 12 cm
3
/mol at 100
GPa. These results agree with the results
reported in previous works [9, 13] which
showed that the molar volume decreases from
25 cm
3
/mol to 16.5 cm
3
/mol [9] and from 20
cm
3
/mol to 16 cm
3
/mol [13] in the 0-20 GPa
pressure range.
Figure 9.The pressure dependence of
silica molar volume.
0 20 40 60 80 100
10
12
14
16
18
20
22
24
SiO
2
(500 K)
M
o
la
r
v
o
lu
m
e
(
c
m
^
3
/m
o
l)
P(GPa)
The short and intermediat range orders in a model of amorphous silica under compression
175
3. Conclusion
Using molecular dynamics simulation, the short and intermadiate range orders in silica model
at 500 K under pressure of 0 - 100 GPa are clarified. Compression causes the coordination number
of Si atoms to increase from 4 to 6. At pressure greater than 20 GPa, the number of SiO6 structural
units in the model is over 94%. The Si-O average bond length of all SiOx is almost unchanged in
the 0-20 GPapressure range and decreases slightly at higher pressure. Specially, we indicated that
the Si-O average bond length of SiO4 is the shortest and that of SiO6 is the longest in the
investigated pressure range. Moreover, we observed that at pressures greater than 20 GPa, the Si-
Si, Si-O and O-O PRDFs have manysharp peaks which show crystalline formation. Obviously,
amorphous silica tends to transform to a crystal structure under compression. The change in the
structural units leads to the change in the linkages. Most of the OSi2 linkages are replaced by the
OSi3 linkages when under apressure of less than 20 GPa. Additionally, we also confirmed that the
first peak spliting the Si-Si PRDF is caused by the redistribution of Si-O-Si bond angles in the
process of structural unit transition under compression. The change of intermediate order is the
main densified mechanism of silica under compression.
Acknowledgements. This research is funded by Vietnam National Foundation for Science and
Technology Development (NAFOSTED) under grant number: 103.05-2017.306.
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