NOC-15070; No of Pages 5
Journal of Non-Crystalline Solids xxx (2010) xxx–xxx
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / j n o n c r y s o l
Structural investigation of iron phosphate glasses using molecular
dynamics simulation
Bushra Al-Hasni ⁎, Gavin Mountjoy
School of Physical Sciences, University of Kent, Canterbury, CT2 7NH, UK
a r t i c l e
i n f o
Article history:
Received 15 September 2010
Received in revised form 9 October 2010
Available online xxxx
Keywords:
Iron phosphate glasses;
Molecular dynamics simulation;
Oxide glasses
a b s t r a c t
The current study reports the first molecular dynamics models of iron phosphate glasses. Models were
made for xFeO–(100 − x)P2O5 glasses with x = 30, 40 and 50 and for xFe2O3–(100 − x)P2O5 glasses with
x = 30 and 40. This study also looks at the effects of mixed Fe2+/Fe3+ contents. The models are in good
agreement with experimental results for nearest-neighbour distances and coordination numbers, and in
reasonable agreement with X-ray and neutron diffraction structure factors. As expected the models contain
a tetrahedral phosphate network with P–O distances of 1.50 ± 0.01 Å. The network connectivity is
dominated by the expected Qn (where n is the number of bridging oxygen) corresponding to the O:P ratio.
These are average Qn of 2.3 for 40FeO and 1.0 for 40Fe2O3 glasses respectively. Interestingly a small amount
of non-network oxygen is found to be present in the 40Fe2O3 glass model. The Fe–O coordination is close to
4.5 in both FeO and Fe2O3 glass models, with Fe–O bond lengths of 2.12 Å and 1.89 Å respectively. The
greater durability of xFe2O3–(100 − x)P2O5 glasses can be attributed to the lower content of P–O–P bonds
and higher bond valence across Fe–O–P bonds. For 40Fe2O3 glass the Fe–Fe correlation shows a main peak
at 5–6 Å in good agreement with the result from magnetic scattering which was interpreted in terms of
speromagnetic order.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Historically, the applications of phosphate glasses have been
limited because of their poor chemical durability [1]. This is attributed
to the abundance of easily hydrated P–O–P bonds [2]. However,
several studies have shown that the addition of iron oxides [2] to
phosphate glasses replaces the P–O–P bonds by more chemically
durable P–O–Fe bonds. The excellent chemical durability of iron
phosphate glass has opened a wide area of potential applications such
as to produce effective laser materials and for vitrification of high level
nuclear waste [3,4]. Both Fe2+ and Fe3+ ions are present in iron
phosphate glasses regardless of the oxidation state in the batch
composition [5,6]. Information about local atomic environment of Fe
in phosphate glasses should assist in understanding the roles of these
two oxidation states [7].
The structure of phosphate glasses has been well studied by Van
Wazer [8], Martin [9] and Brow [10]. It is based on corner-sharing PO4
tetrahedra which may form chains, rings, dimers or isolated PO4
groups. The redox equilibrium and crystallization characteristics of
iron phosphate glasses and glasses containing nuclear waste constituents have been investigated using several techniques such as
⁎ Corresponding author. Tel.: +441227823226.
E-mail address: [email protected] (B. Al-Hasni).
Mossbauer spectroscopy, X-ray absorption spectroscopy, X-ray
diffraction and neutron diffraction [11–14].
During the last two decades, many efforts have been made to
extract more information about phosphate glasses. The structure of
40Fe2O3–60P2O5 glasses has received more attention for their ability
to accommodate large amounts of certain nuclear wastes [11]. In
addition this composition has the highest chemical durability of the
binary phosphate glasses. The high durability of such glasses has
previously been attributed to rigid octahedral Fe3+ sites which link
the short phosphate chains and block migration of other cations [1,2].
In the present work, the local environment around Fe ions in
several iron phosphate glasses has been studied using molecular
dynamics simulation. There is not any previous study in modelling of
iron phosphate glasses. One objective of the present work was to
produce models which agreed with experimental data. In addition,
this study tried to examine the different roles which are played by
Fe2+ or Fe3+ ions.
2. Molecular dynamics method
Molecular dynamics has been used to model xFeO (100 − x) P2O5
glasses with x = 30, 40 and 50 and xFe2O3 (100 − x) P2O5 glasses with
x = 30 and 40. P–O, Fe–O and O–O interactions were described using
rigid ion potentials with Coulomb potential as this allows a large
number of time-steps (necessary for modelling glasses). The
parameters were taken from those derived by Teter [15], which
0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jnoncrysol.2010.10.010
Please cite this article as: B. Al-Hasni, G. Mountjoy, Structural investigation of iron phosphate glasses using molecular dynamics simulation,
J. Non-Cryst. Solids (2010), doi:10.1016/j.jnoncrysol.2010.10.010
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have been effective for modelling phosphate [16], aluminate [17] and
silicate glasses [18] as well. A Buckingham potential is used, as given
in Eq. (1).
Table 2
Details of model compositions.
Model
Fe2O3
(Fe3+)
FeO
(Fe2+)
P2O5
density
g/cm3
O:P ratio
Predicted
P–P CN
30FeO
30FeO/Fe3+
40FeO
40FeO/Fe3+
50FeO
50FeO/Fe3+
30Fe2O3
30Fe2O3/Fe2+
40Fe2O3
40Fe2O3/Fe2+
0
3
0
4
0
5
30
24
40
32
30
24
40
32
50
40
0
12
0
16
70
70
60
60
50
50
70
70
60
60
2.63
2.63
2.72
2.72
2.84
2.84
2.96
2.96
3.04
3.04
2.71
2.74
2.83
2.87
3.00
3.05
3.14
3.10
3.50
3.40
2.58
2.52
2.34
2.26
2.00
1.90
1.72
1.80
1.00
1.20
!
Vij ðr Þ =
qi qj
Cij
−r
+ Aij exp
− 6
ρij
4πε0 r
r
ð1Þ
where Vij is the potential between atoms i and j with separation r, qi is the
effective charge, Aij, ρij and Cij are the potential parameters which are
given in Table 1, and ε0 is a constant equal to 8.85 × 10− 12C2N− 1 m− 2.
In addition, potential parameters for O–P–O and P–O–P bond
bending interactions were taken from [16], and have the form
V iji ðθÞ =
2
1 kiji θ−Θiji
2
ð2Þ
where j is the element type of the central atom, kiji = 3.5 eV and
Θiji = 109.47° for O–P–O, and kiji = 3.0 eV and Θiji = 135.5 for P–O–P.
Each model has a total of ~ 1500 atoms in a cubic box with length
~ 27 Å. Random starting configurations and periodic boundary
conditions were used. The modelling used the DLPOLY program
[19] with time-step of 1 fs. A Berendesn NVT algorithm was used
with a relaxation time of 2 ps. A short-range cut-off of 10 Å was used
for all non-Coulombic parts of the pair potentials. The Coulomb
potential was calculated by using the Ewald sum method with a
precision of 10− 5.
The modelling used six stages. In the first three stages, the system
was equilibrated at temperatures 6000, 3000, and 1500 K, with linear
thermal expansion coefficients of 1.03. 1.015 and 1.005, respectively.
The fourth stage was a temperature quench of 60,000 time-steps from
1500 to 300 K. The quench rate during modelling is 1013 Ks− 1, which
is typical for molecular dynamics simulations of glasses [16–18]. The
final two stages were temperature baths of 40,000 time steps each at
300 K (with and without equilibration, respectively). During the final
stage, the structural parameters were sampled (every 200 time-steps)
to include disorder due to thermal vibrations, which is present in the
experimental results.
Mossbauer spectroscopy and X-ray diffraction studies [20,21] of
FeO–P2O5 glasses indicated that about 5–18% of the Fe ions are Fe3+
ions and the remainder are Fe2+ ions. Parallel to that, it has been
reported [5] that Fe2O3–P2O5 glasses contain two valance states of
approximately 80% Fe3+ and 20% Fe2+. In order to investigate the
effect of the presence of these two valance states, two models have
been made for each composition. One of the models contains 100% of
Fe in one valence state, and the other model contains a minority of 20%
of Fe in the alternate valence state. Details of the model compositions
and densities are given in Table 2. Table 2 also shows the predicted
depolymerisation of the phosphate network. This is represented by the
predicted P–P coordination number (CN) calculated as 8 − 2x O:P ratio
(assuming P has CN = 4 and all oxygens are bonded to phosphorous).
3. Results
Fig. 1 shows images for 40FeO and 40Fe2O3 glasses. The pink
tetrahedral are the PO4 network, the blue spheres are the nonnetwork oxygens (i.e. not bonded to phosphorous), while FeOn
polyhedrons are grey.
Table 1
Potential parameters for two-body interactions used in the current study.
i–j
2+
Fe –O
Fe3+–O
O–O
P–O
qi (e)
Aij(eV)
ρij (Å)
Cij(eV Å− 6)
1.2
1.8
− 1.2
3.0
11,777
19,952
1844
27,722
0.2071
0.1825
0.3436
0.1819
21.642
4.6583
192.58
86.860
The short range order in models can be described using the partial
radial distribution function Tij (r) which is
Tij ðr Þ =
!
Nj
1 1 Ni
∑ ∑ δðr−Rlm Þ = 4πρj rgij ðr Þ
r Ni l = 1 m≠l
ð3Þ
where gij (r) is the partial pair distribution function with gij (r) → 1 as
r → ∞, ρj is the atomic number density of element type j, and ℓ and m
are the indexes over atoms of element type i and j. The functions Tij(r)
are shown in Fig. 2 for the 40FeO and 40Fe2O3 glasses. This clearly
shows the different Fe–O bond lengths of Fe2+ or Fe3+ ions as expected.
The first nearest neighbour peak in Tij(r) is for the P–O correlation.
Oxygen atoms can be classified as having two bonds to P (denoted
bridging or OB) or one bond to P (denoted non-bridging or ONB). The
P–O distance is an average of the P–ONB and P–OB bonds in PO4
tetrahedra. Due to the connectivity between PO4 tetrahedra in 40FeO
glass, the P–O bonds can be distinguished as two distances of 1.46 Å
for P–ONB and 1.53 Å for P–OB as is clear in Fig. 2. These bonds differ by
0.07 Å, whereas differences of ~ 0.1 Å have been reported experimentally [23]. The mean P–O distance is found to be at 1.50 Å. The 40FeO
and 40Fe2O3 models have 98.1% P with four-fold coordination. The
remaining 1.9% of P has five-fold coordination, and this should be
considered as a defect arising from slight inaccuracy in the potentials.
TOO(r) has a first peak at 2.47 Å due to O–P–O configurations with a
coordination number of 4.3 in 40FeO, however, this coordination is
reduced slightly to 3.7 in 40Fe2O3 glass. In comparison, diffraction
studies of 40Fe2O3 glass [12,22] have reported O–O coordination of
5.0 ± 0.5 at a distance of 2.50 ± 0.02 Å. The lower in O–O coordination
in 40Fe2O3 model compared to 40FeO model is due to depolymerisation of the phosphate network when the O:P ratio increases (see
Table 2). TPP (r) has a narrow peak at around 2.97 Å as expected, due
to connected PO4 tetrahedra. Again this peak is reduced due to lower
connectivity in the 40Fe2O3 model.
The most distinctive feature that characterizes each structure of
these glasses is the Fe–O distance. As expected the Fe2+–O distance at
R = 2.12 Å is longer than the Fe3+–O distance which is at 1.89 Å as
seen clearly in the second peak in Tij(r). These distances compare
favourably with distances of 2.12 ± 0.02 Å and 1.94 ± 0.01 Å reported
by Hoppe et al. [21] and Wright et al. [22] for 40FeO and 40Fe2O3
glasses respectively.
Table 3 presents the details of mean Fe2+ and Fe3+ coordination
numbers (CN). They are approximately 4.5 in both 40FeO and 40Fe2O3
models which is slightly lower than the experimental values, 5.0 ± 0.5,
that have been reported by Hoppe et al. [21] and Wright et al. [22] for
these glasses respectively. In order to consider the difference between
this modelling study and diffraction results for coordination numbers
it should be remembered that an Fe–O cut-off distance of 2.72 Å is used
to calculate the number of surrounding oxygens in the models.
The last peak in Tij(r) represents the Fe–P correlation which has
reported at 3.30 Å by Hoppe et al. [21]. This distance is in good
agreement with the results for 40Fe2O3 glass model whereas the
Please cite this article as: B. Al-Hasni, G. Mountjoy, Structural investigation of iron phosphate glasses using molecular dynamics simulation,
J. Non-Cryst. Solids (2010), doi:10.1016/j.jnoncrysol.2010.10.010
B. Al-Hasni, G. Mountjoy / Journal of Non-Crystalline Solids xxx (2010) xxx–xxx
3
Fig. 1. Images of 40FeO glass model to the left and 40Fe2O3 glass model to the right. Larger spheres are "non-network" oxygen which are not bonded to P.
distance is slightly longer at R = 3.46 Å in the 40FeO glass model due
to the longer Fe2+–O bond length.
The bond angle distributions for 40FeO and 40Fe2O3 glass models
are shown in Fig. 3, where the O–P–O bond angle has a well defined
peak at 110°, and P–O–P bond angle has a broad distribution around
151°. (The slight peaks at ~80° and ~125° are defects due to the small
proportion of P with CN of 5). The P–O–P peak shows a decrease in the
connectivity as the amount of Fe is increased. This comes as a result of
increasing the number of non-bridging oxygen when adding more
iron oxide. This can be seen by looking at the O:P ratio and predicted
P–P CN in Table 2. Moreover, the presence of 20% of Fe3+ in FeO–P2O5
glasses decreases the connectivity between PO4 tetrahedra because
the Fe3+ introduces more oxygens.
The connectivity of the phosphate network can be described by the
P–P coordination number (CN) and the Qn distribution as shown in
Table 4 (each P is classified as Qn, where n is equal to the number of
OB). According to the model results 40FeO and 40Fe2O3 are dominated
by Q2 and Q1 respectively. Theoretically this is expected according to
the predicted P–P CN of 2.34 and 1.00 respectively shown in Table 2.
From Table 4 it can be seen that Fe2O3 glasses are more depolymerised
than FeO glasses. Also, the presence of Fe3+ in FeO glasses leads to
slightly more depolymerisation, whereas the presence of Fe2+ in
Fe2O3 glasses leads to slightly less depolymerisation.
The change in the radial distribution function Tij(r) according to
composition x, gives changes in the diffraction structure factor S(Q)
where
Q ðSðQ Þ−1Þ = ∑ wij ðQ Þ∫ Tij ðr Þ−4πrρj sinðQr Þdr
ð4Þ
ij
where Q is the scattering vector, and wij(Q) is the weighting factor for
scattering from the correlations between elements i and j [24]. The
35
PO
OO
PP
FeO
FeP
FeFe
PO
OO
PP
FeO
FeP
FeFe
30
40FeO
Tij ( r )
25
20
40Fe2O3
Fe3+-O
Fe2+-O
15
5
0
1.5
2
2.5
3
3.5
r (Å)
Fig. 2. Radial distribution functions for 40FeO and 40Fe2O3 glass models.
Q
T ðr Þ = 4πρr +
2 max
∫ MðQ Þ⋅Q ⋅ðSðQ Þ−1Þ⋅sinðQrÞ⋅dQ
πQ
ð5Þ
min
It is important to mention that the model T (r) has been obtained
by Fourier transform using Eq. (5), and it has some ripples or small
oscillations even before the first peak as it due to the window
function. The experimental T(r) from X-ray diffraction is characterized
by five main peaks, for P–O, Fe–O, O–O, P–P and Fe–P respectively as
illustrated in Fig. 5. There is a clear decrease of P–O peak intensity and
an increase in Fe–O peak intensity as FeO content increases [21]. The
model results show the same sequence of peaks and the same trends
as FeO content increases, although there are some differences in the
positions and widths of individual peaks.
The models of 30Fe2O3 and 40Fe2O3 glasses show reasonable
agreement with the neutron diffraction S(Q) measured by Wright et
al. [22], and with neutron diffraction and X-ray diffraction S(Q) (but
with small Q range) measured by Karabulut et al. [12], as shown in
Fig. 6.
The neutron data were Fourier transformed to yield the real space
total correlation function T (r) as given in Fig. 7. As expected, three
peaks characterize the total correlation function, P–O, Fe–O and O–O at
1.50 Å, 1.89 Å and 2.47 Å respectively. Again the model results show the
same sequence of peaks and trends as Fe2O3 content increases.
Table 3
Details of mean Fe–O distances and coordination numbers (CN) in models and from
experiments [21,22].
10
1
comparison between X-ray diffraction structure factor measured by
Hoppe et al [21] with the current models is given in Fig. 4 for 30FeO,
40FeO and 50FeO glasses. The model S(Q) are in reasonable agreement with the experimental S(Q). However, it is noticeable that
the experimental S(Q) have peaks with slightly different amplitude
and positions comparison to the model, particularly in the region from
4 to 6 A− 1.
The structure factor S(Q) reveals structural information via a
Fourier transform using a Lorch window function M(Q) to obtain the
total pair correlation function, as in Eq. (5).
4
Model
Fe3+–O
CN
Fe3+–O
distance
Fe2+–O
CN
Fe2+–O
distance
Expt Fe–O
CN
Expt Fe–O
distance
30FeO
30FeO/Fe3+
40FeO
40FeO/Fe3+
50FeO
50FeO/Fe3+
30Fe2O3
30Fe2O3/Fe2+
40Fe2O3
40Fe2O3/Fe2+
–
4.9
–
4.6
–
4.5
4.7
4.7
4.5
4.6
–
1.94
–
1.90
–
1.91
1.89
1.90
1.89
1.90
4.5
4.4
4.5
4.4
4.5
4.5
–
4.3
–
4.5
2.15
2.12
2.09
2.10
2.12
2.11
–
2.08
–
2.10
5
–
5
–
5
–
6
–
5
–
2.12
–
2.12
–
2.10
–
1.96
–
1.94
–
Please cite this article as: B. Al-Hasni, G. Mountjoy, Structural investigation of iron phosphate glasses using molecular dynamics simulation,
J. Non-Cryst. Solids (2010), doi:10.1016/j.jnoncrysol.2010.10.010
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B. Al-Hasni, G. Mountjoy / Journal of Non-Crystalline Solids xxx (2010) xxx–xxx
0.7
0.03
P-O-P
3.00
40FeO
40FeO/Fe3+
O-P-O
2.50
0.025
50FeO
40Fe2O3
0.5
2.00
40Fe2O3/Fe2+
0.02
1.50
40FeO
0.4
40FeO/Fe3+
0.015
40Fe2O3
0.3
40Fe2O3/Fe2+
S(Q )-1
Bond angle disturbution
0.6
40FeO
1.00
0.50
0.01
30FeO
0.2
0.00
0.005
0.1
-0.50
0
0
70
90
110
130
Bond angle (
150
o
-1.00
0.00
170
2.00
4.00
6.00
8.00
10.00
12.00
14.00
Q (1/ Å )
)
Fig. 3. O–P–O and P–O–P bond angle distributions for 40FeO and 40Fe2O3 glass models.
Fig. 4. X-ray diffraction structure factors S(Q) for FeO glasses, the dashed lines represent
the experimental results, while the solid lines are model results, successive curves are
displaced upward by 1.0 for clarity.
4. Discussion
coordination of 4, and all oxygen is bonded to P, i.e. it is either OB or
ONB. For FeO glasses the observed P–P CN is between 99% and 104% of
the expected P–P CN. (The slight excess is due to some defects in the
form of P with CN of 5.) For Fe2O3 glasses the observed P–P CN is
between 108% and 133% of the expected P–P CN. This excess is
possible if some oxygen are not bonded to P and are instead bonded
only to Fe, and since such oxygen are neither OB nor ONB we use the
term “non-network” oxygen. Inspection of the 30Fe2O3 and 40Fe2O3
glass models showed 1.0% (12 of 1100) and 4.2% (44 of 1050) of
oxygens present as non-network oxygens, and some of these are
illustrated in Fig. 1. Non-network oxygens in the models are found to
be bonded to 2.7 Fe3+ ions on average. In comparison, in alpha and
gamma Fe2O3 (respectively) oxygens are bonded to 4.0 and 3.5 Fe3+
ions on average. 31P or 17O NMR would be useful for clarifying
the connectivity of the phosphate network, but unfortunately it is
not possible for iron phosphate glasses due to the magnetic properties of Fe.
Lastly it is of interest to look at the Fe–Fe correlation in the
40Fe2O3 glass model in relation to magnetic scattering studies. One
of the few measurements of magnetic neutron scattering in a glass
was carried out on a sample of 44Fe2O3 56P2O5 glass [25]. The
measured magnetic (i.e. Fe spin) pair distribution function is
reproduced in Fig. 8, and indicates a prominent anti-parallel Fe
spin correlation at a distance of ~ 5.8 Å. This is interpreted as evidence of speromagnetic order in the glass. Fig. 8 shows promising
19.00
50FeO
14.00
T(r)
One aim of the present study was to examine the differences
between Fe2+ and Fe3+ ion sites in iron phosphate glasses. In glasses
with only one Fe valence state present, both Fe2+ and Fe3+ ions tend
to have coordination of around 4.5 but there is a clear difference in Fe–
O bond length as expected. We also studied glasses where there is a
small amount of the alternate valence (e.g. a small amount of Fe3+ in
FeO glasses or a small amount of Fe2+ in Fe2O3 glasses). The results in
Table 3 show that minority Fe2+ and Fe3+ ions tend to keep their
characteristic coordination, and to not have a significant impact on the
majority ions. The Fe–O coordination is slightly affected by the O:P
ratio, showing a slight decrease as the O:P ratio increases (see
Table 4). Although the O:P ratio is increasing, the ratio of ONB to Fe is
actually decreasing. This effect was previously discussed by Hoppe et
al. [21] and can cause cation coordination to decrease.
Another aim of the present study was to look for structural features which might relate to the remarkable chemical durability of iron
phosphate glasses. For example iron pyrophosphate glasses are 1000×
more durable than metaphosphate glasses [10]. There are two
beneficial effects from adding Fe ions to the glass. These effects are
(i) to depolymerise the network and hence remove easily hydrolized
P–O–P bonds, and (ii) to increase the number of stronger Fe–O–P
bonds. Fe2O3 glasses should be stronger than FeO glasses because
(i) they are more depolymerised (see P–P CN in Tables 2 and 4) and
(ii) they have stronger Fe3+–O–P bonds. The latter occurs because Fe
coordination is approximately 4.5 in both FeO and Fe2O3 glasses, and
this gives a higher bond valence of 0.67 for Fe3+–O bonds in Fe2O3
glasses compared to a bond valence of 0.44 for Fe2+–O bonds in FeO
glasses.
One interesting feature found in Fe2O3 glass models is the presence of some “non-network” oxygen. This is apparent when comparing the observed P–P CN from Table 4 with the predicted P–P CN
from Table 2. The latter was calculated assuming that all P has
Table 4
Details of Qn distribution in the models.
Model
Observed P–P CN
Q0%
Q1%
Q2%
Q3%
Q4%
30FeO
30FeO/Fe3+
40FeO
40FeO/Fe3+
50FeO
50FeO/Fe3+
30Fe2O3
30Fe2O3/Fe2+
40Fe2O3
40Fe2O3/Fe2+
2.69
2.50
2.38
2.33
2.06
1.97
1.86
1.91
1.33
1.36
–
–
0.3
–
2
3
5
3
14
13
4
5
10
11
20
26
31
30
46
44
37
39
47
47
53
47
41
42
32
37
47
46
38
33
22
22
20
19
6
4
8
7
4
4
3
1
2
4
1
1
40FeO
9.00
30FeO
4.00
-1.00
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
r(Å)
Fig. 5. Total correlation function T(r) from X-ray diffraction for FeO glasses, the dashed
lines represent the experimental results, while the solid lines are model results,
successive curves are displaced upward by 6 for clarity.
Please cite this article as: B. Al-Hasni, G. Mountjoy, Structural investigation of iron phosphate glasses using molecular dynamics simulation,
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B. Al-Hasni, G. Mountjoy / Journal of Non-Crystalline Solids xxx (2010) xxx–xxx
5
-6
1.5
2.00
40FeO
S(Q)-1
40Fe2O3 (ND)
0.50
30Fe2O3 (ND
0.00
-0.50
-1.00
0.00
-4
0.5
-2
0
2
-0.5
2.00
4.00
6.00
8.00
10.00
12.00
CN Fe-Fe
TFe-Fe(r) from MD models
40Fe2O3 (XRD)
1.00
1
14.00
Q ( 1/Å)
Fig. 6. Neutron and X-ray diffraction structure factors S(Q) for Fe2O3 glasses, the dashed
lines represent the experimental results, while the solid lines are model results,
successive curves are displaced upward by 0.5 for clarity.
10
8 3
6
4
2
0
3
4
5
6
7
4
5
6
7
8
9
0
10
2
DM(r) from Wright et al (2008)
40Fe2O3
1.50
4
-1
r (A)
Fig. 8. Comparison of TFe–Fe(r) in 40FeO and 40Fe2O3 models with the measured
magnetic pair correlation function DM(r) [22]. The inset shows cumulative coordination
numbers for the TFe–Fe(r).
9.00
8.00
60 P2O5 glass can be attributed to the low content of P–O–P bonds and
high bond valence in Fe–O–P bonds. The Fe–Fe correlation in the 40
Fe2O3 glass model shows good agreement with the previous magnetic
scattering observation of speromagnetic order from Wright et al. [25].
40Fe2O3 (ND)
7.00
6.00
T(r)
5.00
30Fe2O3 (ND)
4.00
Acknowledgements
3.00
2.00
We are grateful for Hoppe et al., Wright et al. and Karabulut et al.
for providing X-ray and neutron diffraction data. Thanks go also to the
Ministry of Higher Education in Oman for funding this project.
1.00
0.00
-1.00
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
References
r(Å)
Fig. 7. Total correlation function T(r) from neutron diffraction for Fe2O3 glasses, the
dashed lines represent the experimental results, while the solid lines are model results,
successive curves are displaced upward by 3 for clarity.
agreement between the measurement and the Fe–Fe correlation in
the 40Fe2O3 glass model (in comparison there are only half as many
Fe–Fe correlations in the 40FeO glass model).
5. Conclusion
The glass models presented here are a valuable addition to experimental studies of iron phosphate glasses. In particular, the inclusion
of both Fe2+ and Fe3+ ions is important for better understanding
the roles of these ions. The short range order in the glass models is
realistic, with a well-defined phosphate network, and Fe2+ and Fe3+
sites with notably different bond lengths of 2.12 Å and 1.89 Å
respectively. Good agreement is found between the short range
order in the models and results from neutron and X-ray diffraction,
including reasonable agreement with structure factors. Both Fe2+ and
Fe3+ ions are found to have a typical coordination of ~ 4.5. The xFeO–
(1 − x)P2O5 and xFe2O3–(1 − x) P2O5 glasses have different O:P ratios,
and hence different connectivities. In fact, the observed P–P CN is 2.3
for the 40FeO glass model and is 1.2 for the 40 Fe2O3 glass model.
Interestingly, the latter is higher than predicted due to the presence of
~ 4% of non-network oxygens. The remarkable durability of 40 Fe2O3
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Please cite this article as: B. Al-Hasni, G. Mountjoy, Structural investigation of iron phosphate glasses using molecular dynamics simulation,
J. Non-Cryst. Solids (2010), doi:10.1016/j.jnoncrysol.2010.10.010