Influence of Gd2O3 on thermal stability of oxyfluoride glasses
Influence of Gd2O3 on thermal stability of oxyfluoride glasses
Marta Kasprzyk 0
Marcin S? roda 0
Magdalena Szumera 0
0 Faculty of Materials Science and Technology, AGH University of Science and Technology , Mickiewicza Av. 30, 30-059 Krako ?w , Poland
The aim of the research was to determine the influence of Gd2O3 addition on the thermal stability of glasses from BaF2-NaF-Na2O-Gd2O3-Al2O3-SiO2 system. Thermal analysis was carried out using DSC method. An impact of Gd2O3 on thermal parameters such as transformation temperature (Tg), onset temperature of crystallization (Tx), peak crystallization temperature (Tp), change of specific heat (DCp) and enthalpy of crystallization (DH) was determined. On the basis of thermal analysis, controlled crystallization was conducted. The glass-ceramic materials were identified by XRD. Crystallization kinetics of the glasses was made on the basis of two models: Kissinger and Ozawa. Changes of the glass structure were evaluated in FTIR study. It was determined that the addition of Gd2O3 to the aluminosilicate glass increases the transformation temperature, simultaneously with reducing the Dcp and thermal stability DT values. It results in the decrease of DH. As shown, Gd significantly increases the crystallization activation energy of NaAlSi3O8. Complete substitution of Al2O3 with Gd2O3 leads to forming of Gd9.33(SiO4)6O2. Forming of Na-silicates is not observed. Because of the participation of Gd in forming of fluoride phases (BaGdF5, GdF3), crystallization of Gd9.33(SiO4)6O2 is impeded. Gd2O3
Oxyfluoride aluminosilicate glasses; Glass- ceramics; Thermal analysis; Kinetics; Crystallization
& Marta Kasprzyk
More precise heat treatment than in case of oxide glasses is
required to obtain transparent, low-phonon glass-ceramics
based on oxyfluoride glasses . It is due to the fact that
introducing fluorine atoms into glass structure causes its
weakening and higher ability of crystallization of oxide
framework. On the other hand, the material has to maintain
its transparency and luminescent properties, so that the
glass-ceramics could be used as an active element in
optoelectronic systems. The luminescence properties are
achieved by introducing into the structure of such glass
optically active lanthanides. For receiving higher
luminescence efficiency and greater lifetime, the lanthanides
atoms should be located in low-phonon phase, in this case
the fluoride phase with higher symmetry. For this purpose,
the silicate glasses doped with LaF3 [2, 3], CaF2 [4, 5] and
BaF2 [6, 7] were studied. The results of the research
confirmed the improvement in luminescent properties of the
materials. It is the issue of the material, in which the
fluoride phase would readily host the optically active dopant
into its structure. In this study, influence of Gd2O3
replacing Al2O3 in the glass composition on thermal glass
stability and process of crystallization was studied in the
BaF2?NaF?Na2O?Gd2O3?Al2O3?SiO2 system. The
crystallization process was further studied by heat treatment,
followed by X-ray diffraction experiments and FTIR
Four glasses with varying content of Gd2O3 were prepared
using chemically pure raw materials (Table 1). The glass
was melted in a platinum crucible in an electric furnace in
Table 1 Nominal glass composition in mol%
Glass composition in mol%
air atmosphere holding it at 1400?1470 C for 60 min to
obtain 0.1 mol of glass (ca. 10 g). During melting, the
crucible was covered with a platinum plate to reduce
vaporization losses. The losses of fluorides were
compensated by extra amounts of components calculated from
XRF analysis. The melt was poured onto a brass plate
forming a layer of 2?4 mm thickness and then annealed at
a temperature near the transformation temperature. The
nominal glasses compositions are listed in Table 1.
Thermal stability analysis was performed with DSC
method, using NETZSCH STA 449 F3 equipment and a
platinum crucible in argon atmosphere. Approximately
50 mg of grounded glass below 0.06 mm were used for
DSC measurement from 50 to 1050 C. Crystallization
kinetics analysis was performed for four heating rates: 5,
10, 20 and 30 K min-1 after appropriate equipment
calibration for each rate. The transformation temperature (Tg)
established on inflection point and peak temperature of
crystallization (Tp) were determined from the DSC traces
of the respective glasses. The activation energy (Ea) and
Avrami exponent (n) were calculated according to the
The heat treatment process was carried out in
Nabertherm electric furnace. Phase identification was performed
with X-ray diffraction analysis, using Philips X?Pert
equipment at room temperature with CuKa radiation over
the range of 10 ?70 2h.
The morphologies of the crystalline phases developed
upon the heat treatment of bulk glass samples were
examined with a scanning electron microscope (SEM,
NovaNano SEM 200, FEI Company). SEM tests were
performed with an attachment for the chemical analysis of
specimens in micro-areas with energy dispersive X-ray
spectroscopy (EDAX). The observations were carried out
in the secondary electron mode in low vacuum conditions
(60 Pa) and at an accelerating voltage of 18 kV. The
samples were covered with a carbon layer. The chemical
composition was determined using a WD-XRF Axios Max
spectrometer with Rh 4 kW PANalytical lamp.
FTIR spectra were recorded with a Bruker Company
Vertex 70v spectrometer. Spectra were collected in the
MIR regions (2500?400 cm-1) after 128 scans at 4 cm-1
resolution. Samples were prepared by the standard KBr
Results and discussion
DSC analysis shows that the increasing content of Gd2O3
(replacing Al2O3) influences the thermal stability (Fig. 1).
A monotonic increase in transformation temperature is
observed, together with higher Gd2O3 content (Table 2).
Difference of Tg between samples FGA_0 (with no Gd2O3
content) and FGA_10 (with no Al2O3 content) was found
to be 56 C. Jump-like change of heat capacity (Dcp)
corresponding to the transformation temperature also
shows a significant modification. Addition of 5% mol
Gd2O3 reduces it, and then, Dcp increases along with
complete replacing aluminum atoms with gadolinium.
Glasses (FGA_5 and FGA_7.5) that contain both Gd and
Al in their networks have the lowest Dcp value. It is due to
the fact that those glasses demonstrate lower
thermodynamic fragility and are not prone to strong relaxation
Introducing of Gd2O3 causes crystallization changes,
such as increase in the peak crystallization temperature (Tp)
with simultaneous occurrence of two exothermal effects
(Table 2). Thermal stability, determined as a difference
between onset crystallization temperature (Tx) and Tg
[9?11], has higher values for glasses containing only one
trivalent component, and this parameter has slightly lower
value for glasses containing a mixture of Al2O3 and Gd2O3.
Gadolinium addition also influences the crystallization
enthalpy (DH) value. Comparing these values, it can be
assumed that DH significantly decreases along with higher
Gd2O3 content (Table 2). It indicates a change in
mechanism of the crystallization process, associated with
formation of various crystalline phases in the series of glasses,
as shown in the XRD analysis.
In order to determine changes in the glasses
crystallization kinetics, a non-isothermal DSC analysis was
carried out with four heating rates (Fig. 1).
Crystallization kinetics analysis can be based on one of
the reaction models, developed by Kissinger , Augis?
Bennett  and Ozawa . In the Kissinger method, the
crystallization peak temperature is determined as a function
of the heating rate a as following:
Ea=RTp ? const:
Ozawa and Kissinger plots are commonly used to
calculate the Avrami constant, n, and the crystallization
activation energy, Ea, for glass [12, 15]. In the
Fig. 1 DSC traces of glass: a FGA_0, b FGA_5, c FGA_7.5 and d FGA_7.5
Glass code (content of Gd2O3)
FGA_10 (10 Gd2O3)
isothermal method, the values of the Avrami parameter are
determined using the Ozawa equation:
ln? ln?1 x? ? n ln a ? const: ?2?
where x is the crystallized volume fraction at T for the heating
rate a. It should be taken into account that Kissinger?s
approach does not consider the concept of nucleation and
growth. In this case, transformation under non-isothermal
condition is described as a first-order reaction.
Figure 2 represents the relation of ln(Tp2/an) to 1000/Tp
according to the Kissinger?Matusita model [12, 16, 17]
which is described as:
DT = Tx - Tg/ C
mEa=RTp ? const:
In our calculation, we assume the variable amount of
nucleuses due to the process of formation fluorides as the
first stage of the network rearrangement according to the
XRD results. They can act as a new center of nucleation of
silicates. In this case, m = n - 1. The obtained values
have good linear correlation. The effect of crystallization
was not observed for FGA_10 at heating rate of 5 and
10 C min-1 and could not have been taken to our
calculation. Thus, the values of Ea and Avrami parameter
(n) cannot be estimated exactly for the glass and may
deviate from the proper value. It is presented as the dotted
line in Figs. 2 and 3.
The crystallization activation energy, determined on the
basis of the plots, is presented in Table 3. Analysis of
crystallization kinetics from curves according to Eq. (3)
indicates that the activation energy for aluminosilicate
glass (FGA_0) is slightly higher than Si?O bond
dissociation energy (360?430 kJ mol-1) in silicate glasses . It
may be connected with Al2O3 as a next component of the
framework. There are enough modifier ions to compensate
electric charge to form tetrahedral [AlO4]. The significant
higher values are obtained when Gd2O3 was substituted
Al2O3. The activation energy for the precipitation of BaF2
and BaGdF5 is much lower (250?350 kJ mol-1)  than
those obtained in our analysis. As fluoride phases form
during crystallization, Si?O bond dissociation is not
actually necessary, as fluoride is not bound to silicon . That
is why it can be assumed that obtained in the study values
apply to aluminosilica oxide network crystallization. On
the DSC curves, a separate peak corresponding to fluoride
phase forming was not observed. This statement is based
on the XRD analysis which confirmed the fluoride
crystallize below 800 C and there is no deflection on the
Fig. 2 Plots of ln(a/Tp2) versus 1000/Tp for the oxyfluoride glasses:
a FGA_0, b FGA_5, c FGA_7.5 and d FGA_10
derivative DSC curves in the temperature region between
Tg and 800 C. However, crystallization of fluoride phases
can impact the increase in crystallization activation energy
of silica network by shortage of cations necessary for
silicates crystallization and/or by reinforcement of the oxide
The values of n determined from ln[-ln(1 - x)] versus
ln a plot slope (Fig. 3) are listed in Table 3. It is evident
that it ranges from 1.7 to 1.4, which suggests a change in
the crystallization mechanism . Moreover, the
calculated values of n are not integers, which indicates a
multistage mechanism of crystallization and a complex
crystallization behavior (Fig. 4). Silicate crystallization is
preceded by fluoride phase crystallization as XRD analysis
showed. It can be a result of higher chemical affinity of
modifiers ions toward fluorine and lower strength of
fluorine bonds than oxygen. Size and amount of formed
fluorides can influence the silicate crystallization mechanism.
The results suggest that with increase in Gd2O3 content in
the glass, the crystallization mechanism changes from
twodimensional to one-dimensional growth of silicates (n
parameter is lowering).
1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6
Fig. 3 Plot of ln[-ln(1 - x)] versus ln(a) for the glasses: a FGA_0,
b FGA_5, c FGA_7.5 and d FGA_10
Table 3 Activation energy of crystallization and Avrami parameter
a Values were obtained only from two points on the plot
900 ?C, 12 h
750 ?C, 12 h
750 ?C, 2 h
Fig. 4 XRD patterns of FGA_0 sample after heat treatment
Because of the crystallization effect disappearance in
case of FGA_10 sample heated with 5 and 10 C min-1
rate, it was impossible to carry out a precise crystallization
kinetics analysis of this glass; thus, the Ea and n values
estimated based only on two heating rates can be
inaccurate. However, a lower value of crystallization activation
energy and Avrami parameter, n, can be observed. These
changes, as well as Dcp, DT and n values, are similar to
those corresponding to FGA_0. It indicates a similar role of
Gd2O3 and Al2O3 in the glass structure. Their mutual
substitution in the glass affects the change in thermal
stability of silicate network, which must carry modification of
980 ?C, 30 min, H
900 ?C, 12 h
770 ?C, 2 h
Fig. 5 XRD patterns of FGA_5 sample after heat treatment (H index
indicates the sample was put into a hot furnace directly)
Fig. 7 XRD patterns of FGA_10 sample after heat treatment
XRD analysis revealed FGA_0 glass crystallization as a
two-stage process. Heat treatment of the glass at 750 C for
2 h leads to BaF2 formation as a first phase (Fig. 4). This
effect is not visible on DSC curves, where the
crystallization effect is situated above 800 C. The sample
analysis after 12 h of heat treatment at the same temperature
shows the oxide network crystallization and formation of
NaAlSi3O8 (Fig. 4). Intensity of the peaks corresponding to
this phase is low, which indicates the initial stage of
crystallization process of this phase. Heat treatment at
900 C for 12 h leads to BaF2 phase disappearance
Fig. 6 XRD patterns of FGA_7.5 sample after heat treatment
1070 ?C, 2 h
780 ?C, 2 h
880 ?C, 12 h
780 ?C, 2 h
(Fig. 4). It could be the effect of fluoride phase dissolution
in the glass that becomes a melt in this temperature.
In the case of FGA_5 glass, at the first stage of
crystallization (heat treatment at 770 C for 2 h), BaGdF5 is
formed (Fig. 5). Unlike BaF2, its thermal stability is higher
and it is not disappearing after heat treatment at 900 C for
12 h (Fig. 5). At the same time, similarly to FGA_0 glass,
the silicate network glass crystallization in the form of
albite is observed (Fig. 5). To determine the effect of heat
treatment procedure on both phases crystallization ability,
Fig. 8 SEM image (a) and EDS spectrum of FGA_0 after heat treatment at 750 C for 2 h representing the point #1?crystallites of BaF2
(b) and point #2?glassy matrix (c)
Fig. 9 SEM image (a) and EDS spectrum of FGA_0 after heat treatment at 900 C for 30 min representing the point #1?crystallites of BaF2
(b) and point #2?glassy matrix (c)
an experiment was conducted, in which a glass sample was
placed in the preheated to 980 C furnace and crystallized
for 30 min. XRD analysis showed that under these thermal
conditions, only BaGdF5 was formed (Fig. 5). Such
behavior of BaGdF5 allows to modify the treatment process
in order to obtain a transparent glass-ceramics. It is very
important, since it gives a better opportunity to control
shape and size of BaGdF5 grains (Figs. 12, 13). Similar
effects to FGA_5 are observed for FGA_7.5 glass (Fig. 6),
but the effect of BaGdF5 crystallization is lower and the
additional crystallization of Gd9.33(SiO4)6O2 occurs at
higher temperature. Complete substitution of Al2O3 with
Figures 8 and 9 show the microstructure of FGA_0 sample
after heat treatment at 750 C for 2 h and 900 C for
Fig. 11 SEM image of FGA_0 cross section after heat treatment at
900 C for 30 min
Gd2O3 (FGA_10 glass) leads to change of the course of
crystallization. Despite the 12 h heat treatment, intensity of
corresponding BaGdF5 peaks is very low (Fig. 7). Heat
treatment at 880 C for 12 h leads to small amounts of
GdF3 formation. At the same time, gadolinium silicate
Gd9.33(SiO4)6O2 is formed. However, its ability to
crystallization is lower than NaAlSi3O8.
XRD results show that introducing gadolinium oxide
instead of Al2O3 affects the type of the fluorides to be
formed and benefits the crystallization process. BaGdF5
shows higher thermal stability than BaF2. This observation
may be important for obtaining of glass-ceramics
containing low-phonon fluoride phase with high transparency.
Fig. 12 SEM image (a) and EDS spectrum of FGA_5 after heat treatment at 770 C for 2 h representing the point #1?crystallites of BaGdF5 (b)
and point #2?glassy matrix (c)
30 min. BaF2 is visible in both samples. The crystallites are
larger with much more spherical shape for higher
The sample becomes mostly amorphous with increasing
duration of heat treatment up to 12 h (Fig. 10). It is
confirmed by small thermal effects on DSC curves and
lower intensity of peak on XRD pattern. EDS of the visible
crystallites indicates the silicate phase formation. There is
no sign of spherical BaF2 crystallites that undergo
Fig. 13 SEM image (a) and EDS spectrum of FGA_5 after heat treatment at 900 C for 12 h representing the point #1?crystallites of BaGdF5 (b)
and point #2?glassy matrix (c)
Figure 11 shows glass fracture near the sample surface.
A difference in the cross section between the surface and
the inside is visible. The distinct area of 10 lm in width
with no visible BaF2 crystallites is visible near the surface,
unlike the inner part of the sample (upper part of the
photograph). It may indicate a different chemical
Fig. 14 SEM image (a) and EDS spectrum of FGA_7.5 after heat
treatment at 1070 C for 2 h representing the point #1?phase with
higher concentration of Ga (b), point #2?phase with higher
concentration of Ba (c), point #3?phase with composition
corresponding to Na(AlSi3O8) (d)
Fig. 15 SEM image (a) and EDS spectrum of FGA_10 after heat treatment at 880 C for 12 h representing the point #1?crystallites of BaGdF5 (b)
and point #2?glassy matrix (c)
composition of the surface and the inside of the sample.
Lack of fluoride phase formation in the surface layer may
be a result of vaporization of fluorine during heat
A microstructure of FGA_5 sample after heat treatment
at 770 C for 2 h is presented in Fig. 12. Just comparing
both heat-treated FGA_0 and FGA_5 samples, the
difference in the size of crystallites can be observed. FGA_5
2500 2000 1500
sample heated at even 20 C higher temperature that
FGA_0 sample shows significantly smaller crystallites,
below 150 nm. However, the crystallites are well dispersed
through the glass network and are not forming clusters in
SEM microphotograph reveals the formation of
crystallites in the prism shape after 12 h of heating (Fig. 13).
EDS analysis showed higher concentration of Gd, Ba and F
atoms in this area, which may suggest a fluoride phase.
XRD analysis confirmed formation of BaGdF5 (Fig. 5)
besides Na(AlSi3O8) phase which is also visible as small
Figure 14 presents microstructure of FGA_7.5 sample
after treatment at the temperature 1070 C for 2 h. The
EDS analysis confirms the formation of three kinds of areas
which may correspond to the formation of BaGdF5/GdF3,
Gd9.33(SiO4)6O2 and Na(AlSi3O8).
Figure 15 shows a microphotograph of FGA_10 sample
after heat treatment at 880 C for 12 h. Irregular areas with
higher concentration of Gd precipitated in glassy matrix are
observed (EDS point #1). Comparing these results with
XRD analysis (Fig. 7), it can be concluded that they are
mostly amorphous. The results correspond to the DSC
analysis that shows lower values of DH.
FTIR spectra for as-made glasses are presented in Fig. 16a.
Broad absorption lines confirm the amorphous nature of the
samples. Increasing the fraction of Gd2O3 results in minor
spectral changes. Replacing Al2O3 with Gd2O3 leads to a
shift of the major band maximum toward higher
wavenumbers. This band corresponds to stretching
vibrations of tetrahedrons [SiO4] with various polymerization
degree [22, 23]. Introducing Gd2O3 to the glass does not
shift the bands at 1187 cm-1, which is connected with
asymmetric Si?O?Si Q4 and Si=O bond vibrations . It
can be an evidence that substitution of Al atoms with Gd
does not cause silicate network depolymerization. On the
other hands, the minor shift of 1000 cm-1 band associated
with asymmetric vibrations of Si?O?Si Q2  is observed.
The change in Si?O bond in the glasses doped with
different content Gd2O3 can be assessed by the relationship
between Si?O bond strength and effective frequency as
reported in the literatures [26, 27]. The shift of the peak at
about 1000 cm-1 may result the Si?O bond strength
increase as the doping content of Gd2O3 increase. This
effect may be explained by lower Gd?O bond strength,
which leads to higher strength of Si?O(Gd) bond than Si?
O(Al). To confirm the assumption a deconvolution analysis
is needed. The band at 711 cm-1 is observed for the
FGA_0 glass as a result of Si?O?Si bending vibration. The
band shifts significantly and monotonously toward higher
value of wavenumber when Gd content increases.
Figure 16b shows FTIR spectra for samples heated for
12 h at 900 C (FGA_0, FGA_5, FGA_10) and at 1070 C
(FGA_7.5). For sample with 7.5 and 10 mol% Gd2O3 alone
(spectra c and d), the spectra differ not much to the
asmade glass spectrum. It indicates that the crystallization
ability of Gd9.33(SiO4)6O2 is much lower than NaAlSi3O8.
It should be noted that fluoride phases vibrations, due to the
chemical nature of the bonds, are not active in IR
spectroscopy [28, 29].
Replacing Al2O3 with Gd2O3 in the glass composition
significantly influenced the crystallization behavior of
fluoro-silicate glasses, both in terms of fluoride phase and
silicate network crystallization. The influence of the
mixture of both oxides on crystallization was different than
when we introduced them separately into the glass.
Addition of Gd2O3 to the aluminosilicate glass increases the
transformation temperature, at the same time decreasing
Dcp and thermal stability DT. It also results in decrease of
DH. The study indicates that Gd2O3 admixture significantly
increases the activation energy of NaAlSi3O8
crystallization. Complete replacement of Al2O3 with Gd2O3 leads to
Gd9.33(SiO4)6O2 formation, and Na-silicates crystallization
is not observed. Due to Gd3? ions participation in fluoride
phases (BaGdF5, GdF3) formation, crystallization of
Gd9.33(SiO4)6O2 is impended, which was confirmed by our
study. Thus, Gd2O3 can be used as a facilitating component
for fabricating the low-phonon fluoride phases in the
Acknowledgements This work was supported by AGH University of
Science and Technology Department of Materials Science and
Ceramics AGH number WIMiC No 220.127.116.115 in 2017. We
would like to thank prof. M. Sitarz for his valuable comments.
Open Access This article is distributed under the terms of the
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