Abstract
The glass forming ability was investigated in the codoped Er3+/Yb3+ ternary system NaPO3-WO3-PbF2 with increasing amounts of PbF2. It has been found that glass samples can be obtained for PbF2 contents ranging from 0 mole% to 50 mole% for the codoped samples and an undoped sample containing 60% of lead fluoride could also be vitrified. Thermal properties of the most PbF2 concentrated glasses exhibit strong dependence of the composition with a decrease of the glass transition temperatures and thermal stabilities against devitrification with increasing the lead fluoride content. These glass samples were heat-treated near the crystallization peaks and the X-ray diffraction measurements pointed out that the dominant phase which precipitates from the glass samples containing 40% and 50% of PbF2 is the lead fluorophosphate phase Pb5F(PO4)3 whereas the sample containing 60% of PbF2 exhibits a preferential bulk crystallization of cubic lead fluoride β-PbF2.
glasses; phosphate; lead fluoride
1 Introduction
Phosphate compounds are interesting glass formers and are largely used for their
specific properties with respect to other classical glasses such as silicate, borate
or germanate glasses. Particularly, they are well known for their small liquidus
viscosity, softening temperatures, large UV transparency and high solubility for
other glass modifiers or intermediaries such as alcaline, rare earth or transition
metal compounds11. Klein RM, Kolbeck AG and Quackenbush CL. Glass formation and
properties in aluminum borophosphate system. American Ceramic Society Bulletin.
1978; 57(2):199.
2. Bih L, Allali N, Yacoubi A, Nadiri A, Boudlich D, Haddad M, et
al. Thermal, physical and spectroscopic investigations of P2O5-A(2)MoO(4)-A(2)O
(A=Li, Na) glasses. Physics and Chemistry of Glasses. 1999;
40:229.-33. Brow RK and Tallant DR. Structural design of sealing glasses.
Journal of Non-Crystalline Solids. 1997; 222:396-406.
http://dx.doi.org/10.1016/S0022-3093(97)90142-3.
http://dx.doi.org/10.1016/S0022-3093(97)...
. Another interesting behavior of
phosphate glasses is their ability to incorporate fluoride compounds without
reduction of the glass forming tendency. These vitreous materials are known as
fluorophosphate glasses and are of great interest for rare earth luminescence since
the fluoride compounds strongly reduce the phonon energy of the glass host44. Zhang L, Sun H, Wu H, Wang J, Hu L and Zhang J. Effects of PbF2
on the spectroscopic, lasing and structural properties of Yb3+-doped
fluorophosphate glass. Solid State Communications. 2005; 135(1-2):150-154.
http://dx.doi.org/10.1016/j.ssc.2005.01.053.
http://dx.doi.org/10.1016/j.ssc.2005.01....
-55. Choi JH, Margaryan A, Margaryan A and Shi FG. Optical transition
properties of Yb3+ in new fluorophosphate glasses with high gain coefficient.
Journal of Alloys and Compounds. 2005; 396(1-2):79-85.
http://dx.doi.org/10.1016/j.jallcom.2004.10.076.
http://dx.doi.org/10.1016/j.jallcom.2004...
. Heavy metal fluoride compounds were also
incorporated in several other glass formers and these materials were used to prepare
the well-known ultratransparent glass ceramics66. Wang Y and Ohwaki J. New transparent vitroceramics codoped with
Er3+ and Yb3+ for efficient frequency upconversion. Applied Physics Letters.
1993; 63(24):3268. http://dx.doi.org/10.1063/1.110170.
http://dx.doi.org/10.1063/1.110170...
-77. Hirao K, Tanaka K, Makita M and Soga N. Preparation and optical
properties of transparent glass-ceramics containing β-PbF2:Tm3+. Journal of
Applied Physics. 1995; 78(5):3445.
http://dx.doi.org/10.1063/1.359975.
http://dx.doi.org/10.1063/1.359975...
. These rare earth doped materials exhibit highly
efficient luminescent properties since rare earth ions tend to migrate to the heavy
metal fluoride crystalline phase during heat-treatment. On the other hand, tungsten
phosphate glasses based on the binary system NaPO3-WO3 are
well known for their great glass forming ability and very high thermal stability
against devitrification due to the intermediary behavior of tungsten octahedra
inside the metaphosphate network88. Tick PA, Borrelli NF, Cornelius LK and Newhouse MA. Transparent
glass ceramics for 1300 nm amplifier applications. Journal of Applied Physics.
1995; 78(11):6367. http://dx.doi.org/10.1063/1.360518.
http://dx.doi.org/10.1063/1.360518...
9. Poirier G, Poulain M, Messaddeq Y and L Ribeiro SJ. New tungstate
fluorophosphate glasses. Journal of Non-Crystalline Solids. 2005;
351(4):293-298.
http://dx.doi.org/10.1016/j.jnoncrysol.2004.11.017.
http://dx.doi.org/10.1016/j.jnoncrysol.2...
-1010. Santagneli SH, de Araujo CC, Strojek W, Eckert H, Poirier G,
Ribeiro SJ, et al. Structural studies of NaPO3-MoO3 glasses by solid-state
nuclear magnetic resonance and Raman spectroscopy. The Journal of Physical
Chemistry B. 2007; 111(34):10109-10117. http://dx.doi.org/10.1021/jp072883n.
PMid:17683136
http://dx.doi.org/10.1021/jp072883n...
. In addition, WO3 incorporation decreases the
overall phonon energy of the phosphate glass with formation of WO6
clusters, resulting in interesting optical properties such as optical non linear
absorption1111. Poirier G, Araújo CB, Messaddeq Y, Ribeiro SJL and Poulain M.
Tungstate fluorophosphate glasses as optical limiters. Journal of Applied
Physics. 2002; 91(12):10221., photochromic
effects1212. Poirier G, Nalin M, Cescato L, Messaddeq Y and Ribeiro SJ. Bulk
photochromism in a tungstate-phosphate glass: a new optical memory material? The
Journal of Chemical Physics. 2006; 125(16):161101.
http://dx.doi.org/10.1063/1.2364476. PMid:17092055
http://dx.doi.org/10.1063/1.2364476...
-1313. Poirier G, Nalin M, Ribeiro SJL and Messaddeq Y. Photochromic
properties of tungstate-based glasses. Solid State Ionics. 2007;
178(11-12):871-875.
http://dx.doi.org/10.1016/j.ssi.2007.01.012.
http://dx.doi.org/10.1016/j.ssi.2007.01....
and efficient luminescent
properties when doped with rare-earth1414. Poirier G, Jerez VA, De Araújo CB, Messaddeq Y, Ribeiro SJL and
Poulain M. Optical spectroscopy and frequency upconversion properties of Tm[sup
3+] doped tungstate fluorophosphate glasses. Journal of Applied Physics. 2003;
93(3):1493-1497. http://dx.doi.org/10.1063/1.1536017.
http://dx.doi.org/10.1063/1.1536017...
. Based on these properties, it is suspected that tungsten
phosphate glasses could be a suitable host for lead fluoride incorporation and
preparation of rare-earth doped glass-ceramics containing PbF2
nanocrystals.
For these reasons, this work investigated the glass forming ability of codoped Er3+/Yb3+ samples in the ternary system NaPO3-WO3-PbF2 with a constant NaPO3/WO3 ratio of 3/2 and increasing amounts of PbF2. The thermal properties were investigated by DSC and the most PbF2 concentrated samples were selected for investigation of the crystallization properties. These samples were heat-treated at specific temperatures and the resulting crystalline phases were identified by X-ray diffraction together with the dominant crystallization mechanism (bulk or surface) by DSC.
2 Experimental Part
Investigated compositions were prepared from the starting compounds NH4H2PO4 48% in P2O5, Na2CO3 98%,WO3 99,9%, PbF2 99,9%, Er2O3 99,99% and Yb2O3 99,99%, all from Aldrich. The starting powders were weighted using an analytical balance and grinded in an agate mortar. The resulting powder was transferred to a covered platinum crucible, heated at 400 °C for 1 hour to remove residual moisture and adsorbed gases, at 600 °C for 1 hour to promote NH4H2PO4 and Na2CO3 decomposition and at 850 °C for 10min for melting of these starting powders. The materials were kept at this temperature for only 10min to minimize fluoride losses and the melt was manually homogeneized after 5min and 10min. Finally, the melt was poured in a steel mold preheated 20 °C below Tg and annealed at this temperature for 4 hour before cooling to room temperature inside the furnace. DSC curves were obtained on bulk and powder glass samples of 10mg in aluminum pans between 160 °C and 470 °C at 10 °C/min under N2 atmosphere. These thermal analyzes were obtained using a DSC 200 F3 Maia calorimeter from Netzsch. X-ray diffraction measurements were performed on powder samples using a RigakuUltima IV diffractometer working at 40KV and 30mA between 10° and 70° in continuous mode of 0,02°/s. The crystalline phases were identified according to X-ray powder diffraction patterns (PDF file)1515. International Centre for Diffraction Data - ICDDJoint Committee of Powder Diffraction Standards - JCPDSPowder diffraction file: PDF2000.
3 Results and Discussion
The glass forming ability of Er3+/Yb3+-codoped compositions in
the ternary system (100-x)[0,6NaPO3-0,4WO3]-xPbF2
were investigated by melt-quenching and it was found that glasses can be obtained
from x=0 to x=50 whereas the sample with x=60 crystallized under these cooling
conditions. The molar NaPO3/WO3 ratio of 3/2 was chosen
because of the known high thermal stability of the binary glass
60NaPO3-40WO3 and its suspected high ability to dissolve
lead fluoride. In fact, this assumption could be verified since PbF2
incorporations as high as 50 mole% were achieved for the rare-earth codoped
compositions and 60 mole% for the undoped composition as seen in Table 1 and Figure 1. Samples 0PbEY and 10PbEY also exhibit a strong dark color
whereas samples containing 20% or more of PbF2 are transparent. This
visible absorption has already been described in the binary system
NaPO3-WO3 to be due to tungsten reduction from
W6+ to W5+ in WO6 clusters1616. Poirier G, Ottoboni FS, Cassanjes FC, Remonte A, Messaddeq Y and
Ribeiro SJ. Redox behavior of molybdenum and tungsten in phosphate glasses. The
Journal of Physical Chemistry B. 2008; 112(15):4481-4487.
http://dx.doi.org/10.1021/jp711709r. PMid:18358031
http://dx.doi.org/10.1021/jp711709r...
. Visible and near infrared absorption is related
with both electronic d-d transitions of W5+ and polaron transitions
between W6+ and W5+, resulting in a very broad absorption band
between 500nm and 1300nm.This partial tungsten reduction taking place inside
WO6 clusters is known in tungsten oxide crystalline materials and
amorphous thin films and is commonly attributed to oxygen loss in order to reach
lower free energy. In our case, it is suggested that a similar oxygen loss occurs
during melting but is only possible in high WO3-concentrated glasses
where tungsten oxide clusters are formed. Then, it is assumed that increasing
PbF2 contents progressively breaks the covalent network and W-O-W
bonds from these WO6 clusters and hinders the tungsten reduction. Since
the main objective of this work is the selection of promising glass compositions for
lead fluoride precipitation, the most PbF2 concentrated glass samples
were chosen for further characterizations (40PbEY, 50PbEY and 60Pb). Thermal
properties were investigated by DSC to determine the characteristic temperatures and
thermal stability in function of the composition as shown in Figure 2 and Table 1.
The first general information extracted from these results is that the thermal
behavior of the glass samples is strongly dependent of the lead fluoride content.
The glass transition temperature Tg decreases from 270 °C to 189 °C, which is in
agreement with the expected modifier behavior of lead fluoride in the tungsten
phosphate network. In fact, incorporation of fluoride compounds in covalent vitreous
networks usually breaks the bridging bonds which are converted to terminal bonds. In
our case, it is assumed that terminal P-F and/or W-F bonds are formed and result in
a strong decrease of the network connectivity. Thus, the glass transition
temperature as well as thermal stabilities decrease since this less connected
network is less viscous and tend to crystallize. Another important result extracted
from the DSC curves is the identification of different crystallization peaks for
each sample. Glass 40PbEY exhibits a very weak exothermic event at 470 °C and an
intense peak centered at 530 °C. The baseline shift around 375 °C could not be
related with a crystallization event since heat treatment at 400 °C didn´t induce
any crystallization as shown above in this work. Sample 50PbEY presents to main
crystallization peaks at 325 °C and 396 °C as well as a clear melting event at 432
°C. Finally, the DSC curve of undoped sample 60Pb is characterized by a first
crystallization event at 250 °C, a second exothermic peak at 301 °C with a shoulder
at 327 °C and another weak exothermic event at 396 °C. At higher temperatures, the
baseline rise is attributed to fluoride evaporation. This distinct crystallization
behavior with composition suggests that different crystalline phases can be obtained
in each sample under specific annealing conditions. For these reasons, heat
treatment were performed at specific temperatures for each sample and the X-ray
diffraction measurements of these treated samples are resumed in Figure 3 for composition 40PbEY, Figure 4 for composition 50PbEY and Figure 5 for composition 60Pb. As explained
above, the apparent DSC baseline change for sample Pb40EY is not related with any
crystallization event since this sample heat-treated for 30min at 400 °C remains
completely amorphous (Figure 3). On the other
hand, heat treatment at 520 °C (above the second onset of crystallization
Tx2) for 4 hours promotes crystallization of the lead fluorophosphate
phase Pb5F(PO4)3. Other weak diffraction peaks
observed in the diffraction pattern could not be identified (labelled * in Figure 3) and may be related with precipitation
of another crystalline phase identified by DSC around 470 °C. X-ray diffraction
patterns of sample 50PbEY heat-treated at 300 °C (onset of the first crystallization
event) for 2 hours and 390 °C (onset of the second crystallization event) for 4
hours are presented in Figure 4. The first
heat-treated sample is mainly amorphous since the most intense event is the
diffraction halo. A very weak diffraction peak centered at 30,5° is also observed.
For the other sample heat-treated at higher temperature, intense diffraction peaks
are observed without any residual diffraction halo, indicating the high
crystallinity of this sample. Most of these diffraction peaks were attributed to the
lead fluorophosphate phase Pb5F(PO4)3, as
illustrated in Figure 4. Other weak
diffraction peaks (identified by * in Figure
4) were also observed but could not be attributed to any known phase
containing the elements present in our sample and are not related with any lead
fluoride crystalline phase. This crystalline phase may be precipitated during the
first exothermic event since the corresponding peak is not symmetric and probably
constituted of several elemental components. It can also be observed from Figure 4 that the main diffraction peak of
Pb5F(PO4)3 is centered at 30,5°, suggesting
that the weak diffraction peak observed for the sample annealed at 300 °C is also
due to a early stage of Pb5F(PO4)3 crystallization.
Thus, it can be inferred that the first crystallization peak is mainly related with
precipitation of Pb5F(PO4)3. These results point
out that glass compositions 40PbEY and 50PbEY are not promising for luminescent
glass-ceramics since Pb5F(PO4)3 is not a suitable
crystalline host for rare earth ions due to high phonon energy of phosphate
compounds. X-ray diffraction patterns of undoped sample 60Pb heat-treated at 225 °C
(onset of the first crystallization event) for 1 hour and 300 °C (onset of the
second crystallization event) for 4 hours are presented in Figure 5. For this sample, it could be determined that cubic
lead fluoride is precipitated under heat-treatment in both cases with a higher
crystallinity of the sample for higher heat-treatment temperatures. In addition,
results shown in Figure 5 demonstrated that no
other crystalline phase is precipitated under these conditions, suggesting that
β-PbF2 growth can be easily controlled in the glass sample in order
to achieve the desired crystallite size and distribution. These X-ray diffraction
measurements are helpful to attribute the first crystallization peak to cubic lead
fluoride precipitation but didn´t clarify the origin of the second exothermic event.
Even if the crystallization of the overall glass matrix should be expected, further
specific heat treatments and X-ray characterizations must be performed on this
sample to fully describe the crystallization sequence. Finally, DSC measurements
were performed on the bulk and powder glass samples 50PbEY and 60Pb to access the
main crystallization mechanism which can be induced from nuclei at the surface
(surface crystallization) or in the bulk (volume crystallization). When the
crystallization process is induced from the surface, a large dependence of
crystallization peak intensity and temperature is observed in function of the glass
particle size. On the other hand, a preferential bulk volume crystallization can be
identified by a similar crystallization behavior of the studied crystalline phase in
the powder and bulk form. These results are presented in Figure 6. For sample 50PbEY, the very small shift of the onset
of crystallization for both crystallization events is a strong indication of a
preferential bulk nucleation of the corresponding crystalline phases, the first one
being already attributed to Pb5F(PO4)3. In the case
of composition 60Pb, the first crystallization peak due to cubic lead fluoride
precipitation also suffers a very small temperature shift with particle size,
indicating again a dominant bulk nucleation for this crystalline phase. On the other
hand, a strong temperature shift can be observed for the high temperature
crystallization event, suggesting that this unidentified crystalline phase is
dominantly nucleated by the surface. Since a homogeneous bulk nucleation is required
for control of the crystallite growth and distribution, these thermal results are
promising and suggest that this glass composition is suitable for preparation of
transparent glass-ceramics containing low phonon energy β-PbF2. However,
this glass composition could not be doped with rare earth ions, which is a
requirement for the desired applications but such doping may be achieved for lower
rare earth concentrations or by varying the NaPO3/WO3 ratio
under constant lead fluoride content in order to improve the glass stability against
devitrification.
X-ray diffraction patterns of sample 40PbEY heat-treated at 400ºC for 30 min, 520 °C for 4 hours and crystalline Pb5F(PO4)3 reference.
X-ray diffraction patterns of sample 50PbEY heat-treated at 300ºC for 2 hours, 390 °C for 4 hours and crystalline Pb5F(PO4)3 reference.
X-ray diffraction patterns of sample 60Pb heat-treated at 225 °C for 1 hour, 300 °C for 4 hours and crystalline β-PbF2 reference.
4 Conclusion
Er3+/Yb3+ codoped glass samples could be obtained in the ternary system (100-x)[0,6NaPO3-0,4WO3]-xPbF2 with x ranging from 0 to 50 and an undoped glass sample with x=60 could also be prepared. The most PbF2 concentrated samples (x=40, x=50 and x=60) were selected for thermal and crystallization characterizations. Thermal properties obtained by DSC for all samples pointed out a decrease of the glass transition temperature and glass thermal stability against devitrification related with the modifier behavior of lead fluoride. Heat-treatments performed near the crystallization peaks and X-ray diffraction measurements on the annealed samples indicated that the glass samples containing 40% and 50% of PbF2 preferentially precipitate lead fluorophosphate Pb5F(PO4)3 whereas the glass containing 60% of PbF2 presents a first crystallization event due to β-PbF2 nucleated homogeneously in the bulk and without other undesirable crystalline phase. These results suggest that this glass composition is promising for transparent glass-ceramics containing β-PbF2 nanocrystals for optical applications.
Acknowledgments
The authors would like to thank brazilian funding agencies FAPEMIG, FINEP, CNPq and CAPES for financial support and UNIFAL-MG for the laboratory structure.
References
-
1Klein RM, Kolbeck AG and Quackenbush CL. Glass formation and properties in aluminum borophosphate system. American Ceramic Society Bulletin. 1978; 57(2):199.
-
2Bih L, Allali N, Yacoubi A, Nadiri A, Boudlich D, Haddad M, et al. Thermal, physical and spectroscopic investigations of P2O5-A(2)MoO(4)-A(2)O (A=Li, Na) glasses. Physics and Chemistry of Glasses. 1999; 40:229.
-
3Brow RK and Tallant DR. Structural design of sealing glasses. Journal of Non-Crystalline Solids. 1997; 222:396-406. http://dx.doi.org/10.1016/S0022-3093(97)90142-3.
» http://dx.doi.org/10.1016/S0022-3093(97)90142-3 -
4Zhang L, Sun H, Wu H, Wang J, Hu L and Zhang J. Effects of PbF2 on the spectroscopic, lasing and structural properties of Yb3+-doped fluorophosphate glass. Solid State Communications. 2005; 135(1-2):150-154. http://dx.doi.org/10.1016/j.ssc.2005.01.053.
» http://dx.doi.org/10.1016/j.ssc.2005.01.053 -
5Choi JH, Margaryan A, Margaryan A and Shi FG. Optical transition properties of Yb3+ in new fluorophosphate glasses with high gain coefficient. Journal of Alloys and Compounds. 2005; 396(1-2):79-85. http://dx.doi.org/10.1016/j.jallcom.2004.10.076.
» http://dx.doi.org/10.1016/j.jallcom.2004.10.076 -
6Wang Y and Ohwaki J. New transparent vitroceramics codoped with Er3+ and Yb3+ for efficient frequency upconversion. Applied Physics Letters. 1993; 63(24):3268. http://dx.doi.org/10.1063/1.110170.
» http://dx.doi.org/10.1063/1.110170 -
7Hirao K, Tanaka K, Makita M and Soga N. Preparation and optical properties of transparent glass-ceramics containing β-PbF2:Tm3+. Journal of Applied Physics. 1995; 78(5):3445. http://dx.doi.org/10.1063/1.359975.
» http://dx.doi.org/10.1063/1.359975 -
8Tick PA, Borrelli NF, Cornelius LK and Newhouse MA. Transparent glass ceramics for 1300 nm amplifier applications. Journal of Applied Physics. 1995; 78(11):6367. http://dx.doi.org/10.1063/1.360518.
» http://dx.doi.org/10.1063/1.360518 -
9Poirier G, Poulain M, Messaddeq Y and L Ribeiro SJ. New tungstate fluorophosphate glasses. Journal of Non-Crystalline Solids. 2005; 351(4):293-298. http://dx.doi.org/10.1016/j.jnoncrysol.2004.11.017.
» http://dx.doi.org/10.1016/j.jnoncrysol.2004.11.017 -
10Santagneli SH, de Araujo CC, Strojek W, Eckert H, Poirier G, Ribeiro SJ, et al. Structural studies of NaPO3-MoO3 glasses by solid-state nuclear magnetic resonance and Raman spectroscopy. The Journal of Physical Chemistry B. 2007; 111(34):10109-10117. http://dx.doi.org/10.1021/jp072883n. PMid:17683136
» http://dx.doi.org/10.1021/jp072883n -
11Poirier G, Araújo CB, Messaddeq Y, Ribeiro SJL and Poulain M. Tungstate fluorophosphate glasses as optical limiters. Journal of Applied Physics. 2002; 91(12):10221.
-
12Poirier G, Nalin M, Cescato L, Messaddeq Y and Ribeiro SJ. Bulk photochromism in a tungstate-phosphate glass: a new optical memory material? The Journal of Chemical Physics. 2006; 125(16):161101. http://dx.doi.org/10.1063/1.2364476. PMid:17092055
» http://dx.doi.org/10.1063/1.2364476 -
13Poirier G, Nalin M, Ribeiro SJL and Messaddeq Y. Photochromic properties of tungstate-based glasses. Solid State Ionics. 2007; 178(11-12):871-875. http://dx.doi.org/10.1016/j.ssi.2007.01.012.
» http://dx.doi.org/10.1016/j.ssi.2007.01.012 -
14Poirier G, Jerez VA, De Araújo CB, Messaddeq Y, Ribeiro SJL and Poulain M. Optical spectroscopy and frequency upconversion properties of Tm[sup 3+] doped tungstate fluorophosphate glasses. Journal of Applied Physics. 2003; 93(3):1493-1497. http://dx.doi.org/10.1063/1.1536017.
» http://dx.doi.org/10.1063/1.1536017 -
15International Centre for Diffraction Data - ICDDJoint Committee of Powder Diffraction Standards - JCPDSPowder diffraction file: PDF2000
-
16Poirier G, Ottoboni FS, Cassanjes FC, Remonte A, Messaddeq Y and Ribeiro SJ. Redox behavior of molybdenum and tungsten in phosphate glasses. The Journal of Physical Chemistry B. 2008; 112(15):4481-4487. http://dx.doi.org/10.1021/jp711709r. PMid:18358031
» http://dx.doi.org/10.1021/jp711709r
Publication Dates
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Publication in this collection
Jan-Feb 2015
History
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Received
13 Nov 2014 -
Reviewed
12 Jan 2015