19F MAS-NMR studies of strontium oxyfluoride aluminosilicate glass

Article abstract of DOI:10.1016/j.molstruc.2011.06.021

Local environment of fluorine atoms in the lanthanum oxyfluoride aluminosilicate glasses and glass-ceramics modified by SrO was studied by solid state ¹⁹F MAS-NMR spectroscopy and X-ray diffraction. The effect of strontium concentration on the formation of crystalline LaF₃ phase was determined, as a function of heat treatment conditions. In all glasses studied, the F-Me(n) (where Me = Sr, La), but no Na-F species were observed. The presence of F-La,Sr(n) units, in which fluorine is coordinated by both lanthanum and strontium, was detected in the glass with higher content of SrO. Supplementary XRD analysis of this series confirmed that an increase of strontium contents leads to the formation of Sr_0.69La _0.31F_2.31 and LaSr₂F₇, instead of the pure LaF₃ only.

Full text of DOI:10.1016/j.molstruc.2011.06.021

Journal of Molecular Structure 1001 (2011) 78–82
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Journal of Molecular Structure
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19F MAS-NMR studies of strontium oxyfluoride aluminosilicate glass
a,
b
⇑
´
M. Sroda , Z. Olejniczak
a AGH – University of Science and Technology, Faculty of Materials Science and Ceramics, al. Mickiewicza 30, 30-059 Kraków, Poland
b Institute of Nuclear Physics, Polish Academy of Sciences, Radzikowskiego 152, 31-342 Kraków, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 March 2011
Received in revised form 16 June 2011
Accepted 17 June 2011
Available online 30 June 2011
Local environment of fluorine atoms in the lanthanum oxyfluoride aluminosilicate glasses and glass–
ceramics modified by SrO was studied by solid state 19F MAS-NMR spectroscopy and X-ray diffraction.
The effect of strontium concentration on the formation of crystalline LaF3 phase was determined, as a
function of heat treatment conditions. In all glasses studied, the F–Me(n) (where Me = Sr, La), but no
Na–F species were observed. The presence of F–La,Sr(n) units, in which fluorine is coordinated by both
lanthanum and strontium, was detected in the glass with higher content of SrO. Supplementary XRD
analysis of this series confirmed that an increase of strontium contents leads to the formation of
Sr0.69La0.31F2.31 and LaSr2F7, instead of the pure LaF3 only.
Keywords:
Oxyfluoride glass
SrO
19F MAS-NMR
Glass–ceramics
LaF3
Ó 2011 Elsevier B.V. All rights reserved.
Aluminosilicate glass
1. Introduction
CaF2 system was investigated by Hill et al. [6]. It was found to in-
duce radio-opacity and affect the thermal stability by modifying
Oxyfluoride glasses are intensively studied for their potential
applications in optoelectronic devices as low-loss optical fibers
[1] and optical amplifier materials [2]. An appropriate heat treat-
ment induces the fluoride crystalline phase in the oxide glassy ma-
trix, generating a low phonon structure for optically active ions, i.e.
rare earth elements. The process of cerammization strongly de-
pends on the glass composition, which affects the stability of
amorphous structure as well as the type and order of the formed
crystals. One of the promising low phonon phase is LaF3, and its
crystallization can be induced in alumino-silicate [3] and boro-sil-
icate glassy matrix [4]. A low phonon phase in the glassy matrix
plays a role of the host for optically active lanthanides to increase
the efficiency of luminescence. For this structural configuration the
probability of non-radiative relaxation of excited states is lower, so
that the photon creation is enhanced.
The properties of Sr in the acidic silicate glasses resemble
those of its two neighbors in the periodic table, i.e. Ca and Ba. Be-
cause of similar effective radius, Sr is sometimes compared with
Pb2+, despite their different polarizabilities. In optical glasses, Ca
and Ba are often partially replaced by Sr, which leads to lower
melting temperature and easier refining [5]. The substitution of
CaO by SrO in alumino-silicate glass increases its density and de-
creases viscosity, owing to its greater polarizability. The effect of
strontium substitution for calcium in the SiO2–Al2O3–P2O5–CaO–
the nucleation and crystallization behavior of the glass that is of-
ten used for biomedical application. On the other hand, to our
knowledge, the effect of strontium on the structure and proper-
ties of oxyfluoride aluminosilicate glasses that can be used as a
material for transparent glass–ceramic has not been previously
studied.
Recently, the influence of alkaline and alkaline earth oxides on
the structure of alumino-silicate glasses with LaF3 was analyzed
by DTA, XRD, FT-IR and Raman spectroscopy [7,8]. Their thermal
stability, and the formation of crystalline phases were determined.
The effect of BaO was studied by the Dajneka group using 19F MAS-
NMR method [9]. The modifiers were found to affect the kinetics of
the LaF3 formation during the heat treatment, leading to significant
changes in the micro- and nano-morphology of the materials.
The latter studies showed that both the modifiers and alumi-
num readily coordinate with the Fꢀ ions in the glass structure
[10–12]. This may prevent the formation of LaF3 and/or cause
simultaneous formation of new LaxMexꢀ1Fy phases (where Me is
the alkaline earth metal) during cerammization [7,13]. For LaF3-
doped sodium alumina-silicate, the process of cerammization
was found to induce the NaLaF4 formation [14,15]. The DTA and
FTIR studies showed that the alkaline earth ions (Mg, Ca, Ba) re-
duced the depolymerization of the oxyfluoride network, contrary
to their effect in the pure silicate network [7,8]. In the present
work, we use the 19F MAS-NMR spectroscopy to study the effect
of SrO on the position of Fꢀ ions in the network and on the forma-
tion of LaF3 during the controlled cerammization process.
⇑
Corresponding author.
E-mail address: msroda@agh.edu.pl (M. Sroda).
´
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.06.021

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M. Sroda, Z. Olejniczak / Journal of Molecular Structure 1001 (2011) 78–82
79
Table 1
Nominal composition of the prepared oxyfluoride aluminosilicate glasses.
Glass Composition (mol%)
Al2O3
no.
Na2O þ Na2F2 þ SrO þ 3La2F6
SiO2 Al2O3 SrO
Na2O Na2F2 La2F6
0.4Sr 53.5 15
0.6Sr 66 15
0.8Sr 72.25 15
22.5
10
3.75
3
3
3
3
3
3
3
3
3
0.4
0.6
0.8
Fig. 1. 19F MAS-NMR spectra of polycrystalline NaF, SrF2 and LaF3 at two spinning
speeds: 9 and 13 kHz.
2. Experimental
The compositions of glasses studied are listed in the Table 1.
They were designed to determine the effect of the charge of the
ion modifiers (Na+, Sr2+, La3+) on the framework structure of the
aluminosilicate glass. As the quantitative measure, the ratio:
Al2O3/(Na2O + Na2F2 + SrO + 3La2F6) was chosen, which represents
the relation between the content of Al3+ ion and the value of posi-
tive charge introduced by modifying ions in the glass structure.
This ratio was varied from 0.4 to 0.8 by substituting SrO for SiO2,
with the concentration of all other components kept constant.
Batches were prepared by mixing appropriate quantities of chem-
ically pure reagents: SiO2, Al2O3, Na2CO3, NaF and LaF3. The mix-
ture in 20 g portions was placed in platinum crucible in an
electric furnace and melted at 1450 °C in the air atmosphere. Dur-
ing melting the crucible was covered to minimize the fluorine loss.
The molten glass was poured out onto a stainless steel plate form-
ing a layer of ca. 3 mm thickness. The obtained glasses were trans-
parent with a small degree of opalescence for glass with the
highest content of SrO (0.4Sr sample), which could be eliminated
by faster cooling.
Fig. 2. 19F MAS-NMR spectra of Sr-oxyfluoride glass as-made (a) and after 700 °C/
2 h treatment (b).
High resolution, solid state 19F Magic-Angle-Spinning Nuclear
Magnetic Resonance (MAS-NMR) spectra were measured on a Tec-
mag APOLLO pulse NMR spectrometer at the magnetic field of 7.05
Tesla produced by the Magnex wide-bore superconducting mag-
net. The resonance frequency for 19F was 282.7 MHz. A Bruker
HP-WB high-speed MAS probe equipped with the 4 mm zirconia

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80
2 s, and 256 scans were acquired. The frequency scale in ppm
was referenced to the 19F resonance of 1 M solution of NaF, which
is located at ꢀ120 ppm from CFCl3 [16].
The Bruker HP-WB MAS probe that was used in the measure-
ments exhibits a significant fluorine background, which becomes
dominant in the case of samples containing small amounts of the
element. Therefore it was necessary to eliminate it, by measuring
the signal from the empty rotor and subtracting it digitally.
Powder X-ray diffraction (XRD) was measured on a X’Pert Phi-
lips using Cu K
the Scherrer equation: the value of the shape factor, K, was taken to
be 0.9 and -Al2O3 was used as an internal standard.
a radiation. The crystallite size was calculated from
a
3. Results and discussion
The 19F MAS-NMR spectra of model crystalline SrF2, NaF and
LaF3 samples measured at two spinning speeds are shown in
Fig. 1. The spinning sidebands are marked by stars and they are
separated by 31.8 and 46.0 ppm, for the spinning speeds equal to
9 and 13 kHz, respectively. The chemical shift observed in these
model compounds moves towards more positive values with the
decrease of ionicity (i): F–Na 83.3%, F–Sr 75.6%, F–La 67.8% [17],
i.e. with the increase of the fluorine–cation bond strength.
A single central band at ꢀ225 ppm observed in the NaF spec-
trum corresponds to the fluorine atom surrounded by six sodium
ions F–Na(6) [18]. The peak at the same position was also reported
by Zeng and Stebbins in the Na–silicate glass [16]. It should be
noted that no such line was found in any of glass or glass–ceramics
samples studied in this work. It means that in oxyfluoride alumino-
silicate glasses modified by alkaline earth oxides the fluorine ions
do not make bonds similar to that existing in NaF.
The 19F MAS-NMR spectrum of SrF2 exhibits a single central
band at ꢀ90 ppm. This is close to ꢀ84 ppm reported by Miller
[19], who assigned it to the fluorine anion interacting with four
strontium ions: F–Sr(4). Two central bands at +25 and ꢀ23 ppm
are observed in the spectrum of crystalline LaF3. They originate
from fluorine ions interacting with the local structure of the first
coordination sphere F–La(3), and with three near and one more
distant lanthanum neighbors F–La(3 + 1), respectively [20]. In fact,
the central band at +25 ppm is a superposition of two lines at +26
and +19 ppm, which were identified in experiments performed at
higher magnetic field of 11.7 Tesla [9]. An additional broadening
Fig. 3. 19F MAS-NMR spectra of 0.8Sr glass after heat treatment at 760 °C and
990 °C for 2 h.
rotor and KEL-F cap was used to record the MAS spectra at the
spinning speed varying from 9 to 13 kHz. Each spectrum was mea-
sured at least twice using different spinning speeds, in order to
identify central bands in the spectrum. The spectra were measured
using a single 2
ls radio-frequency pulse, corresponding to p/2
flipping angle. The acquisition delay used in accumulation was
Fig. 4. XRD analysis of low strontium content glass (0.8Sr) after heat treatment at 760 °C (top) and 990 °C (bottom).

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81
Table 2
of this band that is visible on Fig. 1 is caused by a partial overlap of
the spinning sideband.
LaF3 crystallite sizes of the 0.8Sr glass after
heat treatment.
The 19F MAS-NMR spectra of as-made glasses measured at 9 and
13 kHz spinning speeds are shown in Fig. 2a. In the sample con-
taining the smallest amount of strontium (0.8Sr) two central bands
were found, at ꢀ23 and +22 ppm, the second partially overlapping
with the spinning sideband belonging to the first. They originate
from fluorine ions interacting with the local structure of the first
coordination sphere F–La(3), and with three near and one more
Temperature of
heat
treatment, time
2 h (°C)
Average
crystallite
size (nm)
760
900
990
12
180
176
Fig. 5. XRD analysis of 0.6Sr glass after heat treatment at 760 °C.
Fig. 6. XRD analysis of high strontium content glass (0.4Sr): as-made (top) and after heat treatment at 760 °C (bottom).

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82
distant lanthanum neighbors F–La(3 + 1), respectively [20]. With
the increase of strontium concentration (sample 0.6Sr) these lines
become more pronounced and wider, but they almost completely
disappear in the 0.4Sr sample, having the highest strontium con-
centration. This is the evidence of decreasing number of fluorine
ions surrounded by lanthanum atoms. At the same time, new cen-
tral bands appear already in the spectrum of 0.6Sr sample at ꢀ65
and ꢀ90 ppm, the latter becoming dominant in the case of 0.4Sr
sample. The observed changes in the NMR spectrum with the in-
crease of strontium content result from the increasing coordination
of fluorine ions by Sr2+ ions. The line at ꢀ90 ppm can be assigned to
the local structure of F–Sr(4) type, and its formation is accompa-
nied by the decrease of the number of fluorine atoms that are close
to La3+ ions. Using similar arguments, we can assign the line at
ꢀ65 ppm to the local structure of F–Sr(4 ꢀ n)La(n) type, where
fluorine is coordinated by lanthanum and strontium ions simulta-
neously. The result show that the increase of strontium content
causes significant changes occurring in the fluoride sub-network
of the glass. Comparing to other bivalent modifiers, i.e. Mg, Ca,
Ba used in oxyfluoride glasses [10], Sr has the highest affinity to
fluorine ions. Therefore it will strongly compete with lanthanum
for fluorine.
are in good agreement with the changes observed in the NMR
spectra.
The above results suggest that the central band observed in the
range from ꢀ68 to ꢀ65 ppm in the 19F MAS-NMR spectrum origi-
nates from the local structure of F–[Sr(4 ꢀ n),La(n)] type, in which
fluorine anions coordinate with both lanthanum and strontium
cations.
4. Conclusions
The 19F MAS-NMR studies demonstrate the strong influence of
SrO on the position and coordination of fluorine atoms in the
glass framework. It causes significant changes in the first crystal-
lization step: LaF3 only, LaF3 and Sr0.69La0.31F2.31, and LaSr2F7 only
fluoride phases are formed for 0.8Sr, 0.6Sr, and 0.4Sr glasses,
respectively. Furthermore, the NMR results unambiguously ex-
clude the occurrence of fluorine–sodium bonds in the oxyfluoride
aluminosilicate glasses containing LaF3 and modified by both
Na2O and SrO. Instead, the fluorine atoms prefer the bonding with
lanthanum and strontium. The mixed strontium–lanthanum fluo-
rides are formed in the cerammization process, when the SrO
concentration increases. This is manifested by the appearance of
the new central band in the 19F MAS-NMR spectrum at about
ꢀ65 ppm. The above results show that the addition of strontium
affects strongly the LaF3 crystallization kinetics and the structure
of the oxyfluoride glass. This way the process of forming the
nanocrystalline low-phonon-energy phase can be controlled to a
larger extent.
The 19F MAS-NMR spectra of glasses that were heat-treated at
700 °C for 2 h are shown in Fig. 2b. The positions of central bands
are very similar to that found in the original glasses, but the peaks
amplitudes are bigger while their FWHM widths much smaller.
This is the effect of cerammization process, which causes progres-
sive ordering of the structure around the fluorine ions. In the case
of the 0.4Sr sample, the process already began while the melt was
undercooled, which can be seen in Fig. 2a. For the samples contain-
ing smaller amount of Sr (0.6Sr and 0.8Sr), the thermal treatment
caused a significant intensity increase of +25 and ꢀ23 ppm lines,
corresponding to crystalline LaF3 phase. For larger Sr content
(0.4Sr), the pure crystalline LaF3 phase is not observed. Instead,
the mixed structures of La–F–Sr type and the Sr–F bonds are
formed, as manifested by the lines at ꢀ65 and ꢀ90 ppm, respec-
tively. Such effect was not seen in the glass samples containing
other modifiers, like MgO and CaO [10]. The origin of a weak cen-
tral band at ꢀ187 ppm that is present in the NMR spectra of all
heat-treated samples is not known at the moment.
Acknowledgements
The work was supported by AGH – University of Science and
Technology, Faculty of Material Engineering and Ceramics, Grant
No. 11.11.160.365/2011.
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