Journal of Non-Crystalline Solids 358 (2012) 1333–1338
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Journal of Non-Crystalline Solids
journal homepage: www.elsevier.com/ locate/ jnoncrysol
Electronic states of SnO-ZnO–P2O5 glasses and photoluminescence properties
Hiroyo Segawa a,⁎, Satoru Inoue a, Kiyoshi Nomura b
a
b
Optical and Electronic Materials Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
Dept. of Applied Chemistry, School of Engineering, University of Tokyo, Hongo 7-3-1, Bunkyoku, Tokyo 113-8656, Japan
a r t i c l e
i n f o
Article history:
Received 13 September 2011
Received in revised form 24 February 2012
Available online 30 March 2012
Keywords:
SnO-ZnO–P2O5 glasses;
Glass transition temperature;
Refractive index;
Mössbauer spectroscopy;
Photoluminescence
a b s t r a c t
SnO–ZnO–P2O5 glasses with 30 and 40 mol% P2O5 were prepared by a melting process in an air atmosphere.
The glass transition temperature, refractive index, and photoluminescence of the glasses were investigated.
The electronic states of Sn(II) and Sn(IV) were determined by Mössbauer spectroscopy. The PO4 units were
investigated by Raman spectroscopy. The glass transition temperature was lower than 450 °C, and decreased
as the Sn concentrations increased, so that the minimum was about 250 °C. The refractive index increased as
the Sn concentration increased. The emission spectra of the glasses peaked at around 2.0–3.0 eV and depended
on the glass compositions.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Glasses with low melting temperatures are interesting for many
applications, such as the molding of optical elements without the use
of plastics and sealing materials to join metals and ceramics.
SnO–P2O5 glasses are glass systems prepared at low melting temperatures and are known to have high refractive indices. In particular, the
nD of glasses containing 60–70 mol% SnO was larger than 1.75 [1].
However, SnO–P2O5 glasses exhibited poor chemical stabilities, and
their hygroscopic nature makes them unsuitable for many commercial
applications. Furthermore, SnO is easily oxidized to SnO2 in air, and
the atmosphere during the melting process must be controlled in
order to obtain glasses containing high SnO. On the other hand, ZnO is
known to be added in large quantities to P2O5 glass systems and to
form glass networks [2]. The addition of ZnO might be effective for
improving the chemical durability of the glasses.
SnO–ZnO–P2O5 glasses were obtained at low melting temperatures
and are potential alternatives to Pb-based sealing frits [3]. Their thermal
properties and chemical durability have not been investigated yet.
Recently, Masai et al. reported that SnO–ZnO–P2O5 glasses showed
photoluminescence (PL) properties at around 2.5–3.5 eV by irradiation
with UV light, and that the quantum efficiency values were high [4].
From the data on the ratio of Sn(II) to Sn(IV), they suggested that the
PL occurred by Sn(II). However, the existence of Sn(IV) could demonstrate the presence of the PL in SnO2–SiO2 glasses [5]. The photoluminescence at 2.5–3.5 eV of the Sn-doped silica glasses is attributed to a
⁎ Corresponding author. Tel.: + 81 29 860 4601; fax: + 81 29 854 9060.
E-mail address: [email protected] (H. Segawa).
0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jnoncrysol.2012.03.001
radiative T1 → S0 transition of Sn-related neutral oxygen vacancies
[6,7]. In the case of SnO2 crystal, a broad peak at around 2.5 eV was
observed [8] and was assigned to oxygen vacancies [9]. The coexistence
of Sn(II) and Sn(IV) in glasses might affect the PL spectra.
In this paper, SnO–ZnO–P2O5 glasses were fabricated in air and the
glass-forming region was investigated and the electronic state of Sn
was investigated by Mössbauer spectroscopy. The thermal and optical
properties of the glasses were measured and the dependence of the
glass composition was investigated.
2. Experimental procedure
Glasses of SnO–ZnO–P2O5 were prepared from reagent-grade SnO,
ZnO, and NH4H2PO4 (Wako Pure Chemical Industries) using a combinatorial glass melting apparatus that could melt 18 samples at a time [10].
The raw materials were mixed, 5 g batches were prepared, and the
batches were melted in SiO2 crucibles in air for 1 h at 1000 °C.
The melts were poured onto a stainless plate and annealed for 1 h at
Tg+ 10 °C. Amorphous states were confirmed by observation of the
externals.
The glass transition temperature, Tg, was measured using TG/DTA
(Bruker). TG/DTA was measured for 20 mg crushed glasses, using
alumina powder as a reference, and at the heating rate of 10 °C/min.
Glass compositions were determined by the use of inductively
coupled plasma (ICP) after the glass powders were dissolved in HCl
and HF aqueous solution at 105 °C. The concentrations of the OH groups
were determined by a peak at around 3000 cm− 1 measured for the
1-mm-thick glass samples by FT-IR spectrometry (Perkin Elmer,
Spectrum GX), because the water generally remained in the phosphate
glasses.
H. Segawa et al. / Journal of Non-Crystalline Solids 358 (2012) 1333–1338
0
1334
0.2
1
0.4
0.8
0.6
0.6
0.8
0.4
1
0.2
0
0.2
0.4
0.6
0.8
1
Fig. 1. Glass forming regions described by batch compositions.
The ratio of Sn(II) to Sn(IV) was estimated by 119Sn transmission
Mössbauer spectroscopy. The 119Sn Mössbauer spectra were measured
at room temperature using a NaI scintillation counter, where 5 mCi
Ca119mSnO3 was used as the 23.8 keV γ-ray source. The isomer shift
(IS) was quantitatively referred to as a CaSnO3 absorber. Doppler
velocity was calibrated using a 10 mCi 57Co(Rh) source and α-Fe foil
as an absorber. The spectra were fitted as the sum of two or three
doublet peaks (component peaks) composed of Lorentzian curves by
the nonlinear least squares method.
Refractive indices of the glass samples polished to a thickness of
1 mm were measured at 633 nm by a prism coupler (Metricon).
PL and PL excitation (PLE) spectra from the polished surface of the
glasses were measured by a fluorescence spectrometer (Hitachi) at
room temperature. The PL and PLE spectra were measured for 320
and 480 nm, respectively.
Errors of the data were estimated by a few times of measurements.
3. Results
Fig. 1 shows the glass-forming region, in which the compositions of
the batches are plotted. Glasses containing over 40 mol% P2O5 were not
prepared because their refractive indices would not have been high. All
glasses containing 40 mol% P2O5 were fabricated and were transparent.
The glasses containing 20 mol% P2O5 were not poured due to their high
viscosity. Glasses containing 30 mol% P2O5 were fabricated at ZnO
concentrations between 40 and 60 mol%. The ZnO–P2O5 and SnO–
P2O5 binary glasses were devitrified partially. The other glasses containing 30 mol% P2O5 were not poured. When these glasses were kept in air
atmosphere in a few months, most of the glasses containing 40 mol%
P2O5 showed some crystals due to the less chemical durability, although
the glasses containing 30 mol% P2O5 did not change. The crystal volume
decreased with increase of ZnO concentration. This suggests that the
decrease of P2O5 is effective to improve the chemical durability and
the addition of ZnO improves it slightly.
The analyzed compositions of the glasses are summarized in Table 1
wherein the sample names are used. The water in the glasses was
neglected, because the OH group concentration was lower than 1%.
The SiO2 was contaminated from the crucibles. The contamination of
SiO2 in the 30 mol% P2O5 glasses was greater than that in the 40 mol%
P2O5 glasses. The compositions of SnO and SnO2 were determined
from the Mössbauer spectra. The P2O5 concentration increased
with the increasing addition of ZnO. This might mean that the ZnO evaporated more than the P2O5, resulting in the change in the P2O5
concentration.
Fig. 2 shows 119Sn Mössbauer spectra of (a) the glasses containing
30 mol% P2O5 and (b) those containing 40 mol% P2O5. The spectra
were deconvoluted by three doublet peaks. The fitting curves are
shown in Fig. 2. The peaks near IS ∼ 0 mm/s, which is called C1, were
assigned to a resonant doublet by Sn(IV) because the IS values were
determined for the IS of CaSnO3. The peaks near IS ∼ 0 mm/s look like
a singlet, and the quadrupole splitting (QS) values are almost 0. The
QS represents the distortion of the environment of Sn form cubic
symmetry [11]. The asymmetric peaks at 3 mm/s and 4 mm/s were
decomposed into the two doublets, called C2 and C3, respectively,
which are assigned to be due to Sn(II). The ratio of the doublet areas
of C1 to (C2 + C3) represents the ratio of Sn(IV) to Sn(II). In Fig. 2, the
Sn(IV) quantities in 30 mol% P2O5 glasses were larger than those in
40 mol% P2O5 glasses. The Sn(II) volume increased with the increase
in the Sn concentration in the 30 mol% and 40 mol% P2O5 glass systems,
respectively. From the results of the deconvolution of 119Sn Mössbauer
spectra of each glass, the molar ratios of Sn(II) and Sn(IV) were calculated and the analytical concentrations of SnO and SnO2 were obtained as
shown in Table 1.
Fig. 3 plots the Sn(IV) concentration for the total concentration of
Sn. For the glasses containing 40 mol% P2O5, the ratio of Sn(IV)
increased almost up to 10% as the Sn concentration increased. For
the glasses containing 30 mol% P2O5, the ratio of Sn(IV) was much larger than that of the glasses with 40 mol% P2O5 and decreased as the
Sn concentration increased. This means that the Sn(II) changed to
Sn(IV) and that the changes were caused easily when the P2O5 concentration was small. The redox of the melts is expressed as,
2þ
4þ
−
Sn ↔Sn þ 2e ;
2NH3 þ 7=2O2 ↔2NO2 þ 3H2 O:
From these equations, it was found that Sn(IV) is formed by the
oxidation of Sn(II). The ammonium ions from the P2O5 source change
to NO2 due to the reduction. Thus, Sn(IV) was formed in 30 mol% P2O5
glasses more than in the 40 mol% glasses.
Table 1
Compositions of batches and glasses analyzed by ICP.
Sample
Batch (mol%)
Glass (mol%)
ZnO
SnO
P2O5
ZnO
Error
SnO
Error
SnO2
Error
SnO + SnO2
P2O5
Error
SiO2
Error
4Z3S3P
5Z2S3P
6Z1S3P
6S4P
1Z5S4P
2Z4S4P
3Z3S4P
4Z2S4P
5Z1S4P
6Z4P
40
50
60
0
10
20
30
40
50
60
30
20
10
60
50
40
30
20
10
0
30
30
30
40
40
40
40
40
40
40
27.0
35.7
45.7
0.0
7.2
12.2
19.7
27.5
35.9
45.2
0.1
0.1
0.1
0.0
0.0
0.0
0.0
0.1
0.1
0.0
25.0
16.5
7.9
53.2
45.0
38.6
31.2
21.5
12.3
0.0
0.1
0.0
0.0
0.2
0.2
0.2
0.0
0.1
0.1
0.0
10.3
8.2
5.1
3.7
2.8
3.3
1.7
1.4
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
35.4
24.7
13.1
56.9
47.9
41.9
32.9
22.9
12.3
0.0
36.9
39.0
41.0
39.9
42.5
44.1
46.4
49.0
51.4
54.4
0.2
0.0
0.1
0.2
0.2
0.2
0.1
0.0
0.1
0.0
0.8
0.6
0.3
3.1
2.4
1.9
1.0
0.6
0.3
0.3
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
H. Segawa et al. / Journal of Non-Crystalline Solids 358 (2012) 1333–1338
1335
12
a
Sn(IV) (mol%)
10
8
6
4
2
0
0
10
20
30
40
50
60
Analyzed Sn (mol%)
Fig. 3. Plots of Sn(IV) concentration for total concentration of Sn. Closed circles: 30 mol%
P2O5 series; open circles: 40 mol% P2O5 series. All error magnitude was included in the
symbols.
b
IS values were larger than those of amorphous SnO (2.87 mm/s [12])
and the covalency of Sn\O is larger than that of the SnO. This means
that the covalency of the Sn\O bond increased as the Sn concentration
a
3.7
Isomer shift (mm/s)
3.6
3.5
3.4
3.3
3.2
0
10
20
30
40
50
60
50
60
Total Sn (mol%)
b
2.2
Q splitting (mm/s)
2
Fig. 2. 119Sn Mössbauer spectra of (a) the glasses containing 30 mol% P2O5 and (b) 40 mol%
P2O5. Circles: measured data. Solid lines: fitting curves for C1 (green), C2 (blue), and
C3 (red).
Fig. 4 shows (a) IS and (b) QS values of C2 and C3 peaks for Sn
concentration. The IS decreased as the Sn concentration increased. The
IS values reflect the s-electron density around the Sn nucleus [11]. The
1.8
1.6
1.4
1.2
1
0
10
20
30
40
Total Sn (mol%)
Fig. 4. (a) IS and (b) QS values of C2 (diamonds) and C3 (circles) curves for Sn concentration; closed: 30 mol% P2O5 series and open: 40 mol% P2O5 series.
H. Segawa et al. / Journal of Non-Crystalline Solids 358 (2012) 1333–1338
Isomer shift (mm/s)
1336
-0.1
450
-0.2
400
-0.3
350
-0.4
300
-0.5
250
-0.6
0
10
20
30
40
50
60
200
0
10
20
30
40
50
60
Total Sn (mol%)
Fig. 5. IS values of C1 curve for Sn concentration. Closed circles: 30 mol% P2O5 series; open
circles: 40 mol% P2O5 series. Error magnitude of some data was included in the symbols.
Fig. 7. Glass transition temperature versus Sn concentration. Closed circles: 30 mol% P2O5
series; open circles: 40 mol% P2O5 series. All error magnitude was included in the symbols.
increased. The QS did not depend on the Sn concentration. This indicates that the distortion of the Sn environment did not change even if
the glass composition changed.
Fig. 5 shows the IS values of the C1 doublet peak for the Sn concentration. The IS values of C1 were about −0.4 mm/s and did not depend on
the glass compositions. The IS was lower than the SnO2. The shifts mean
that the electron of Sn is less than that of SnO2; that is, the electrons on
the Sn atoms were attracted by oxygen atoms around the Sn(IV).
Fig. 6 shows Raman spectra of (a) 30 mol% P2O5 glasses and (b)
those of 40 mol% glasses. The band at the range from 1000 to
1200 cm− 1, B1, is assigned to the P\O stretch of non-bridging oxygens
in PO4 units [2]. The band at around 700 cm− 1, B2, is assigned to the
P\O\P stretch in PO4 units. In Fig. 6(a), the peaks around 1150 cm− 1
in B1 band are assigned to the P\O stretch in Q 1 species, in which a
PO4 unit has one bridging oxygen and three non-bridging oxygens.
This means that most of the PO4 units are terminated in the glass. The
spectra did not depend on the glass compositions, although the B1
band shifted slightly lower with increase of ZnO concentration. It was
found that the PO4 units exist in similar environments in 30 mol%
P2O5 glasses. In Fig. 6(b), the B2 band did not depend on the glass compositions; however, the B1 band shifted lower and the peak shifted from
1120 to 1200 cm− 1 when the ZnO concentration increased. The B1
band was composed of two peaks, in which a peak at around
1140 cm− 1 and that at around 1200 cm− 1 are assigned Q1 and Q 2 [2].
The shift of B1 band is caused by the increase of the Q 2 peak and
decrease of Q1 peak as the ZnO increases. This means that the PO4
units change from Q1 to Q2 and form a chain network as ZnO increases.
a
b
1.75
1.70
1.65
1.60
1.55
Fig. 6. Raman spectra of (a) 30 mol% P2O5 glasses and (b) 40 mol% glasses.
0
10
20
30
40
50
60
Fig. 8. Refractive index versus Sn concentration. Closed circles: 30 mol% P2O5 series;
open circles: 40 mol% P2O5 series. All error magnitude was included in the symbols.
H. Segawa et al. / Journal of Non-Crystalline Solids 358 (2012) 1333–1338
a
a
1337
1.8
1.7
Intensity (a.u.)
1.6
1.5
1.4
1.3
1.2
1.1
1.5
2
2.5
3
3.5
4
4.5
1
Photon energy (eV)
b
b
0
10
20
30
40
50
60
0
2
4
6
8
10
12
1.8
Intensity (a.u.)
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.5
2
2.5
3
3.5
4
4.5
Photon energy (eV)
1
Fig. 9. PL (ex = 320 nm) and PLE (em = 480 nm) spectra of (a) 30 mol% P2O5 glasses
and (b) 40 mol%.
Fig. 10. Stokes shift versus (a) SnO concentration and (b) SnO2 concentraion. Closed
circles: 30 mol% P2O5 series; open circles: 40 mol% P2O5 series.
In the 40 mol% P2O5 glasses, P2O5 increased as the ZnO concentration
increased, as shown in Table 1, resulting in the formation of a chain
network of PO4 units containing more than 50 mol% P2O5.
Fig. 7 shows the glass transition temperature versus the Sn concentration. Tg decreases as Sn concentration increases. This means that the
addition of Sn is effective for lowering Tg. SnO has lower melting point
than ZnO. Thus, decrease of Tg was caused by addition of SnO.
The refractive indices at 633 nm are plotted for the Sn concentrations in Fig. 8. The refractive index increased as the Sn concentration
increased. The addition of Sn is effective for increasing the refractive
index, the same as in the SnO–P2O5 glass system [1].
Fig. 9 shows PL and PLE spectra of (a) 30 mol% P2O5 glasses and
(b) 40 mol%. The spectra were normalized by the PLE spectra in order
to compare them. In Fig. 9(a), the PLE peaks shifted to the high energy
side with increasing ZnO concentration. The PL band depended on the
glass compositions; however, the peak position is between 2.5 and
3.0 eV. In Fig. 9(b), the PLE peaks shifted to the high energy side with
increasing ZnO concentration, depending on the band edges of the
glasses. The intensities of the PL peaks of the 4Z2S4P and 5Z1S4P glasses
were smaller than the others because the Sn concentration was low. The
peak positions of 3Z3S4P, 4Z2S4P, and 5Z1S4P glasses were in the range
of 2.5 to 3.0 eV, and those of 6S4P, 1Z5S4P, and 2Z4S4P glasses were
about 2.0 to 2.5 eV, smaller than the others.
4. Discussion
4.1. Chemical bonding in glass
Mössbauer data showed that the covalency of the Sn\O bond
increased as the Sn concentration increased. In the glass containing
40 mol% P2O5, Raman shifts were not clear because the B1 band
composed of two peaks assigned to Q 1 and Q 2. However, in the glass
containing 30 mol% P2O5, Raman band assigned to Q 1 units was shifted
to long wavenumber with increase of Sn concentration. This suggests
that the P\O bond was strengthened by addition of Sn. The changes
of chemical bonding in glass are caused instead by the addition of Zn
and Sn.
The field strength is expressed as Z / r2 (Z: cation charge and r: ionic
radius). The ionic radius of Sn(II), Sn(IV) and Zn(II) is 0.93, 0.71 and 0.74
[13], respectively. The field strength of Sn(II) is smaller than that of
Zn(II). This means that the electron on oxygen is attracted more
strongly to Zn ions than to Sn ions. Thus, the covalency of Sn\O was larger than that of Zn\O. The electron on oxygen was attracted to Zn
when ZnO was added instead of SnO. Thus, the electrons between
P\O networks were attracted to Zn and the P\O network might be
weak and the Raman band assigned to Q1 shifted to lower wavenumber.
However, the shift was not large, because the formation of Sn(IV),
1338
H. Segawa et al. / Journal of Non-Crystalline Solids 358 (2012) 1333–1338
which has large field strength, might weaken the effect of Zn. The
increase of the Sn\O covalency might be effective to increase the
refractive index.
4.2. Effects of the ratio of Sn(II) and Sn(IV) on PL spectra
From Table 1, the Sn concentrations and the ratio of Sn(II) to Sn(IV)
of 3Z3S4P are similar to those of 4Z3S3P, resulting in similar PL spectra.
Although the PL spectra are known to be related to the defect in the
oxygen around Sn in the SiO2 glasses [5–7], it is difficult to deconvolute
the band into a few peaks. In particular, the deconvolution of the PL
spectra assigned to Sn(II) and Sn(IV) might be impossible. Thus, Stokes
shift was plotted in Fig. 10 to discuss the effect of Sn(IV) on the PL. In
Fig. 10(a), the Stokes shift plotted for SnO concentration. Stokes shift
in the glass containing 40 mol% P2O5 increased with increasing the
SnO concentration. However, Stokes shift decreased in the case of the
glass containing 30 mol% P2O5. The glass containing 30 mol% P2O5
contained much more SnO2 than the 40 mol% P2O5 glass. Thus,
Fig. 10(b) shows the relationship between Stokes shift and SnO2 concentrations. The Stokes shift increased until 5 mol% and decreased
with the increase of SnO2. This suggests that the Sn(IV) affects the PL
spectra.
5. Conclusions
The thermal and optical properties of SnO–ZnO–P2O5 glasses with 30
and 40 mol% P2O5 prepared by a melting process in air atmosphere were
investigated. Sn(IV) was formed when the P2O5 concentration was
30 mol%. The covalency bonds between Sn and oxygen increased as the
Sn concentration increased. Most of the PO4 units were terminated in
the glasses. The glass transition temperature decreased and the refractive
index increased as the Sn concentration increased. This means that the
addition of Sn was effective for the preparation of glasses with high
refractive indices at a low melting temperature. The emission spectra of
the glasses peaked at around 2.0–3.0 eV and changed depending on the
glasses. The Stokes shift depended on the SnO2 concentration.
References
[1] J. Cha, T. Kubo, H. Takebe, M. Kuwabara, J. Ceram. Soc. Jpn. 116 (2008) 915–919.
[2] Richard K. Brow, David R. Tallant, Sharon T. Myers, Carol C. Phifer, J. Non-Cryst.
Solids 191 (1995) 45–55.
[3] R. Morena, J. Non-Cryst. Solids 263&264 (2000) 382–387.
[4] H. Masai, Y. Takahashi, T. Fujiwara, S. Matsumoto, T. Yoko, Appl. Phys. Express 3
(2010) 082102.
[5] T. Hayakawa, T. Enomoto, M. Nogami, Jpn. J. Appl. Phys. 45 (2006) 5078–5083.
[6] L. Skuja, J. Non-Cryst. Solids 149 (1992) 77–95.
[7] L. Rebohle, J. von Borany, W. Skorupa, H. Fröb, S. Niedermeier, Appl. Phys. Lett. 77
(2000) 969–971.
[8] G. Blattner, C. Klingshirn, R. Helbig, Solid State Commun. 33 (1980) 341–344.
[9] J. Jeong, S.-P. Choi, C.I. Chang, D.C. Shin, J.S. Park, B.-T. Lee, Y.-J. Park, H.-J. Song,
Solid State Commun. 127 (2003) 595–597.
[10] S. Inoue, S. Tosoroki, T. Konishi, T. Araki, T. Tsuchiya, Appl. Surf. Sci. 223 (2004)
233–237.
[11] T. Nishida, M. Katada, Y. Takashima, Bull. Chem. Soc. Jpn. 57 (1984) 3566–3570.
[12] G.S. Collins, T. Kachnowski, N. Benczer-Koller, M. Pasternak, Phys. Rev. B 19
(1979) 1369–1373.
[13] R.D. Shannon, C.T. Prewitt, Acta Crystallogr. B25 (1969) 925–946.