Luminescence of Sn2+ center in ZnO-B2O3 glasses melted in air and Ar conditions

Article abstract of DOI:10.1246/bcsj.20150145

The authors report on luminescence of SnO-doped 60ZnO-40B₂O₃ (SZB) glasses melted in air and Ar atmosphere. In photoluminescence (PL), excitation spectra consist of two excitation bands, and the lower excitation band, emerging at low

Full text of DOI:10.1246/bcsj.20150145

Selected Paper
Luminescence of Sn2+ Center in ZnO-B2O3 Glasses Melted
in Air and Ar Conditions
Hirokazu Masai,*1 Yuto Suzuki,1 Takayuki Yanagida,2 and Ko Mibu3
1Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011
2Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0101
3Department of Engineering Physics, Electronics, and Mechanics, Graduate School of Engineering,
Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Aichi 466-8555
E-mail: masai_h@scl.kyoto-u.ac.jp
Received: April 22, 2015; Accepted: May 12, 2015; Web Released: May 18, 2015
The authors report on luminescence of SnO-doped 60ZnO-40B2O3 (SZB) glasses melted in air and Ar atmosphere.
In photoluminescence (PL), excitation spectra consist of two excitation bands, and the lower excitation band, emerging at
lower energy with increasing amount of SnO, affects the optical absorption edge. The present SZB glasses can exhibit
light emission by irradiation of near UV light, which is different from the zinc phosphate glass system. X-ray induced
scintillation spectra indicate that concentration of Sn2+ mainly affects the intensity. 119Sn Mössbauer spectra show that a
certain amount of Sn2+ centers in the SZB glass was oxidized to Sn4+ even when melted under the inert atmosphere,
which is one of the reasons for the low PL quantum efficiencies, approximately 60%. The 119Sn Mössbauer spectra also
suggest site dispersion of Sn2+ centers, whose local coordination states affect the optical absorption and luminescent
properties.
Oxide glasses possessing transparency, chemical durability,
and wide chemical composition range are one of the funda-
mental materials in our daily lives. Because glasses have no
structural ordering in the long range, the good formability is
also an advantage for practical applications. Although such
oxide glasses generally exhibit little light emission without a
strong laser light source, fiber-type lasers using glass-based
devices have been used by doping with rare earth (RE) cations
possessing a sharp emission band. On the contrary, non-rare
earth cations exhibiting light emission have attracted atten-
tion not only from ubiquitous presence but also from unique
emission properties. The ns2-type cation (n ½ 4) is an emission
center that gives strong photoluminescence (PL) because of the
phosphor. In addition, we have also demonstrated white light
emission from MnO-codoped SnO-ZnO-P2O5 glasses using
energy transfer from Sn2+ to Mn2+ centers.10 Thus, the emis-
sion consisting of broad bands can be tailored by addition of
another emission center, or by changing the local coordination
field of the Sn2+ center (i.e. by changing chemical composition
of the mother glasses). On the other hand, radioluminescence
(RL) properties of the same glass system have been report-
ed.13,14 More recently, we have demonstrated UV-excited light
emission of Sn2+-doped strontium borate glasses, and dis-
cussed the energy diagram by comparison with oxide crystal
possessing stoichiometric chemical composition.15 In a pre-
vious report,15 it was found that borate systems can also exhibit
high QE values around 80%. From these reports, we have
emphasized that the Sn2+ center is a fascinating non-RE center
in glasses.
In the present study, we examined luminescence of Sn-doped
zinc borate glasses by UV and X-ray irradiation. Various kinds
of ZnO-B2O3 glasses have been examined as a host material
for luminescent applications.16-21 Although it is easily expected
that zinc borate glasses can be a good host of the Sn2+ center
for PL and RL applications, the number of previous reports
on Sn2+ in zinc borate glasses is limited. It therefore follows
that examination of the luminescence is worthwhile. Here, the
chemical composition of the mother glass was selected as
60ZnO-40B2O3 for comparison with the previous phosphate
60ZnO-40P2O5 system.9,22 To examine the effect of basicity of
oxide matrix on the emission properties of the Sn2+ emission
parity allowed transition.1,2 The emission centers, such as Sn2+
,
Sb3+, Tl+, and Pb2+, which possess electrons in the outermost
shell in both the ground state (ns2) and the excited state
(ns1np1), exhibit a broad emission with a lifetime of the
microsecond order,1 and the PL intensity of the emission center
is generally stronger than that of transition metal emission
centers whose emission is inherently forbidden. For example,
Sb3+, Mn2+-codoped halo-calcium phosphate crystal has been
used for practical white fluorescent lamps,1,3 and several
authors have been focused on the ns2-type centers in glass.4-13
Recently, the authors reported PL of Sn2+-doped ZnO-P2O5
low-melting glasses.9-12 The glasses showed blue light emis-
sion with high quantum efficiency (QE) by deep UV irradia-
tion. It is notable that the transparent Sn2+-containing glasses
show intense UV-excited emission comparable to crystal
Bull. Chem. Soc. Jpn. 2015, 88, 1047–1053 | doi:10.1246/bcsj.20150145
© 2015 The Chemical Society of Japan | 1047

center is also meaningful from the viewpoint of the local
structure in the glass matrix. In order to clarify the oxidation
reaction during the melting in air, we have prepared zinc borate
glasses melted in air and Ar. By studying the luminescence
properties of the glasses, this report provides a valuable guide-
line for preparation of activator-doped inorganic borate glasses
by melt quenching.
2.0 Ar
1.0 Ar
0.5 Ar
0.1 Ar
x = 0
Experimental
0.1 air
0.5 air
1.0 air
2.0 air
The xSnO-(60 ¹ x)ZnO-40B2O3 (xSZB) glasses were pre-
pared according to a conventional melt quenching method
using a platinum crucible.22 Batches consisting of SnO (99.5%),
ZnO (99.9%), and B2O3 (99.9%), which were purchased from
Kojundo Chemical Laboratory Co., Ltd. and stored in a
desiccator, were mixed and melted at 1100 °C for 30 min in
air and Ar (99.999%) atmosphere. The glass melt was quenched
on a stainless plate maintained at 200 °C and then annealed at
the glass-transition temperature, Tg, as measured by differential
thermal analysis (DTA), for 1 h. After cutting (10 mm © 10
mm © 1 mm), the glass samples were polished with aqueous
diamond slurries. Because the sample size for measurement
was fixed, we can quantitatively compare the luminescence
intensity among these samples.
450 500 550 600 650 700 750 800
Temperature/°C
Figure 1. Powder DTA patterns of the xSZB glasses with
varying Sn contents. Tg of each sample is shown using
arrows. Triangle and dashed line indicate the first crys-
tallization peak of each glass.
The absorption spectra were measured using an U3500
UV-vis-NIR spectrophotometer (Hitachi, Japan). The PL and
PL excitation (PLE) spectra were measured at room temper-
ature using an F7000 fluorescence spectrophotometer (Hitachi,
Japan). On PL measurements, band pass filters were used for
the excitation (2.5 nm) and the emission (2.5 nm). The emission
decay at room temperature was measured using a Quantaurus-
Tau (Hamamatsu Photonics, Japan). The excitation light source
for emission decay measurement was an LED operated at a
photon energy of 4.43 eV and a frequency of 10 kHz. The
internal quantum efficiency (QY) was evaluated using a
Quantaurus-QY (Hamamatsu Photonics, Japan). RL spectra
by X-ray radiation at room temperature were measured using
the monochromator equipped charge coupled device (CCD,
Andor DU-420-BU2).23 The supplied bias voltage and tube
current were 40 kV and 0.052-5.2 mA, respectively. The irra-
diated dose was calibrated by using an ionization chamber.
119Sn Mössbauer spectra, i.e., absorption spectra of γ-rays by
the 119Sn nuclei in the samples, were measured in conventional
transmission geometry using a Ca119mSnO3 source at room
temperature. The energy of the γ-rays from the source was
modulated by the Doppler effect using a velocity transducer
with a constant acceleration mode, and the abscissa of the
spectra was expressed with the unit of the Doppler velocity as
in the literature. The valence states of the Sn atoms, which are
sensitively reflected as the peak positions in 119Sn Mössbauer
spectra,24 were deduced by fitting the measured spectra using
standard software Normos (made by R. A. Brand, commer-
cially available from WissEl GmbH).
these glasses were in the range of 545 « 3 °C, and no signifi-
cant difference was observed in Tg based on the chemical com-
positions. We can find that temperature of the first crystal-
lization peak Tp increases with increasing amount of Sn as
shown in Figure 1. The quantity ¦T, Tp ¹ Tg, is defined as the
thermal stability parameter for crystallization of the glass. The
change of ¦T indicates that the glass network is stabilized
by addition of SnO. Figure 2a shows the optical absorption
spectra of xSZB glasses (x = 0, 0.1, 0.5, 1.0, and 2.0) melted
in air. Since the origin of the absorption edge is Sn2+ cation
(intra-atomic 5s2-5s15p1 transition), we hereby treat as a direct
transition.25 The absorption edge is red-shifted with increasing
the amount of SnO. It was found that the absorption edge of the
SnO-doped oxide glasses correlated with the local coordination
state of the Sn2+ emission center.11,12 Based on the previous
reports,11,24 we introduce the optical band edge Eg opt which is
evaluated by the extrapolation of the linear portion of the
absorption spectra as shown in Figure 2a. Figure 2b shows the
differential Eg opt (¦Eg opt) values from the nondoped glass as a
function of Sn amount. By fitting an exponential function, it is
found that the slope of ¦Eg opt of the xSZB glasses melted in
Ar is 1.3 times steeper than that of the glasses melted in air.
In our previous reports,12 we have found that the shift of opti-
cal absorption also correlates with the actual concentration of
the Sn2+ center, and that almost 100% of tin species can exist
as Sn2+ valence states in ZnO-P2O5 glasses melted under Ar.
Therefore, it suggests that the observed difference between the
glass melted in Ar and one melted in air is due to the real
amount of Sn2+ center in each glass, and it also indicates an
oxidation reaction of Sn2+ into Sn4+ during the melting in air.
Figure 3 shows the PL-PLE contour plots of the xSZB
glasses: x = 0.1 and 1.0 using an intensity axis on a linear
scale. These intensities are normalized using the peak in order
to compare these spectrum shapes. The PL peak intensities of
the xSZB glasses melted in Ar are approximately 1.2 times as
Results and Discussion
PL Properties of xSZB Glasses Melted in Air and in Ar.
In the composition range of x = 0-2.0, the obtained xSZB
glasses were colorless and transparent independently of the
preparing atmosphere. Figure 1 shows the DTA curves of the
xSZB glasses with varying Sn contents. The values of Tg for
1048 | Bull. Chem. Soc. Jpn. 2015, 88, 1047–1053 | doi:10.1246/bcsj.20150145
© 2015 The Chemical Society of Japan

(a)
obtained by scanning of the peak energy of PL spectra.
Although band overlap of PL and PLE is observed around
3.5 eV, we confirm that the SZB glass shows light emission by
near-UV light (365 nm) shown in Figure 4b. Because no emis-
sion was observed in the zinc phosphate system in the same
irradiation condition, it is an advantage of the borate glasses
in comparison with phosphate glasses. Figure 4c shows the
concentration dependence of peak energies of PL and PLE
bands of the SZB glasses. With increasing Sn amount, both the
excitation peaks and the emission peaks red-shift. Comparing
Figure 4c with optical absorption edge shown in Figure 2b, we
have found that the change of PLE peak does not correspond to
the change of the optical absorption edge. Such correlation
between the optical absorption edge and the PLE peak was
observed in the SrO-B2O3 glass system,15 but not observed in
the ZnO-P2O5 glass system.9,11,12 Thus, we think that the emer-
gence of excitation band at lower photon energy by addition
of SnO, whose intensity is lower than that of the excitation
peak band, is characteristic of the borate glass system. Based on
the obtained results as shown above, the emerging band affects
the optical absorption edge and the light conversion of near-UV
light, which is different from the phosphate glass system.
The emission decay curves monitored at 2.95 eV for the
xSZB glasses melted in air are shown in Figure 5, where the
samples were irradiated using UV light of 4.43 eV. It was
reported that the Sn2+ center possessing C2v symmetry in SiO2
glass exhibits the S1 excitation band at 4.9 eV.6 On the other
hand, we recently reported that the higher-energy band is due
to 2-coordinated Sn2+ whereas the lower band is due to Sn2+
center possessing high coordination number in phosphate
glass.12 The decay curves consist of two components: one is
faster (lifetime, ¸¤1/e, of nanoseconds), and the other is slower
(¸¤¤1/e of microseconds). Considering previous decay constants
of the Sn2+ center in oxide crystals or oxide glasses, the shown
decay with the order of lifetimes of microseconds is due to the
T1¼S0 radiative transition, and the ¸1/e lifetime is calculated
as ca. 5.8 « 0.2 ¯s, which is almost independent of the SnO
amount. The decay constant is comparable to that in other
borate glass system.15 On the other hand, the origin of faster
decay is not clarified. Because a lifetime of the F+ or F cen-
ter in oxide glasses is reported to be ca. 10¹2-10 s,26,27 the
possibility is negligible. Although the details have not been
clarified yet, plausible origins are (1) the defect center in the
host zinc borate glass,21 or (2) S1-S0 relaxation of the Sn2+
center.1,15
70
60
50
40
30
20
10
0
x = 0
0.1
0.5
1.0
2.0
Eg opt
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Photon energy/eV
(b)
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
E
g opt (air)
Eg opt (Ar)
0.0
0.5
1.0
1.5
2.0
Sn amount/mol%
Figure 2. (a) Optical absorption spectra of xSZB glasses
(x = 0, 0.1, 0.5, 1.0, and 2.0) melted in air. (b) Differential
Eg opt (¦Eg opt) values from the nondoped ZB glass as a
function of Sn amount.
large as those of the glasses melted in air. As shown in
Figure 3, an additional PLE band appears in the excitation
in the lower energy region in the xSZB glasses melted in Ar,
and the intensity is enhanced by the inert preparation. These
figures indicate that the higher the excitation energy becomes,
the higher the emission peak energy becomes. The intensity of
the emission takes the maximum at a composition of x = 1.0,
which is much larger in comparison with SnO-doped strontium
borate glass system containing 75 mol % of B2O3.15 Although
the present divalent cation, Zn2+, is different from that in
the previous report (Sr2+), we can speculate that the network
forming B2O3 concentration affects the dispersion of dopant
cation. Future study is needed to examine the correlation
between chemical composition and dispersion of doped emis-
sion centers.
Figure 6 shows the relationship between the SnO concen-
tration and internal quantum efficiency (QE) of the xSZB
glasses melted in air and in Ar. The magnitudes of the glasses
melted in Ar are higher than that melted in air, and the QE
values of the glass melted in Ar atmosphere are around 60%,
and slightly decrease in the case of 2.0 mol % addition. It is
notable that the maximum QE values of the ZnO-B2O3 glasses
are at least 20% lower than those of ZnO-P2O5 glasses.11 On the
other hand, the zinc borate system can possess light emission by
excitation of near-UV light (365 nm), although the efficiency
is not so large. Although the mechanism has not been clarified
yet, one of the plausible reasons is the basicity of the host
matrix which affects the radiative/nonradiative relaxation
rates.
In all chemical compositions, emission intensity of PL-PLE
spectra of the SZB glass melted in Ar was higher than that
of the glass melted in air. Normalized PL-PLE spectra of the
0.1SZB glasses prepared in different atmospheres are shown
in Figure 4a. The PL spectra were obtained by excitation of
the peak energy of PLE spectra whereas the PLE spectra were
Bull. Chem. Soc. Jpn. 2015, 88, 1047–1053 | doi:10.1246/bcsj.20150145
© 2015 The Chemical Society of Japan | 1049

air
Ar
6.0
5.5
5.0
4.5
4.0
PLE
6.0
5.5
5.0
4.5
4.0
PLE
0.1
0.1
PL
PL
2.0
2.5
3.0
3.5
4.0
2.0
2.5
3.0
3.5
4.0
Emission energy/eV
Emission energy/eV
6.0
5.5
5.0
4.5
4.0
6.0
5.5
5.0
4.5
4.0
PLE
PLE
1.0
1.0
PL
PL
2.0
2.5
3.0
3.5
4.0
2.0
2.5
3.0
3.5
4.0
Emission energy/eV
Emission energy/eV
Intensity (arb. units)
low
high
Figure 3. PL-PLE 3D contour plots of the xSZB glasses prepared in air and Ar. Intensities are normalized using the peak intensity in
order to compare these spectra shapes. Dashed lines indicate the peak energies of each excitation and emission energy.
X-ray-Induced Scintillation Properties of xSZB Glasses
Melted in Air and in Ar. It was reported that Sn2+-doped
oxide glasses exhibit scintillation by X-ray irradiation.13,14
Therefore, it is natural to assume that the present zinc borate
glasses also show the emission. Figure 7a shows X-ray-
induced scintillation spectra of the xSZB glasses (x = 0, 0.1,
0.5, 1.0, and 2.0) melted under Ar. In Sn-free glass, broad
emission attributable to defects in the glass is observed. On
the other hand, the broad emission due to Sn2+ centers is
observed from Sn2+-doped samples. The peak energies of
the xSZB glasses prepared in both conditions are red-shifted
with increasing Sn amount, corresponding to the PL peak shift
shown in Figure 4b. Although the peak energies of scintillation
are similar to those of PL, which suggests the emission-related
energy levels of PL and scintillation are the same, the emission
intensity roughly increases with increasing the SnO amount.
Thus, in the X-ray-induced scintillation process, in which
secondary electrons generated from the host matrix by the
exposure activate the Sn2+ centers, the highest scintillation
intensity was obtained from the xSZB glasses containing much
higher SnO amount in comparison with that in the PL process
even though some concentration quenching may occur.
Figure 7b shows the correlation between the irradiated dose
and the scintillation intensity of the xSZB glasses melted in Ar.
In the Sn2+ concentration range (0 ¯ x ¯ 2), a linear depend-
ence of the absorbed dose is observed, suggesting a consid-
erable defect is not generated in the glass during the X-ray
irradiation. On the other hand, Figure 7c shows the correla-
tion between the scintillation intensity and additive amount of
the Sn species in the glasses. There is a deviation from the
linear dependence of the emission intensity at higher Sn-added
region. The deviation from the linear change suggests the
concentration quenching of the Sn2+ center. It is found that
(1) values of Eg opt reflect the Sn2+ concentration,12 and (2)
scintillation intensity also depends on the concentration of
activator,28 whose concentration quenching occurred in higher
amount compared with the photoluminescence process. There-
fore, we focus on the relationship between Eg opt and scin-
tillation intensity in order to estimate the amount of Sn2+ center
in the SZB glass. Figure 7d shows correlation between the
scintillation intensity and Eg opt of the SZB glasses prepared
in Ar. It has been previously suggested that the value of Eg opt
can be a standard for evaluation of Sn2+ species.11 As shown
in Figure 7d, there is a linear relationship between the scin-
tillation intensity and Eg opt. The correlation also suggests that
X-ray-induced scintillation intensity of the SZB glasses is
mostly governed by the amount of Sn2+ emission center.
Relationship between Sn2+ Concentration of xSZB
Glasses and the Luminescence. In order to examine the
correlation between the real Sn2+ content and optical properties
in these glasses, we evaluate the valence state of tin using 119Sn
Mössbauer spectroscopy. Figure 8a shows 119Sn Mössbauer
spectra of the 1SZB glasses prepared in air and in Ar along
with that of the 0.1SZB glass prepared in Ar. The peaks
1050 | Bull. Chem. Soc. Jpn. 2015, 88, 1047–1053 | doi:10.1246/bcsj.20150145
© 2015 The Chemical Society of Japan

(a)
0
–1
–2
–3
–4
0
–1
–2
–3
–4
x = 0.1
0.5
x = 1.0
Ar
Air
Ar
1.0
2.0
0
5000
Time/ns
10000
Air
PL
PLE
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
0
2000 4000 6000 8000 10000
Time/ns
Photon energy/eV
Figure 5. PL decay curves of the xSZB glasses melted in
Ar. Inset shows PL decay curves of the 0.5SZB glasses
prepared in Ar and air.
(b)
2SnO-60ZnO-
40P2O5
2.0SZB
70
60
50
40
(c) 5.0
4.5
air
Ar
PLE
4.0
¤ QE (Ar)
30
3.5
˜ QE (air)
air
20
3.0
0.0
0.5
1.0
1.5
2.0
Sn amount/mol %
Ar
PL
2.5
Figure 6. QE values of the xSZB glasses prepared in air
and Ar as a function of Sn amount.
0.0
0.5
1.0
1.5
2.0
Sn amount/mol %
Figure 4. (a) Normalized PL-PLE spectra of 0.1SZB
glasses prepared in air and Ar. The PL spectra were
obtained by excitation of the peak energy of PLE spectra
whereas the PLE spectra were obtained by scanning the
peak energy of PL spectra. (b) Photograph of 2.0SZB
glass together with 2SnO-60ZnO-40P2O5 glass under UV
(365 nm) irradiation. (c) Concentration dependence of peak
energies of PL and PLE bands of the SZB glasses as a
function of Sn amount.
correction factors. Considering the purity of Ar gas (99.999%),
the purged melting condition, and previous studies on zinc
phosphate glass system, we speculate about plausible reasons
that (1) the basicity of glass affects the Sn2+/Sn4+ ratio and/or
(2) the starting materials contain an oxidization component.
Since the optical basicity of B2O3 and P2O5 are 0.42, and 0.33,
respectively,31 we hereby think that the origin is mainly
attributable to basicity of the host glass, and that the low Sn2+
concentration is one of the origins of the lower QE values
(Figure 6) in comparison with a zinc phosphate system. How-
ever, another possibility cannot be denied, because the 0.1SZB
glass prepared under Ar contained Sn2+ ratio of 23% (40% by
using the correction factors). Since the values of basicity of
these host glasses take similar values, the Sn2+ ratio of these
glasses under Ar preparation suggests a hypothesis that some
cation-free components were contained in the starting materials
to affect the valence state of tin. Although it is not clarified yet,
one plausible factor is a hydroxyl group (like as H3BO3) or
H2O molecules connected to B2O3, because no clear contam-
ination could be observed in the glass system. Since we have
used commercially available B2O3 powder without any heat
around 0 mm s¹1 correspond to the Sn4+ species, whereas those
around 2 and 4 mm s¹1 correspond to the Sn2+ species.29 These
spectra show that certain amounts of Sn2+ in these glasses were
oxidized to Sn4+, even though the glass was melted in an inert
atmosphere. After peak deconvolution, the Sn2+ ratios of the
1SZB glass melted in air and that melted in Ar are calculated
to be 14%, and 45%, respectively. On the other hand, if correc-
tion of Debye-Waller factors (Sn2+: 0.22, and Sn4+: 0.49)12,30
are applied, the Sn2+ ratios of these glasses become 26%, and
65%, respectively. It is notable that approximately a half of
Sn2+ centers were oxidized during inert melting, even after the
Bull. Chem. Soc. Jpn. 2015, 88, 1047–1053 | doi:10.1246/bcsj.20150145
© 2015 The Chemical Society of Japan | 1051

(a)
(c)
1.0
0.8
0.6
0.4
0.2
0.0
x = 0
0.1
0.5
1.0
2.0
Ar
air
2.0
2.5
3.0
3.5
4.0
4.5
0.0
0.5
1.0
1.5
2.0
Photon energy/eV
Sn amount/mol %
800
600
400
200
0
(b)
(d)
1.0
0.8
0.6
0.4
0.2
0.0
2.0
1.0
0.5
0.1
0
4.0
4.4
4.8
5.2
0
2
4
6
8
10
Eg opt/eV
Dose/Gy
Figure 7. (a) X-ray-induced scintillation spectra of the xSZB glasses prepared under Ar. The dose was 1 Gy. (b) Correlation between
the scintillation intensity and the irradiated dose of the xSZB glasses prepared in Ar. (c) Normalized scintillation intensities of the
xSZB glasses as a function of Sn amount. The dose of X-ray was 1 Gy. (d) Correlation between the scintillation intensity and Eg opt
of the xSZB glasses prepared in Ar.
treatment, it is expected that OH group or H2O of starting mate-
rials may affect the valence of tin. Here, we have emphasized
that the quadrupole doublet of Sn2+ in the 1.0SZB glasses is
depicted as an asymmetric shape. Since the Sn2+ center possess
no specific crystallographic orientation in the random glass
network, we fitted the spectrum with an assumption that there
are two Sn2+ sites in the 1.0SZB glasses, whereas only one
Sn2+ site is observed in the 0.1SZB glass. It is reported that at
least two different sites exist in the SnO-ZnO-P2O5 glasses.12
As shown in Figure 4a, an emergence of an excitation band
suggests the possibility of additional Sn2+ sites. It is, therefore,
suggested that the asymmetric peaks attributable to Sn2+
species reflect the different local coordination states. Figure 8b
shows normalized scintillation intensity and Eg opt of the SZB
glasses melted in Ar shown in Figure 8a as a function of the
Sn2+ amount, which is obtained by 119Sn Mössbauer analysis
without the correction of the Debye-Waller factors. Although
the Sn2+ amount may be underestimated, the relationship
between scintillation intensity and Eg opt can be discussed. Both
scintillation energy and value of Eg opt are rapidly changed
at low concentration, and the degree of increase becomes
gradual with increasing the amount of Sn2+. Such saturation
curves correspond to an emerging of different coordination
states, whose existence is suggested by the results of optical
absorption, PL, and Mössbauer spectroscopy.
Conclusion
We have demonstrated the UV-excited photoluminescence
and X-ray-induced scintillation of Sn2+-doped zinc borate
(SZB) glasses. It is found that oxidation of Sn2+ to Sn4+ during
glass melting can be detected from the optical absorption edge
Eg opt. Although PL characteristic of the Sn2+ center in borate
glasses was observed, the PL of the zinc borate glasses is
different from that of zinc phosphate glasses from the view-
point of peak energy shift and the potential of luminescence by
near UV irradiation. The maximum QE of the present SZB
glasses was approximately 60%, which was affected by both
Sn2+ ratio and basicity of the host glasses. X-ray-induced
scintillation intensity of the SZB glasses roughly depends
on the real Sn2+ amount, which also affects the Eg opt value.
The asymmetric peaks attributable to Sn2+ species in 119Sn
Mössbauer spectra reflect the different local coordination states
of Sn2+ species, which is also suggested by other results of the
present study.
1052 | Bull. Chem. Soc. Jpn. 2015, 88, 1047–1053 | doi:10.1246/bcsj.20150145
© 2015 The Chemical Society of Japan

(a)
3
4
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0.1SZB-Ar
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–8 –6 –4 –2
0
2
4
6
8
Velocity/mm s-1
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(b)
ΔEg opt
Scintillation
intensity
0.0
0.1
0.2
0.3
0.4
0.5
Sn2+ amount/mol %
Figure 8. (a) 119Sn Mössbauer spectra of the 1SZB glasses
prepared in Ar and air along with that of the 0.1SZB glass
prepared in Ar. Dotted lines indicate each Sn compo-
nent after peak deconvolution. (b) Normalized scintillation
intensity and ¦Eg opt of the glass melted in Ar as a func-
tion of Sn2+ amount obtained by Mössbauer spectroscopy.
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This work was partially supported by the JSPS KAKENHI
Grant-in-Aid for Young Scientists (A) Number 26709048.
Collaborative Research Program of I.C.R., Kyoto University
(grant #2014-31), and the SPRITS program, Kyoto University.
The 119Sn Mössbauer measurement was performed at Nagoya
Institute of Technology under the Nanotechnology Platform
Program of MEXT, Japan.
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