Physical properties of oxide glasses depend on the cooling process from the super-cooled liquid state. It is demonstrated that the Sn2+ species possessing electrons in the outermost shell can function as a qualitative probe cation to evaluate the randomness of the host glass network. With slower cooling, the formation of a more dense glass network is observed as confirmed by the heat capacity, elastic modulus, 11B NMR, first sharp diffraction peak of X-ray diffraction, and Boson peak results. The optical absorption, photoluminescence (PL), and Sn K-edge X-ray absorption fine structure data strongly suggest that the aggregation of Sn2+ results from the slow-cooling of the melt, and that PL properties of Sn2+ can be affected by the ordering of the transient state of super-cooled liquid. The results highlight that the structure of the transient state of glass is important for the functionalization of optical glasses.
© 2016 Optical Society of America
Since oxide glass is a solidified supercooled liquid, the structure and physical properties of the glass, e.g., the valence and local coordination states of the constituent cations and the homogeneity of the glass/melt, depend on the preparation process even though the nominal chemical composition remains fixed [1–8]. The relationship between the cooling rate and the frozen state of the supercooled liquid, i.e., the fictive temperature, has previously been investigated [9,10]. The physical properties of oxide glass depend on the treatment of this supercooled liquid above the glass transition temperature Tg, a threshold for the diffusion of constituent atoms [11,12]. Since the supercooled liquid exhibits a higher Gibbs free energy than a crystal constructed of a periodic structure, the structural rearrangement of the glass occurs near the thermodynamically stable crystalline state above Tg. It is notable that such structural stabilization still exists in an as-prepared glass with the much wider variety of local coordination compared with that in corresponding crystals. Although it is difficult to examine all the local coordination states in glass because of the microscopic homogeneity, such topological homogeneity affects many parameters, such as phonon vibration, energy propagation, and thermal stability of glasses. Thus, it is important for the advancement of glass science from both scientific and industrial perspectives [11–15].
To evaluate the change of local structure, i.e., structural rearrangement, we used ns2-type cations (n≥4) , which have previously been used as a probe of the optical basicity of the glass [17–19]. Since ns2-type cations possess an electron in the outermost shell in both the ground (ns2) and excited states (ns1np1), the emission of ns2-type cation is strongly affected by the local coordination state. These cations exhibit strong photoluminescence (PL) intensity because of the parity-allowed transitions [20,21]. Even in a random (glass) matrix, the quantum efficiencies exhibit a value of approximately 80%, which is comparable to those of crystal phosphors [22–24]. Therefore, we expect that the local coordination state of the ns2-type emission centre, which can be estimated from the PL and PL excitation (PLE) spectra, can function as a probe to evaluate the randomness of the host matrix. The correlation between the emission properties and the randomness of the oxide matrix is beneficial from the perspective of the local structure of the emission centre.
We recently reported on the PL properties of xSnO-(25-x)SrO-75B2O3 glass ; a stoichiometric composition of a Sn-doped SrB6O10 crystal . Here, the chemical composition of the mother glass was selected as 0.5SnO-24.5SrO-75.0B2O3, the stoichiometric chemical composition of the (Sr0.98,Sn0.02)B6O10 crystal, to compare the physical properties of a glass and a corresponding crystal. Although concentration quenching of the Sn2+ emission centre in the glass is observed with the Sn2+ concentration , we selected this concentration to investigate the variation of the local coordination states of Sn2+. To examine the effect of the cooling condition on the thermal and optical properties, several glasses were prepared with different cooling temperatures. The relationship between the structural ordering of the Sn-doped SrO-B2O3 glass and the PL properties of the Sn2+ centre was examined using these glasses.
Preparation of SnO-doped strontium borate glasses
The 0.5SnO-24.5SrO-75.0B2O3 glass was prepared by employing a platinum crucible . Batches consisting of SnO (99.5%), SrCO3 (99.9%), and B2O3 (99.9%) were mixed and melted at a temperature of 1373 K for 20 min in air. The three glass samples were prepared by different methods. One sample was prepared according to a conventional melt-quenching method in which the glass melt, whose volume was typically 4 cm3, was quenched on a stainless steel (SUS) plate (20 × 30 × 1 cm3) maintained at room temperature (300 K) with 5mm-thick plate made of SUS for pressing. The other two samples were prepared by pouring the glass melt onto a Pt plate (54 mm-diameter, 0.2mm-thick) in an electric furnace at temperatures of 573 K, and 773 K. The door of the electric furnace was closed as soon as the glass melt was poured, and the glass samples were maintained at the elevated temperature for 1 h in air. Following cutting, the glass samples were polished with aqueous diamond slurry.
Preparation of SnO-doped strontium borate crystal
For comparison, the (Sr0.98Sn0.02)B6O10 crystal was prepared by sintering. Following previous reports , SnO, SrCO3, and B2O3 were mixed and heat-treated twice at a temperature of 1073 K in a N2 atmosphere. A pellet of the sintered (Sr0.98Sn0.02)B6O10 crystal of 10 mm in diameter was used for the measurements.
Tg was determined by a differential thermal analysis (DTA) system operating at a heating rate of 10 K/min using TG8120 (Rigaku, Japan). The specific heat capacity of the samples was measured by differential scanning calorimetry (DSC) operating at a heating rate of 10 K/min using DSC8230 (Rigaku, Japan). The PL, PLE, and absorption spectra were measured using an 850 fluorescence spectrophotometer (Hitachi, Japan) at room temperature. The QE was measured using a Quantaurus-QY (Hamamatsu Photonics, Japan). The densities were measured using the Archimedes method with water at room temperature. We measured the refractive index of the samples using a prism coupler with 473, 633, 1319, and 1553 nm light sources (Metericon, N.J. USA.). The error of the measurement was 1 × 10−4.
The 11B magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of the glasses were acquired using a JEOL DELTA 600 spectrometer (14.10 T) at 192.6 MHz using a 4 mm double resonance MAS NMR probe with a ZrO2 rotor. For each sample, 256 acquisitions were obtained with a pulse delay of 3 s and a pulse width of 0.3 μs, with a tip angle of 15°. The 11B MAS spectra were corrected and referenced against a 1M H3BO3 aqueous solution at 19.6 ppm. In order to estimate the population and NMR parameters of the boron species, spectral deconvolution was performed using the DmFit 2002 program with a “Q-mas 1/2” model  including the assumption of the presence of three boron species. The chemical shift of 11B was mainly influenced by its first coordination number, i.e., BO3 and BO4. It was necessary to introduce two three-coordinated boron species for ring and non-ring structures (BO3 ring, and BO3 non-ring), and four-coordinated boron (BO4).
The Sn K-edge (29.3 keV) XAFS spectra were measured at the BL01B1 beamline of SPring-8 (Hyogo, Japan). The energy of the storage ring was 8 GeV with a typical current of 100 mA. The measurements were performed using a Si (311) double crystal monochromator in transmission mode (Quick Scan method) at room temperature. Pellet samples for the measurements were prepared by mixing the granular sample with boron nitride. The analyses were performed using the REX2000 software (Rigaku).
The Brillouin shifts νB of the glasses were measured using the high-resolution modification of a Sandercock FP system . The excitation laser is a frequency doubled diode-pumped solid state (DPSS) Nd: yttrium-aluminium-garnet laser oscillating in a single longitudinal mode at 532 nm (Oxxius SLIM-532 300 mW). The longitudinal sound velocity, VL, is calculated using the equation VL = νBλ/2n532, where νB, λ, and n532, are the Brillouin shift, wavelength of incident light ( = 532 nm), and refractive index at 532 nm, respectively. The n532 values were calculated from the Cauchy equation, using refractive indices at different wavelengths. The longitudinal elastic modulus c11 is calculated using the equation c11 = ρVL2, where ρ is density.
Confocal micro-Raman measurements with a backscattering geometry were performed to analyse Boson peaks. We employed the same excitation laser as Brillouin scattering measurements and attenuated to 20 mW at the specimen. A home-built microscope consisting of ultra-narrow band notch filters (OptiGrate) and a 20 × objective lens (Mitsutoyo, M Plan APO SL20, NA = 0.28) was used for focusing the excitation laser and collecting Raman-scattered light. The scattered light was analysed by a single monochromator (Jovin-Yvon, HR320, 1200 grooves/mm) equipped with a charge-coupled-device camera (Andor, DU420).
The high-energy X-ray diffraction (XRD) experiment was performed at the BL04B2 beamline of the SPring-8 synchrotron radiation facility, using a two-axis diffractometer dedicated to the study of disordered materials . The energy of the incident X-rays was 61.4 keV. The raw data were corrected for polarization, absorption, and background, and the contribution of Compton scattering was subtracted using standard data analysis software .
3. Results and discussion
3.1. Structural rearrangement of the host glass depending on the cooling rate
Initially, we focused on the structural change of the host matrix. Figure 1 shows the specific heat capacity curves of the 0.5SnO-24.5SrO-75B2O3 glasses prepared with different quenching conditions. It is found that as the quenching temperature decreased, the Tg and heat capacity values increased. The larger heat capacity suggests that a larger free volume exists in the glass matrix. Figure 2(a) shows Brillouin scattering spectra of the 0.5SnO-24.5SrO-75B2O3 glasses prepared at different quenching temperatures: quenching at 300 K, 573 K, and 773 K. The 0.5SnO-24.5SrO-75B2O3 glass quenching at 300 K shows the smallest Brillouin shift (absolute value), indicating soft glass network. The molar volumes and longitudinal elastic modulus c11 of the 0.5SnO-24.5SrO-75B2O3 glasses as a function of the quenching temperatures are shown in Fig. 2(b). It is found that as the quenching rate increased, the molar volume increased and c11 decreased. The increase in the molar volume is consistent with that of the free volume, as mentioned above. Furthermore, c11 increased as Tg decreased. The relationship between c11 and Tg exhibits an opposite trend to that reported for silica glass . As suggested, there is a positive correlation between c11 and the mean coordination in the three dimensional network . We attribute this difference to the fact that boron adopts either a three- (BO3) or four- (BO4) coordination state [11,12,31,32], which differs from the SiO4 tetrahedra . Figure 3(a) shows the normalized 11B MAS NMR spectra of the 0.5SnO-24.5SrO-75B2O3 glasses quenching at 300 K and 773 K. The sharp peak at 0 ppm can be assigned to the formation four-coordinated boron BO4, whereas the other non-symmetric peak can be attributed to three-coordinated boron BO3 [11,12,31,32], where the three-coordinated boron species consist of ring and non-ring BO3 structures. The ratio of BO3 and BO4 in the 0.5SnO-24.5SrO-75B2O3 glasses as a function of the quenching temperature is shown in Fig. 3(b). The 11B MAS spectra can be deconvoluted into three boron units demonstrating that the glass produced by quenching at low cooling temperature contains a larger amount of three-coordinated boron, which is inline with previous reports [11,12] and our prediction from the elastic modulus. In the case of the four-coordinated BO4- state, a counter cation is required to compensate for the excess negative charge. As the elastic properties are related to the atomic packing characteristics , it is assumed that denser packing can be achieved by quenching at higher cooling temperature, while it is known that dense packing can result in a smaller free volume and a lower heat capacity.
To reveal the structural changes observed in the NMR spectra, we used two approaches: measurement of low energy excitation by Raman spectroscopy and pair distribution function analysis by high-energy XRD technique. Figure 4(a) shows the Raman scattering spectrum of the 0.5SnO-24.5SrO-75B2O3 glasses in the “Boson-peak (BP)” region [34–39] with the fitted log-normal function . It is known that the peak frequency of the BP, νBPmax is associated with a characteristic local structural length, ξ, which is responsible for the emergence of the Boson peak, such that 2πνBPmax = V/ξ, where V is the average sound velocity . Our Brillouin scattering measurement showed approximately 1.4% increase of the longitudinal sound velocity with increasing quenching temperature, whereas the observed increase in νBPmax is approximately 4.0% as shown in Fig. 4(b). This indicates that ξ should decrease by 2.9% with increasing quenching temperature. Since a shorter ξ represents a narrower distribution of structures, it is likely that higher quenching temperature results in a narrower distribution of structures. This is supported by the fact that the thermodynamically stable BO4 units were generated by quenching at high cooling temperature as we presented earlier in this section. Figure 4(c) shows the total structure factor S(Q) of the 0.5SnO-24.5SrO-75B2O3 glasses prepared with different quenching rates, with quenching at 300 K and 773 K. The first sharp diffraction peak (FSDP) is observed at around Q = 1.2 Å−1 (see inset). Although the curves are similar, a distinction between them can be observed, especially in the FSDP region. It is well know that FSDP reflects the structural ordering of the glass matrix [41–43], therefore, it is natural to assume that a stronger intensity suggests a more ordered structure of the random matrix. Thus X-ray structure factors clearly indicate that the 0.5SnO-24.5SrO-75B2O3 glass quenched at higher cooling temperature exhibits a more ordered network structure. The total correlation function, T(r), of these glasses is shown in Fig. 4(d). The peak at approximately 1.4 Å is due to B-O correlation [1,44], and two spectra exhibit similar shape. On the contrary, around the 1.9 Å region, the intensity of the glass quenched at higher cooling temperature is higher than that quenched at lower cooling temperature. Considering previous reports on bond distance between cation and oxygen [1,44–47], the peak observed at 1.9 Å can be assignable to Sn-O correlation. The result suggests that distribution of Sn cation also depends on the cooling rate. The following session, we discuss the relationship between cooling rate of the glass and structural arrangement of Sn2+ centre.
3.2. Structural rearrangement of the Sn2+ centre depending on the cooling rate
We studied the structural change of the guest Sn2+ emission centre. Figure 5 shows Sn K-edge XANES spectrum of 0.5SnO-24.5SrO-75B2O3 glass with the spectra for SnO and SnO2 also provided for reference. If we take the absorption edge energy, E0, to be the energy at the zero-intercept of the second derivative, we can evaluate the oxidation state of the Sn cation from the E0 value; |Δ(E0(SnO)-E0(glass))| being calculated as less than 1.6 eV in all instances. Considering the resolution of the measurement (ΔE/E ~6 × 10−5: ~1.75 eV), it is assumed that the difference between values is insignificant. Thus, the Sn K-edge XANES spectra suggest that the percentage of Sn2+ to total Sn in 0.5SnO-24.5SrO-75B2O3 glass is nearly 100%. Figure 6(a) shows optical absorption spectra of the 0.5SnO-24.5SrO-75.0B2O5 glasses prepared with different cooling conditions. As the cooling temperature increases, the absorption edge is red-shifted. As the optical absorption edge of the 25.0SrO-75.0B2O5 glass is greater than 6.0 eV , the observed change can be attributed to the local coordination change of the Sn2+ species, whose absorption properties are significantly affected by the local coordination field. As it was previously reported that the absorption band red-shifts with an increase in the amount of SnO of the glass , it is expected that the narrowing of the band edge with higher cooling temperature is as a result of the decrease in the distance between the Sn2+ species.
The PL and PLE spectra of the 0.5SnO-24.5SrO-75B2O3 glasses prepared with different quenching conditions are shown in Fig. 6(b) together with the spectrum of the (Sr0.98Sn0.02)B6O10 crystal. It is observed that both the emission and excitation spectra could change depending on the cooling conditions; the excitation peak energies exhibit a red-shift while the emission peaks exhibit a blue-shift. From these spectra, it is suggested that the higher excitation band at approximately 5 eV was supressed by quenching at higher cooling temperature, and that the lower excitation band survived. Comparing the absorption spectrum with the PLE spectrum, we note that the lower excitation band mainly affects the optical band edge. The decrease of the Stokes shift between the peaks of PL and PLE can also be observed in the Sn2+-doped oxide glass by increasing the amount of SnO [47,48]. Therefore, this suggests that a local highly concentrated state of Sn2+, i.e., an aggregation of Sn2+, is generated in the glass prepared by quenching at higher cooling temperature.
Figures 6c demonstrate the PL-PLE contour plots of the 0.5SnO-24.5SrO-75B2O3 glasses prepared with different quenching conditions. The contour plot of the (Sr0.98Sn0.02)B6O10 crystal is also shown for comparison. As can be seen in the figure, the emission intensity decreases by quenching at higher cooling temperature, which strongly suggests that the suppression of the Sn2+ emission occurred as a result of the aggregation. However, only small part of the excitation band, which exhibits a similar excitation energy to the S0-T1 forbidden transition, survives at lower photon energy levels. It is notable that the residual excitation band at lower excitation energy values affects the highest internal quantum efficiency for all the samples, whose efficiencies are approximately 60%.
Here we discuss the possible reason for the aggregation of Sn2+ ions at higher temperatures. One of the possibilities is the effect due to the thermal treatment at high temperature done on the samples cooled on the platinum plate. However, in our experimental method, the Pt plate is quickly heated to the melt temperature because Pt is highly conductive and the heat capacity of the Pt plate was less than only 19% than that of the glass melt. Thus, the thermal treatment conditions for the quenchings at 773 K and 573 K are almost the same, and the difference in the results for 773 K and 573 K quenching cannot be accounted for.
We consider that the cooling rate of the host network most affects the aggregation of the guest Sn ions. Since the structural rearrangement of the constituents should be prohibited below the glass transition temperature, a longer duration for the rearrangement in the melt should allow the SrB6O10 host network to substantially exclude the guest SnO, leading to the aggregation of the Sn2+ cations. In the following, we qualitatively compare the cooling rates of the glass melts for each quenching temperature in our experimental condition.
The hot glass melt was cooled by (i) heat transfer due to convection of the ambient in the furnace, (ii) heat transfer due to conduction with the metal plates, which were Pt or SUS, and (iii) radiation (Stefan-Boltzmann law). Thus the time derivative of the glass-melt temperature can be expressed as the sum of these processes as ,49], and setting the initial melt temperature to be T0 = 1373 K, we estimate the initial cooling rate (Ṫ(0)) of the glass melt quenched at 573 K to be about 15% faster than Ṫ(0) for 773 K quenching. The cooling rate at 300 K, on the other hand, has a non-negligible additional contribution due to the conduction with the SUS plate (heat bath). Although the actual value of hcond between the glass melt and the SUS plate is not known accurately, it is obvious that the cooling rate quenched at 300 K is even more rapid than those for 773 K and 573 K because not only of the more rapid convective and radiative cooling, but also of the additional conductive cooling.
From the above analysis, it is likely that the glass melt quenched at 773 K underwent the glass transition much more slowly than it did in the 300 K case. Thus, the aggregation of the Sn2+ cations observed at higher quenching temperature can be attributed to the slower cooling rate of the glass melt.
The local coordination state of the Sn species was estimated from Sn K-edge EXAFS spectra. Figure 7 shows the k3χ(k) data for the Sn of the 0.5SnO-24.5SrO-75B2O3 glasses prepared with different quenching rates in addition to the spectrum for the (Sr0.98Sn0.02)B6O10 crystal. Comparing the spectra for the three 0.5SnO-24.5SrO-75B2O3 glasses, we observed the following: (1) The amplitude of the glass quenched at 300K higher than compared with that of the glass with the higher cooling temperatures. (2) The oscillation and amplitude of the 0.5SnO-24.5SrO-75B2O3 glass became similar to that of the (Sr0.98Sn0.02)B6O10 crystal prepared in N2. The Sn K-edge XANES spectra and the XAFS data suggest that as the quenching rate of the 0.5SnO-24.5SrO-75B2O3 glass-melt decreases, the local coordination states of the glass become more similar.
It is demonstrated that the Sn2+ emission centre can function as a probe to evaluate the preparation conditions of strontium borate glasses. It is likely that the electrons in the outermost shell of the Sn2+ emission centre are affected by the local coordination state, and hence can change the optical and PL spectra. It is suggested from the measurements of heat capacity and 11B NMR that the densification of the glass network. Furthermore EXAFS data implies that elastic modulus also induces an aggregation of the Sn2+ centre and results in the decrease of the emission intensity. Furthermore, it is worth mentioning that this is a valid approach in order to improve the emission intensity of the parity-allowed emission centre existing in the cooling process of the supercooled liquid. We are confident that the control of the local coordination field of the emission centre will be an important factor in a random network matrix for industrial optical applications.
This work was partially supported by the Grant-in-Aid for Young Scientists (A) 26709048, and by the Collaborative Research Program of Institute for Chemical Research, Kyoto University (grant #2015-69). The authors also thank Professor Masahide Takahashi (Osaka Prefecture University) for allowing the author to use the prism coupler.
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