Titania is widely considered as an alternative constituent for replacing heavy metal oxides in optical glasses. Its effect on optical properties, however, is complex. This is due to the dielectric properties of the prevalent ionic species, Ti4+, the potential co-existence of trivalent titanium, Ti3+, giving rise to intrinsic and extrinsic charge transfer reactions, and the existence of different coordination polyhedra, depending on matrix composition. Here, we present a systematic study of the optical properties of the soda-lime-silicate glass system as a function of TiO2 addition. We consider the silica-rich region of the SiO2-Na2O-CaO-TiO2 quaternary, which may be taken as model for a variety of technical glasses. Trends are described in the refractive index, the Abbe number, the optical bandgap and the Urbach energy. The addition of TiO2 increases the refractive index and the optical dispersion while it lowers the optical bandgap and the Urbach Energy. Results are discussed in relation to relevant literature data towards using titania silicate glasses as high-index replacements for heavy metal – containing oxide glasses.
© 2016 Optical Society of America
Many applications of glass rely on their optical properties, where glasses are among the few solid materials which transmit visible light. This is because without the specific incorporation of localized electronic states with low activation energy, most inorganic glassy materials are electronic isolators which do not absorb light in the visible spectral range, i.e., without going into details, the electronic orbitals, atoms, molecules or chemical bonds in glassy materials do not resonate with visible photons . This in itself, however, is not a unique feature of glasses. More importantly, glassy materials present a degree of homogeneity which covers a very wide range of length scales, and glasses are usually ideally isotropic. These properties lead to the practical absence of visible light scattering, and also to the absence of birefringence. Unless glass is dedicatedly doped with specific coloring (or scattering) agents, glass-light interaction is, in a simplistic view, dominated alone by the polarization of dipoles with very low dipole interaction (symmetry) and, hence, low light refraction as compared to a hypothetic single crystal of identical composition. As long as the glass composition does not undergo dramatic changes (i.e., staying in the area of oxide or even silicate glasses), the resonance function of this polarization remains relatively similar in shape, and compositional variations act mainly on the position of the resonance frequency. This aspect has led to an empirical understanding where optical refraction and optical dispersion (Abbe number) are roughly correlated, leading to the so-called banana-shape of the data distribution in the Abbe diagram . Adding TiO2 to a glass has been considered as a way to at least partially push these borders, and TiO2-doped optical glasses have consequently found applications which rely on the high field strength and polarizability of the relatively small tetravalent titanium ion when placed within in a large coordination polyhedron.
TiO2 is a semiconductor with a typical band gap of 3.20 eV and 3.03 eV for the anatase and rutile structure, respectively . This means that doping a glass with TiO2 changes the optical bandgap of the glass, so that the electronic resonance in the ultraviolet spectral range shifts to higher wavelength. The change of the UV absorption edge results in a decrease of the Abbe number in the visible spectral range, typically to below 50, i.e., to the region of so-called flint glasses. Typical titania containing optical glasses are light flints such as LF5HTi and LLF1HTi, but also deep flints (TF) such as high-TiO2 containing fluorophosphate or fluorosilicate-based glasses . Several of the Kodak crown glasses developed by Morey also had high TiO2 contents [4–6], other titania containing crown glasses are e.g. BK7HTi and FKSHTi. Titania was also used in combination with silica in the core of the first low-loss optical fiber (<20dB/km) , and it is extensively used in highly nonlinear optical glasses (e.g. the TiO2-Nb2O5-Na2O-SiO2 . The low Abbe number at relatively high refractive index (the comparably strong dependence of refractive index on wavelength), has - besides optical applications - made TiO2 an interesting component in stemware, glass jewelry, crystal ware and tableware, where the combination of these two properties leads to higher Fresnel reflection with high wavelength-dependence and, thus, more appealing appearance. However, while several studies have dealt with the effect of TiO2 on the optical properties in the ternary system of TiO2-Na2O-SiO2 [9–15], only little has been published in the scientific literature on the technically more relevant quaternary system of soda-lime-silicates doped with TiO2 [16, 17], as it is most often used in glass jewelry, crystal ware and tableware. In the present paper, we therefore provide a comprehensive study of the effect of TiO2 on the optical properties in the soda-lime-silicate system. The glass series studied were composed according to the quaternary diagram in Fig. 1 while the Na2O content was kept constant.
Glass samples as shown in Fig. 1 were melted at 1450 °C for 18 h, followed by a homogenization step of 1 h and a conditioning step of 2 h at 1500 °C in Pt/Rh10 crucibles. The homogenization consisted of stirring with a Pt/Rh10 flag at 8 rpm (about 48-50 Nm). Employed raw materials are listed in Tab. 1. We did not visually observe any Rh or Pt dissolution in the glasses (which would generate a characteristic violet-brownish coloration). Annealing was performed for 1h at 580-550 °C before the glass was cooled with initially 0.5 °C/min until 430-400 °C, and subsequently more rapid cooling to room temperature (approximately 2 °C/min). In total, three glass series were prepared as stated above.
The actual chemical compositions of the melted glasses were determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) without internal standard. Data are given in Table 2. The laser ablation unit was a LSX-213G2 + , CETAC and the ICP-MS was an X-series II, Thermo Fischer Scientific. The chemical composition was determined from the average of five analyses per sample. The uncertainty was determined from the standard deviation; it was ≤0.3 for Na2O, ≤1.3 for SiO2, ≤0.3 for CaO and ≤0.2 for TiO2 (all in mol.%). As can be seen from Tab. 2, the total oxygen content was kept roughly constant over all three glass series.
Refractive indices were measured using a V-block Pulfrich refractometer PR2, Carl Zeiss, with 90° polished specimens. The refractive indices nC' at 643.8 nm (red Cd line), nd at 587.6 nm (yellow He line), ne at 546.1 nm (green Hg line), nF' at 480.0 nm (blue Cd line) and ng at 435.8 nm (violet Hg line) were recorded. The molar electronic polarizability (αm) was calculated from the refractive index of the glass according to Eq. (1), also known as the Lorentz-Lorenz equation as originally derived by Lorentz  and Lorenz . Here, Vm is the molar volume, N is the amount of polarizable ions per mole, normally assumed to be equal to Avogadro’s number and n is the refractive index (in the present work nd has been used). The molar electronic polarizability per oxide atom was calculated by dividing αm with the total number of oxygen (-II) atoms in the glass .
An exemplary powder X-ray diffraction (XRD) pattern of specimen 1.4 was obtained on a Siemens Kristalloflex D500 diffractometer over a 2θ range of 15–75°, step size 0.02°. XRD was performed in order to prove the amorphous nature of the specimen.
UV-Vis-NIR spectra were recorded on an Agilent Cary 5000 double-beam spectrophotometer with integration sphere. The recorded data interval was 1 nm, a scan rate of 600 nm/min and a spectral bandwidth (SBW) of 2 nm was used. The source changeover was at 350 nm and the detector as well as grating changeover was at 800 nm. A masking aperture of 10 mm2 was used for all specimens.
The optical absorption coefficient (A) was derived by normalizing the absorption to the thickness (t), according to Eq. (4), where ln (I0/IT) is the absorbance and I0 and IT are the intensities of the incident and transmitted light, respectively.
All specimens were polished to 10 mm ( ± <0.1 mm) thickness, which means that for the present study, the absorption is roughly the same as the absorption coefficient. Noteworthy, in this consideration, we neglect eventual scattering losses, assuming that within the experimental regime of interest, the glass samples are ideally homogeneous.
The optical absorption of topologically disordered materials follows Tauc’s rule, Eq. (5), as was first shown by Tauc et al.  and further developed by Mott and Davis . Here, hc/λ is the incident photon energy, Eopt is the optical bandgap and r denotes the nature of the transition. It is common to use r = 2 which corresponds to indirectly allowed transitions, as suggested by Rao et al . Rearranging Eq. (5) into Eq. (6) suggests plotting left side of Eq. (6) as function of hc/λ. By linear extrapolating, the value of Eopt can subsequently be derived. This plot is also known as Tauc’s plot.
The Urbach law is given by Eq. (7) . Plotting ln A(λ) as function of the incident photon energy hc/λ enables determination of the Urbach Energy (ΔE) from the inverse of the slope of the linear region .
The cut-off wavelength was determined from the maximum of each absorption spectrum, in some cases, the first maximum when approaching from longer wavelengths.
Structural data of selected samples were determined using X-ray Absorption Fine Structure Spectroscopy (EXAFS) at the I811 Beamline of Max-II at Maxlab, Lund University in Sweden . The Max-II storage ring was operating at 1.5 GeV with positron currents in the range of 280-100 mA. The EXAFS Ti K-edge (4966 eV) was measured in fluorescence mode, collecting data up to 5700 eV. The incident ion chamber was filled with a mixture of about 85 vol.% He and 15 vol.% N2 gas. The sample holder chamber was filled with pure He gas. A 6 μm metallic Ti foil reference sample was measured between each sample. The selected samples that were measured were 1.4, 2.7, 3.3, 3.5 and 3.7 (Table 2). They were ground and sieved to < 45μm powder. Each sample was then uniformly dispersed between two x-ray transparent Kapton adhesive tapes in a plastic sample cell. Five scans were collected for each sample and each scan was performed over 180 s. The scans were averaged in order to increase the signal-to-noise ratio. To reveal local structural information on the Ti environment, in particular, the Ti-O coordination number, the corrected EXAFS data were fit to a TiO2 (rutile) model with a space group of P42/mnm using the ARTEMIS software as implemented in the IFEFFIT package [26, 27].
3. Results and Discussion
Besides a small amount of light scattering which was observed when pointing a laser through the glasses, especially in the first series, all samples were visually transparent as well as essentially colorless and homogeneous to the naked eye. Some very slight yellow coloration is discussed in section 3.2. The scattering intensity can be seen in the inset of Fig. 2, especially for the specimen 1.2 and 1.4. It is jointly attributed to the presence of tiny bubbles distributed in the glassy matrix, and to eventual liquid-liquid phase separation [28, 29]. XRD was employed to verify the presence of any crystalline components in the samples, revealing that at least sample 1.4 was fully amorphous within the detection limits of the laboratory XRD at the employed scanning rate (i.e., to a crystallite size of < 2nm). Therefore we anticipate all samples to be fully amorphous. Phase separation was not further investigated in this study as it is not relevant for applications where thin glasses are used, e.g., stemware or tableware.
3.1 Refractive index and optical dispersion
The TiO2 content has a large impact on the refractive index, as can be seen in Fig. 3. Overall, the data correspond fairly well to each other, with the exception of series 1 (where TiO2 is systematically replacing CaO). Here, the effect of TiO2 is absent, indicating that also CaO contributes to the refractive index, thus compensating the action of TiO2. As seen from the analyzed compositional data in Tab. 2, the TiO2 content varies between 0 and 5.5 mol%, while CaO varies between 11.2 and 0.6 mol% in series 1, which means that the partial molar effect of TiO2 on the refractive index is in fact larger than that of CaO. Series 1 is an excellent example on the possibility to reduce the Abbe number while keeping the refractive index constant (see Tab. 3). Refractive indices at other wavelengths, given in Tab. 3, show similar behavior as a function of the TiO2 content.
As has been discussed in detail by Duffy , the refractive index of a glass (i.e., a system of low symmetry) is derived from the molar electronic polarizability of the constituent ions through the Lorentz-Lorenz equation, c.f. Equation (1). In most cases (oxide glasses) the oxide ion polarizability has a much larger effect on the refractive index than the molar electronic polarizability of the cations except for those elements with unusually high polarizability such as, e.g., Pb2+ and Ba2+. Duffy  showed that the effect of TiO2 on the refractive index is similarly dominated by the molar electronic polarizability of Ti4+. The conclusion drawn by Duffy  reflects in the plot of the refractive index as a function of the molar electronic polarizability per oxide atom (Fig. 4), that is, through normalizing the oxide ion effect on the refractive index. The fact that Fig. 4 shows a fairly linear relationship confirms Duffy’s reasoning that Ti4+ has a large molar electronic polarizability relative to the other constituents of the present glasses (Duffy calculated it to be 1.6 Å3, comparable to Ba2+ with a value of 1.55 Å3 ). As can be seen in Fig. 4, some of the determined refractive indices are doubtful, e.g., some of the values in the series from Villegas  that has increasing molar electronic polarizability per oxide atom but not increasing refractive index.
The optical dispersion is calculated from the difference between nF’ and nC’, c.f. Eq. (2). For the visible spectral range, it is usually expressed as the Abbe number. Data for the optical dispersion and the Abbe number are shown in Tab. 3. As can be seen in Fig. 5, all series follow linear trends. With increasing TiO2 content, the glasses change from crown (Abbe number > 50) to flint (Abbe number < 50). The TiO2 content at the crossover is approximately 5.5-6 mol%. Abdel-Baki et al.  reported Abbe numbers in the series xTiO2-(60-x)SiO2-40Na2O (x = 0, 5, 15 and 20 wt%) ranging between 62.5 and 14.9 with TiO2 content of 0 to 16 mol%. The value of 62.5 corresponds relatively well to the value of 58.5 found with no added TiO2; however, with increasing TiO2 content the reported values by Abdel-Baki et al. are considerably lower. In the absence of phase separation or other distorting effects, these lower values can be explained only with the considerably lower SiO2 content (below 60.7 mol%) so that Ti4+ is most likely acting as a network former. The considerably lower Abbe number also reflects the considerably higher estimate for the optical non-linearity of the glasses of Abdel-Baki et al , as given by the values of second-order index of refraction (n2)  and the third order non-linear susceptibility (χ3) . The estimated values of n2 (10−13esu) range from 0.82 for the base glass to 6.78, and χ3 (10−13 esu) ranges from 0.03 to 0.26. On the other side, using the same approach for their estimation in the present study, we obtain ranges from 1.26 to 1.91 and from 0.05 to 0.08, respectively (the values were calculated according to semi-empirical relation of Boling et al.  for n2, see Eq. (8) and Eq. (9) given by Vogel et al.  for χ3, where K in Eq. (8) is an empirical constant equal to 391 ).
3.2 Optical absorption and absorption edge
The optical clarity of the glass specimens is displayed in Fig. 7 as the absorption in the visible range, 380 to 780 nm, and the average transmission of the same range as a function of the TiO2 content in the inset. As can be seen in Fig. 7, the absorption spectra confirm that the glasses do not contain any significant amount of Ti3+ (which would give rise to blue to violet coloration ). The presence of Ti3+ could be determined by, e.g., electron spin resonance (ESR) and the Ti3+/Ti4+ ratio by e.g. x-ray photoelectron spectroscopy (XPS). From Fig. 7, it can be seen that there is some apparent difference in the absorbance, and that the average T% becomes lower with increasing TiO2 content. The latter observation is due to increased Fresnel reflection. As mentioned previously, a slight yellowish coloration was observed by the naked eye which is attributed to the Fe2+-Ti4+charge transfer reaction [36, 37]. It increases as the TiO2 (and also the impurity Fe2O3) content increase, as has been shown by Chavoutier et al. .
The optical band-gap of TiO2 is low in comparison to all other oxide constituents of the present glass (depending on configuration, i.e., anatase ~3.2 eV , as compared to amorphous SiO2, ~9.3 eV ). Therefore, when TiO2 is added to a glass composition that mainly consists of SiO2, it changes the absorption edge. Here, the absorption edge shifts to higher wavelength (Figs. 8(a)-8(c). The exact location of the absorption edge is usually determined in different ways such as by determining the cut-off wavelength or by determining the optical bandgap; data on both properties are given in Tab. 4. The cut-off wavelength ranges from 254 nm for zero TiO2 content to 319 nm for 12 mol% TiO2 content. The optical bandgap is a more conventional way to determine the absorption edge. It was determined by extrapolating the linear region of the Tauc’s plot, a guide for the eye is given in the insets of Figs. 8(a)-8(c). The determined optical bandgaps ranges from 3.86 eV for zero TiO2 content to 3.55 eV for 12 mol% TiO2 content.
The optical bandgap data as a function of TiO2 content are given in Fig. 9 along with literature data [13, 16, 17]. There is a clear trend, although it is not linear. The determined optical bandgap data in the present work is considerably higher than those reported in the literature for both the TiO2-Na2O-CaO-SiO2 and the TiO2-Na2O-SiO2 glass systems. Higazy et al. [16, 17] studied the optical bandgap in TiO2 doped soda-lime-silicate glass as a function of temperature and as a function of the TiO2 content. The data provided by Higazy et al. at 298 K is recalled in Fig. 9, following a rather linear trend with increasing TiO2 content. Furthermore, it was found in this previous study that the optical bandgap as a function of temperature also red-shifts linearly . The optical bandgap value of Higazy et al. [16, 17] is somewhat lower than the values in the present work. However, as Higazy et al. do not reveal any other component amounts besides the TiO2 content, it is difficult to deduce the origin of the lower values. In fact, the optical bandgap of fused silica is reported by Wemple  to decrease with the addition of other oxides, an effect which is increasing in the order CaO<BaO<Na2O<PbO. Optical bandgap data in the TiO2-Na2O-SiO2 glass system have been reported by Abdel-Baki et al.  (shown in Fig. 9) as well as Hogarth and Khan , both somewhat lower than the values we report in the present work and without any obvious trend with the TiO2 content. Abdel-Baki et al.  explained the higher values (see Fig. 9) of the optical bandgap to be due to the higher glass stability in the terms of the average bond strength of the glass. However, using Sun’s  calculated single bond strength values to determine the average bond strength of the glasses according to Dimitrov and Komatsu , one can see that the average bond strength decreases in the series of Abdel-Baki et al. . In line with this information, Abdel-Baki et al. , in other words, suggest that the structure somehow rearranges as the TiO2 content increases since the calculated bond strength values of Sun  do not take into account variations in cation coordination. The reviews on the structure of silicate glasses (including TiO2 containing glasses) by Henderson  and also by Farges et al. [44–47] suggest that there are changes in the coordination environment of Ti4+ with increasing TiO2 content. The published information of the Ti4+ coordination in the literature, though, seems contradictory in that some authors suggest the presence of 4- and 6-coordinated Ti4+ while others suggest also 5-coordinated entities .
In conclusion, the low optical bandgap values which are found in the literature can be explained by the larger amount of network modifier species variations in the coordination environment of the titania species and the eventual presence of Ti3+ (which may trigger Ti3+-O2- charge transfer). Plotting the optical bandgap as a function of the SiO2 content for the present data confirms that these arguments where the decrease in the SiO2 content lowers the optical bandgap (Series 2 in Tab. 4).
The Urbach tail is the tail of the conduction band and the valence band, originating from localized states in disordered solids as originally introduced by Franz Urbach in 1953 . The width of the Urbach tail is frequently called the Urbach energy and can be determined from the exponential part of the absorption edge, see Figs. 8(a)-8(c) and Eq. (6). The glasses investigated in the present work were found to follow Urbach’s rule, i.e., the absorption edge has an exponential part. In Fig. 10, the Urbach Energy shows a clear trend with increasing TiO2 content. Samples from the 2nd Series deviate a little from series 1-3, indicating that CaO might affect the Urbach energy, as well. Kumar et al  also found that the absorption edge obeys the Urbach rule at low TiO2 additions (0.1 wt%), however, no Urbach energy data were given. The Urbach energy has not frequently been determined, however, Abdel-Baki et al. , determined the Urbach energy values for some glasses in the TiO2-Na2O-SiO2 ternary. However, except for the Ti-free specimen, we do not find any collation between these data and our own analyses.
Notable work has been performed on the optical properties of oxide glasses, culminating in the pioneering work by Duffy and Ingram [49, 50]. The understanding of optical properties and its relation to composition has been extensively reviewed by Dimitrov and Komatsu , and it has been shown that many of the properties relate to each other, at least theoretically. The determined values of the optical bandgap and the Urbach energy in the soda-lime-silicate glass system upon addition of TiO2 derive from the molar electron polarizability. Its values, determined according to the Lorentz-Lorenz equation, Eq. (1), are given in Tab. 4 and visualized in Fig. 11. The relation is not very surprising as the molar electronic polarizability is linearly related to the TiO2 content.
As has been described by Henderson  and further by Farges et al. [44–47], Ti4+can be coordinated differently in different glass hosts and the study by Ponader et al.  showed that the coordination of Ti4+ changes upon changes in the Na2O/TiO2 ratio, and also upon changes in the Na2O/SiO2 ratio, in ternary TiO2-Na2O-SiO2 glasses. The glass series in the present work have a fairly constant Na2O/SiO2 ratio, however, the ratio of the Na2O/TiO2 ratio changes. As can be seen in Figs. 12(a)-12(b), plotting the optical bandgap and the Urbach Energy as a function of Na2O/TiO2 reveals distinct regimes. In accordance with the reasoning of Ponader et al.  and Duffy , the change in the Na2O/TiO2 ratio leads to a change in the coordination number. This subsequently reflects in the optical bandgap and in the Urbach energy, which also depend on the coordination of Ti4+. The data of Figs. 12(a)-12(b) suggest that there is a coordination change of Ti4+ when the Na2O/TiO2 ratio is about 3, which corresponds to a TiO2 content of about 5-6 mol%. However, the different regimes of the optical bandgap and Urbach Energy do not reveal any direct information of the coordination states of Ti4+. Ponader et al.  suggest that upon decrease in the Na2O/TiO2, the proportion of six-coordinated relative to the proportion five-coordinated Ti4+ is increased. Duffy  on the other hand suggests that upon decreasing the Na2O/TiO2 ratio, the Ti4+ is continuously transformed into the four-coordinated state (from the state of fivefold-coordination). Fargin et al.  showed from ab-initio calculations based on the Lines theory  that the single oxide molar electronic polarizability is higher for [TiO6]−8 than for [TiO5]−6. The results by Fargin et al. indicate that there is a change in optical properties with the coordination change of Ti4+.
To clarify this controversy, EXAFS analyses were performed on the present samples (see Fig. 13 and Table 5). These clearly indicated that as the Na2O/TiO2 ratio is below 3, the average coordination number is closer to 4, and as the ratio is above 3, the average CN is closer to 5. This suggests that the conclusions which were drawn by Duffy  are correct. Here, one should also bear in mind that the previous studies were performed in the ternary TiO2-Na2O-SiO2 glass system, as compared to the presently studied quaternary glass system.
The data derived by EXAFS, together with the information given in Figs. 12(a)-12(b) enable a clear understanding of the absorption edge and its relation to the coordination of Ti4+. Similar to Figs. 12(a)-12(b) one can find different regimes when plotting the refractive index, the molar volume and the Abbe number as a function of Na2O/TiO2 ratio. Our data show that the single oxide polarizability and, evidently, its impact on the refractive index depend on the size of the polyhedron, i.e., on the Ti-O distance. This experimentally verifies the Lines theory  and the ab-initio calculations made by Fargin et al. .
We provide a comprehensive study of the effect of TiO2 on optical properties of glasses of the technically relevant soda-lime-silicate system. Compositional trends are obtained for the refractive index, the optical dispersion (Abbe number), the overall transmission, the optical bandgap and the Urbach energy. It is revealed how these trends are deriving from both the Ti4+ polarizability and the chemically induced change in the coordination environment of Ti4+ ion species. Our findings also experimentally verify that the single oxide polarizability depends not only on the properties of the cation but also on the cage size of the cation polyhedron, i.e., the M-O distance and the number of M-O bonds per unit volume.
The Marie-Curie Fellowship granted to Dr. Stefan Karlsson via the VINNMER programme (Vinnova) co-funded by Marie Curie Actions FP7-PEOPLE-2011-COFUND (GROWTH 291795) is gratefully acknowledged. Portions of this research were carried out at beamline I811, MAX-lab synchrotron radiation source, Lund University, Sweden. Funding for the beamline I811 project was kindly provided by The Swedish Research Council and The Knut och Alice Wallenbergs Stiftelse. This work was partially supported by the German Science Foundation through grant no. WO1220/10-1 within the scope of the priority program 1594. XRD measurements performed by Sindy Fuhrmann and refractive indices measurements performed by Christian Zeidler of the Otto Schott Institute of Materials Research, Friedrich-Schiller University of Jena, are gratefully acknowledged. Also, Sharafat Ali of Linnaeus University and Stefan Carlson of Max-lab at Lund University are gratefully acknowledged for their help during the EXAFS measurements.
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