Study on the Optical Properties of High Refractive Index TeO 2 -PbO-ZnO-BaF 2 Glass System

The tellurite 60TeO 2 -20PbO-(20-x)ZnO-xBaF 2 ( x � 1, 3, 5, 6, 7, 9, and 11) series glass samples were prepared. The samples’ SEM, XRD, DSC, density, refractive index, and absorption spectra were tested. A self-made optical platform was used to measure and calibrate the refractive index. The relationship between glass composition and crystallinity was discussed. The eﬀects of density, molar refractive index, and metal standard value on the refractive index of glass samples were studied. The results showed that TPZBF glass has a high refractive index, and the refractive index increases with the increase in BaF 2


Introduction
With the rapid development of precision optical instruments, optical information communication, optoelectronic products, and other fields, the demand for optical glass with excellent performance has increased, and the requirements are getting higher and higher. In optical design and communication, high-quality optical glass with a refractive index between 1.9 and 2.2 is significant to simplify the optical system and improve the imaging quality. It plays a vital role in the miniaturization of mobile phones, digital cameras, infrared applications, and the development of optical communication technology [1][2][3].
e TeO 2 -PbO glass system has been extensively studied. is system has a large glass-forming area, good chemical stability, and thermal properties and is suitable for further research based on this system [7,8]. Adding the transition metal zinc oxide to the glass system can extend the transparent area of the glass in the ultraviolet region. It can improve the moisture resistance, chemical durability, and thermal stability of the glass to a certain extent and comprehensively enhance the strength of the glass so that the glass can be a stable application under complicated working conditions [9][10][11]. Barium is the alkaline earth metal element with the most significant atomic number, largest ionic radius, and strongest basicity. erefore, adding barium to the tellurite glass matrix adjusts the glass's structure and improves the glass's refractive index, density, gloss, and chemical stability. At the same time, a small amount of BaO will also accelerate the glass's melting and promote the glass's clarification [12,13]. At present, studies have shown that TeO 2 -PbO-ZnO system glass has excellent thermal stability, high mechanical strength, high absorption and emission cross section, broad effective fluorescence linewidth, long fluorescence lifetime, and low threshold. It is an ideal gain medium for realizing the high-energy ultrashort tunable laser [14,15]. In addition, there are studies on adding the TeO 2 to the B 2 O 3 -PbO-ZnO glass system to obtain boron tellurite glasses with high rare-earth ion doping concentrations. ese glasses can be used in optical devices such as visible light lasers and fibre amplifiers. However, the composition of these glasses contains more B 2 O 3 , resulting in a lower refractive index of these glasses [9,16]. erefore, it is of great significance to study the factors affecting the refractive index of the TeO 2 -PBO-ZnO-BaF 2 glass system for finding new optical materials. e purpose of this research was to study the TeO 2 -PbO-ZnO-BaF 2 glass system by measuring its scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), density, refractive index, absorption spectrum, and so on. e relationship between refractive index and glass composition is discussed.

Fabrication of Glass.
e conventional melt quenching technique was adopted to synthesise 60TeO2-20PbO-(20-x) ZnO-xBaF2 (where x � 1, 3, 5, 6, 7, 9, and 11 mol %) glasses, named TPZBF−1,−3, −5, −6, −7, −9, and −11, respectively. We used reagent-grade chemicals as raw materials and mixed them evenly after weighing. Each batch was melted in a pure Al 2 O 3 ceramic crucible in the range of 850°C for 1.5 h and then cast onto the 200-250°C preheated brass mould. e obtained bulk glass was annealed and cooled to room temperature from 250°C to 300°C at a cooling rate of 1°C/ min. e cooled glass was cut into glass pieces of about 20 × 10 × 1 mm, and the surfaces of 20 × 10 mm and 20 × 1 mm were optically polished.

Refractive Index Test of Glass.
e refractive index was measured using a self-made optical platform. e structure diagram of the platform is shown in Figure 1. e light source was provided by a He-Ne laser (LSR650NL-100) with a wavelength of 650 nm. e beam emitted from the laser was linearly polarised light with a diameter of about 3 mm. e beam was thicker and had low intensity; therefore, two convex optical lenses were used to focus the beam. As shown in Figure 1, the second convex lens located was twice the focal length of the first convex lens. A high-intensity parallel beam with a diameter of about 0.8 mm was obtained through the convex lens's focusing effect. e end of the optical platform had a unique inclination angle, where it could install adjustment blocks of different angles (30°, 45°, and 60°). We used screws to fix the angle adjustment block and platform together and then inserted baffles into the groove of the platform. On the one hand, the baffle could firmly lock the angle adjustment block and the convex lens on the platform, and on the other hand, it could help locate the angle and reduce the error in the measurement. e position of the light beam passing through the glass was recorded on a unique piece of paper. We marked the paper with the tangent, normal, and positioning lines on the stage. e paper sheet is shown in Figure 2(a). e positioning lines on the paper sheet coincided with the edge of the stage. In addition, the height of the stage could be adjusted. en, the glass was placed on the stage, the edge of the 20 × 1 mm surface of the glass was bonded to the tangent line, and the glass and the stage were glued together. After that the stage was pressed onto the angle adjustment block's oblique side to find a convenient position for observation and recording, as shown in Figure 2(b). We beamed the light across the glass at the intersection of the tangent line and normal line on the paper sheet, fixed the stage firmly on the base, adjusted it to the appropriate height, and started the test.
It should be noted that after starting the test, except for the replacement of the paper sheet and angle adjustment block, other parts of the platform were not disassembled to avoid other errors.
After starting the test, we could observe the light passing through the glass and refracting, as shown in Figure 2(c). We marked the position where the light exited from the glass with a needle and pierced a spot as small as possible on the paper. After the test, we scanned the paper with a printer and uploaded it to the computer to accurately measure the refraction angle. We used computer aided design software to open the scanned picture, connected the incident point and the exit point, and measured the angle between the normal line and this line. is angle is the angle of refraction, as shown in Figure 2(d). We measured six data at each grade (30°, 45°, and 60°), found the average value, and put it into the following formula: where n is the refractive index, α is the angle of incidence, and β is the angle of refraction.
In the case of incident angles of 30°, 45°, and 60°, test the samples to find the average refraction angle at each incident angle and put the average refraction angle into equation (1) to find the corresponding refraction rate. en, take the average of the three refractive indexes measured at the incident angle of 30°, 45°, and 60°, and the average value (4) (6) obtained is the measured refractive index n test obtained by the optical platform.
To eliminate error, the following formula was used to correct the measured refractive index n test : where n theory is the theoretical refractive index and a and b are two correction coefficients. To obtain a and b, the standard optical quartz glass and an ellipsometer were used for calibration. e refractive index of the standard optical quartz glass under the irradiation of visible light with a wavelength of 650 nm was 1.4565. e self-made optical platform was used to test the standard optical quartz glass under the same conditions, taking its test and theoretical values as conditions for finding a and b.
en, the refractive index of TPBZF-1 glass was measured with an ellipsometer (J. A. Woollam Co., Inc. M2000XF), and the theoretical and measured values of TPBZF-1 glass were used as the other conditions for finding a and b. Bringing these two conditions into equation (2), getting the values of a and b, and then bringing the measured values of other glasses into equation (2) got the corresponding modified glass refractive index n.

Other Characterisations of the Glass.
e prepared glasses were subjected to different characterisations: SEM was performed using a high-resolution environmental scanning electron microscope (FEI Quanta 650). SEM images were obtained in a high vacuum mode and with an acceleration of 500 kV. To eliminate the charging effect, the sample surface was sprayed with gold; X-ray diffraction (Ultima IV) patterns were determined at room temperature with a diffraction angle of 2θ � 10°-100°and at a rate of 0.02°/ min; glass transition temperature and crystallisation temperature were determined by DSC (NETZSCH STA 449F3). A 10 mg sample was heated in Al 2 O 3 crucible at a heating rate of 30°C/min in N 2 gas atmosphere; the density was measured according to the Archimedes principle using pure water (ρ � 0.99980 g/cm 3 , 16°C) as immersion liquid at room temperature; the absorption spectra were measured in the wavelength range of 190 nm to 900 nm using a UV-VIS spectrophotometer (PerkinElmer Lambda 650) with optically polished samples of approximately 5 mm thickness.

Scanning Electron Microscope (SEM).
e fabricated TPZBF1-7 glass was clear, bright, and light green glass. TPZBF-9 glass was also relatively clear but had obvious crystallisation points. TPZBF-11 glass was completely crystallised and could not transmit light.
SEM images for the TPZBF-7 glasses are presented in Figure 3(a). ese images show a glassy surface with no crystal precipitate since only the common constituents were present. TPZBF1-7 sample profiles were found to be similar. e TPZBF-9 sample had obvious crystallisation points on the surface; the SEM image of the crystallisation point position is shown in Figure 3(b).

X-Ray Diffraction (XRD).
e XRD diffraction pattern of TPZBF glass is shown in Figure 4. It can be seen from Figure 4 that when x � 0 and 7, the X-ray pattern of the glass powder is basically the same. ey have a wide diffraction peak near 2θ � 30°and 2θ � 50°, respectively, in the diffraction pattern.
ere is no very sharp diffraction peak, which is a very typical diffraction pattern of an amorphous substance. ese show that when the mole percentage of BaF 2 is between 1% and 7% (corresponding to the mole percentage of ZnO between 19% and 13%), the product obtained after the smelting of the TPZBF glass system is a stable glass structure without precipitation of crystalline phases [17].
In the glass melting process, a part of BaF 2 combines with O 2 in the environment and is transformed into BaO that enters the glass network as a modifier and F 2 that escapes to the atmosphere; the other part of BaF 2 breaks the MO bond (M � Te, Pb, Zn) and enters the glass network to form an oxyfluoride glass structure. Hence, the crystallisation tendency of the glass increases sharply and crystalline phases begin to appear. When the mole percentage of BaF 2 reaches 11%, the glass is completely crystallised. e crystalline phases observed in the TPZBF-9 sample were identified as PbF 2 and Ba 2 ZnF 6 . We found that a chemical reaction occurs in the melt that forms the glass, the content of which is as follows: e addition of excess F element replaces the O element in the glass network structure, leading to these PbF 2 and Ba 2 ZnF 6 crystalline phases. In Figure 4, the phase peaks of the PbF 2 crystal are located at 2θ � 28.9°, 42.7°, 47.0°, and 48.2°, respectively, while the phase peaks of the Ba 2 ZnF 6 crystal are located at 2θ � 39.2°. Figure 5 is the DSC spectrum of TPZBF-7 glass. From the figure, the glass transition temperature T g , the onset of crystallisation temperature T c , and the difference ΔT of the glass can be estimated. e values of T g , T c , and ΔT are 266, 325, and 59°C, respectively. e ΔT value can be used to evaluate the thermal stability of the glass. e larger the ΔT, the wider the operable range without devitrification during glass melting and the better the thermal stability. e ΔT value of the TPZBF-7 sample is 59°C, which is a small value in tellurite glass. It can also be seen from Figure 5 that the first crystallisation peak of the glass is next to the glass transition temperature peak, so the series of glasses are relatively easy to crystallize [18][19][20].

Density.
e composition and density of the glass are shown in Table 1. As the TPZBF-11 component glass was devitrified, the properties of TPZBF-11 are not studied below. It can be seen from Table 1 that as BaF 2 gradually replaced ZnO, the density of the glass gradually increased from 6.243 g/cm 3 to 6.327 g/cm 3 . e density of the glass depends not only on the mass of the atoms constituting the glass but also on the tightness of the atomic packing and the coordination number. e following two aspects mainly determine the gradual increase in density of TPZBF series glass. On the one hand, the relative atomic mass of Ba is 137.32, while the relative atomic mass of Zn is 65.38, which is much lower than Ba. On the other hand, according to the classification of oxides in the glass network, BaO and ZnO belong to modified bodies and intermediates, respectively. As a modified body, BaO can enter the structural gaps of the glass network, making the structure of the glass more compact, thereby increasing the density. Since the polarisability of Zn 2+ ions is lower than that of Ba 2+ and Pb 2+ ions, ZnO tends to play the role of network structure in the glass network (for example, in the TeO 2 -ZnF 2 -ZnO glass system, Zn 2+ participates in the formation of the glass network in the form of zinc oxide tetrahedrons [21]). e decrease in ZnO weakens the construction of the glass network, which reduces the molecular volume and increases the glass density. erefore, under the combined effect of the above two reasons, when BaF 2 replaces ZnO, the density of the glass increases significantly.
In addition, although the density of the glass increases, the increase is not apparent.
is is because the glass composition contains 20% PbO (molar ratio). PbO is also a glass network modification body, and the molecular weight of Pb is greater than that of Ba, so the addition of BaF 2 can only slightly increase the density of the glass.

Refractive Index and Factors Affecting the Refractive Index.
e refractive index n theory of TPZBF-1 glass and standard optical quartz glass was measured with an ellipsometer. e test refractive index n test of TPZBF1-9 glass was measured with a self-made optical platform. e n theory and n test of the two were brought into equation (2) and calculated as a and b:    Table 2.
e self-made optical platform had a refractive index error of 0.0824% for the standard optical quartz glass and 0.5521% for glass with a composition of TPZBF-1. e values of these two errors were below 1%. erefore, the measurement result is relatively accurate. e refractive index and other physical properties of the glass are shown in Table 3.
It can be seen from Table 3 that as BaF 2 replaced ZnO, the refractive index of the glass increased. e refractive index of glass depends on the density of the glass and the polarisability of internal ions.
H. N. Ritland's research shows that the refractive index of glass is a single-valued function of the density [22]. Generally, the greater the density of the glass, the denser the glass network and the more compact the structure and the slower the propagation of light in the glass, so the refractive index increases accordingly. Conversely, the lower the density, the smaller the refractive index. e density and refractive index of TPZBF series glass also conform to this rule.
According to Maxwell's electromagnetic field theory, the propagation speed of light in a medium should be e expression of n is inferred from the following: where c is the speed of light in a vacuum, μ is the magnetic permeability of the medium, ε is the dielectric constant of the medium, μ 0 is the magnetic permeability in a vacuum, ε 0 is the dielectric constant in a vacuum, μ r is the relative permeability of the medium, and ε r is the relative dielectric of the medium electric constant. In dielectrics such as inorganic materials, μ r ≈ 1, so n ≈ ε r 1/2.
is shows that the refractive index of the medium increases with the increase in its dielectric constant. e dielectric constant is related to the polarisation of the medium. e higher the polarisation of the medium, the greater the refractive index.
When light passes through the glass, it causes the polarisation of the ions inside the glass. e polarisation of the ions, in turn, affects the propagation speed of the light wave in the glass. e greater the polarisability, the greater the energy absorbed after the light wave passes, the more significant the reduction in propagation speed, and the greater the refractive index. e polarisation process is divided into passive polarisation and active polarisation [23]. Passive polarisation means that an ion is polarised under an external electric field generated by other ions. e polarisation rate α expresses its magnitude. Active polarisation means that an ion uses its electric field to act on surrounding ions to polarise other ions. Generally, the anion (O 2-, F − ) has a large radius and is easy to deform and polarise but the active polarisation ability is low. e cation radius (Te 4+ , Pb 2+ , Ba 2+ , and Zn 2+ ) is relatively small, and when the electricity price is high, its active polarisation effect is large but the polarisation degree is low. e radius and polarisability of the ions contained in the TPZBF series glass are shown in Table 4. e polarisability of Ba 2+ ions is much higher than that of Zn 2+ ions, so as BaF 2 replaces ZnO, the refractive index of the glass increases.
It can be seen from Table 4 that the ionic radius of Ba 2+ ions was also larger than that of Zn 2+ ions. When the radius of cations with the same atomic valence increased (Ba 2+ ⟶Zn 2+ ), the molar volume and molar refraction of the glass increased at the same time, and the increase in the molar volume reduced the refraction of the glass. e increase in the molar refraction increased the refractive index of the glass, so there was no linear relationship between the refractive index of the glass and the ionic radius. However, it can be demonstrated from other angles that the large ion radius of Ba 2+ ions can provide glass with a higher refractive index.
e molar volume R m of the glass can be calculated using the following formula [24]: where M is the molar mass of the glass, d is the density, and n is the refractive index of the glass. e refractive index of glass can be regarded as the sum of its linear and nonlinear refractive indexes, as shown in the following formula [25]: where n 0 is the linear refractive index, n 2 is the nonlinear refractive index, and E is the field strength of the beam. is means that the higher the nonlinear refractive index n 2 , the higher the refractive index n of the glass. According to the research conclusion of Nasu [26], the nonlinear refractive index n 2 of glass is directly proportional to the ratio of molar refractive index to molar volume (R m / V m ). e molar volume V m , molar refractive index R m , and their ratio of the glass are shown in Table 3. As the content of BaF 2 increased, the value of R m /V m of the glass increased from 0.6033 to 0.6295. erefore, the replacement of ZnO by BaF 2 increased the refractive index of the glass. Figure 6 shows the absorption spectra of the TPZBF glasses. It can be seen from the figure that as BaF 2 gradually replaced ZnO, the absorption boundary of the glass in the ultraviolet region gradually shifted to the long-wavelength direction. is phenomenon is mainly because the polarisability of Ba 2+ ions is higher than that of Zn 2+ , which leads to stronger absorption of ultraviolet light by the glass, which shifts the absorption boundary to the long-wavelength direction.

Absorption Spectrum.
e TPZBF-9 sample underwent a certain degree of crystallisation, so its transmittance was also affected accordingly and was significantly lower than that of other glasses in the system.   Figure 6: Optical absorbance spectra of TPZNF glasses in the 300-900 nm range (a) and in the 500-700 nm range (b).
Advances in Materials Science and Engineering e small picture on the upper righthand corner of Figure 6 is a partial picture of the 500-700 nm segment. It can be seen from the picture that as BaF 2 gradually replaced ZnO, the absorption strength of the glass decreased. is is because the following formula exists in the transparent area of the glass [27]: where OD is the optical density and n is the refractive index of glass. If OD is regarded as a function of n, when n > 1, OD monotonically decreases, that is to say, the higher the refractive index, the lower the absorption intensity. e absorption intensity of TPZBF series glass in this area decreases with the increase of BaF2 content. is phenomenon is consistent with the change rule of refractive index.

Conclusions
60TeO 2 -20PbO-(20-x)ZnO-xBaF 2 tellurite series glass was prepared, and SEM, XRD, DSC, density, refractive index, and absorption spectrum tests were carried out. A refractive index test platform for bulk solids was established, and the error was found to be within 1% after testing with standard optical quartz glass and TPZBF series glass.
rough the test and calculation of the prepared TPZBF series glass, the following regular was found. As the BaF 2 in the glass composition replaces ZnO, the density and refractive index of the glass gradually increase. e absorption edge in the absorption spectrum shifts to the long-wavelength direction, and the ultraviolet absorption increases. When the molar ratio of the BaF 2 component exceeds 9%, the glass begins to show a more obvious tendency to crystallize and the transmittance is significantly reduced.
Data Availability e data in the article are available if necessary.

Conflicts of Interest
e authors declare that they have no conflicts of interest.