Owing to the miniaturization and lightweight of optical devices, the requirement of optical glasses with high refractive index has increased gradually. New heavy-metal oxide glass materials exhibit high thermal stability and high refractive index, which have been widely studied and applied in lenses and optical windows^[1]. Glasses with high refractive index can effectively reduce the volume of optical components and increase the viewing angle^[2]. In optical design, optical glasses with refractive index between 1.9 and 2.2 are of great significance for simplifying optical systems, improving imaging quality, and miniaturizing devices. In addition, high refractive index glasses can be used in short-wave infrared detection equipment^[3], semiconductor integrated circuit chip short-wave lithography^[4], camera lenses, tank fire control system driver perimeter mirrors^[5], etc. Therefore, new heavy-metal oxide glasses with high refractive index and large size are paid more and more attention.

La₂O₃-TiO₂ glasses fabricated by aerodynamic levitation are widely studied for high refractive index. TiO₂ shows high refractive index and plays an important role in the field of high refractive-index glasses. The advantages of lanthanum-containing glasses are the unique combination of high refractive index, low dispersion, and high transparency over a wide range of wavelengths^[6]. In 2008, Arai, et al.^[6] fabricated La₄Ti₉O₂₄ glass spheres with refractive index of 2.37 at 587 nm. It is proved that high refractive index of La₄Ti₉O₂₄ is due to high concentration of titanate and high volume ratio of TiO₅ clusters. In the La₄Ti₉O₂₄ glass structure, 65% of the TiO₅ polyhedra and 35% of the TiO₆ polyhedra coexist. Compared with the TiO₆ polyhedra, the TiO₅ polyhedra in the glasses greatly improves the polarizability, which endows the La₄Ti₉O₂₄ glass with high refractive index properties^[7]. Titanium dioxide exists in complex forms in glasses, with the forms of 4, 5, and 6 coordination in the glass network structure^[8]. In addition, Farges, et al.^[9] revealed the existence of oxygen quadruple, quintuple, and hexaplet clusters in titanium silicate compositions. Adding titanium dioxide to glasses can improve optical property, even obtain ultra-high refractive index^[10-11]. Titanium dioxide may play a leading role in the optical properties and structure of titanate-based glass.

La₂O₃-TiO₂ glasses with refractive index more than 2.0 are mostly prepared by containerless processing methods^[12-13]. However, the glasses prepared by such methods are too small to be applied. Generally, the diameter of glass spheres prepared by containerless methods does not exceed 10 mm^[14], which limits the applications in large-size optical lenses, tank perimetry, infrared windows, etc. By hot-pressed sintering, large- size lanthanum titanium glass can be prepared. However, because the glass prepared by this method is too colored and has a maximum transmittance of 54% in the visible light region^[14], it cannot be used as a material for the preparation of optical lenses and windows. Therefore, in order to obtain large-size high-refractive index lanthanum titanium optical glass, it is meaningful to find a suitable network former and locate a balance point between size and high refractive index, and prepare large-size high-refractive index lanthanum titanium optical glass by high-temperature melting method. To date, large-size La₂O₃-TiO₂ based glasses with high refractive index by high temperature melt quenching method has not been reported^[15]. Based on the containerless lanthanum-titanium binary glass system, the glass forming ability can be improved by introducing GeO₂. Germanate glasses can dissolve a large number of rare earth ions^[16]. Moreover, GeO₂ exhibits a high refractive index^[17]. Therefore, GeO₂-La₂O₃-TiO₂ glasses with large size and high refractive index can be prepared by high temperature melt quenching method. Structural, optical and thermal property of the new glasses would be studied.

The compositions of new glasses are determined to be (0.74-x)GeO₂-0.26La₂O₃-xTiO₂ (x=0.45, 0.47, 0.49, 0.51, 0.53), labled as GLT-(1-5). High purity GeO₂ (99.99%, in mass), La₂O₃ (99.99%, in mass), and TiO₂ (99.99%, in mass) powders were weighted, and fully mixed according to stoichiometric composition. 10% (in mol) H₃BO₃ (99.99%, in mass) was added into the mixture, as a melting aid. EDS results show that the boric acid can volatilize completely at this melting temperature and holding time. For each sample, a 20 g batch of raw materials was thoroughly mixed and then melted in muffle furnace (VSF1700) at 1300 ℃ for 5 h in platinum crucible. Then, the melt was cast into a copper mold at high temperature to obtain bulk glasses. The glass samples were then placed in a muffle furnace pre-heated to about 50 ℃ below the glass transition temperature T_g (about 790 ℃) for 5 h to eliminate the internal residual stress and oxygen vacancy. Glass samples with diameter of larger than 20 mm and thickness greater than 5 mm were successfully prepared. The glasses were then reduced to a thickness of 1.5 mm by double-sided polishing for follow-up tests.

The glass transition and initial crystallization temperatures were measured by a thermal analysis mass spectrometer STA449C (DTA, STA449C, FEI, USA) at a heating rate of 10 ℃/min. Raman spectra were obtained by a laser Micro-Raman spectrometer (Thermo Scientific DXR) excited by a 633 nm incident laser. X-ray Diffraction (XRD) was performed using an X-ray diffractometer (Brooke D8ADVANCE, Germany). The density was measured in a gas pycnometer (Micromeritics AccupycII1340, Atlanta, Georgia). Optical transmittance spectra were measured using an ultraviolet spectrophotometer (Cary 5000) in the wavelength range from 250 to 2000 nm. Transmittance spectra in the range of 2000-10000 nm were detected by a Fourier-transform infrared spectrometer (EQUlNOX55). Refractive index curves were measured and fitted by ellipsometry (J.A. Woollam M2000) in the range of 210-1600 nm.

The thermal stability can be indicated by the temperature difference ΔT=T_x-T_g, where T_x is the crystallization onset temperature and T_g is the glass transition temperature^[18]. ΔT reflects the resistance of the glass phase to crystallization during the heat-treatment^[19]. The DTA curves of GLT-(1-5) glasses are shown in Fig. 1(a). Owing to the existence of a large exothermic peak, the glass transition temperature T_g and crystallization peak T_p can be easily obtained from the results. Fig. 1(b) depicts an enlarged view of the T_g areas of the GLT glasses. T_g, T_x, and ΔT are evaluated from DTA curves. The curves are plotted with the content of TiO₂ as variable in Fig. 1(c). As shown in Fig. 1(c), T_g grows gradually as TiO₂ is added. The maximum ΔT is about 236.1 ℃ at 45% (in mol) TiO₂, as shown in the inset of Fig. 1(c).

The results show that the glass forming ability and thermal stability can be weakened by increasing the content of TiO₂. As TiO₂ content is more than 54% (in mol), or less than 45% (in mol), GLT glasses cannot be prepared. Therefore, 49% (in mol) TiO₂ is the optimal addition for the glass stability.

A series of GLT glasses with different titanium contents were prepared by high temperature melting. The XRD patterns of the glass samples obtained by traditional melting method are presented in Fig. 2, indicating that the glasses are colorless and transparent. The absence of characteristic peaks in the XRD patterns indicates amorphous structure. In high temperature environment at 1300 ℃, a redox equilibrium reaction of titanium ions occurs in glasses, which is TiO₂ → TiO_2-X + X/2O₂ + XO^2-. This coloration-induced equilibrium is influenced by the acidity and alkalinity of the glass, melting atmosphere, and melting temperature. The presence of Ti⁴⁺ in the sample causes Ti⁴⁺ ion to strongly absorb ultraviolet light and to absorb the purple-blue part of the band in the visible region, which can make the glass yellowish coloration^[20].

The network structure is studied by Raman spectra. Fig. 3 shows the Raman spectra of the GLT-(1-5) glasses. The peak characteristics of different chemical bonds are marked in the spectra, near 120, 300 and 760 cm^-1. As the content of TiO₂ increasing, the peak around 300 cm^-1 shifts from 294 to 315 cm^-1, and then moves back to 298 cm^-1. The peak intensity decreases after GLT-4 reaches maximum. The laws of the peaks near 120 and 760 cm^-1 are similar.

The peaks at 120 cm^-1 may correspond to the boson peak, which is a universal feature of disordered materials^[21-22]. In addition, the wide peak at 300 cm^-1 corresponds to the vibration mode of the interconnected GeO₆ polyhedra^[23]. The hump at 760 cm^-1 is caused by two non-bridging oxygen atoms (Ge-O-NBO) of the GeO₄ tetrahedron^[24] and TiO_n including titanium oxide quadruple, quintuple, and hexaplet clusters^[8]. The lower peak of the binding energy is NBO, and the higher binding energy is bridging oxygen (Bridging Oxygens, BOs). The lower the amount of non-bridging oxygen, the higher the stability of the glass network structure and the tighter the connectivity of the structural units are. In general, three valence states of Ti are present in TiO_n clusters simultaneously. Because the proportion of the TiO₄ polyhedra is much lower than those of TiO₅ and TiO₆, it contributes little to the overall structural change. In addition, the proportion of TiO₆ decreases with an increase in the Ti content, and a portion of TiO₆ is transformed into the TiO₅ structure. By increasing the length of the Ti-O bond and the proportion of TiO₅, the TiO_n polyhedral structure becomes denser^[18].

Fig. 4 shows the optical transmittance spectra of GLT-(1-5) glass in the visible to mid-infrared range. The glasses show almost no absorption in the visible region, and the maximum transmittance of each sample is greater than 72%. The highest transmittance reaches 78% in GLT-1 glasses. The theoretical transmittance can be calculated by the formula T=(1−R)² and R=(n−1)²/(n+1)², where R is the reflection coefficient and n is the refractive index. The actual test value of 78.01% is completely consistent with the theoretical transmittance of 78.36% calculated by the above formula. The maximum transmittance of glasses decreases slightly with the increase of TiO₂ content, which is due to the difference in the reflection coefficient caused by different refractive indices. With refractive index increasing, the corresponding transmittance decreases. Fig. 4(b) shows the infrared transmission spectra of GLT glasses. The cutoff wavelengths determined at 5% transmission of GLT-(1-5) glasses are 3.733.77 μm.

The main absorption band near 2.9 μm may be caused by the existence of free hydroxyl (OH) groups^[25]. The calculation formula of hydroxyl absorption coefficient is α_OH=ln(T₀/T)/L, where L is the thickness of the glass sample, T is the transmittance of the absorption band at 3000 nm, and T₀ is the maximum infrared transmittance of the glass sample.

The calculated absorption coefficients of the GLT-(1-5) glasses are 3.36, 3.22, 3.20, 3.32, and 3.33 cm^-1. The uncertainty for absorption coefficient is affected by the test accuracy of transmittance and sample thickness. The uncertainty is ±0.01.

The cutoff wavelength at 5% transmittance in the IR region was slightly changed from 3.75 μm for GLT-1 glass to 3.73 μm for GLT-5 glass. Compared with silicate and borate glasses, the cutoff wavelength of GLT glasses are shorter. These changes can be attributed to the vibrational energy of phonons. The maximum phonon energy of the sample is 765 cm^-1, so that the infrared absorption edge of the samples are shorter than that of other oxide glasses such as the silicate and borate glasses^[26-27].

The refractive index increases gradually to the maximum value of 2.06 and then tends to stabilize. When TiO₂ replaces GeO₂, the refractive index increases because TiO₂ with refractive index of 2.55 is higher than GeO₂. The refractive index of GeO₂ is only 1.99^[28]. Since the high refractive index of titanium dioxide has a great influence on the performance, the refractive index of the samples increases rapidly when the molar fraction of titanium dioxide varies from 45% to 53%.

The v_d (Abbe number) of the glasses can be calculated according to the following formula: v_d=(n_d-1)/(n_F-n_C), where n_d, n_F, and n_C are the refractive indices at 587.6, 486.1, and 656.3 nm, respectively. The value of v_d initially increases and reaches a maximum value of 22.56 for GLT-4 glass. For the GLT-(1-5) glasses, v_d shows a downward trend. With an increase in the refractive index, the Abbe number shows an opposite trend.

Density, an important physical property of glasses, is closely related to the structure and composition. It reflects parameters such as the molecular weight, coordination number of atoms, and packing density of the glasses. The density is measured using the gas expansion replacement method. The volume of the glass samples is measured. The volume is obtained after 15 times of test. The samples’ density ρ can be obtained after weighing the mass of the samples.

According to Fig. 6, the density of GLT glasses decreases gradually with the increase in TiO₂ content. The density decreases from 5.06 to 5.00 g/cm³. The content of TiO₂ is increased by 6%(in mol), while the density is decreased by 0.06 g/cm³, indicating a significant effect on the density. The effect of composition on the density can be explained in three aspects.

Firstly, the molecular weight of GeO₂ is 104.64. As TiO₂ shows a comparatively small molecular weight of only 79.87， the density decreases after TiO₂ replaces GeO₂. Secondly, from the view of atomic stacking, a different oxygen content can result in a different molecular structure. The tighter network structure can cause a higher atomic packing density, showing a higher density. The last aspect is the coordination number of metal ions, which is an important structural feature of glasses. The coordination number of La³⁺ is 9^[29], which is higher than that of general Ge⁴⁺ (4 or 6)^[30]. A denser glass network structure can be obtained by a higher coordination number so that the density increases with an increase in La³⁺ and decrease in Ge²⁺. Generally, Ti⁴⁺ ions have 4, 6 coordination number, which is similar to germanium. The contribution to the density of the network structure is roughly the same. However, its relative molecular weight is small. Therefore, the density of the GLT-(1-5) glasses decreases with an increase in the TiO₂ content.

The variation law of the refractive index is discussed in relation with the content of metal ions, according to Lorentz-Lorenz formula^[31]:

where M is the relative molecular weight of the oxide component and ρ is the density of the glass sample. n represents the refractive index of the material, N_i represents the number of i atoms, and α_i represents the electron polarizability of i atoms. And V_m and R_m represent the molar volume of the material and the molecular refractive index of the material, respectively. The formula shows that the molar volume V_m and electronic polarizability α_i are two important factors affecting the refractive index^[32]. In the oxide system, the polarizability of the metal ions is very weak compared to that of oxygen ions. The effect on the refractive index is negligible. Therefore, we only discuss the electronic polarizability of oxygen ions. When the TiO₂ content is more than 49% (in mol), the calculated molar volume of the GLT glasses, listed in Table 1, is increased with the increase of TiO₂ content. Then, the electron polarization rate of oxygen, α_O, is obtained by the deformed derivation of the Lorentz-Lorenz formula^[33-34]. The calculation formula of α_O is as follows:

In GLT glasses, the polarizabilities of metal cationic La³⁺, Ge⁴⁺, and Ti⁴⁺ are 1.04, 0.143, and 0.185^[2], respectively. The uncertainty of molar volume V_m and electronic polarizability α_i is affected by refractive index and density. The uncertainty is ±0.015.

Clearly, the molar volume V_m and oxygen electron polarizability α_i gradually increase with an increase in TiO₂, similarly to the refractive index of the glass samples.

GeO₂-La₂O₃-TiO₂ glasses with large size and high refractive index were successfully prepared by high- temperature melting-quenching method. Thermal and optical property of the glasses were researched. The maximum ΔT can reach 236 ℃, indicating high glass forming ability and strong resistance to crystallization. The maximum refractive index n_d and Abbe number reach 2.06 and 23, respectively. 49% (in mol) is the optimal TiO₂ concentration for the glass stability. The glass forming ability and thermal stability can be weakened by increasing the content of TiO₂ in GLT-(1-5). In contrast, the refractive index has been significantly improved. The refractive index is affected by glass compositions, coordination number of metal ions, polarizability of oxygen ions, and structure of the glass network. The molar volume V_m and oxygen electron polarizability α_i have the same variation trend as the refractive index of the glass samples. For lanthanum titanium glass prepared by high-temperature melting method, the increase of titanium content can effectively increase the refractive index of the glass, and still maintain a high transmittance in the visible light band. As a result of the good thermal and optical property, the GLT glasses can be used in optical windows and lenses under high temperature conditions.