Figure 1 shows the Differential Thermal Analysis (DTA) curve for the 54Al 2 O 3 -46Ta 2 O 5 glass. The glass transition temperature T g is located at 858 °C and the first T P1 and second T P2 crystallization peak are observed at 912 °C and 1054 °C, respectively. The difference between T P1 and T g (ΔT = T P1 − T g ) a measure of the thermal stability of the glass, is 54 °C, indicating the difficulty for vitrifying the glass using a conventional melting process. X-ray Diffraction (XRD) analysis confirmed that glass was totally amorphous and that the main phase of the crystallized sample after DTA was AlTaO 4 . The density of the annealed glass was ρ = 6.01 g/cm3. The composition of the glass samples measured by x-ray fluorescence (XRF) showed that the changes with respect to the nominal composition were less than 1 mol%. The microstructure of the fabricated glasses investigated through high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) is shown in Fig. 2. Observation through the HAADF-STEM has the advantage of achieving chemical contrast at the nanometric scale because it is very sensitive to the atomic number21. From the figure it can be observed that the glass is homogeneous at different scales and no phase-separation is observed. The randomly distributed bright points at the highest magnification are associated with the Ta atoms which have a much larger atomic number compared with the Al atoms (dark regions).

Figure 1 DTA curve for the 54Al 2 O 3 -46Ta 2 O 5 glass showing T g and the first (T P1 ) and second crystallization peaks (T P2 ). Full size image

Figure 2 HAADF-STEM images at different magnifications for the 54Al 2 O 3 -46Ta 2 O 5 glasses. Full size image

Figure 3 shows the transmittance spectrum of the 54Al 2 O 3 -46Ta 2 O 5 glass in the ultraviolet-visible (UV/vis) region. The glass was transparent in the visible region and had a maximum apparent transmission of 81%. The maximum theoretical transmittance was also estimated to be 81% using the equation R max = 1−[2R′/(1 + R′)], where R′ = [(n d − 1)/(n d + 1)]2 and the experimental refractive index n d value of the glass which was found to be 1.94. The estimated value was similar as that of the experimental result, indicating that the apparent transmittance value was to the result of losses only due to sample surface reflection and no light scattering occurred in the glass22. As observed in the inset of Fig. 3, the glass is colorless and transparent, which confirms that the valence state for all of the Ta ions is five and no Ta4+ ions are present23. The optical bandgap energy was estimated to be 4.3 eV using the UV absorption edge located at 288 nm.

Figure 3 Transmittance spectrum of the 54Al 2 O 3 -46Ta 2 O 5 glass in the UV/vis region. The inset picture shows the glass sample used for the transmittance experiment. Full size image

The measured longitudinal velocity V P and transversal velocity V S of the 54Al 2 O 3 -46Ta 2 O 5 glass were 5.86 km/s and 3.20 km/s, respectively. From these values and the experimental density, it was found that the Young’s modulus E was 158.3 GPa, the bulk modulus K was 124.1 GPa, the shear modulus G was 61.5 GPa and the Poisson’s ratio v was 0.29. These values for the elastic moduli are considerably high and comparable to the maximum values in oxide glasses such as 40Y 2 O 3 -55Al 2 O 3 -5SiO 2 and 28.5La 2 O 3 -71.5Al 2 O 3 , whose Young’s moduli were determined using Brillouin spectroscopy (169 GPa); however our measurement system showed that the Young’s modulus of those glasses were 145.5 GPa and 123 GPa respectively1,9,10. The Vickers hardness of the 54Al 2 O 3 -46Ta 2 O 5 glass was 9.10 ± 0.05 GPa, which is also comparable to the maximum values reported for the oxide glasses; 81.8Al 2 O 3 -18.2Y 2 O 3 (~9 GPa) and 29.3Al 2 O 3 -50.2SiO 2 -20.5Sc 2 O 3 (9.4 GPa)20,24. Figure 4 shows indentation imprint for the 54Al 2 O 3 -46Ta 2 O 5 glass at a load of 2.942 N. Extensive lines due to shear deformation on each face of the imprints are observed. In addition, at the same load, some of the imprints exhibited radial crack behavior25,26. No cracks were observed in any indentation below 1 N. The indentation cracking resistance (CR) was estimated to be 2.50 ± 0.13 N, which is comparable to a commercial Vycor glass27.

Figure 4 Vickers indentation imprint at 2.943 N for the 54Al 2 O 3 -46Ta 2 O 5 glass. Full size image

The 27Al MAS NMR spectrum of the 54Al 2 O 3 -46Ta 2 O 5 glass is presented in Fig. 5. Although the spectrum is broad due to the amorphous nature of the glass, two distinctive peaks and a small shoulder were observed. These peaks and the shoulder were assigned to 4-coordinated Al (Al[4]), 5-coordinated Al (Al[5]) and 6-coordinated (Al[6]), respectively28,29,30. The spectrum was decomposed into the three components using the “dmfit” program applying a simple Czjzek model31,32. The thin dotted lines in the spectrum correspond to each of the components. The fitting, values for δ iso (isotropic chemical shift), dCSA (width of the Gaussian distribution of δ iso ) and vQ* (quadrupolar product in kHz) were determined to be 64.8 ppm, 15 ppm and 1134 kHz for Al[4]; 36.7 ppm, 12 ppm and 985 kHz for Al[5]; and 10.3 ppm, 15 ppm and 973 kHz for Al[6], respectively33. Based on the integration of the peak areas, the fractions of Al[4], Al[5] and Al[6] were estimated to be 44.1%, 41.9% and 14.0%, respectively. The estimated average oxygen coordination number for Al was 4.7. The fractions of Al[5] and Al[6] were considerably larger than those observed in other aluminate glasses; Al typically forms AlO 4 tetrahedra in MO-Al 2 O 3 (M = Ca, Sr and Ba) glasses30. While Al[5] and Al[6] have been observed in some Al 2 O 3 -containing glasses, such as R 2 O 3 -Al 2 O 3 (R is a rare earth ion or Y), R 2 O 3 -Al 2 O 3 -SiO 2 and CaO-Al 2 O 3 -SiO 2 , the fraction of Al[5] has generally ranged from 3 to 30% and that of Al[6] from 1 to 2%24,34,35. The structure of the 54Al 2 O 3 -46Ta 2 O 5 glass may therefore be due to not only the presence of AlO 4 networks but also result in part from the high oxygen coordination of Al. The mechanism of glass formation with retention of large fractions of Al[5] and Al[6] is interesting and thus will be the subject of further investigations.