Nanoindentation, High-Temperature Behavior, and Crystallographic/Spectroscopic Characterization of the High-Refractive-Index Materials TiTa₂O₇ and TiNb₂O₇

Abstract

Colorless single crystals, as well as polycrystalline samples of TiTa2O7 and TiNb2O7, were grown directly from the melt and prepared by solid-state reactions, respectively, at various temperatures between 1598 K and 1983 K. The chemical composition of the crystals was confirmed by wavelength-dispersive X-ray spectroscopy, and the crystal structures were determined using single-crystal X-ray diffraction. Structural investigations of the isostructural compounds resulted in the following basic crystallographic data: monoclinic symmetry, space group I2/m (No. 12), a = 17.6624(12) Å, b = 3.8012(3) Å, c = 11.8290(9) Å, β = 95.135(7)°, V = 790.99(10) Å(3) for TiTa2O7 and a = 17.6719(13) Å, b = 3.8006(2) Å, c = 11.8924(9) Å, β = 95.295(7)°, V = 795.33(10) Å(3), respectively, for TiNb2O7, Z = 6. Rietveld refinement analyses of the powder X-ray diffraction patterns and Raman spectroscopy were carried out to complement the structural investigations. In addition, in situ high-temperature powder X-ray diffraction experiments over the temperature range of 323-1323 K enabled the study of the thermal expansion tensors of TiTa2O7 and TiNb2O7. To determine the hardness (H), and elastic moduli (E) of the chemical compounds, nanoindentation experiments have been performed with a Berkovich diamond indenter tip. Analyses of the load-displacement curves resulted in a hardness of H = 9.0 ± 0.5 GPa and a reduced elastic modulus of Er = 170 ± 7 GPa for TiTa2O7. TiNb2O7 showed a slightly lower hardness of H = 8.7 ± 0.3 GPa and a reduced elastic modulus of Er = 159 ± 4 GPa. Spectroscopic ellipsometry of the polished specimens was employed for the determination of the optical constants n and k. TiNb2O7 as well as TiTa2O7 exhibit a very high average refractive index of nD = 2.37 and nD = 2.29, respectively, at λ = 589 nm, similar to that of diamond (nD = 2.42).

Ti‐Based Oxide Anode Materials for Advanced Electrochemical Energy Storage: Lithium/Sodium Ion Batteries and Hybrid Pseudocapacitors

“…Considering the special growth tendency of similar oxides, our group successfully prepared the 1D nanostructured TiNb 2 O 7 with diameters of 100–300 nm and lengths of 300–600 nm ( Figure a–d), Interestingly, we first found the prior growth direction of the TiNb 2 O 7 nanorod was along the (003) plane, where the specific orientation benefited to lithium‐ion intercalation/de‐intercalation. By reason of unique structure, the TiNb 2 O 7 nanorods exhibited a high reversible capacity of 264.6 mA h g −1 at 0.1 C, and indeed achieved high capacity of 140.4 and 83.9 mA h g −1 at the charge/discharge rates of 20 C and 50 C, respectively.…”

Superionic Lithium Intercalation through 2 × 2 nm2 Columns in the Crystallographic Shear Phase Nb18W8O69

“…26,27 A similar trend was observed in the TiO2•Nb2O5 block structures; in this case, though, Nb 5+ is the higher charged species. Cheetham and Von Dreele21,28 used neutrons to show that Nb 5+ prefers the central octahedron in (3 ´ 3)¥ TiNb2O7-recently confirmed with X-rays for this compound and the Ta analogue29 . The same phenomenon has been reported in the orthorhombic polymorph of (¥ Ti2Nb10O2921,28 and in (3 ´ 4)2 TiNb24O62 13 .…”

Sol‐gel derived TiNb2O7 dielectric thin films for transparent electronic applications

“…For a normal Schottky conduction, the geometry of vs should be straight lines. According to the literature, the refractive index of the TNO film at 589 nm is about 2.37 . Then, we can estimate the reasonable slope of fitting line to be 6.1935 × 10 −4 .…”

“…According to the literature, the refractive index of the TNO film at 589 nm is about 2.37. 21 Then, we can estimate the reasonable slope of fitting line to be 6.1935 9 10 À4 . After fitting the Schottky conduction lines, we can obtain that the y-intercept is an upward trend with rising annealing temperature; that is to say, the Schottky barrier height is a downward trend.…”