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

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) angstrom, b = 3.8012(3) angstrom, c = 11.8290(9) A, beta= 95.135(7)degrees, V = 790.99(10) angstrom(3) for TiTa2O7 and a = 17.6719(13) angstrom, b = 3.8006(2) angstrom, c = 11.8924(9) angstrom, beta = 95.295(7)degrees, V = 795.33(10) angstrom(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 E-r = 170 +/- 7 GPa for TiTa2O7. TiNb2O7 showed a slightly lower hardness of H = 8.7 +/- 0.3 GPa and a reduced elastic modulus of E-r = 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 n(D) = 2.37 and n(D) = 2.29, respectively, at lambda = 589 nm, similar to that of diamond (n(D) = 2.42).