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) A, b = 3.8012(3) A, c = 11.8290(9) A, β = 95.135(7)°, V = 790.99(10) A3 for TiTa2O7 and a = 17.6719(13) A, b = 3.8006(2) A, c = 11.8924(9) A, β = 95.295(7)°, V = 795.33(10) A3, 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 experime...