Micro-Raman scattering and x-ray diffraction studies of (Ta2O5)1−x(TiO2)x ceramics

Abstract

High dielectric constant $({\mathrm{Ta}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}{)}_{\text{1−x}}({\mathrm{TiO}}_{\mathrm{2}}{)}_x$ ceramics for $\text{x=0.00,}$ 0.05, 0.08, and 0.11 were prepared and studied utilizing micro-Raman scattering and x-ray diffraction techniques. The modification in the lowest frequency phonon mode of pure ${\mathrm{Ta}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ with increasing ${\mathrm{TiO}}_{\mathrm{2}}$ content in these ceramics was assigned as Ta–Ti vibrations originating from the interaction between ${\mathrm{TiO}}_{\mathrm{6}}$ octahedra and ${\mathrm{TaO}}_n^{\text{5-2n}}$ or ${\mathrm{Ta}}_{\mathrm{6}}{\mathrm{O}}_{\mathrm{12}}^{\mathrm{+6}}$ clusters. Raman spectra, in the range −200–900 °C, reveal a softening of this mode with increasing temperature followed by a structural transformation. A triclinic to monoclinic phase ${\mathrm{(H}}_{\mathrm{mon}}^{\prime })$ transition was observed at about 300, 360, 450, and 540 °C for $\text{x=0.00,}$ 0.05, 0.08, and 0.11, respectively. It was also found that reannealing at 1150 °C for 12 h transforms these ceramics from triclinic to a metastable monoclinic phase $H_{\mathrm{mon}},$ which was found to be an irreversible process.