Raman scattering study of a phase transition in tantalum pentoxide

Ta2O5 ceramics and crystals were prepared and analyzed using Raman spectroscopy in the temperature range 25–873 K. The low-energy phonon bands (ν < 100 cm−1) of Ta2O5 are assigned to external modes originating from the interaction between different Ta polyhedra and TaOn5 − 2n Ta6O126+ clusters. The temperature and polarization dependence of external modes suggests the existence of clusters in the material. A softening of the low-energy bands was observed with increasing temperature. The spectral evolutions show a soft mode-derived second-order monoclinic to orthorhombic phase transition at about 600 K in Ta2O5.