In situ Observation of 15 mol% Ba-Substituted Strontium Tantalate up to 1773 K by Ultraviolet Laser Raman Spectroscopy

Raman spectra of 15 mol% Ba-substituted Sr2Ta2O7 have been successfully obtained at temperatures up to 1773 K by a continuous-wave ultraviolet Raman spectroscopic system. The Raman spectra do not change significantly with an increase in temperature, despite the P21  Cmcm transition. The present result is described in terms of the parameter related to temperature variation at constant pressure, defined in a similar way to the Gruneisen parameter. Raman bands located above 220 cm show smaller values of the parameter than lower frequency modes, suggesting that these higher frequency modes are assigned to TaO6 internal modes. Due to the strong Ta—O chemical bonds, the dipole moments in the TaO6 octahedra and the ferroelectric polarization in the whole crystal lattice can be induced by the Ba substitution without large deviation of the Ta—O—Ta bond angles from 180, which enhances the photocatalytic performance.