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

Abstract

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^-1 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.