Quantitatively Analyzing Pressure Induced Phase Transformation by Photoluminescence Spectra in Eu3+-doped Sodium Potassium Bismuth Titanate

Abstract

(Na_0.8,K_0.2)_0.5Bi_0.497Eu_0.003TiO₃ (NKBET20) piezoelectric ceramic powders were prepared by the solid-reaction method. The phase structures of the NKBET20 powders under various pressures were investigated by photoluminescence (PL) spectra and X-ray diffraction (XRD). The PL spectra of the doped Eu³⁺ ions suggest a pressure induced transformation from the tetragonal to rhombohedral phase (R phase), and the phase transformations were confirmed by XRD analyses. Furthermore, the fluorescence intensity ratio of the $\mathrm{D}_0^5\to \mathrm{F}_2^7$ transition to the $\mathrm{D}_0^5\to \mathrm{F}_1^7$ transition (FIR₂₁) could be utilized for the quantitative analyses of the phase transformation. The results from the PL method show that as the pressure increases from 0 to 500 MPa, the fractions of the R phase of the NKBET20 powders increase from about 11% to 58%, while the fractions of the tetragonal phase (T phase) decrease from about 89% to 42%, which are consistent with the XRD Rietveld refinement. Unlike the ceramic bulks, the pressure induced phase transformation in the ceramic powders shows no obvious trigger point and is much gentler. This work suggests a different viewpoint to study the pressure induced phase transformation qualitatively and quantitatively, which can be used for more phase analyses.