Crystal phase modified blue upconversion on Tm3+/Yb3+:BCZT ceramic phosphor benefits multifunctionality in white-light applications

Abstract

Rare earth (RE) doped perovskite oxide hosts especially titanates, are promising phosphor materials in terms of white-light emission owing to their extraordinary properties such as an exceptional hosting environment for RE-ions and a switchable crystal phase near the phase boundary. Here, we report a new strategy of crystal phase modification to enhance the blue upconversion (UC) efficiency to such an extent that the combinational mixing of blue and green/red-emitting phosphor gives intense white emission. The Lead free (Ba_0.85Ca_0.15)(Zr,Ti)O₃ ceramics were synthesised at different sintering temperatures by incorporation of Tm³⁺/Yb³⁺ ions as dopants. The UC quantum efficiency of the Tm³⁺/Yb³⁺:BCZT sample sintered at 1300 °C was recorded at different excitation power densities. It was observed that the crystal phase transformation from tetragonal to rhombohedral symmetry in the sample near the phase boundary plays a cruicial role in improving the quantum efficiency. White-light emission applications were demonstrated by preparing biphasic samples with powder mixing of a BCZT:Tm³⁺/Yb³⁺ (blue-emitting) + BCZT:Er³⁺/Yb³⁺ (green/red-emitting) phosphor, and their composition were optimised at a mixed ratio. Thereafter, photometric characterization (CIE chromaticity, colour purity and corelated colour temperatures) was performed, and it indicated the suitability of the current biphasic samples in direct white-light (cooler) applications on an industrial scale. Crystal phase modified blue emission efficiency enhancement is a key feature of this work, which helps to generate approximately pure white-light with ideal chromacity (∼0.333, 0.343) emission when Tm³⁺/Yb³⁺:BCZT is mixed with a green emitting BCZT:Er³⁺/Yb³⁺ phosphor.