Researchers from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences have developed a new way to significantly enhance upconversion luminescence in oxide perovskites, a class of materials known for their thermal and chemical stability, but has limited optical efficiency.

Led by Prof. JIANG Changlong, the researchers introduced a dual-cation substitution strategy in titanate perovskites by precisely adjusting the sodium-to-lithium ratio at the A-site of the crystal. This controlled substitution triggers a structural transition that improves energy transfer between rare-earth ions, resulting in a marked increase in luminescence intensity and quantum yield.

The findings were published in Journal of Alloys and Compounds.

Efficient and stable luminescent materials are in high demand for applications such as solid-state lighting, full-color displays, and secure anti-counterfeiting measures. While fluoride-based upconversion materials can be highly efficient, they often lack long-term stability. Oxide perovskites are more robust, but their use has been limited due to low efficiency and thermal quenching.

In this study, the researchers synthesized a series of dual-cation titanate perovskites with the formula Li_(1-x)NaxLaTi₂O₆, which were doped with Yb³⁺/Er³⁺ or Yb³⁺/Tm³⁺ ion pairs. Replacing Li⁺ with Na⁺ induces a phase transition from a tetragonal to a rhombohedral structure, altering the local crystal field around the luminescent ions and enabling more efficient energy transfer.

Of the tested materials, Li_0.1Na_0.9LaTi₂O₆:Yb³⁺/Er³⁺ exhibited the best performance. Compared with the baseline compositions, the optimized phosphor achieved up to a 32-fold enhancement in upconversion intensity and a 70-fold improvement in quantum yield while maintaining excellent thermal stability.

Under 980-nm laser excitation, the phosphors emit bright and tunable green and blue light across a wide temperature range, highlighting their potential for use in high-performance lighting devices and durable anti-counterfeiting labels.

Schematic overview of structure-guided Na⁺/Li⁺ substitution strategy in titanate perovskites for enhanced upconversion emission and applications in NIR-triggered light-emitting diodes and anti-counterfeiting. (Image by JIANG Changlong)