Anomalous thermally enhanced upconversion blue emission of Ho3+ ions in co-doped rare-earth-based halide perovskites

The energy transfer upconversion from Yb 3+ to Ho 3+ is inherently inefficient due to energy level mismatch, typically yielding green and red emission. Achieving efficient blue emission from Ho 3+ remains challenging. Here we co‑dope a rare-earth-based perovskite Cs 3 GdCl 6 with Yb 3+ /Ho 3+ and O 2− to construct a crystal‑field‑tunable lattice. In situ variable‑temperature X-ray diffraction patterns and Raman spectroscopy confirm compressive lattice distortion at room temperature and expansive distortion at elevated temperature. Crystal field variations induce Stark splitting of the degenerate 4 f energy levels of Yb 3+ and Ho 3+ . The Stark effect improves energy matching among the sensitizer, activator, and 975 nm excitation, thereby enhancing both energy transfer upconversion and excited‑state absorption. The improved energy‑level alignment promotes population of the Ho 3+ 5 F 2,3 / 3 K 8 and 5 G 5 levels, leading to a 134‑fold enhancement in blue emission. The Cs 3 GdO x Cl 6-x :Yb 3+ /Ho 3+ crystals exhibit green‑to‑blue upconversion luminescence under 975 nm excitation, showing potential for high‑security anticounterfeiting. This work significantly enriches Ho 3+ upconversion kinetics.