Pressure-driven cross-relaxation for photoluminescence tuning in Er³⁺-doped Cs₂NaYbCl₆ double perovskites

Lanthanide-doped lead-free double perovskites (DP) are promising phosphors due to their tunable photoluminescence (PL) properties. However, achieving tunable emission in a single material remains a significant challenge. Herein, dynamic PL tuning from green to red is achieved through downshifting (DS) and upconversion (UC) processes in Er³⁺-Cs₂NaYbCl₆ DP under pressure, respectively. Evidently, the red-to-green emission ratio reaches up to 99% through pressure-regulated Yb³⁺-Er³⁺ cross-relaxation (CR) in the DS process, compared with 80% emission ratio through regulation of Er³⁺-Er³⁺ CR in the UC process. In situ high-pressure Raman spectroscopy combined with density functional theory (DFT) calculations further reveals that the pressure-induced lattice contraction increases the phonon energy, promotes the phonon-assisted CR process, significantly enhances the red light emission intensity, and dynamically tunes the red-to-green emission ratio, particularly in the DS process.