Cr3+-Doped Sr1– n Ba n HfO3 Perovskites: Distinct Evolution of Average Structure and Local Coordination for Tunable Near-Infrared Emission

Abstract

Cr3+-activated perovskite phosphors are promising near-infrared (NIR) emitters, yet their luminescent behaviors are often interpreted primarily in terms of average crystal field effects. Herein, we investigate Sr1–nBanHfO3:Cr3+ as a model system to elucidate the role of local coordination. While synchrotron diffraction reveals a continuous structural evolution from orthorhombic to cubic symmetry with increasing Ba content, Cr K-edge XANES/EXAFS, DFT-assisted local structure analysis, and spectral simulations reveal that the Cr-centered local coordination environment evolves differently from the average crystallographic framework. This difference between average and local structural evolution strongly influences the luminescence behavior, including excitation intensity ratios, R-line evolution, and thermal coupling between the 4T2 and 2E states. Quantitative analysis shows a nonmonotonic variation of C/B, a continuous nephelauxetic trend, and a systematic decrease in both ΔE(E–T) and thermal activation energy ΔEa, indicating that local coordination variation plays a critical role in the excited-state dynamics beyond a simple average crystal-field description. The Sr-rich composition exhibits superior thermal stability and emission efficiency, enabling proof-of-concept phosphor-converted NIR LED (pc-NIR LED) demonstrations for short-range NIR illumination, imaging, and material identification. This work highlights local coordination engineering as a key strategy for designing high-performance NIR-emitting materials.