A Combined Experimental and Computational Study on the Broadening Mechanism of the Luminescence in Narrow-Band Eu²⁺-Doped Phosphors

In this work, we present a comprehensive study of the luminescence relaxation mechanism and the associated spectral broadening in a series of Eu2+-doped narrow-band phosphors. It is highlighted that the commonly used full-width at half-maximum (fwhm) is no longer a sensitive measure for quantifying the emission bandwidth of these materials. A thorough understanding of the factors contributing to the narrow bandwidth requires an explicit treatment of the magnetic structure of the ground and emissive excited state manifolds. This requires incorporating spin–orbit coupling effects using wave function-based methods such as the complete active space self-consistent field combined with second-order N-electron valence state perturbation theory (CASSCF/NEVPT2). In addition, for the associated excited state dynamics calculations, one needs to consider vibronic coupling interactions on the basis of Franck–Condon (FC), Herzberg–Teller (HT), and, when necessary, pseudo Jahn–Teller (PJT) coupling effects. Our analysis underscores that understanding and controlling the synergistic roles of these "static" and "dynamic" effects are essential for accurately assessing the narrow band emission relaxation in these systems. We show that these results can, in principle, be generalized to an arbitrary set of narrow-band phosphor candidates and can potentially aid the experimental efforts toward developing novel phosphors with enhanced luminescent properties.