The recently discovered Sr[Li2Al2O2N2]:Eu2+ red phosphor, candidate for the next generation of eco‐efficient white light‐emitting diodes, exhibits excellent emission spectral position and exceptionally small linewidth. It belongs to the UCr4C4‐structure family of phosphors containing many potential candidates for commercial phosphors, whose small linewidth, tentatively ascribed to the high‐symmetry cuboid environment of the doping site, has drawn the attention of researchers in the last five years. Density functional theory, ΔSCF method, and configuration coordinate models (CCM) are used to provide a complete characterization of this material. Using a multi‐dimensional CCM, an accurate description of the coupling of the vibronic structure with the electronic 5d→4f transition is obtained, including the partial Huang–Rhys factors and frequency of the dominant modes. It is shown that, in addition to the first‐coordination shell cuboid deformation mode, low‐frequency phonon modes involving chains of strontium atoms along the tetragonal axis shape the emission linewidth in Sr[Li2Al2O2N2]:Eu2+. This finding sheds new light on the emission properties of UCr4C4‐structure phosphors, possessing similar Ca/Sr/Ba channel. The approach provides a robust theoretical framework to systematically study the emission spectra of such Eu‐doped phosphors, and predict candidates with expected similar or even sharper linewidth.

White light-emitting diodes are gaining popularity and are set to become the most common light source in the U.S. by 2025. However, their performance is still limited by the lack of an efficient red-emitting component with a narrow band emission. The red phosphor SrLiAl3N4:Eu2+ is among the first promising phosphors with a small bandwidth for next-generation lighting, but the microscopic origin of this narrow emission remains elusive. In the present work, density functional theory, the ΔSCF-constrained occupation method, and a generalized Huang–Rhys theory are used to provide an accurate description of the vibronic processes occurring at the two Sr2+ sites that the Eu2+ activator can occupy. The emission band shape of Eu­(Sr1), with a zero-phonon line at 1.906 eV and a high luminescence intensity, is shown to be controlled by the coupling between the 5d z 2 –4f electronic transition and the low-frequency phonon modes associated with the Sr and Eu displacements along the Sr channel. The good agreement between our computations and experimental results allows us to provide a structural assignment of the observed total spectrum. By computing explicitly the effect of the thermal expansion on zero-phonon line energies, the agreement is extended to the temperature-dependent spectrum. These results provide insight into the electron–phonon coupling that accompanies the 5d–4f transition in similar UCr4C4-type phosphors. Furthermore, these results highlight the importance of the Sr channel in shaping the narrow emission of SrLiAl3N4:Eu2+, and they shed new light on the structure–property relations of such phosphors.