Uncovering the origin of emission wavelength shift and anomalous intensity enhancement at elevated temperature in phosphors

White light emitting diodes (LEDs) are the primary source of lighting in this modern era, but it has not been understood why their emission light becomes inconsistent with increasing temperature. To investigate the origin of this problem, we studied chloroborate phosphors, i.e., Ba₂Ln(BO₃)₂Cl:Eu²⁺ (Ln = Y, Gd, Lu), based on first-principle DFT calculations. Using the frozen phonon approach, we analysed the effect of raising temperature on the 5d to 4f transition energies of Eu²⁺ doped at a Ba1²⁺ site in a 2 × 2 × 2 supercell of Ba₂Ln(BO₃)₂Cl. Our results suggest that at high temperatures the interactions of the Eu²⁺ ion with its surrounding anions in Ba₂Ln(BO₃)₂Cl:Eu²⁺ become strong, causing the Eu²⁺ ions to vibrate more vigorously from its equilibrium position and sharply increasing the emission energy (Δ_f-d), which leads to a pronounced heat-induced blue shift (HIB) phenomenon in the phosphors.