The ${\mathrm{Sb}}^{3+}$ ion has attracted significant interest due to its effectiveness in boosting the optical properties of host materials. Among the interesting phenomena is the commonly observed dual-band emission, which has often been interpreted by adopting the phenomenological model that explains the dual-band emission (“ultraviolet band” and “visible band”) in Sb-doped $L{\mathrm{PO}}_4$ ($L$ = Sc, Y, Lu). However, the model for the Sb-doped $L{\mathrm{PO}}_4$ series itself has not been well understood theoretically. In this work, we employ first-principles calculations combined with group-theory analysis to clarify the underlying physical mechanism behind dual-band emission in Sb-doped $L{\mathrm{PO}}_4$ series. We demonstrate that the dual band arises from two excited-state equilibrium structures; one exhibits a relatively small distortion with respect to the ground-state equilibrium structure, while the other displays a significantly larger distortion, characteristic of an “off-center” configuration. The deviations from the ground-state configuration are dominated by two distinct vibrational modes, $b_2$ and $e$ modes, involving the Jahn-Teller effect and the pseudo-Jahn-Teller effect, respectively. Furthermore, charge transition levels and energy barriers calculated using the climbing image nudged elastic band method have aided in the understanding of the relaxation between the two excited-state configurations and the property changes across the Sc, Y, and Lu series. These insights provide a basis for understanding the exotic properties of ${\mathrm{Sb}}^{3+}$ in other hosts and may facilitate the design of optical materials in a broader range of systems involving ${\mathrm{Sb}}^{3+}$ ions.

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• Luminescence
• Perovskites
• Wide band gap systems
• Density functional calculations