Cr3+ and Cr4+ coexistence in CaAl12O19: origin and mechanism of near and shortwave infrared luminescence

Abstract

Achieving simultaneous near-infrared (NIR) and shortwave infrared (SWIR) emissions from single-phase phosphors is challenging because of the distinct electronic configurations and local coordination environments required for these emissions. Multivalence transition metal doping enables diverse optical transitions from coexisting oxidation states. In this study, we demonstrated that the coexistence of Cr3+ and Cr4+ ions in a CaAl12O19 host leads to broad NIR and SWIR emissions, respectively. The Cr3+ ions were stabilized in octahedral sites and exhibited broadband NIR emission via the spin-allowed 4T2(4F) → 4A2(4F) transition, while the Cr4+ ions occupied tetrahedral or distorted coordination sites and contributed broadband SWIR emission through the 3T2→3A2 transition. Crystal field parameters derived from a Tanabe– Sugano diagram analysis (Dq/B values of 2.1 and 1.6 for Cr3+and Cr4+, respectively) confirmed the octahedral and tetrahedral environments and the origin of the dual emissions. We propose an emission mechanism where the Cr3+/Cr4+ interplay enables efficient dual-band NIR–SWIR luminescence.