EPR-Guided Interpretation of NIR-I Luminescence in Cr 3+ -Doped Spinel Mg(Ga,Al) 2 O 4

The combination of electron paramagnetic resonance (EPR) and optical spectroscopy enables a comprehensive characterization of the local environment of luminescent Cr 3+ centers. Optical absorption and photoluminescence measurements provide direct information on the crystal-field strength through the energetic positions of the 4 T 2 and 2 E excited states, whereas EPR spectroscopy is highly sensitive to local symmetry, lattice distortions, and magnetic interactions involving isolated ions, exchange-coupled pairs, and clusters. Together, these complementary techniques allow for a detailed description of the nearest-neighbor environment of luminescent centers, which plays a decisive role in determining the spectral shape, bandwidth, and energy position of the Cr 3+ -related emission. In this work, I analyze the EPR spectra of Cr 3+ -doped spinel materials based on two host matrices, MgGa 2 O 4 and MgAl 2 O 4, forming the mixed spinel system Mg(Ga,Al) 2 O 4:0.06Cr 3+ . Understanding the relationship between the local structure of Cr 3+ centers and their spectroscopic signatures is essential for the rational design of broadband near-infrared emitting materials. Spinel hosts activated with Cr 3+ ions are particularly attractive for applications in broadband NIR phosphors, optical amplifiers, and bioimaging probes due to their structural stability and tunable crystal-field environment. By combining EPR and optical spectroscopy, this work provides insight into the structural origin of isolated ions, Cr–Cr pairs, and cluster-like centers in Mg(Ga,Al) 2 O 4:0.06Cr 3+, offering a spectroscopic framework for the design and optimization of Cr 3+ -activated NIR luminescent materials.