The combination of electron paramagnetic resonance (EPR) and optical spectroscopy enables a comprehensive characterization of the local environment of luminescent Cr3+ centers. Optical absorption and photoluminescence measurements provide direct information on the crystal-field strength through the energetic positions of the 4T2 and 2E 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 Cr3+-related emission. In this work, I analyze the EPR spectra of Cr3+-doped spinel materials based on two host matrices, MgGa2O4 and MgAl2O4, forming the mixed spinel system Mg(Ga,Al)2O4:0.06Cr3+. Understanding the relationship between the local structure of Cr3+ centers and their spectroscopic signatures is essential for the rational design of broadband near-infrared emitting materials. Spinel hosts activated with Cr3+ 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)2O4:0.06Cr3+, offering a spectroscopic framework for the design and optimization of Cr3+-activated NIR luminescent materials.
Grzegorz Leniec (2026) studied this question.