Mn–Mn Magnetic Coupling Interaction-Induced Red Emission in a Tetra-Coordinated Lattice

Strong magnetic coupling interactions originating from a short Mn–Mn distance in some heavy Mn2+-doped matrices have important effects on luminescence of single Mn2+ ions and close-knit Mn2+–Mn2+ pairs. However, the intrinsic mechanism of controlling spectral regulation remains elusive since the underlying relationship among Mn–Mn distances, magnetic coupling interactions, and optical properties is unclear. Herein, we create an unusual red emission (620 nm) of Mn2+ in a typical tetra-coordinated lattice of Li2CdGeO4 by simply enhancing the Mn2+-doped level in addition to regular green emission (528 nm). Although the dual emission peaks occupy the same tetra-coordinated crystallographic lattice of CdO4, different temperature-dependent emission behaviors are observed, which expands a possibility in optical thermometry sensors. Detailed Mn–Mn distances are calculated via Rietveld refinement analysis, and their effects on the Mn–Mn coupling interactions are evaluated. Furthermore, the Mn–Mn coupling interaction types are identified through electron spin resonance and magnetic measurements. The continuously decreasing distances between Mn2+ ions strengthen the dipole–dipole coupling effect, resulting in the atypical red emission in a tetra-coordinated lattice environment. These findings elucidate the spectral regulation mechanism from the perspective of magnetic coupling interactions, providing a new pathway to regulate Mn2+-related emission.