Unveiling the luminescence property of Li2MgGeO4:Mn4+ featuring the tetrahedral crystallographic-site occupancy of Mn4+

Abstract

Owing to the occupying tendency of Mn⁴⁺ at octahedral sites, doping Mn⁴⁺ activators in tetrahedral structures poses challenges and hence is seldom reported. In this work, tetrahedrally sited Mn⁴⁺ phosphors were studied. By combining X-ray diffraction (XRD) data with Rietveld refinement analysis, the location of Mn⁴⁺ was determined. It was found that by adding excessive raw MgO, the phosphor synthesis temperature can be improved, enhancing the crystallinity of the crystal and thus improving the emission performance of the phosphor. In addition, excessive raw MgO forms a second phase in an LMGO matrix, which does not change the doping site for Mn⁴⁺. The Tanabe–Sugano diagram of Mn⁴⁺ in the tetrahedral field and the energy-level diagram of these phosphors were constructed for the first time, and the excitation and emission mechanisms are discussed in detail. With 1.2-fold excess of raw MgO, the prepared sample (LMGO-Mn-1.2) shows the best luminescence, demonstrating red emissions peaked at 656 nm and affording an emission intensity enhancement of over 50 times compared to a stoichiometric LMGO:Mn⁴⁺ system. At 150 °C, LMGO-Mn-1.2 keeps 90% emission intensity compared to that at room temperature. Finally, a high-efficiency warm white light–emitting diode was built. This work provides new insights into the study of Mn⁴⁺-activated phosphors in a tetrahedron crystal field.