Site Preference-Driven Mn⁴⁺ Stabilization in Double Perovskite Phosphor Regulating Quantum Efficiency from Zero to Champion

Abstract

The tetravalent-state stability of manganese is of primary importance for Mn4+ luminescence. Double perovskite-structured A2B′B″O6:Mn4+ has been recently prevalent, and the manganese ions are assumed to substitute for the B″(IV–VI)­O6 site to stabilize at the tetravalent charge state to generate far-red emissions. However, some Mn-doped A2B′B″O6-type materials show no or weak luminescence such as typical Ca2MgWO6:Mn. In this work, a cation-pair co-substitution strategy is proposed to replace 2Ca2+ by Na+–La3+ to form Ca2–2x Na x La x MgWO6:Mn. The significant structural distortion appears in the solid solution lattices with the contraction of [MgO6] but enlargement of [WO6] octahedron. We hypothesize that the site occupancy preference of Mn migrates from Mg2+ to W6+ sites. As a result, the effective Mn4+/Mn2+ concentration enhances remarkably to regulate nonluminescence to highly efficient Mn4+-related far-red emission. The optimal CaNa0.5La0.5MgWO6:0.9%Mn4+ shows an internal quantum efficiency of 94% and external quantum efficiency of 82%, reaching up to the top values in Mn4+-doped oxide phosphors. This work may provide a new perspective for the rational design of Mn4+-activated red phosphors, primarily considering the site occupancy modification and tetravalent-state stability of Mn.

Enriching the Deep-Red Emission in (Mg, Ba)3M2GeO8: Mn4+ (M = Al, Ga) Compositions for Light-Emitting Diodes

“…14−16,41−43 Since the 3d−3d forbidden transition probability of Mn 4+ ions with the 3d 3 configurations is largely determined by the local symmetry of [MnO 6 ] octahedrons, the decrease of octahedral symmetry usually leads to the relaxation of the selection rule of Mn 4+ ions occupied at these octahedral sites. Similar results are reported by Wang et al 20 Here, the luminous intensity of Ca 2 MgWO 6 : Mn 4+ is reported to be increased by the larger cation pair (Na + − La 3+ ) substitution in Ca 2+ sites. This substitution makes severe lattice distortion in the [WO 6 ] sites (preferential for Mn 4+ occupation) by changing the [W−O] bond length, which led to a higher emission.…”

“…Here, Mn 4+ ions preferentially occupy any of the three octahedrons provided by (Mg1/Al1), (Mg2/Al2), and (Mg3/Al3) sites, as shown in Figure 1c, due to the strong ligand field stabilization energy of Mn 4+ ions in the octahedral sites. 19,20 Meanwhile, the Rietveld analysis of Mg 3 Ga 2 GeO 8 (MGG) indicated that this host is crystallized in the orthorhombic arrangement with the space group of Imma (Figure 1b). A small fraction of impurity-phase o-MGO is also observed in this structure.…”

“…20 The average [(Mg/Al)−O] bond lengths are calculated to be 2.039 and 2.047 Å for MAG: Mn 4+ and MAG: Mn 4+ , 0.03Ba 2+ samples, respectively (Table S4). Wang et al 20 also reported a similar kind of larger ionic substitution in Ca 2 MgWO 6 , which resulted in increased [W−O] bond lengths of [WO 6 ] octahedrons. The stretching vibration corresponds to the [WO 6 ] octahedrons reportedly becoming stronger due to the above substitution, as visible in the present case also.…”

“…This might be because of the reduced symmetry of structurally distorted [(Mg/Al)O 6 ] octahedrons and increased [(Mg/Al)− O] bond lengths due to Ba 2+ codoping as discussed in the XRD analysis of MAG: Mn 4+ and MAG: Mn 4+ , 0.03Ba 2+ samples previously. 20 The average [(Mg/Al)−O] bond lengths are calculated to be 2.039 and 2.047 Å for MAG: Mn 4+ and MAG: Mn 4+ , 0.03Ba 2+ samples, respectively (Table S4). Wang et al 20 also reported a similar kind of larger ionic substitution in Ca 2 MgWO 6 , which resulted in increased [W−O] bond lengths of [WO 6 ] octahedrons.…”

Broadband UV-Excitation and Red/Far-Red Emission Materials for Plant Growth: Tunable Spectrum Conversion in Eu3+,Mn4+ Co-doped LaAl0.7Ga0.3O3 Phosphors

“…The optimal concentration of Eu 3+ ions in La 1– x AlO 3 : x Eu 3+ was explored and are reported in Figure S1 in the Supporting Information. The luminescent film laminated glasses (LFLGs) were fabricated according to our previous work. , …”