The exploration of transition metal Mn²⁺-activated luminescent materials is gaining increasing interests due to their diverse uses. Herein, Mn²⁺-incorporated fluorphlogopite (FP; KMg₃AlSi₃O₁₀F₂) mica ceramics were successfully prepared by a high-temperature solid-state reaction process, in which Mn²⁺ occupied the Mg²⁺ site through isomorphic substitution. The FP itself and derived Mn-mica (KMg_2.5Mn_0.5AlSi₃O₁₀F₂) possess negligible luminescence under ultraviolet (UV) excitation. However, the doping of rare earth Eu²⁺ into Mn-mica generates an evident deep-far-red broadband emission at approximately 620–860 nm, peaking at 720 nm when excited with 240–360 nm, which is ascribed to the intrinsic ⁴T₁ → ⁶A₁ transition of Mn²⁺, and the maximum spectral enhancement reaches approximately 22-fold when excited with 320 nm. The more dramatic result is that the complete substitution of Na⁺ for K⁺ in Mn-mica (NaMg_2.5Mn_0.5AlSi₃O₁₀F₂) not only results in an enlarged excitation range toward the near-UV region but also greatly enhances the deep-far-red emission (more than 12-fold) under 365 nm excitation. After optimization, the luminescence internal quantum yield (QY) is 87.4%, and the emission intensity at 423 K retains 78% of that at ambient temperature, indicating that the modified mica ceramics have superior luminescence and thermal stability through the cooperative effects of isomorphic cation substitution and doping, which is applicable for plant cultivation lighting and latent fingerprint identification.

Red-light-emitting phosphors capable of being well excited with blue light are highly desirable in solid-state lighting. In this work, a novel Eu³⁺-activated high-entropy rare earth oxide Ce_0.2La_0.2Gd_0.2Y_0.2Lu_0.2O_1.6:xEu³⁺ (x = 4–16 mol%) phosphor was successfully prepared by solution combustion reaction for the first time. The multi-composition rare earth oxide has a specific cubic fluorite structure, which is almost the same as that of the pure CeO₂ despite the tiny ceria composition in the sample, demonstrating the formation of a high-entropy composite solid solution. To our surprise, the high-entropy phosphor exhibits extremely intense red emission at 613 nm, corresponding to the ⁵D₀→⁷F₂ characteristic transition of Eu³⁺ under the excitation of blue light at 466 nm. The luminescence internal quantum yield (QY) for the optimal high-entropy phosphor (x = 12 mol%) reaches nearly 50% and can further increase to 67.8% through a subsequent heat-treatment process at 1400 °C. The QY result is much superior to that of previously reported Eu³⁺-activated CeO₂ as well as Y₂Ce₂O₇ and La₂Ce₂O₇ low-entropy composite oxides (QYs are approximately 10%–20%). Moreover, the high-entropy oxide phosphor also shows better luminescence thermal stability than low-entropy oxides, as confirmed from the temperature-dependent photoluminescence emission spectra. The tremendous improvement in optical properties depends closely upon the high-entropy and other related effects. The novel high-entropy rare earth oxide phosphor is beneficial to be used in the field of solid-state lighting owing to the coincidence of excitation of blue light with the emission of InGaN light-emitting diode (LED) chips.