Mn-Doped Ca₁₄Mg₄Ga₁₂O₃₆ with the Tululite Mineral Structure for Color-Tunable Emission

In this study, the tululite mineral, Ca 14 Mg 4 Ga 12 O 36 ( CMG ), was synthesized by employing high-temperature solid-state reactions (1250 °C) and characterized by powder X-ray diffraction (PXRD). The structure consists of a network of (Ga/Mg)O 4 tetrahedral units, forming a three-dimensional structure along with (Ga/Mg)O 6 units and CaO 6 and CaO 7 units. This compound was employed as a host to prepare Mn-based phosphors. Two different synthetic protocols were employed: (i) an ambient atmosphere (air) and (ii) a reduced atmosphere (10% H 2 /90% Ar). The prepared samples were characterized by employing a variety of techniques that include PXRD, XPS, UV–Vis absorption spectra, steady-state and time-resolved photoluminescence (PL) spectra, PL lifetime, and PL quantum yield (PLQY) measurements. The ambient condition-prepared samples contain Mn in the +4 oxidation state. The samples prepared under a reducing atmosphere contain Mn in the +2 oxidation state. The optical studies indicate that the Mn 4+ ions are present in an octahedral environment and Mn 2+ ions in a tetrahedral environment. The Mn 4+ -containing compounds exhibit a deep red emission (716 nm) upon excitation at 484 nm. The PLQY was found to be 51.1%, with a lifetime of 2.78 ms. The Mn 2+ -containing compounds exhibit an excitation wavelength-dependent (230–284 nm) tunable emission at 400 and 524 nm. The 400 nm emission was attributed to the possible oxygen vacancies in the reduced sample with the observed lifetime of ∼110 μs. The green emission (524 nm) can be attributed to the Mn 2+ ions having a PLQY of 9.28% with a lifetime of 4.61 ms (λ ex = 284 nm). The concentration-caused quenching, radiative and nonradiative decay behaviors, spectroscopic parameters B, C, and D q / B, and the nephelauxetic ratio β were calculated for both the Mn 4+ and Mn 2+ ions in the compounds to understand the origin and possible mechanism of the observed emission behavior. The afterglow (>50 s) nature of Mn 2+ -containing compounds was investigated toward possible text encryption-related coding/decoding applications.