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

In this study, the tululite mineral, Ca14Mg4Ga12O36 (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)O4 tetrahedral units, forming a three-dimensional structure along with (Ga/Mg)O6 units and CaO6 and CaO7 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% H2/90% 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 Mn4+ ions are present in an octahedral environment and Mn2+ ions in a tetrahedral environment. The Mn4+-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 Mn2+-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 Mn2+ ions having a PLQY of 9.28% with a lifetime of 4.61 ms (λex = 284 nm). The concentration-caused quenching, radiative and nonradiative decay behaviors, spectroscopic parameters B, C, and Dq/B, and the nephelauxetic ratio β were calculated for both the Mn4+ and Mn2+ ions in the compounds to understand the origin and possible mechanism of the observed emission behavior. The afterglow (>50 s) nature of Mn2+-containing compounds was investigated toward possible text encryption-related coding/decoding applications.