Mg₄InSbO₈ Spinel: Double 2:1 Ordering in Tetrahedral and Octahedral Sublattices and Perspective Application as an Optical Thermometer via Mn^2+/4+ Doping

Abstract

AB2O4-type spinel oxides were extensively investigated as prospective magnetic, photocatalytic, cathode, and host materials. Herein, a new Mg4InSbO8 spinel was rationally designed and structurally characterized by high-resolution X-ray powder diffraction (XRPD) in combination with neutron powder diffraction. Mg4InSbO8 crystallizes in the Imma-superstructure with lattice dimensions of a = √2/2·a C, b = 3√2/2·a C, and c = a C, where a C is the lattice parameter of the cubic (Fd3̅m) archetype spinel. The formation of this hitherto unreported Imma-spinel superstructure is a result of 2:1 Mg2+/Sb5+ ordering in the octahedral sublattice, which further creates two crystallographically independent tetrahedral sites with distinctive sizes and thereby resulting in a site-selective 2:1 occupation for the Mg2+ and In3+ cations. Mg4InSbO8 is the first spinel superstructure exhibiting double 2:1 cationic ordering in both octahedral and tetrahedral sublattices. Compared with other ordered spinel structures, we found that the ionic radius and charge differences, coordination geometry preference, and chemical bonding character are all responsible for the formation of this unique doubly ordered spinel superstructure of Mg4InSbO8. Theoretical calculations, together with diffuse reflectance spectroscopy, revealed that Mg4InSbO8 is an indirect semiconductor. Moreover, the multiple tetrahedral and octahedral sites in Mg4InSbO8 are appropriate for green- and deep-red-emitting Mn2+ and Mn4+ activators, respectively. Tunable color emission from green to white and then to deep red can be achieved for Mg4InSbO8/Mn2+/4+ phosphors by adjusting the excitation energy. More importantly, Mn2+ and Mn4+ activators experience distinctive thermal quenching responses, thereby resulting in a vivid color change from green to red for Mg4InSbO8/Mn2+/4+ upon heating. The highest sensitivity for color change is S r = 2.10% at 453 K, manifesting potential applications in temperature sensing. Our findings pave the way to designing cation-ordered spinel superstructures and exploring and expanding their prospective applications.

Structure and photoluminescence of Mn2+/4+-activated doubly ordered spinel Mg4(Ga/Al)SbO8: site-selective Al3+-to-Ga3+ substitution enabling Mn4+ accumulation, excellent anti-thermal quenching of Mn2+ green emission, and optical thermometry

“…The highest S r value of 1.22% K −1 is obtained for MGSO:0.02Mn at room temperature (303 K) when excited at a wavelength of 280 nm. Such a high S r value is comparable to other Mn 2+ /Mn 4+ -based spinel-type optical thermometers such as 0.1 wt%Li-MgGa 2 O 4 :Mn 2+ /Mn 4+ ( S r = 1.10% K −1 ) 50 and Mg 4 InSbO 8 :Mn 2+ /Mn 4+ ( S r = 2.10% K −1 ), 45 and superior to other optical thermometric materials, such as Li 2 ZnSiO 4 :Mn 2+ ( S r = 0.682% K −1 ), 65 Sr 1.95+ x Li 1− x Si 1− x Al x O 4 F:Eu 2+/3+ ( S r = 0.83 K −1 ), 66 BaMgP 2 O 7 :Eu 2+/3+ (0.66% K −1 ), 67 La 2 LiSbO 6 :Eu 3+ /Mn 4+ ( S r = 0.89% K −1 ), 68 and Ca 2 Y 8 (SiO 4 ) 6 O 2 :Bi 3+ /Eu 3+ ( S r = 0.958% K −1 ). 69 According to the evolution trend of the S r – T curve for MGSO:0.02Mn, a much higher S r value could be expected below room temperature, manifesting its perspective application in remote and noninvasive temperature sensing at low temperatures.…”

“…74). 45 Le-Bail fitting to the XRPD patterns demonstrated that the systematic extinction conditions of our newly designed spinels are also consistent with this Imma -symmetry. Hence, Mg 4 InSbO 8 was used as an initial model in Rietveld refinements for MGSO, MGA 0.5 SO, and MASO.…”

“…Our previous crystal structure investigation on Mg 4 InSbO 8 unravelled that the 2 : 1 tetrahedral ordering is coupled with the 2 : 1 Mg 2+ /Sb 5+ ordering in the octahedral sublattice, which creates two different tetrahedral sites with distinct sizes and thus site-selective Mg 2+ /In 3+ occupation in the tetrahedral sublattice. 45 Herein, the 2 : 1 Mg 2+ /Sb 5+ ordering is maintained across the MGSO–MASO full solid-solution, and the 2 : 1 Mg 2+ /(Ga 3+ /Al 3+ ) ordering in the tetrahedral sublattice is thus maintained expectedly. We also noticed that there is a small anti-site Mg 2+ /In 3+ and Mg 2+ /Ga 3+ disordering in Mg 4 InSbO 8 and MGSO, respectively, while Mg 2+ /Al 3+ is completely ordered in MASO.…”

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

“…The Mn 4+ ion, in octahedral coordination, has been shown to exhibit deep red to NIR emission. − The Mn 2+ ion, on the other hand, exhibits green and orange-red emissions that depend on the coordination. The tetrahedral Mn 2+ ions exhibit a green emission, while octahedral Mn 2+ ions exhibit an orange-red emission. − Recently, the use of Mn 2+ and Mn 4+ ions in a spinel lattice has been shown to exhibit a tunable emission …”

“…32−38 Recently, the use of Mn 2+ and Mn 4+ ions in a spinel lattice has been shown to exhibit a tunable emission. 39 We have been investigating mineral structures to generate new colored compounds 40−43 and for white-light emission. 44 In continuation of this theme, we have now taken up the study of compounds based on the tululite mineral, Ca 14 Zn 6 A 10 O 35+x (A = Al and Ga; 0 ≤ x ≤ 1).…”