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

Abstract

Mineral structure-stimulated material design has made great success in the development of excellent phosphor materials. Herein, spinel-type oxides Mg₄Ga_1−yAl_ySbO₈ (MGA_ySO) with a double 2 : 1 ordering of Mg/(Ga/Al) and Mg/Sb cations in tetrahedral and octahedral sublattices, respectively, were rationally designed and structurally characterized by combined Rietveld refinements against high-resolution X-ray powder diffraction (XRPD) data and neutron powder diffraction (NPD) data. A joint hybrid density functional theory (DFT) calculation and crystal orbital Hamilton population (COHP) analysis demonstrated that these new spinels are direct semiconductors with band gap values increasing along with the Al³⁺ content due to the lift of anti-bonding states from the Sb2–O pairs. Mn-activated MGSO exhibited dual emissions from multiple green-emitting Mn²⁺ and red-emitting Mn⁴⁺ activators due to the facile occurrence of Mn⁴⁺-to-Mn²⁺ self-reduction, which is inevitable in Mn-doped spinel-type phosphors. This self-reduction can be effectively inhibited by the site-selective Al³⁺-to-Ga³⁺ substitution in MGA_ySO:Mn^2+/4+, thereby resulting in an accumulation of Mn ions in the octahedrally coordinated sites and a tunable emission colour from green to yellow and then to deep-red. Interestingly, Mn²⁺ green emissions presented excellent anti-thermal quenching (165.4% at 463 K) in a very wide temperature range (303–463 K), whereas severe thermal quenching was observed for the Mn⁴⁺ red emissions. This distinctive thermal response could be applied in temperature sensing, as demonstrated by a high relative sensitivity (S_r) of 1.22% K⁻¹ at room temperature (303 K), which is superior to many reported optical thermometry materials. Our findings not only offer structural insight into new doubly ordered spinels, but also provide an effective strategy for regulating the valence states of Mn ions for potential application in light-emitting diodes and temperature sensing.