ZnGa_2-AlO₄:Mn²⁺,Mn⁴⁺ Thermochromic Phosphors: Valence State Control and Optical Temperature Sensing

Dual-emitting and thermochromic manganese ion single doped ZnGa_2-AlO₄ phosphors were prepared by solid-state reaction. The regulation of the valence state and the luminescent properties, especially the luminescent thermal stability of manganese ions in ZnGa_2-AlO₄, are discussed in detail. When excited by ultraviolet (UV) light, the emission spectra of ZnGa₂O₄:Mn²⁺,Mn⁴⁺ present an ultranarrow green emission band at 503 nm with a fwhm of 22 nm, which derives from the Mn²⁺ ions formed by the self-reduction of doped Mn⁴⁺, and a red emission band of the Mn⁴⁺ ions at 669 nm. In addition, a ZnGa_2-AlO₄:Mn²⁺,Mn⁴⁺ solid solution was designed and synthesized by Al³⁺ replacing Ga³⁺. The doping of Al³⁺ effectively inhibited the degree of Mn⁴⁺ self-reduction to Mn²⁺, thus realizing the regulation of valence state of manganese ions. Interestingly, the thermal stability of luminescence shows that the response of Mn²⁺ and Mn⁴⁺ to temperature is obviously different in ZnGa_2-AlO₄, implying the potential of the prepared phosphors as optical thermometers. Subsequently, three kinds of optical thermometers with superior color discrimination and high relative sensitivity (S_r) based on the fluorescence intensity ratio (FIR) technique were realized in 100-475 K. The S_r value of ZGO:0.005Mn/ZGA_0.5O:0.005Mn/ZGAO:0.005Mn phosphors can be as high as 4.345%/4.001%/3.488% K^-1 (at 350/325/400 K), reflecting the great potential of ZnGa₂O₄:Mn²⁺,Mn⁴⁺ for optical thermometry applications.