Observing high dependence of red luminescence on localized symmetry in Mn⁴⁺ activated inverse spinel phosphors

Localized symmetry has been shown to significantly impact the luminescence behavior of Mn⁴⁺ ions through the electron-phonon coupling process. Building on this characteristic, three types of inverse spinel structure oxides (Mg₂XO₄, where X = Ti, Ti/Ge, Ge) doped with Mn⁴⁺ were developed, exhibiting strong red emission when exposed to UV and blue light. A thorough examination reveals that the symmetric improvement of the Mn⁴⁺ sites within the octahedral environment leads to significant changes in their luminescence behavior, including a suppression of zero-phonon-line (ZPL) emission, a blueshift, and an extension of the luminescence lifetime. Moreover, variable-temperature PL spectra of phosphors are carefully measured. Low-temperature PL spectra demonstrate three distinct sharp emission peaks for Mg₂GeO₄:Mn⁴⁺, while Mg₂TiO₄:Mn⁴⁺ exhibits a broad emission band. The average primary phonon energies involved in the vibronic processes are determined through theoretical fitting of temperature-dependent PL intensities. Lastly, the luminescence dynamics associated with anti-Stokes, ZPL, and Stokes emissions are analyzed. The observed increase in luminescence lifetime indicates a significant impact of the localized environment on luminescence properties.