Color-tunable emissions and multi-mode optical thermometer in Mg₃Gd₂Ge₃O₁₂: Tb³⁺, Eu³⁺ phosphors

A series of Mg₃Gd₂Ge₃O₁₂ phosphors co‐doped with Tb³⁺ and Eu³⁺ was produced using a solid‐state method. The phosphors doped solely with Tb³⁺ have blue emission (383 nm) and green emission (544 nm), and the addition of Eu³⁺ can produce red emission (610 nm). The transformation of green‐orange could be achieved by adjusting the doping amount of Eu³⁺ because of the energy transfer (ET) from Tb³⁺ to Eu³⁺. The ET process was attributed to dipole−dipole interaction. In addition, the emission intensities of three levels and the lifetimes of ⁵D₃ decrease with increasing temperature, and color changes are produced. According to these interesting phenomena, an optical thermometer with different modes was designed utilizing the fluorescence intensity ratio (FIR) and the fluorescence lifetime (FL) model. The maximum values of relative sensitivity (Sr) were found to be 1.24% K⁻¹ at 494 K (FIR = I₃₈₃/I₆₁₀), 0.43% K⁻¹ at 423 K (FIR = I₅₄₄/I₆₁₀), and 1.38% ms K⁻¹ at 465 K (FL). The temperature sensing performance of the sample demonstrates good repeatability through cyclic testing. The above results demonstrate that Mg₃Gd₂Ge₃O₁₂: Tb³⁺, Eu³⁺ samples are potential materials for visual optical temperature measurement.