Co-substitution of Zn2+/Ge4+ in Er3+-activated garnet phosphors to enhance luminescence properties and optical thermometry

Abstract

The gadolinium-gallium garnet has emerged as a promising candidate for preparing efficient luminescent materials; however, the luminescence properties of rare-earth activators in its solid solution phosphors are scarcely explored via crystal field engineering. In this work, Er³⁺-doped Gd₃Zn_xGa_5−2xGe_xO₁₂ (GZG_5−2xG:Er³⁺) solid solutions were prepared by a high-temperature solid-state reaction, where a heterovalent co-substitution strategy was implemented by introducing the Zn²⁺/Ge⁴⁺ pair to replace 2 Ga³⁺ sites. Rietveld refinements of X-ray diffraction data confirmed that all samples crystallized in the pure garnet phase. Upon 381 nm excitation, the Er³⁺ concentration was spectrally optimized to be about 10%, substituting for the Gd³⁺ sites, and via Zn²⁺/Ge⁴⁺ co-substitution, the luminescence intensity of GZG₂G:10%Er³⁺ increased by more than threefold compared with that of the baseline GZGG:10%Er³⁺ sample. All the GZG_5−2xG:10%Er³⁺ solid solution samples showed a bright green color under illumination using an ultraviolet lamp at ∼365 nm. The thermometric properties of the thermally coupled levels of Er^{3+ 2}H_11/2-⁴S_3/2 were systematically studied on the basis of the fluorescence intensity ratio principle. The GZG_5−2xG:10%Er³⁺ samples can perform temperature sensing over a wide temperature range of 298–573 K, with a maximum relative sensitivity (S_r) above 1.20% K⁻¹ and an applicable minimum temperature resolution (δ_T) around 0.4 K. Continuous exploration of solid solution-manipulating luminescence properties could lead to the development of novel thermometry probes (particularly for activators with spin-allowed transitions) with higher S_r and smaller δ_T for practical optical temperature sensing.