Sm3+-Doped Y2Ge2O7 Orange-Red Phosphor Prepared via the Sol–Gel Method for Multimodal Applications: A Study of of the Effect of Dopant Concentration on Structural and Optical Properties

Abstract This work reports, for the first time, the synthesis of single-phase trivalent samarium-doped yttrium pyrogermanate (Y2Ge2O7:xSm3+) phosphors via the sol–gel method. The crystalline structure, photoluminescence mechanisms, energy transfer dynamics, and optical thermometry properties were systematically investigated. The experimental optical bandgap (∼5.99–6.10 eV) shows good agreement with those obtained from the density functional theory (DFT) calculations. Upon Sm3+ incorporation, a concentration quenching effect reduces the quantum yield from 8.1% to 0.3%, correlated with a reduction in the average lifetime from 2.08 to 0.61 ms. Analysis of the decay kinetics via the Van Uitert model indicates a dipole–dipole interaction governing the energy transfer, achieving a cross-relaxation efficiency of ∼73% at 4.0 mol %. Remarkably, the CIE coordinates, correlated color temperature (CCT), and color purity (CP) remain independent of the dopant content, exhibiting an exceptional red color purity above 95% for all samples. These results demonstrate that Y2Ge2O7xSm3+ phosphors are promising candidates for solid-state lighting and multifunctional optical applications. Additionally, the ratio between emission sub-bands from 4G5/2 → 6H9/2 to 4G5/2 → 6H5/2 provides a relative sensitivity value (SR) for the sample with 4.0 mol % up to 0.23%/K, indicating a potential use as an optical sensing thermometer.