1：Experimental Design and Method Selection:

The hydrothermal method was selected for the synthesis of phosphor particles due to its advantages such as low processing temperature, high purity, homogeneous mixing, large yield, and ease of preparation. The pH of the synthesis precursors was adjusted to 9.

2：Sample Selection and Data Sources:

1. 2. Sample Selection and Data Sources: The synthesizing reagents included gadolinium oxide (Gd2O3), holmium oxide (Ho2O3), ytterbium oxide (Yb2O3), HCl, ZnO, and CaCO3. The co-dopants concentration was varied in the range of 0-20 mol % with an interval of 5 mol %.

3：The co-dopants concentration was varied in the range of 0-20 mol % with an interval of 5 mol %. List of Experimental Equipment and Materials:

3. List of Experimental Equipment and Materials: Equipment used included a Kratos Axis Ultra HAS XPS equipment for elemental composition analysis, Bruker D8 advanced X-ray diffractometer for XRD analysis, JEOL JSM 6301F for FESEM, CCD spectrometer (Model: ULS2048X64, Avantes, USA) for upconversion spectra, Hitachi fluorescence spectrometer (model F-2500) for downshifting photoluminescence measurement, and SQUID magnetometer (Model: Quantum design Ref: MPMS-5S) for paramagnetic behavior study.

4：Experimental Procedures and Operational Workflow:

The synthesis involved dissolving rare earth oxides in HCl, adjusting the pH to 9.1 with NH4OH, hydrothermal treatment at 200°C for 2 h, centrifugation, washing, drying, and annealing at 1000°C for 3h.

5：1 with NH4OH, hydrothermal treatment at 200°C for 2 h, centrifugation, washing, drying, and annealing at 1000°C for 3h. Data Analysis Methods:

5. Data Analysis Methods: The data analysis included XRD for crystalline nature confirmation, XPS for chemical state determination, FE-SEM for morphology and size determination, FTIR for impurity confirmation, and luminescence spectra analysis for optical properties.