1：Experimental Design and Method Selection:

The study involved synthesizing K2Tb

2：5Ta5(PO4)3 using a high-temperature flux method for single crystals and a solid-state reaction for polycrystalline samples. Eu3+-doped samples were prepared similarly. Characterization included single-crystal X-ray diffraction, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), IR spectroscopy, UV-Vis spectroscopy, and photoluminescence (PL) measurements. Sample Selection and Data Sources:

Samples included undoped K2Tb

3：5Ta5(PO4)3 and doped K2Tb5–xTa5(PO4)

xEu3+ (x=

4：01, 03, 05, 07, 10). Raw materials were purchased from Sinopharm Chemical Reagent Shanghai Limited Company. List of Experimental Equipment and Materials:

Equipment included a Bruker SMART APEXII CCD diffractometer for X-ray diffraction, Rigaku DMax2500 diffractometer for PXRD, Carl Zeiss AG Merlin Compact SEM, Bruker V70 FT-IR spectrophotometer, Hitachi UH4150 spectrophotometer for UV-Vis, and FLS1000 Edinburgh Analytical Instrument for PL. Materials were K2CO3, Tb4O7, Eu2O3, Ta2O5, NH4H2PO

5：Experimental Procedures and Operational Workflow:

For single crystals, reactants were mixed, pretreated at 500°C, heated to 900°C, cooled slowly to 650°C, and washed. For polycrystalline samples, stoichiometric mixtures were calcined at 950°C with intermediate grindings. PL measurements used a xenon lamp for excitation and a flash lamp for lifetime measurements.

6：Data Analysis Methods:

Data were analyzed using software like APEX2, SAINT, SHELXS97, SHELXL2014 for crystallography. PL decay curves were fitted with non-exponential functions, and energy transfer efficiency was calculated.