1：Experimental Design and Method Selection:

A solid solution series of Na3(Rb1-xEux)(Mg1-x/7Lix/7)7(PO4)6:Eu2+ with 0 ≤ x ≤ 0.3 was prepared by conventional solid state reaction. Phase purity was controlled by powder X-ray diffraction (XRD). Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded. Temperature-dependent measurements from 77 to 500 K were performed. Diffuse reflectance (DR) spectra were recorded. External quantum efficiencies (eQE) were obtained. Scanning electron microscopy (SEM) micrographs were recorded. Particle size distribution was determined. An LED prototype was manufactured using the phosphor blend.

2：3 was prepared by conventional solid state reaction. Phase purity was controlled by powder X-ray diffraction (XRD). Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded. Temperature-dependent measurements from 77 to 500 K were performed. Diffuse reflectance (DR) spectra were recorded. External quantum efficiencies (eQE) were obtained. Scanning electron microscopy (SEM) micrographs were recorded. Particle size distribution was determined. An LED prototype was manufactured using the phosphor blend. Sample Selection and Data Sources:

2. Sample Selection and Data Sources: High purity reagents were used. Samples with varying Eu2+ concentrations were synthesized.

3：List of Experimental Equipment and Materials:

Panalytical X’Pert PRO MPD diffractometer, Edinburgh Instruments FLS920 spectrometer, Xe arc lamp, Peltier cooled single-photon counting photomultiplier (Hamamatsu R2658P), nano-second pulsed EPL375 ps Laser, Oxford Instruments cryostat MicrostatN2, Oxford Instruments Optistat AC-V 12 closed cycle He-cryostat, Zeiss EVO MA10 SEM, Horiba LA-950-V2 organic particle sizer, Osram LED Color Calculator

4：49, reagents including MgCO3, Li2CO3, (NH4)H2PO4, Eu2O3, Na2CO3, Rb2CO3, BaSO4, LaPO

Ce3+, Lu3Al5O12:Ce3+, Ba2Si5N8:Eu2+, Silbione RT Gel 4317 A and B, 390 nm LED (ATI-005HUV3904-C2).

5：2). Experimental Procedures and Operational Workflow:

4. Experimental Procedures and Operational Workflow: Educts were mixed in an agate mortar with acetone, fired at 900 °C for 12 h under forming gas. XRD patterns recorded. PL and PLE spectra recorded with correction. Time-resolved spectroscopy with laser. Temperature-dependent measurements with cryostats. DR spectra recorded with integration sphere and vacuum conditions. eQE measured. SEM micrographs taken. Particle size distribution measured. LED prototype made by blending phosphors in silicone, curing, and placing on LED.

6：Data Analysis Methods:

Rietveld analysis attempted. Kubelka-Munk function and Tauc plot for band gap. Blasse’s formula for concentration quenching. Fitting of decay times with specific function. Franck-Condon factors calculated from DFT and Ligand Field Theory.