*2.1. Experimental Procedure*

The Mn4+doped CaSb2O6 (CSO: Mn4+) phosphors with different concentrations (CaSb2-xO6: *x*Mn4+ (*x* = 0.25, 0.5, 0.75, 1, 2, and 3 mol %)) were prepared by a conventional solid-state reaction method. All analytical grade chemicals, i.e., calcium carbonate (CaCO3), antimony oxide (Sb2O3), and manganese carbonate (MnCO3) were purchased from Sigma Aldrich Co, Gangnam-gu, Seoul, Korea. Raw materials were used as received without further purification. Initially, the stoichiometric ratios of the desired raw materials were weighed and put in an agate mortar for grinding with the support of the pestle. After homogeneous mixing, the powders were carefully transferred to alumina crucibles and calcined at 1000 ◦C for 6 h. After the completion of the calcination process, the obtained powders were ground smooth for characterizations.
