| abstract |
A mixing step of mixing at least one of aluminum oxide or silicon oxide, silicon nitride, aluminum nitride, and a europium compound, and a firing step of firing the mixture after the mixing step in a condition of more than 1950 ° C. and not more than 2200 ° C. for 10 hours or more. And a heat treatment step of heat treatment in an atmosphere of an inert gas other than nitrogen of 1300 ° C or higher and 1600 ° C or lower and partial pressure of 10 kPa or lower after the firing step. |