Hexagonal was found to be an efficient green‐emitting phosphor with infrared excitation. It was prepared by firing a mixture of and coprecipitated at 630°C in an argon atmosphere. The optimum firing condition of was revealed to lie in the region where (a) the decomposition reaction is complete, (b) liquid and hexagonal are coexistent, and (c) the phase transition of from hexagonal to cubic does not occur. When pumped by a Si‐doped diode, properly prepared is 4 ∼ 5 times as bright as the commercially available .