Improvement on the linearity of blue phosphor: Evaluation of hexagonal ZnS:Ag,Al

— ZnS:Ag,Al phosphors were prepared in an H 2 S flow without using flux at various temperatures. A cubic structure (a zinc‐blende structure) was obtained for the sample prepared at 900°C, and a hexagonal structure (a wurzite structure) was obtained at 1050–1200°C. The life was improved simply with increasing preparation temperature. The luminous efficacy and the current coefficient had their optimum temperature at 1050°C. They showed similar tendencies related to the preparation temperature. The diffuse reflectance also showed the similar tendency of the preparation temperature dependence. The thermoluminescence measurement showed that the traps at a depths of 1.3 eV (observed below 1050°C) and 1.15 eV (observed above 1050°C) disappeared at a preparation temperature of 1050°C.