The invention is directed a composition represented by the chemical formula EuxA1−xF2+x−2yOy wherein A is alkaline earth, 0.002≦x≦0.20, and 0≦y≦x; the composition exhibiting a luminescence spectrum characterized by peaks at 592±2 nm and 627±2 nm, wherein the ratio of the peak intensity at 592±2 nm to that at 627±2 nm is at least 5% larger than the corresponding peak intensity ratio at the same wavelengths of a first corresponding reference composition with the same value of x that has not been exposed to a temperature above 100° C., and wherein said ratio of the peak intensity at 592±2 nm to that at 627±2 nm is at least 5% smaller than the corresponding peak intensity ratio at the same wavelengths of a second corresponding reference composition with the same value of x that has been subject to heating to 900° C. for 6 hours.