The invention is directed a composition represented by the chemical formula Eu.sub.xA.sub.1-xF.sub.2+x-2yO.sub.y wherein A is alkaline earth, 0.002.ltoreq.x.ltoreq.0.20, and 0.ltoreq.y.ltoreq.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.degree. 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.degree. C. for 6 hours.