elected from the group of metals consisting of zirconium, titanium, haf: nium, and thorium, Y is an element selected from the group of elements consisting of silicon and germanium, the molar ratio of U is from 1/99 to 99, the molar ratio of is from 1/3 to 2, and the sum of 11+?) is equal to than one hundred to one at one second following cessation of excitation to fluorescence. 2.