Methods have been devised for the preparation of pure and doped (S, Se, Te) chlorosodalites. Optical data are presented which are at variance with the accepted mechanism for photochromism in these materials (i.e. the presence of an S22- ion). Intrinsic and extrinsic mechanisms which depend upon the formation of an F centre in conjunction with an O2- ion on a chlorine vacancy site are proposed. Similar mechanisms involving S, Se and Te may occur, and it is possible that more complex ions are involved.
during firing. The dark Cu~S inclusions and their correlation with the emitting "spots" inside the particles can be seen directly by microscope. ~ Dark Cu.~S inclusions are also indirectly indicated by the somewhat darker body color of the phosphor, which cannot be lightened even by a very thorough cyanide wash. These cubic particles also are well electroluminescent. On the other hand, the particles of a hexagonal phosphor are highly perfect; i.e., there are
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