Studies on the Concept of Large Activator Centers in Crystal Phosphors

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Abstract
The explanation of the variation of efficiency with concentration of activator in terms of a large center, given by Ewles, was oversimplified. The recent suggestion of Johnson and Williams that an activator can only function as a luminescent center if no other activator occupies any of lattice sites surrounding it, leads to the same expression for the number of centers as that originally given by Ewles and is effectively based on the same idea. Making use of the experimental findings reported in the present work that lattice defects can act as luminescent centers, and taking into account the competition of these centers in the luminescent process, a modification of Ewles' theory of efficiency based on the idea of a large center leads to the expression for the efficiency. The preparation of two series of calcium oxide phosphors from highly purified and annealed calcium oxide is described. The modified theory is shown to agree well with efficiency measurements on these phosphors and with those quoted by Johnson and Williams, except at high concentrations. Estimates of the “radial extent” of the centers are given.

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