Faulting and Electroluminescence in Hexagonal ZnS Phosphors

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Abstract
Controlled experiments have been conducted to isolate the processing parameters which produce bright electroluminescence in hexagonal ZnS:Cu, Cl phosphors. Crystal structure, in particular, was studied as it is affected by the mechanical, thermal, and chemical treatments of this processing. The correlation between electroluminescent brightness and stacking disorder which has often been suggested has been more quantitatively established. It was possible only partially to separate the effects of the entry of activator (Cu) from those of faulting upon electroluminescence. It was found that these two phenomena are intimately related in their importance for high brightness. The nature and extent of the disorder has been investigated by precision x‐ray diffractometry of the powders and by single‐crystal x‐ray diagrams of individual granules from them. Each granule has been found to be heterogeneous to xrays so that the analysis of the powder patterns is not straightforward. Much, or most of the disorder is obscured in the background of powder patterns and shows itself clearly only on single‐crystal diagrams. The latter have been analyzed according to Jagodzinski's model based on two independent fault parameters; the fits obtained with this model between calculated and observed x‐ray scattering are impaired by heterogeneity. The analysis discloses that the disorder in the homogeneously scattering‐faulted regions may be an order of magnitude larger than indicated by the line breadth of the parent phase, which indicate faulting frequencies seldom exceeding 3%. Coherence in both the transformed and residual parent phase is favored by faulting in the presence of activator during the processing which produces bright EL. This coherence is lacking, even when faulting is extensive, if the disorder occurs without activator; under such conditions EL is practically not observed. Conclusions reached in the past about the effect of structure upon EL, and also upon birefringence in ZnS phorphors, should be re‐examined for the validity of the x‐ray diffraction work on which they may have been based, inasmuch as in the course of the present work it has been shown that high precision diffractometry and single granule diagrams are needed to obtain an adequate picture of the disorder. Film diffraction may be entirely misleading.