I think this 1962 patent has interesting knowledge.
luminescent phosphor layers on a crt, separated by non-luminescent layers.
What's interesting is the composition of the non-luminescent layers, and the "poisoning agents" (in addition to the phosphor layers)
There's 4 groups of materials: phosphors, activating agents for the phosphors, separator material, and "poisoning agents". Apparently up to 3 phosphor layers, and then a layer of aluminum
I'm not exactly why the phosphor layers are subjected to "poisoning". But the key thing is the use of additional odd elements to to do that. They mention salts there.
read the full patent to get the gist of it
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By Way of example, this invention is described with reference to FIGS. 1-3 as embodied in luminescent screens which include three different phosphors, for example, red-emitting, green-emitting, and blue-emitting. However, the invention may instead be embodied in -screens harving a different number of diierent phosphors or of phosphors having different color emission characteristics. For example, the invention may be embodied in a two phosphor screen having orange and cyan emissions.
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In preparing the screen 22, the first phosphor particles .of the layer 24 are preferably first surface-poisoned as in preparation of the multiple coated particles 8. These surface-poisoned particles are then deposited in a layer on a suitable substrate such as by settling, slurrying, dusting, or adsorption deposition. Alternatively, the phosphor particles forming the layer 24 may first be deposited on the substrate and then surface-poisoned. The second and third phosphor layers 26 and 30, and the second nonluminescent separator layer 28 are superimposed on the surface-poisoned particles of the base layer 24 in their appropriate order, such as by settling, slurrying, dusting, evaporating or adsorption deposition.
If desired, a suitable metal backing layer (not shown), such as aluminum, may be applied over the third phosphor layer 30 `by a process known in the art.
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This invention is not limited to any particular type or kind of phosphor, separator material, or poisoning agent. Any of the known phosphors from families such as the sulfides, oxides, selenides, phosphates, or silicates of metals such as zinc, cadmium, manganese, magnesium or calcium may be employed. Such materials may be activated by any of the known suitable activators such as silver, manganese, or copper.
Suitable nonluminescent separator materials include, for example, silica, titanium dioxide, mica, bentonite, talc, vanadium oxide and many other materials which are nonluminescent, are compatible with a suitable method of deposition, and are capable of forming relatively compact, uniform layers.
The poisoning agent may be any material which will act to poison the particular phosphor being used and will not otherwise adversely affect the cathode ray tube into which the screen is incorporated. Suitable poisoning agents include metals such as iron, cobalt, and nickel. These agents may be provided in the form of salts thereof such as sulfates, chlorides, or nitrates.