How can I prepare a luminescent alkaline earth sulfide?

Calcium sulfide isn't very soluble in water. According to the CRC Handbook of Chemistry and Physics, the solubility of calcium sulfide is only 21 mg per 100 mL for cold water (15°C) and 48 mg per 100 mL for hot water (60°C).

Separation of calcium sulfate and sulfide by filtration is ineffective. Calcium sulfate (and calcium sulfate dihydrate) have solubilities about 10 times greater than that of calcium sulfide, so any unreacted calcium sulfate is going to heavily contaminate your product.

The luminescence might be quenched by contaminants or water. Luminescence is a multistep process. First, the powder absorbs high-energy light, which puts some of its electrons into a higher energy ("excited") state. The electrons can lose the excess energy in several different ways. Emission of energy in the form of light is what you're looking for, but collisions with water molecules or impurities in the powder can carry away the excess energy very quickly. If the powder is wet, you might not be seeing any luminescence because the energy of the excited state is being entirely dissipated ("quenched") by collisions with water molecules.

Certain contaminants are required to see the luminescence. Ions of certain metals act as 'activators' in luminescence. For example, the luminescence of natural CaS minerals (oldhamite) is due to manganese and cerium ions. You might try deliberately contaminating your product with a small amount of a manganese salt.

Try a new recipe. Strontium sulfide is the active ingredient in some luminous paints; like CaS, it has a yellow luminescence. If you really want the calcium sulfide just for its luminescence you might try adapting the following recipe for strontium sulfide phosphor (taken from Alyea and Dutton's Tested Demonstrations in Chemistry). Notice the small amount of manganese(II) chloride added as an activator.

Provide 10 g SrCO₃, 10 g roll sulfur, 0.05 g KCl, 0.05 g NaCl, and 0.04 g MnCl₂·4 H₂O. Thoroughly mix the above ingredients by grinding them together in a mortar. Fire the mixture for about 30 minutes at 800°C.

References

Tested Demonstrations in Chemistry, H. R. Alyea, F. B. Dutton, Journal of Chemical Education, Easton, Pa, 1965

Contains recipes for other luminescent powders.

Physics and Chemistry of II-VI Luminescent Semiconductors, D.R. Vij and N. Singh (eds.), Nova Science Publishers, 1997. (ISBN 1-56072-433-1).

Chapter 7 deals specifically with luminescence in alkaline earth metal sulfides.

Author: Fred Senese senese@antoine.frostburg.edu