Luminescence

There are many different kinds of luminescence. The mechanism is different in each case, but some materials fit into more than one category. Some luminescence categories are are listed in the following table:

Possibly the first luminescent material ever synthesized was barium sulfide (BaS). It was discovered by an alchemist called Vincenzo Cascariolo in 1603. Much later, calcium sulfide was found to give much brighter luminescence. It was first created by mixing ground up oyster shells with sulfur and heating the mixture (2CaCO3 + 3S -> 2CaS + 2CO2 + SO2).

Compounds such as CaS rely upon the inclusion of small concentrations of impurity atoms for luminescence. In fact, if the CaS was completely pure, it would not exhibit luminescence. These impurity atoms are incorporated into the crystal lattice of the host material and are called a dopant. The emission characteristics are more characteristic of the dopant atoms than of the host lattice.

To make CaS, I mixed equal quantities of calcium carbonate and sulfur powder in a test tube. I also mixed in a very tiny amount (less than 1%) of 'Lo Salt' to act as the dopant. Lo Salt is manufactured by Klinge and is 66% potassium chloride and 33% sodium chloride.

At this point the mixture, which is in the bottom of the right hand test tube, is not luminescent.

I heated this mixture gently over a gas burner until vapor stopped coming off and I was left with a grey lump in the bottom of the test tube. This lump shows weak luminescence under UV. I then transferred it to a ceramic dish, ground and mixed it again and then heated it until it glowed orange. When cool, the compound glows with a mixture of pale pink and green patches under UV and has an after-glow.

If you decide to try this, don't use large quantities of the chemicals. A by-product of the reaction is sulfur dioxide gas, which is toxic.

I also tried using strontium carbonate instead of calcium carbonate. This results in strontium sulfide (SrS). SrS seems to be much brighter than CaS. It glows a greenish colour under UV and has a fairly strong after-glow.

The picture on the left shows the SrS in the ceramic dish after heating.

This is the SrS illuminated with a soft UV lamp. It fluoresces a pale yellow / green colour. It is quite bright and can be seen glowing under UV with the room lights on.

In complete darkness, the after-glow is visible for about 15 minutes

Strangely, after storing it in a polythene bag for a few months, it seems to have become inactive.

Modern luminescent compounds such as zinc sulfide (ZnS) include a controlled quantity of dopant such as copper. This is written as ZnS:Cu. ZnS:Cu fluoresces brilliant green under UV light. It is also sensitive to X-Rays and electrons.