Lead azide

 was an explosive chemical compound Jeff Stone once used to give the Daleks a very bad headache on Venus.

Because he was a mineralogist, Jeff was soon pressed into service by the Daleks to help them find a solution to the rust problem they were having on that moist world. They quickly discovered that a new lead-enriched paint would protect their outer casings. Stone worked out that if he combined an amount of lead salt with sodium azide — both of which happened to be around the lead mine where he was working for the Daleks — he would get lead azide.

By adding a tiny amount to the paint that was being sprayed onto the Daleks, he would be coating them with a layer of very fine crystals. When the crystals fully hardened as the paint dried, they would eventually break, because the heat of the Sun was intense enough on Venus to cause significant expansion. Each time one of these tiny crystals broke, it unleashed a tiny explosion. But because there were millions of these lead azide crystals in the paint, the cumulative effect was hugely disruptive to an individual Dalek. (DWAN: Red for Danger)