Cadmium cyanide. This is made by combining saturated solutions of cadmium sulfate and an alkali cyanide. Cadmium cyanide precipitates out on mixing. It is photosensitive and decomposes to a brown substance rapidly even in indirect sunlight. However, low level white light that's sans UV seems to be OK. I don't know if it's UV light, intense white light, or both that cause the photodecomposition. Cadmium cyanide is nearly as toxic as thallium cyanide although so far as I am aware cadmium poisoning is treatable and you might even be able to have a somewhat normal life afterwards. Thallium is not so forgiving.