Question: What Is Red Mercury?
Answer: The science newsgroups have been a-buzz with tales of a 2-kiloton yield Russian red mercury fusion device, theoretically in the possession of terrorists. This, of course, prompts the question: What Is Red Mercury? The answer to this question depends largely on whom you ask. Is red mercury real? Absolutely, but definitions vary. If you had asked me before I did a bit of Internet research, I would have given you the standard cinnabar/vermillion answer. However, the Russian tritium fusion bomb is more interesting...
Cinnabar is naturally-occurring mercuric sulfide (HgS), while vermillion is the name given to the red pigment derived from either natural or manufactured cinnabar.
Mercury (II) Iodide
The alpha crystalline form of mercury (II) iodide is called 'red mercury', which changes to the yellow beta form at 127°C.
Any Red-Colored Mercury Compound Originating in Russia
as in the cold war definition of 'Red'. I doubt anyone is using 'red mercury' in this manner, but it's a possible interpretation.
A Ballotechnic Mercury Compound
Presumably red in color. Ballotechnics are substances which react very energetically in response to high-pressure shock compression. Google's Sci.Chem group has had a lively ongoing discussion about the possiblity of a an explosive form of mercury antimony oxide. According to some reports, red mercury is a cherry red semi-liquid which is produced by irradiating elemental mercury with mercury antimony oxide in a Russian nuclear reactor. Some people think that red mercury is so explosive that it can be used to trigger a fusion reaction in tritium or deuterium-tritium mixture. Pure fusion devices don't require fissionable material, so it's easier to get the materials needed to make one and easier to transport said materials from one place to another. Other reports refer to a documentary in which is was possible to read a report on Hg2Sb207, in which the compound had a density of 20.20 Kg/dm3 (!). Personally, I find it plausible that mercury antimony oxide, as a low density (nonradioactive?) powder, may be of interest as a ballotechnic material. The high-density material seems unlikely. It would also seem unreasonably dangerous (to the maker) to use a ballotechnic material in a fusion device. One intriguing source mentions a liquid explosive, HgSbO, made by Du Pont laboratories and listed in the international chemical register as number 20720-76-7. Anyone care to look it up?
A Military Code Name for a New Nuclear Material
As I understand it, this definition originates from the extraordinarily high prices commanded and paid for a substance called 'red mercury', which was manufactured in Russia. The price ($200-300K per kilogram) and trade restrictions were consistent with a nuclear material as opposed to cinnabar.
Looking for more information? Here is an insightful reader response to this FAQ, written by Wm. Yerkes.