Seven distinct families of superconductors with critical temperatures at ambient pressure that equal or surpass the historic 23 K limit for Nb3Ge have been discovered in the last 25 years. Each family is reviewed briefly and their common chemical features are discussed. High temperature superconductors are distinguished by having a high (>= 50%) content of nonmetallic elements and fall into two broad classes. 'Metal-nonmetal' superconductors require a specific combination of elements such as Cu-O and Fe-As which give rise to the highest known T-c's, probably through a magnetic pairing mechanism. 'Nonmetal-bonded' materials contain covalently bonded nonmetal anion networks and are BCS-like superconductors. Fitting an extreme value function to the distribution of T-c values for the known high-T-c families suggests that the probability of a newly discovered superconductor family having maximum T-c > 100 K is similar to 0.1 to 1%, decreasing to similar to 0.02 to 0.2% for room temperature superconductivity.
- QUATERNARY BOROCARBIDES
- HAFNIUM NITRIDE