Chemical stability of binary Coulomb compounds

Chemical stability of binary Coulomb compounds is studied through extensive calculation of their Madelung energies. A remarkable finding is that the c ompounds with disordered ionic arrangements in a certain range of the compo sition are stable against chemical separation beyond the charge ratio aroun d 6; clearly the entropic effect due to finite temperature promotes the mix ing of ions of different species. For the ordered AB(3)-type compounds, the re exists a crystalline structure which is more chemically bounded than tha t of Dyson (1971). Relative stability of the ordered and disordered compoun ds is also discussed.