Pairing of vibronic small excitons due to enhanced polarizability in a crystalline polarizable nomnetallic medium

Parity breaking at a centrosymmetric site, resulting in the appearance of a n inversion electric dipole moment, is shown to enhance the polarizability and thus add an additional binding energy for the pairing of vibronic excit ons into biexcitons. Implications of such formation of biexcitons in polari zable media like alkali halides are discussed. The possibility of electron- hole binding in layered perovskites, where the electron and hole move in th e neighboring conducting planes, while binding is mediated by a highly pola rizable partner in the intercalated insulating layer, is also considered. [ S0163-1829(98)02035-9].