Mobility of an electron in a multimode polar lattice

The interaction of a free electron with a polar lattice possessing more than one infrared-active opticalphonon mode is considered. From the full Lagrangian describing the electron-lattice system in the presence of an applied field we derive an effective electron-phonon coupling constant and an effective longitudinal opticalphonon frequency that we argue give accurate predictions when used in the extensive, existing polaron theories. We apply this formalism to the strongly coupled large polaron of Bi12SiO20 , where the Boltzmann equation cannot apply. We calculate a theoretical prediction for the large polaron mobility as a function of temperature which gives good agreement with experiment. We determine the temperature dependence of Feynman’s variational parameters v and w to assist in our predictions. @S0163-1829~99!01426-5#