Mobility of an electron in a multimode polar lattice

The interaction of a free electron with a polar lattice possessing more tha n one infrared-active optical-phonon mode is considered. From the full Lagr angian describing the electron-lattice system in the presence of an applied field we derive an effective electron-phonon coupling constant and an effe ctive longitudinal optical-phonon frequency that we argue give accurate pre dictions when used in the extensive, existing polaron theories. We apply th is 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 ag reement with experiment. We determine the temperature dependence of Feynman 's variational parameters upsilon and w to assist in our predictions. [S016 3-1829(99)01426-5].