Band mobility of photoexcited electrons in Bi12SiO20

We determine the band mobility of photoexcited electrons in cubic n-type Bi12SiO20. We measure a room-temperature mobility of 3.3 +/- 0.5 cm(2)/(V s) that decreases monotonically to 1.7 +/- 0.3 cm(2)/(V s) as the temperature is increased to 200 degrees C. We show that electrons in Bi12SiO20 form large polarons. Our results are predicted by strong coupling polaron theory if the band mass of the electrons is chosen to be 2.0 +/- 0.1 electron masses. We determine the electron-phonon coupling constant and effective longitudinal optical phonon frequency required for this prediction from the available infrared reflectivity spectrum of Bi12SiO20.