Electron Hall Mobility in the Alkali Halides

New results will be presented on the mobility of electrons in alkali-halide crystals down to the temperature of liquid helium. An improved technique was used to observe the transient Hall effect for electrons released from color centers by light. Optical-mode scattering with mobility dependent upon $\exp \left(\frac{\Theta }{T}\right)$ was generally observed above 40°K. The Debye temperature $\Theta$ can be compared with the optical-mode frequency of the material, whereas the magnitude of the scattering can be used to estimate band and polaron mass parameters. Mobilities as high as 15 000 ${\mathrm{cm}}^2$/V-sec, depending upon $F$-center density, were observed for lightly colored crystals of KBr at 7°K. Similar high mobilities were found for KI and a sample of zone-refined KCl, but the largest mobility observed in NaCl was 1200 ${\mathrm{cm}}^2$/V-sec. The low-temperature mobility in KBr could be reduced an order of magnitude by optically converting $F$ centers (neutral defects) to $F^{\prime }$ and $\alpha$ centers (charged defects). The original mobility could then be restored by illumination in the $F^{\prime }$ band at low temperature.