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 $\mathrm{exp}(\frac{\ensuremath{\Theta}}{T})$ was generally observed above 40\ifmmode^\circ\else\textdegree\fi{}K. The Debye temperature $\ensuremath{\Theta}$ 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\ifmmode^\circ\else\textdegree\fi{}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 ${F}^{\ensuremath{'}}$ and $\ensuremath{\alpha}$ centers (charged defects). The original mobility could then be restored by illumination in the ${F}^{\ensuremath{'}}$ band at low temperature.