Photoemissive, Photoconductive, and Optical Absorption Studies of Alkali-Antimony Compounds

By means of absorption and photoconductivity measurements, the following band gaps were found: ${\mathrm{Na}}_3$Sb—1.1 ev; ${\mathrm{K}}_3$Sb—1.1 ev; ${\mathrm{Rb}}_3$Sb—1.0 ev; ${\mathrm{Cs}}_3$Sb—1.6 ev; and ${\mathrm{}\left(\mathrm{NaK}\right)\mathrm{}}_3$Sb—1.0 ev. A model has been derived for the photoemission from these materials which fits the three last materials listed above in addition to the ${\mathrm{}\left(\mathrm{NaK}\right)\mathrm{}}_3$Sb with surface layers of Cs or Rb added. The electron affinities found from the photoemissive data using this model were as follows: ${\mathrm{Rb}}_3$Sb—1.2 ev; ${\mathrm{Cs}}_3$Sb—0.45 ev; ${\mathrm{}\left(\mathrm{NaK}\right)\mathrm{}}_3$Sb—1.0 ev; [Rb]${\mathrm{}\left(\mathrm{NaK}\right)\mathrm{}}_3$Sb—0.70 ev; and [Cs]${\mathrm{}\left(\mathrm{NaK}\right)\mathrm{}}_3$Sb—0.55 ev. The electron affinity of ${\mathrm{K}}_3$Sb was estimated to be between 1.1 and 1.8 ev and that of ${\mathrm{Na}}_3$Sb to be between 2.0 and 2.4 ev. By means of the temperature dependence of the photoemission, ${\mathrm{Rb}}_3$Sb, ${\mathrm{Cs}}_3$Sb, and the multi-alkali materials were found to have $p$-type conductivity; whereas ${\mathrm{K}}_3$Sb and ${\mathrm{Na}}_3$Sb were found to have $n$-type conductivity.