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By means of absorption and photoconductivity measurements, the following band gaps were found: Na₃Sb---1. 1 ev; K₃Sb---1. 1 ev; Rb₃Sb---1. 0 ev; Cs₃Sb---1. 6 ev; and (NaK) ₃Sb---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 (NaK) ₃Sb with surface layers of Cs or Rb added. The electron affinities found from the photoemissive data using this model were as follows: Rb₃Sb---1. 2 ev; Cs₃Sb---0. 45 ev; (NaK) ₃Sb---1. 0 ev; Rb (NaK) ₃Sb---0. 70 ev; and Cs (NaK) ₃Sb---0. 55 ev. The electron affinity of K₃Sb was estimated to be between 1. 1 and 1. 8 ev and that of Na₃Sb to be between 2. 0 and 2. 4 ev. By means of the temperature dependence of the photoemission, Rb₃Sb, Cs₃Sb, and the multi-alkali materials were found to have p-type conductivity; whereas K₃Sb and Na₃Sb were found to have n-type conductivity.
W. E. Spicer (1958) studied this question.