## Abstract

Although caesium antimonide is the most efficient photo-emitter known, no adequate explanation has been offered for its unique properties. All previous attempts at its chemical and structural characterization have been unsuccessful. The present X-ray investigation shows that it is a 'normal valency' intermetallic compound with a small range of homogeneity near to the composition Cs$_{3}$Sb. The atomic arrangement is pseudo-body-centred cubic with a defect structure based upon the B 32 sodium thallide type (NaTl). The cubic unit cell (a = 9$\cdot $14 to 9$\cdot $19 angstrom) contains sixteen atoms at special positions of space group Fd3m-O$_{h}^{7}$. Eight equivalent sites at (0, 0, 0; 0, ${\textstyle\frac{1}{2}}$, ${\textstyle\frac{1}{2}}$; ${\textstyle\frac{1}{2}}$, 0, ${\textstyle\frac{1}{2}}$; ${\textstyle\frac{1}{2}}$, ${\textstyle\frac{1}{2}}$, 0) + 0, 0, 0; ${\textstyle\frac{1}{4}}$, ${\textstyle\frac{1}{4}}$, ${\textstyle\frac{1}{4}}$ are occupied solely by caesium atoms and eight other sites, at () + ${\textstyle\frac{1}{2}}$, ${\textstyle\frac{1}{2}}$, ${\textstyle\frac{1}{2}}$; ${\textstyle\frac{3}{4}}$, ${\textstyle\frac{3}{4}}$, ${\textstyle\frac{3}{4}}$, are occupied randomly by four caesium and four antimony atoms. The structure may be described as consisting of two interpenetrating diamond-type lattices, one of caesium atoms and the other of equal numbers of caesium and antimony atoms. The semi-conducting properties of the material are explained by a filled Brillouin zone containing two electrons per atom. The crystal structure of caesium antinomide also suggests which of the previously proposed mechanisms for its photo-emissivity is acceptable. The observed X-ray diffraction effects are accounted for only if the amplitudes of thermal vibrations are very much greater for caesium atoms than for antimony atoms. In caesium antimonide the root-mean-square displacements of the different atoms from their mean lattice positions (0$\cdot $52 angstrom for Cs and 0$\cdot $25 angstrom for Sb) are approximately the same as the corresponding displacements in metallic caesium and metallic antimony. This unexpectedly large difference between the Debye temperature factors of the different atoms causes the appearance of X-ray superlattice reflexions which were at first thought to be due to specific shifts of some or all of the atoms from special positions. Although the assignment of separate temperature factors to different atoms is well known, the need for this is illustrated quite strikingly by the caesium antimonide diffraction pattern.