Does Cesium Auride have any interesting electrical properties or is anything known about its electrical conductivity?

Question

I have read that this remarkable compound has relativistic electron orbitals. But is there any data about the energy bandgap structure? Or moreover, is it known if this material has conductive/semiconductive properties?

Answer 1

I'm not sure quite how 'remarkable' the CsAu compound is - there are many alloys among the heavier elements. Whether the electrons are 'relativistic' or not has little to do with how their orbitals behave.

But, on the Au-Cs phase diagram (see A.D. Pelton in Binary Alloy Phase Diagrams, II edition, Ed. T.B. Massalski), both CsAu and CsAu$_{5}$ compounds exist. The CsAu phase is in the CsCl structure, with interpenetrating simple cubic sublattices. This phase is transparent and non-metallic. Even the liquid at this composition shows a dramatic reduction in conductivity (and a large decrease in volume) indicating a molten-salt like behavior.

Measurements by W.E Spicer in Physical Review indicate that CsAu is indeed a semiconductor with a bandgap of about 2.6eV, and it is likely a direct gap. These experimental results were not far off of calculations by V.E. Wood and J.R. Reitz in 1962. It is considered a highly ionic compound as confirmed by XPS measurements by G.K. Wertheim et al.