Electronic structure and the metal-nonmetal transition in liquid CsAu andCs3Sb

The electronic structure and ionicity of some alkali-gold and alkali-pnictide compounds has been calculated and used to investigate the type of chemical ordering and the nature of the metal-nonmetal transition (MNMT) in their liquid semiconducting phases. The gold compounds are found to extremely high ionicities, averaging 0.96, while the pnictides are much lower, ranging from 0.60 for ${\mathrm{Cs}}_3$Sb to 0.05 for ${\mathrm{Li}}_3$Sb. The thermochemical, structural, and transport properties of the liquid alloys are similar, but the range of composition for which $l$-CsAu is semiconducting is larger than in the other systems. This is shown to be wider than is consistent with band-filling models of the MNMT and is further evidence that the transition occurs by carrier localization at defects such as $F$ centers. The pnictides and Pb alloys have more usual MNMT's and presumably $F$ centers are not stable at their much lower ionicities. The unifying feature of the bonding in these systems is that the lowest valence orbital of the cation is above the highest occupied anion orbital in their free atoms. They may therefore be classed as charge-transfer compounds. However, their bonding cannot always be truly termed ionic as the net ionic charges and ionicities are often very low, as in the Li salts.