Thermoelectric properties and site-selective Rb+/K+ distribution in the K2-xRbxBi8Se13 series

beta-K2Bi8Se13 possesses promising thermoelectric properties whereas Rb2Bi8Se13 does not. The formation of solid solutions between these two compounds was attempted to study the alkali metal distribution in the structure and its influence in the electronic properties. The structure of the K2-xRbxBi8Se13 series of solid solutions was studied and the members with 0 < x less than or equal to 1 were found to adopt the beta-K2Bi8Se13 structure. Detailed crystallographic studies were performed for the x = 1.0 composition to examine in detail the K/Rb distribution in the lattice and the effect on the K/Bi disorder that exists in the beta-K2Bi8Se13 structure. The Rb+ ions occupy exclusively one crystallographic site replacing K+. This defines RbKBi8Se13 as a new quaternary compound. Lattice parameters and band gaps are reported for the different compositions. Charge transport measurements including electrical conductivity, thermoelectric power, and Hall effect show that the materials are n-type semiconductors with increasing carrier concentration as a function of composition x.