Thermoelectric Properties and Site-Selective Rb⁺/K⁺ Distribution in the K_2-xRb_xBi₈Se₁₃ Series

β-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 K_2-xRb_xBi₈Se₁₃ series of solid solutions was studied and the members with 0 < x ≤ 1 were found to adopt the β-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 β-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.