K7.62(1)Si46 and Rb6.15(2)Si46: Two Structure I Clathrates with Fully Occupied Framework Sites

We report here the synthesis and structural characterization of two Structure I clathrates in the K-Si and Rb-Si systems. As observed previously for Na8Si46, the alkali-Si clathrates are fully stoichiometric at the framework sites, i.e., devoid of framework vacancies. This is in sharp contrast to the analogous K-Ge, Rb-Ge, and Rb-Sn, (K,Cs)-Sn, and Cs-Sn systems, where two vacancies are formed predominantly at one-third of the crystallographic 6c tetrahedral sites (the formation of vacancies is rationalized to remove the tetrahedral atom of its hypervalency). This result is understood generally in terms of weaker Tt-Tt (Tt (tetrelide)=Si, Ge, Sn) bonding as one descends the periodic table. Also observed are metallic conductivities for both K8Si46 and Rb6Si46, substantiating further the absence of vacancies -- the "extra" electrons from the guest atoms participate in the conduction bands of the Si network, resulting in conductivities typical of semimetals. This is contrasted with the semiconducting behavior of the vacancy bearing K8Ge44 (K8Ge44square, open2).