Deviation from guest dominated glass like lattice dynamics in prototypical ternary Bafilled Ni substituted Ge clathrates

Intermetallic guest filled clathrate cages have been identified as promising materials for thermoelectric applications. The structure, electronic structure, and phonon dynamics of type I Ba filled Ni substituted Ge clathrates are explored using density-functional theory. The formation energy of these type I clathrates (with tetrahedral vacancies) is calculated as a function of Ni substitution (x) in the framework. Ni substitution destabilized the framework vacancies resulting in a framework devoid of vacancies beyond a particular concentration of Ni substituition (for x≥3). By tuning the concentration of Ni in the framework, n type to p type doping can be achieved while retaining the compositional homogeinity. Furthermore, Ni substitution in the framework lowered the thermal conductivity of these compounds. Results of molecular dynamics simulations showed that with the increase in temperature the guest, the substitutional host as well as the host atoms rattled to collectively lower the lattice thermal conductivity of these clathrates. This is found to be contradictory to the concept of guest dominated glass like phonon dynamics in these compounds.