Rattler Site Selectivity and Covalency Effects in Type-I Clathrates

Type-I germanium clathrates comprised of Ga/Ge tetrakaidecahedra and dodecahedra cages that host alkaline earth atoms exemplify the properties of good thermoelectrics where the guest-atom anharmonic vibrations play a major role. However, while the host lattice--guest atom interaction is essential for this class of materials it is at the same time difficult to investigate due to the random Ga/Ge site occupation in the host lattice. In the present 71 Ga NMR study of Sr 8 Ga 16 Ge 30 and Ba 8 Ga 16 Ge 30 samples we extract different Ga crystallographic site contributions from otherwise complex NMR spectra by exploring differences in their spin--lattice relaxation rates. Such approach opens a unique possibility for a site-selective study that directly proves non-negligible interaction of the anisotropic rattling motions of endohedral Sr with Ga atoms occupying the specific $24k$ sites of larger tetrakaidecahedral cages. This interaction affects electron--phonon coupling and modifies the local chemical environment --- possibly even through weak covalent bonding of Sr to the cage. Our results are thus in agreement with claims that the conventional picture of purely ionic interactions between the rattling guest atoms and the cage is only approximate and that covalent effects should be taken into account in clathrates or similar thermoelectric cage materials.