Germanium isotope effect induced guest rattling and cage distortion in clathrates

Intermetallic clathrates are materials characterized by a large cage structure where guest atoms can move anharmonically, providing these materials exotic thermoelectric properties. Unfortunately, the dynamical and atomic nature of the rattling phonons, and their interactions with the electronic structure, are not fully understood. Here, we report that a germanium isotope effect can trigger an inherent guest rattling and cage distortion in clathrate Ba8Ga16Ge30 (BGG). Raman-scattering spectroscopy and advanced electron microscopy demonstrate that the atomic germanium isotope effect induces an off-centre rattling at the 6d sites as well as a tetrakaidecahedron deformation which is anisotropic for n-type BGG but isotropic for p-type BGG. The present findings indicate that the large n-type germanium isotope effect arises from the strong electron-phonon coupling, which opens up a novel avenue for manipulating dynamical motions of phonons via atomic isotope engineering.