A crystal-chemistry view of guest-framework interactions in clathrate-I and skutterudite materials

Crystalline cage compounds such as type-I clathrates and filled skutterudites represent classical model systems that form compositions with low lattice thermal conductivity. Although they are structurally ordered crystalline solids, several type-I clathrates exhibit glass-like thermal conductivity, including a suppressed low-temperature phonon peak and a plateau. Their thermal transport has been rationalized by the phonon-glass electron-crystal concept, in which weakly bound guest atoms undergo localized "rattling" vibrations that scatter heat-carrying acoustic phonons. In this perspective, we reconsider this interpretation from a crystallographic and chemical viewpoint. By comparing type-I clathrates and skutterudites, we show that the guest-site potential is not determined by cage size alone, but by the topology, distortion, and chemical polarity of the framework. In type-I clathrates, framework vacancies and electronegative d-block substituents act as symmetry-breaking centers that chemically structure the guest potential. In such cases, low thermal conductivity is better described in terms of coupled guest-framework phonon modes, reduced phonon group velocities, and modified phonon dispersion. In contrast, chemically more homogeneous clathrates such as Sr₈Ga₁₆Ge₃₀ approach the limiting case of a weakly structured potential and display glass-like thermal conductivity. Filled skutterudites, although chemically more uniform, possess compact dodecahedral cages that intrinsically couple the guest atom to the framework resulting in lower thermal conductivity as compared to unfilled skutterudites. Thus, rattling is not a universal mechanism in cage compounds, but one case within a broader structure-chemistry relationship.