Interstitial-Electron-Induced Topological Molecular Crystals

Topological phases usually are unreachable in molecular solids, which are characteristic of weakly dispersed energy bands with a large gap, in contrast to topological materials. In this work, however, we propose that nontrivial electronic topology may ubiquitously emerge in a class of molecular crystals that contain interstitial electronic states, the bands of which are prone to be inverted with those of molecular orbitals. We provide guidelines hunt for such interstitial-electron-induced topological molecular crystals, especially in the topological insulating state. They exhibit a variety of exceptional qualities, as brought about by the intrinsic interplay of molecular crystals, interstitial electrons, and topological nature: (1) They may host cleavable surfaces along multiple orientations, with pronounced topological boundary states free from dangling bonds. (2) Strong response to moderate mechanical perturbations, whereby topological phase transition would occur under relatively low pressure. (3) Inherent high-efficiency thermoelectricity as jointly contributed by the non-parabolic band structure (therewith high thermopower), highly mobile interstitial electrons (high electrical conductivity), and soft phonons (small lattice thermal conductivity). (4) Ultralow work function owing to the active interstitial electrons. We utilize first-principles calculations to demonstrate these properties with the representative candidate K$_4$Ba$_2$[SnBi$_4$]. Our work suggests a pathway of realizing topological phases in bulk molecular systems, which may advance the interdisciplinary research between topological and molecular materials.