  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.
