Spontaneous Electronic Band Formation and Switchable Behaviors in a Phase-Rich Superatomic Crystal

Structural phase transitions run
in families of crystalline solids.
Perovskites, for example, feature a remarkable number of structural
transformations that produce a wealth of exotic behaviors, including
ferroelectricity, magnetoresistance, metal–insulator transitions
and superconductivity. In superatomic crystals and other such materials
assembled from programmable building blocks, phase transitions offer
pathways to new properties that are both tunable and switchable. Here
we describe [Co6Te8(PEt3)6]­[C70]2, a novel superatomic crystal with two
separate phase transitions that drastically transform the collective
material properties. A coupled structural-electronic phase transition
triggers the emergence of a new electronic band in the fullerene sublattice
of the crystal, increasing its electrical conductivity by 2 orders
of magnitude, while narrowing its optical gap and increasing its spin
density. Independently, an order–disorder transition transforms
[Co6Te8(PEt3)6]­[C70]2 from a phonon crystal to a phonon glass. These
results introduce a family of materials in which functional phase
transformations may be manipulated by varying the constituent building
blocks.