  The phenomenon of frustration, which gives rise to many fascinating
phenomena, is conventionally associated with the topology of non-bipartite
lattices, where nearest-neighbor (nn) interactions and global connectivity
compete in the lowering of energy. The issue of rattling atoms in spacious
lattice sites is a separate occurrence that can also lead to a high density of
low energy states (unusual low temperature thermodynamics) and to practical
applications such as in improved thermoelectric materials. In this letter we
address a unique situation where both phenomena arise: a four-fold single-site
instability leads to rattling of cations on a diamond structure sublattice
where nn interactions frustrate simple ordering of the displacements. The
system deals with this coupling of rattling+frustration by commensurate
ordering. Such a disorder-order transition may account for the second phase
transition seen in KOs2O6 within the superconducting state, and the unusual
low-energy dynamics and associated electron-phonon coupling can account for the
qualitative differences in physical properties of KOs2O6 compared to RbOs2O6
and CsOs2O6, all of which have essentially identical average crystal and
electronic structures.
