  A solid with larger sound speeds exhibits higher lattice thermal conductivity
(k_{lat}). Diamond is a prominent instance where its mean sound speed is 14400
m s-1 and k_{lat} is 2300 W m-1 K-1. Here, we report an extreme exception that
CuP2 has quite large mean sound speeds of 4155 m s-1, comparable to GaAs, but
the single crystals show a very low lattice thermal conductivity of about 4 W
m-1 K-1 at room temperature, one order of magnitude smaller than GaAs. To
understand such a puzzling thermal transport behavior, we have thoroughly
investigated the atomic structure and lattice dynamics by combining neutron
scattering techniques with first-principles simulations. Cu atoms form dimers
sandwiched in between the layered P atomic networks and the dimers vibrate as a
rattling mode with frequency around 11 meV. This mode is manifested to be
remarkably anharmonic and strongly scatters acoustic phonons to achieve the low
k_{lat}. Such a dimer rattling behavior in layered structures might offer an
unprecedented strategy for suppressing thermal conduction without involving
atomic disorder.
