  High performance thermoelectric devices requires materials with low lattice
thermal conductivities. Many strategies, such as phonon engineering, have been
made to reduce lattice thermal conductivity without simultaneously decrease of
the charge transport performance. It is a simple and effective approach to use
materials with heavy element to reduce the lattice thermal conductivity. Here,
based on the first-principles calculations and phonon Boltzmann transport
equations, we find the replacement of Zr with heavy element Hf in ZrNCl doesn't
reduce the lattice thermal conductivity, instead, it surprisingly increases by
about 4 times at 300K. This unusual lattice thermal conductivity is mainly
attributed to the dramatic enhancement in phonon lifetimes in Hf compound,
originating from the strong interatomic bonding due to lanthanide contraction.
Our findings unveil the microscopic mechanisms of high thermal transport
properties in materials with heavy element, providing an alternative strategy
in materials design with low lattice thermal conductivity for thermoelectric
applications such as power restoration and generation.
