  In spite of intensive studies on the chalcogenides as conventional
thermoelectrics, it remains a challenge to find a proper material with high
electrical but low thermal conductivities. In this work, we introduced a new
class of thermoelectrics, Ge-based inorganic halide perovskites \ce{CsGeX3} (X
= I, Br, Cl), which were already known as a promising candidate for
photovoltaic applications. By performing the lattice-dynamics calculations and
solving the Boltzmann transport equation, we revealed that these perovskites
have ultralow thermal conductivities below 0.18 W m$^{-1}$ K$^{-1}$ while very
high carrier mobilities above 860 cm$^2$ V$^{-1}$ s$^{-1}$, being much superior
to the conventional thermoelectrics of chalcogenides. These results highlight
the way of searching high-performance and low-cost thermoelectrics based on
inorganic halide perovskites.
