  Thermoelectric devices can directly convert waste heat into electricity,
which makes them an important clean energy technology. The underlying materials
performance can be evaluated by the dimensionless figure of merit ZT. Metal
halides are attractive candidates due to their chemical flexibility and ease of
processing; however, the maximum ZT realized (ZT = 0.15) falls far below the
level needed for commercialization (ZT > 1). Using a first-principles procedure
we assess the thermoelectric potential of copper halide CsCu2I3, which features
1D Cu-I connectivity. The n-type crystal is predicted to exhibit a maximum ZT
of 2.2 at 600 K along the b-axis. The strong phonon anharmonicity of this
system is shown by locally stable non-centrosymmetric Amm2 structures that are
averaged to form the observed centrosymmetric Cmcm space group. Our work
provides insights into the structure-property relations in metal halide
thermoelectrics and suggests a path forward to engineer higher-performance
heat-to-electricity conversion.
