Effect
of Isovalent Substitution on the Thermoelectric
Properties of the Cu<sub>2</sub>ZnGeSe<sub>4–<i>x</i></sub>S<sub><i>x</i></sub> Series of Solid Solutions
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Abstract
Knowledge
of structure–property relationships is a key feature
of materials design. The control of thermal transport has proven to
be crucial for the optimization of thermoelectric materials. We report
the synthesis, chemical characterization, thermoelectric transport
properties, and thermal transport calculations of the complete solid
solution series Cu<sub>2</sub>ZnGeSe<sub>4–<i>x</i></sub>S<sub><i>x</i></sub> (<i>x</i> = 0–4).
Throughout the substitution series a continuous Vegard-like behavior
of the lattice parameters, bond distances, optical band gap energies,
and sound velocities are found, which enables the tuning of these
properties adjusting the initial composition. Refinements of the special
chalcogen positions revealed a change in bonding angles, resulting
in crystallographic strain possibly affecting transport properties.
Thermal transport measurements showed a reduction in the room-temperature
thermal conductivity of 42% triggered by the introduced disorder.
Thermal transport calculations of mass and strain contrast revealed
that 34% of the reduction in thermal conductivity is due to the mass
contrast only and 8% is due to crystallographic strain