Crystal chemistry and thermoelectric transport of layered AM2X2 compounds

Abstract

Compounds that crystallize in the layered CaAl₂Si₂ structural pattern have rapidly emerged as an exciting class of thermoelectric materials with attractive n- and p-type properties. More than 100 AM₂X₂ compounds that form this structure type – characterized by anionic M₂X₂ slabs sandwiched between layers of octahedrally coordinated A cations – provide numerous potential paths to chemically tune every aspect of thermoelectric transport. This review highlights the chemical diversity of this structure type, discusses the rules governing its formation and stability relative to competing AM₂X₂ structures (e.g., ThCr₂Si₂ and BaCu₂S₂), and attempts to bring some of the most recently discovered compounds into the spotlight. The discussion of thermoelectric transport properties in AM₂X₂ compounds focuses primarily on the intrinsic parameters that determine the potential for a high figure of merit: the band gap, effective mass, degeneracy, carrier relaxation time, and lattice thermal conductivity. We also discuss routes that have been used to successfully control the carrier concentration, including controlling the cation vacancy concentration, doping, and isoelectronic alloying (approaches that are highly interdependent). Finally, we discuss recent progress made towards n-type doping in this system, highlight opportunities for further improvements, as well as open questions that still remain.