Organizing Chaos: Boosting Thermoelectric Properties by Ordering the Clathrate Framework of Ba₈Cu₁₆As₃₀

Abstract

The type I clathrate, Ba 8 Cu 16 As 30 , is reinvestigated and found to have a low-temperature polymorph mP108-Ba 8 Cu 16 As 30 with ordered Cu and As sites. In situ temperature-dependent powder X-ray diffraction experiments guided synthetic efforts toward the synthesis of the ordered monoclinic (mP108) and disordered cubic (cP54) polymorphs with high phase purity. While a transition from mP108-Ba 8 Cu 16 As 30 to cP54-Ba 8 Cu 16 As 30 is not directly observed, cP54-Ba 8 Cu 16 As 30 is stabilized through quenching from high temperatures and is confirmed through high-resolution synchrotron powder X-ray diffraction. Combined theoretical predictions and experimental observations of the thermoelectric properties of both polymorphs reveal that the ordering of Cu and As atoms in the clathrate framework simultaneously enhances the Seebeck coefficient and electronic conductivity by increasing the hole effective mass and reducing the electronic scattering events. Consequently, the zT of mP108-Ba 8 Cu 16 As 30 reaches a maximum of 0.2 at 575 K, an order of magnitude higher than that of cP54-Ba 8 Cu 16 As 30 .