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

The type I clathrate, Ba8Cu16As30, is reinvestigated and found to have a low-temperature polymorph mP108-Ba8Cu16As30 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-Ba8Cu16As30 to cP54-Ba8Cu16As30 is not directly observed, cP54-Ba8Cu16As30 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-Ba8Cu16As30 reaches a maximum of 0.2 at 575 K, an order of magnitude higher than that of cP54-Ba8Cu16As30.