Comparing the role of annealing on the transport properties of polymorphous AgBiSe₂ and monophase AgSbSe₂

AgBiSe₂ and AgSbSe₂, two typical examples of Te-free I-V-VI₂ chalcogenides, are drawing much attention due to their promising thermoelectric performance. Both compounds were synthesized via melting and consolidated by spark plasma sintering. The role of annealing on the transport properties of polymorphous AgBiSe₂ and monophase AgSbSe₂ was studied. Annealing has a greater impact on AgBiSe₂ than AgSbSe₂, which is ascribed to the temperature dependent phase transition of AgBiSe₂. Unannealed AgBiSe₂ shows p-n switching, but annealed AgBiSe₂ exhibits n-type semiconducting behavior over the whole measurement temperature range. By performing high-temperature Hall measurements, we attribute this intriguing variation to the change in the amount of Ag vacancies and mid-temperature rhombohedral phase after annealing. Both AgBiSe₂ and AgSbSe₂ exhibit low thermal conductivity values, which are ∼0.40-0.50 W m^-1 K^-1 for AgSbSe₂ and ∼0.45-0.70 W m^-1 K^-1 for AgBiSe₂, respectively. The maximum ZT value of AgBiSe₂ is enhanced from 0.18 to 0.21 after annealing. Pristine AgSbSe₂ presents a ZT value as high as 0.60 at 623 K, although slight deterioration emerges after annealing.