Synthesis and Transport Properties of ZnSnP_2-yAs_y Chalcopyrite Solid Solutions

This work focuses on the synthesis and properties of quaternary ZnSnP_2-yAs_y chalcopyrite solid solutions. Full miscibility of the solid solution is achieved using ball milling followed by hot press sintering. The measured electrical conductivity increases substantially with As substitution from 0.03 S cm^-1 for ZnSnP₂ to 10.3 S cm^-1 for ZnSnAs₂ at 715 K. Band gaps calculated from the activation energies show a steady decrease with increasing As concentration from 1.4 eV for ZnSnP₂ to 0.7 eV for ZnSnAs₂. The Seebeck coefficient decreases significantly with As substitution from nearly 1000 μV K^-1 for ZnSnP₂ to -100 μV K^-1 for ZnSnAs₂ at 650 K. Thermal conductivity is decreased for the solid solutions due to alloy phonon scattering, compared to the end members with y = 0 and y = 2, with the y = 0.5 and y = 1.0 samples exhibiting the lowest values of 1.4 W m^-1 K^-1 at 825 K. Figure of merit values are increased for the undoped solid solutions at lower temperatures when compared to the end members due to the decreased thermal conductivity, with the y = 0.5 sample reaching zT = 1.6 × 10^-3 and y = 1 reaching 2.1 × 10^-3 at 700 K. The largest values of the figure of merit zT for the undoped series was found for y = 2 with zT = 2.8 × 10^-3 at 700 K due to the increasing n-type Seebeck coefficient. Boltztrap calculations reveal that p-doping could yield zT values above unity at 800 K in case of ZnSnAs₂, comparable with ZnSnP₂.