Thermoelectric and piezoelectric properties of the predicted Al_xIn_1-xN composites based on ab initio calculations

Theoretical investigations of the thermoelectric and piezoelectric characteristics in the Al_xIn_1-xN system have been carried out based on a first principles approach in combination with the semi-classical Boltzmann transport concept and density functional perturbation theory. Based on our previous work, herein, the study specimens Al₅InN₆, Al₆In₂N₈, Al₄In₂N₆, Al₃In₃N₆, Al₂In₄N₆, and AlIn₇N₈ have been predicted to be stable phases. These novel phases intrinsically exhibit moderate positive Seebeck curves (199.1-284.6 μV K^-1) and a ZT close to unity that varies marginally over a broad temperature range of 200-800 K, demonstrating the sign of good bipolar effect tolerance. Addition of heftier elements, such as In, results in lower thermal conductivity, which in turn generates a high power factor (0.019-0.345 W m^-1 K^-2) in these alloys. While hole doping enhances the peak Seebeck coefficient of each phase, the electrical conductivity has been greatly compromised, resulting in a lower power factor. These composites also exhibit large piezoelectric constants, in which their respective largest piezoelectric tensor is several orders higher than that of quartz. The decomposition process shows that In and N are the main contributors of the internal piezoelectric term. Overall results indicate that Al_xIn_1-xN show bright prospects in thermoelectric and piezoelectric applications.