Electronic structures and thermoelectric properties of polytype phases of bismuth

Abstract

In this study, first principles calculations demonstrated that the cohesive energies, atomic volumes, or bulk moduli of the rhombohedral (A7), monoclinic (M), and triclinic (T1 and T2) structures of bismuth are very close to each other, thereby suggesting that these four structures of bismuth will probably coexist with each other, which agrees well with previously reported experimental and theoretical observations. The band structures showed that the A7, M, and T1 structures are semimetals with a small overlap, whereas T2 is a direct semiconductor with a narrow band gap. We also found that the T2 structure has much higher Seebeck coefficients than the A7, M, and T1 structures, mainly because of its larger effective mass and lower carrier density. In addition, the figure of merit was determined as highest for T2 among the four structures, and thus the T2 structure of bismuth should be preferred in actual situations in terms of the thermoelectric properties.