Strained Lamellar Structures Leading to Improved Thermoelectric Performance in Mg₃Sb_1.5Bi_0.5

Mg₃Sb_2-xBi_x solid-solutions represent an important class of thermoelectric (TE) materials due to their high efficiency and variable operating temperature range. Of particular significance for midtemperature applications is the Mg₃Sb_1.5Bi_0.5 composition whose superior thermoelectric (TE) performance is attributed to the complex conduction band edge in conjunction with alloy dominated phonon scattering. In this work, we show that microstructure also plays a significant role in lowering the lattice thermal conductivity which in turn affects the overall TE performance (change in peak zT values between 1.1 and 1.4 have been observed). Temperature dependent TE properties of Mg_3+xSb_1.5Bi_0.5 compositions with varying nominal Mg content (x = 0.2, 0.3, 0.4) have been studied. A marked reduction of the lattice thermal conductivity (κ_L) is observed in compositions with low nominal Mg content (x = 0.2), which is due to the presence of lamellar structures within the grains. These lamellar regions are isostructural to the matrix with a low misfit angle and represent compositional fluctuations in the Bi to Sb ratio. Both the size (200 nm-500 nm) and the interfacial strain contribute to the enhanced phonon scattering. A quantitative estimate of κ_L reduction due to these structures have been carried out using a mean free path (MFP) spectrum analysis which reveal a good match with experiments at room temperature. Further, the electrical properties are not influenced by these lamellar structures as observed from the similar power-factor (S²σ) and weighted mobilities in all of the compositions. This is due to their similar orientation to the adjacent matrix region. Thus, the zT parameter in the various compositions with similar carrier concentration can be significantly altered (∼25%) by adjusting the nominal Mg content. The results demonstrate that preferential phonon scattering by microstructure modification can be a new route for property improvement in Mg_3+xSb_2-yBi_y solid-solutions.