$\mathit{Abinitio}$ determination of crystal structures of the thermoelectric material MgAgSb

Materials with the half-Heusler structure possess interesting electrical and magnetic properties, including potential for thermoelectric applications. MgAgSb is compositionally and structurally related to many half-Heusler materials but has not been extensively studied. This work presents the high-temperature x-ray diffraction analysis of MgAgSb between 27 and 420 ${}^{\circ }$C, complemented with thermoelectric property measurements. MgAgSb is found to exist in three different crystal structures in this temperature region, taking the half-Heusler structure at high temperatures, a Cu${}_2$Sb-related structure at intermediate temperatures, and a previously unreported tetragonal structure at room temperature. All three structures are related by a distorted Mg-Sb rocksalt-type sublattice, differing primarily in the Ag location among the available tetrahedral sites. Transition temperatures between the three phases correlate well with discontinuities in the Seebeck coefficient and electrical conductivity; the best performance occurs with the novel room temperature phase. For application of MgAgSb as a thermoelectric material, it may be desirable to develop methods to stabilize the room temperature phase at higher temperatures.

Figure 1

Detail of nonambient XRD data collected on MgAgSb over two cycles of heating and cooling between 30 and 420 ${}^{\circ }$C, showing the room temperature, intermediate, and high-temperature MgAgSb phases ($\alpha$-, $\beta$-, and $\gamma$-MgAgSb, respectively) and secondary phases of Sb and Ag${}_3$Sb.

Figure 2

Representative HTXRD scans collected at 30 ${}^{\circ }$C (top), 330 ${}^{\circ }$C (middle), and 420 ${}^{\circ }$C (bottom), along with the peak positions for the room temperature, intermediate, and high-temperature MgAgSb phases ($\alpha$-, $\beta$-, and $\gamma$-MgAgSb, respectively) and the two secondary phases of Sb and Ag${}_3$Sb underneath.

Figure 3

Crystal structure of the high-temperature $\gamma$-MgAgSb phase, with the Mg shown in dark blue, the Sb in light tan, and the Ag in silver inside the polyhedra.

Figure 4

Crystal structure of the intermediate $\beta$-MgAgSb phase, drawn to include atoms outside the unit cell in order to emphasize the distorted rocksalt lattice of Mg and Sb atoms, with the Mg shown in dark blue, the Sb in light tan, and the Ag in silver inside the polyhedra.

Figure 5

Crystal structure of the room temperature $\alpha$-MgAgSb phase, looking along the (a) $\langle 001\rangle$ and (b) $\langle 110\rangle$ directions, drawn to include atoms outside the unit cell in order to emphasize the distorted rocksalt lattice of Mg and Sb atoms, with the Mg shown in dark blue, the Sb in light tan, and the Ag in silver inside the polyhedra.

Figure 6

Lattice parameters of the Mg-Sb rocksalt sublattice of the three MgAgSb phases, determined for the $\alpha$- and $\beta$-phases from the tetragonal lattice parameters, as a function of temperature. Error bars, based on estimated standard deviations, are smaller than the symbols.

Figure 7

Seebeck coefficient, $S$, and electrical conductivity, $\sigma$, (top) and thermoelectric power factor, $S^2\sigma$, (bottom) of MgAgSb between 30 and 410 ${}^{\circ }$C.

Figure 8

Thermal conductivity, $\lambda$, (top) and thermoelectric figure of merit, $ZT$, (bottom) of MgAgSb.