Elaborating the Crystal Structures of MgAgSb Thermoelectric Compound: Polymorphs and Atomic Disorders

Abstract

Gaining insight into crystal structure is essential for understanding thermoelectric transport mechanisms and predicting thermoelectric properties. The main challenge in studying thermoelectric mechanisms is often imprecise or wrong models of the crystal structure. This work examines the structure modifications observed in MgAgSb thermoelectric materials by multi-temperature high-resolution synchrotron radiation powder X-ray diffraction (SR-PXRD). Rietveld refinement reveals large atomic displacement parameters (ADPs) of the Ag1 atoms at the 4a position indicating possible atomic disorder, which may contribute to the low thermal conductivity observed in α-MgAgSb. The temperature dependence of anisotropic structural parameters indicates a tendency of increasing structural symmetry in α-MgAgSb with increasing temperature, largely contributing to the temperature evolution of the thermoelectric properties. Two MgAgSb polymorphs (β-MgAgSb and γ-MgAgSb) coexist at 700 K, and only the γ-MgAgSb crystalline phase is found at high temperatures (800-1000 K). The content of γ-MgAgSb phase decreases with temperature due to the increase of liquid impurities, and the sample is only 43.8% crystalline at 1000 K. At 800 K, the high resolution powder data are fitted equally well using type I (with Mg, Ag, and Sb on the 4b, 4c, and 4a sites, respectively) and type II (with Mg, Ag, and Sb on the 4a, 4b, and 4c sites, respectively) half-Heusler crystal structure models. Nonetheless, Maximum Entropy Method (MEM) analysis carried out on the extracted factors shows that the type II structure gives a more physically sound MEM electron density. The disorder in γ-MgAgSb consists of mixed sites of Mg and Ag as well as vacancies, and the strong disorder of the cation sublattice contributes to the low thermal conductivity.

High-Performance Mg 3 Sb 2- x Bi x Thermoelectrics: Progress and Perspective

“…The hierarchical chemical bonding in α -MgAgSb leads to strong anharmonicity and intrinsically low sound velocity [ 59 ], which are responsible for the low thermal conductivity κ and high zT near RT [ 45 , 46 , 57 ]. However, MgAgSb has a complex phase transition process when cooling from its liquid state with two phase transition points at around 360°C and 300°C, respectively [ 60 ]. Additionally, the binary impurity phase, for instance, Ag 3 Sb, is also easily formed during the synthesis.…”

Ultralow thermal conductivity from transverse acoustic phonon suppression in distorted crystalline α-MgAgSb

“…Thus far, in addition to transport property measurements, experimental characterizations of the MgAgSb-based materials have been mainly based on X-ray diffraction and electron microscopy, which have provided microscopic insight into their crystalline structures 15,26,30,32 . In contrast, numerous theoretical characterizations have focused on their local crystal structure, electronic band structure, chemical bonding, and atomic dynamics, etc.…”

“…Here, we use neutron scattering measurements together with systematic ab initio simulations to study the crystalline structure and the atomic dynamics of two α-MgAgSb-based materials, MgAg 0.97 Sb 0.99 and MgAg 0.965 Ni 0.005 Sb 0.99 (the sample of Ni ptype substitution for Ag has a higher anomalous electrical resistivity 15 ), the α-phase of which exhibits the highest thermoelectric performance 32 . We find that their ultralow κ lat is induced both by static local structure distortion suppression of the transverse acoustic phonons and the phonon anharmonicity.…”

Insight on the Interplay between Synthesis Conditions and Thermoelectric Properties of α-MgAgSb

“…Like for dyscrasite, the evolution of mobility was probably mostly related to the difference in grain boundaries scattering. The nature of γ-MgAgSb being unclear (metallic or narrow band gap semiconductor depending on calculations [42]), it is difficult to explain the evolution we see in carrier concentration. This impurity has not been found much in α-MgAgSb publications and the phase has not been studied on the temperature range below 300 °C.…”