High‐Density Frenkel Defects as Origin of N‐Type Thermoelectric Performance and Low Thermal Conductivity in Mg₃Sb₂‐Based Materials

Abstract

Mg3Sb2‐based intermetallic compounds with exceptionally high thermoelectric performance exhibit unconventional n‐type dopability and anomalously low thermal conductivity, attracting much attention to the underlying mechanisms. To date, investigations have been limited to first‐principle calculations and thermodynamic analysis of defect formation, and detailed experimental analysis on crystal structure and phonon modes has not been achieved. Here, a synchrotron X‐ray diffraction study clarifies that, against a previous view of a simple crystal structure with a small unit cell, Mg3Sb2 is inherently a heavily disordered material with Frenkel defects, charge‐neutral defect complexes of cation vacancies and interstitials. Ionic charge neutrality preserved in Mg3Sb2 is responsible for exotic n‐type dopability, which is unachievable for other Zintl phase materials. The thermal conductivity of Mg3Sb2 exhibits deviation from the standard T−1 temperature dependency with strongly limited phonon transport due to a strain field. Inelastic X‐ray scattering measurement reveals enhanced phonon scattering induced by disorder. The results will draw renewed attention to crystal defects and disorder as means to explore new high‐performance thermoelectric materials.

Understanding the High Thermoelectric Performance of Mg3Sb2‐Mg3Bi2 Alloys

“…[ 32–34 ] For instance, no interstitial Mg—part of the Frenkel defect complex—presence was detected using the advanced imaging technique integrated differential phase contrast. [ 35 ] Furthermore, the n‐type dopability of attributed to the charge neutrality of the Frenkel defect complex [ 18 ] does not explain the persistent p‐type behavior of samples under Sb‐excess conditions. Experimental and computational efforts will be necessary to understand the detailed effects of various defects, not limited to Mg vacancies, on thermal and electronic properties.…”

“…[10,[29][30][31] In addition to the dominant nature of Mg vacancies, the presence of complex Frenkel defects-which is the pair formation of vacancies at Mg(1) site and Mg interstitial-and their impact on the thermal and electrical properties are also discussed in recent works. Kanno et al [18] report a high density of Frenkel defects using synchrotron X-ray diffraction measurements. Structural analysis reveals that the lattice strain accompanying this heavily disordered structure can be an additional reason for the exceptionally low lattice thermal conductivity.…”

“…In addition, the alloy shows promisingly low lattice thermal conductivity, which is attributed to the instability of the Mg atom in the layered position (Mg(1) in Figure 1a) [ 17 ] and its highly disordered structure. [ 18 ] The combining advance in the thermal and electrical transport properties leads to a high thermoelectric figure of merit 1.5 at 700 K [ 1,19 ] or 1.2 at 450 K, [ 6 ] much higher than that of p‐type counterpart ( = 0.6 at 700 K) ( Figure ). Beyond its intrinsic properties, the microstructure of the ‐based compounds has a critical impact on the performance, such as grain boundary resistance at near room temperature, [ 20 ] dislocations, [ 21 ] grain boundary engineering, [ 22 ] and so on.…”

Chemical-Pressure-Induced Point Defects Enable Low Thermal Conductivity for Mg2Sn and Mg2Si Single Crystals

“…Here, κ tot is the sum of the electronic thermal conductivity, κ el ; bipolar thermal conductivity, κ bip ; and lattice thermal conductivity, κ lat . In Mg-based compounds, there can be Mg-related point defects due to the high vapor pressure of Mg. ,− ,− These point defects have critical effects on the TE performance as they not only generate electronic transport carriers but also scatter electronic/thermal transport carriers. Thus, the control of the point defects is vital to further enhance the TE performance.…”

Effects of Voigt diffraction peak profiles on the pair distribution function

“…The second sample broadening effect is microstrain. To first approximation, it arises when the unit-cell size is not identical for every cell, which can be observed in crystalline structures with defects, such as interstitial atoms or dislocations (Rodrı ´guez-Carvajal et al, 1991;Kanno et al, 2021). In this case, the diffraction conditions will be fulfilled in slightly different directions in different regions of the crystallite, resulting in peak broadening with a tan dependency (Dinnebier & Billinge, 2008).…”