Improved Thermoelectric Properties of N-Type Mg₃Sb₂ through Cation-Site Doping with Gd or Ho

Abstract

The success of n-type doping has attracted strong research interest for exploring effective n-type dopants for Mg3Sb2 thermoelectrics. Herein, we experimentally study Gd and Ho as n-type dopants for Mg3Sb2 thermoelectrics. The synthesis, structural characterization, and thermoelectric properties of Gd-doped, Ho-doped, (Gd, Te)-codoped, and (Ho, Te)-codoped Mg3Sb2 samples are reported. It is found that Gd and Ho are effective n-type cation-site dopants showing a higher doping efficiency as well as a superior carrier concentration in comparison with anion-site doping with Te, consistent with the previous theoretical prediction. For n-type Mg3Sb2 samples doped with Gd or Ho, optimal thermoelectric figure of merit zT values of ∼1.26 and ∼0.94 at 725 K are obtained, respectively, in Mg3.5Gd0.04Sb2 and Mg3.5Ho0.04Sb2, which are superior to many reported Te-doped Mg3Sb2 without alloying with Mg3Bi2. By codoping with Gd (or Ho) and Te, reduced thermal conductivity and enhanced power factor values are achieved at high temperatures, which results in enhanced peak zT values well above unity at 725 K. This work reveals Gd and Ho as effective n-type dopants for Mg3Sb2 thermoelectric materials.

Understanding the High Thermoelectric Performance of Mg3Sb2‐Mg3Bi2 Alloys

“…[30] Recently, many other cation site substitutions are reported with higher doping efficiency and thermal stability compared to the anion alternatives [10,[12][13][14][15]24,30,[44][45][46][47] (Figure 4). Defect formation energy calculations predict that all of these cation site dopants are expected to substitute the Mg(1) site, [24,[47][48][49][50] while a neutron scattering experiment shows the Yttrium (Y) substitution on the Mg(2) site. [44] Further studies will be required; however, it might be more reasonable to expect the preferential cation substitution on Mg(1) site as the Mg(1) site is more flexible in size compared to Mg(2) as Mg( 1) is a Mg atom residing between [Mg 2 Sb 2 ] À2 slabs.…”

“…Possible reasons may be the size difference of Y atom and La atom or the suppression of Mg vacancies with Y [44] ; however, further understanding of the effects of various cation dopants should be important as the potential future work to utilize the positive aspect of cation site doping. In addition, Pr, [48] Nd, [47,53] Tm, [47] Ce, [54] Gd, [49,50] and Ho [49] were reported as the effective dopant at least to control the carrier concentration.…”

Understanding the influence of Bi/Sb substitution on carrier concentration in Mg3Sb2-based materials: decreasing bandgap enhances the degree of impurity ionization

“…Many elements have been reported to be effective donor impurities for Mg 3 Sb 2based materials, such as chalcogen (Te, Se and S) and lanthanides (La, Nd, Pr, etc. ), [10][11][12][13][14][15][16] which can achieve a high electron concentration from 10 19 to 10 20 cm À3 . Poor low-temperature thermoelectric performance usually limits the efficiency of a thermoelectric device.…”

Large improvement in thermoelectric performance of pressure-tuned Mg3Sb2

“…Polvani et al 32 reported that the zT value increased twofold in p-type Sb 1.5 -Bi 0.5 Te 3 upon pressure tuning. For improving the thermoelectric performance of Mg 3 Sb 2 , optimizing carrier concentration 21,[33][34][35] or tuning carrier scattering mechanism 10,26 have been proved to be effective methods by a plenty of literatures, however, there are few theoretical reports about the method of pressure tuning. Therefore, it is necessary and important to investigate the effect of pressure on the thermoelectric properties of Mg 3 Sb 2 , which can provide signicant guides for the experimental researchers.…”