Na-doping enables both dislocations and holes in EuMg2Sb2 for thermoelectric enhancements

Abstract

A high conduction band degeneracy (N_v = 6) leads to an extraordinary figure of merit (zT) in n-type Mg₃Sb₂ thermoelectrics, while the p-type ones show a much inferior zT due to its lower N_v of only 1 owing to the large energy offset (ΔE ∼ 0.4 eV) between the two valence bands. The derivatives of Mg₃Sb₂, ternary AB₂C₂ (A = Eu, Yb, Ca, Sr, Ba; B = Mg, Zn, Cd; C = Sb, Bi) Zintls, show a similar valence band structure but with a much smaller ΔE of <0.2 eV, leading to potential convergence of both valence bands for a high overall N_v of 3. This band feature promotes AB₂C₂ ternaries to be potential p-type thermoelectrics matching with n-type Mg₃Sb₂ for efficient thermoelectric applications, thus motivating the current work focusing on p-type EuMg₂Sb₂ thermoelectrics. Interestingly, Na-doping at the Eu site is found to be particularly effective for creating dense in-grain dislocations, which results in a remarkable reduction in lattice thermal conductivity to ∼0.5 W m⁻¹ K⁻¹. Moreover, Na-doping increases the hole concentration up to 6.0 × 10¹⁹ cm⁻³, leading to an effective involvement of multiple transporting valence bands. The synergistic effects of dense dislocations and multi-band transport, both enabled by Na-doping, led EuMg₂Sb₂ to show the highest zT among known doped p-type ternary AMg₂C₂ Zintl thermoelectrics.