Mg2Si1−xSnx codoped with Bi and Cr : Enhanced thermoelectric performance and effect on magnetic order.
1Department of Physics, Indian Institute of Technology, Powai, Mumbai 400 076, India
2Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, USA
3Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa 50011, USA
Mg2Si based compounds have always been favorable thermoelectric materials due to abundance and cost effectiveness of the constituent elements. We have explored the stability of Mg2Si with chromium (1.85, 3.7, 5.55, and 6.25% Cr) and tin (12.5 and 50% Sn) doping. Three doped Mg2Si compounds (Sn, Sn+Bi and Sn+Bi+Cr doped) are then chosen to explain the effect on thermoelectric performance. Even though Cr is antiferromagnetic in nature in its elemental state, Mg2Si is nonmagnetic for ≤2%Cr doping but becomes ferromagnetic with further increase in the Cr doping concentration (≤12.96% Cr). Sn doesnot affect the magnetic properties of Mg2Si, keeping it an non-magnetic indirect band gap semi-conductor whereas a small amount of Bi (3.7%) raises the Fermi level into the conduction bands, making it a degenerate semiconductor. A highest figure of merit (ZT) of 1.75 and power factor of 7.04 mWm−1K−2 is observed for Mg2Si0.296Sn0.666Bi0.037 at 700 K. When a small amount of Cr is added, it decreases the ZT and power factor to 0.78 and 7.04 mWm−1K−2 respectively due to (i) uniform doping acting as an electron acceptor, decreasing conduction, and (ii) the loss of low-lying conduction band degeneracy with doping, decreasing the Seebeck coefficients. A study of different configurations of Cr suggests that it prefers to form clusters inside the lattice, which might play a crucial role in tuning the magnetic and TE performance of doped Mg2Si compounds.
References:
[1] Vikram, Duane D. Johnson & Aftab Alam. Enhanced thermoelectric performance of Mg2Si1−xSnx codoped with Bi and Cr. Phys. Rev. B 98, 115204 (2018).