Effect of AFM ordering on thermoelectric responses of Mg₃X₂(X: C, Si, Ge) monolayers : a DFT insight

Two-dimensional materials have gained a lot of attention in the last few decades due to their potential applications in thermoelectric and nano-electronic devices. This study systematically presents the mechanical, electronic and thermoelectric characteristics of two-dimensional honeycomb-kagomeMg3X2(X:C,Si,Ge) structures in the framework of density functional theory computations and by solving semiclassical Boltzmann transport equation. The geometrical stability of these structures is validated by phonon spectrum and molecular dynamics simulations. Following the elastic constants, we have inferred that all the systems are mechanically stable and brittle in nature. Lower values of Debye temperature of all structures suggest thatMg3X2monolayers should have lower values of lattice thermal conductivity compared to graphene. Electronic structure calculations indicate that these materials are semimetallic in their nonmagnetic phase. All the structures display remarkably low lattice thermal conductivity (0.9-1.5 W (mK)^-1) due to a large scattering factor and higher anharmonicity. The presence of sharp density of states peaks close to the Fermi level, arising from nearly flat and dispersionless band in the antiferromagnetic (AFM) arrangement, is poised to enhance the Seebeck coefficient, thereby potentially boosting the thermoelectric performance. The estimated values of thermoelectric figure of merit (ZT) are around 0.78 and 0.67 forMg3Si2andMg3Ge2structure respectively in AFM phase atT= 700 K. These outcomes of our findings suggest thatMg3X2monolayers exhibit substantial promise for thermoelectric device application.