Janus breathing-Kagome lattice Nb3TeX7 (X = Cl, Br and I) with low thermal conductivity and high thermoelectric figure of merit

Kagome lattices have attracted much attention because of their potential in superconductivity and topological applications. In this work, we proposed three two-dimensional Janus breathing-Kagome lattices, Nb3TeCl7, Nb3TeBr7 and Nb3TeI7, and further investigated their stability, electronic and thermoelectric (TE) transport performance. We illustrated that the three materials all possess high stability, and low cleavage energy of 0.21-0.22 J/m2. Besides, they exhibit common features of Kagome lattices, such as fat bands and van Hove singularities near band-edges, which lead to high Seebeck coefficient (0.37-0.57 mV/K) and TE power factor of (2.79 -3.71 mW/K2m). Further, owing to their asymmetric Janus structures, the strong coupling between acoustic and optical phonons, as well as low frequency for lying-phonons, the three lattices also possess low lattice thermal conductivity of 1.06-2.76 W/mK. Ultimately, they deliver promising TE figure of merit of {0.28, 0.39, 0.43} at 300 K, and increase further to {1.09, 1.39, 1.40} at 700 K, demonstrating breathing-Kagome lattice Nb3TeX7 are suitable candidates in TE application.