Ultralow Lattice Thermal Conductivity with an Outstanding Figure of Merit of Predicted Zintl Phases: XIn₂C₂ (X = Sr, Ba)

Searching for a new thermoelectric (TE) material with a high figure of merit (ZT), at least 2.5, has a strong commercial motivation. Zintl phases are considered to be the most promising candidates for TE energy conversion through the recovery of waste heat. In the scope of the present research, we employed a combination of the first-principles calculations with Boltzmann transport theory to study the transport properties of two predicted compounds, SrIn 2 C 2 and BaIn 2 C 2, and compared the results with those of the previously synthesized MgAl 2 C 2 . We carried out geometric optimization for stable phase development and assessed their stability through various criteria, including formation enthalpies, reaction energy, decomposition energy, cohesive energy, and study of high-temperature stability via ab initio molecular dynamics calculations. Additionally, we investigate their dynamic stability via phonon dispersion and phonon density of states, indicating short phonon lifetimes, low phonon group velocities, and large scattering rates. The band gap values for MgAl 2 C 2, SrIn 2 C 2, and BaIn 2 C 2 compounds are 2.46, 0.76, and 0.93 eV, respectively. It is noticeable that the total thermal conductivities (in the unit, W m –1 K –1 ) of MgAl 2 C 2, SrIn 2 C 2, and BaIn 2 C 2 are 10.957, 1.538, and 0.383. The figures of merit of these three compounds at 300 K are 0.06, 0.19, and 1.10, respectively. The ultralow lattice thermal conductivity (0.24 W m –1 K –1 ) and high Seebeck coefficient (225 μV K –1 ) at 300 K are the prime contributors to achieving the high ZT value of BaIn 2 C 2 . The unprecedented ZT value is 2.86 for BaIn 2 C 2 and 1.93 for SrIn 2 C 2 at a high temperature of 1000 K. Furthermore, the ductile nature and high melting point of BaIn 2 C 2 render it highly suitable for practical applications.