First principles study of multifunctional properties of Ga- and Tl-based quaternary materials for energy applications

The present study used density functional theory and presents a thorough first principles investigation of the structural, electronic, mechanical, optical, and thermoelectric properties of novel CaGaCu₃Se₄ and CaTlCu₃Se₄ quaternary chalcogenides. With strong bonding networks and advantageous thermoelectric properties, these materials have a cubic P4̄3m space group. Compared to CaTlCu₃Se_4, CaGaCu₃Se₄ has a more negative cohesive energy of -5.13 eV per atom and formation energy of -3.97 eV per atom, signifying its greater stability. Due to the Ga and Tl substitution effects on the conduction band nature, the band dispersion profile study demonstrates a direct band gap semiconducting nature at the Γ-point, with CaGaCu₃Se₄ having a large gap of 1.23 eV and CaTlCu₃Se₄ having a gap of 0.31 eV using the TB-mBJ approach. CaTlCu₃Se₄ shows an improved low-energy absorption, large static dielectric constant (6.0), and sharper ε ₂(ω) transitions, while the optical investigation discloses a strong visible-UV absorption and a substantial dielectric response. Both these materials are mechanically strong and ductile, with Pugh's ratios greater than 2.8 and a moderate anisotropic value (A = 1.3). CaGaCu₃Se₄ gives larger stiffness with E = 94.94 GPa and G = 34.67 GPa, while CaTlCu₃Se₄ shows an improved shear agreement. CaTlCu₃Se₄ is a better performer according to the thermoelectric calculations. However, being more conductive, CaGaCu₃Se₄ loses more heat because of the greater κ _e. CaTlCu₃Se₄ displays promise as a good thermoelectric candidate for energy-conversion applications requiring mid-to-high temperatures.