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 CaTlCu3Se4, 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.