Role of Cu Substitute in AgGaTe₂ in Structural Stabilities, Electronic Property, and High-Pressure Phase Transitions: A DFT Investigation

The Cu–Ag–Ga–Te system has garnered significant attention due to its promising properties, including excellent photoelectric and thermoelectric characteristics. This study investigates the effects of substitutional Cu doping into AgGaTe2 at a 1:1 ratio of Ag and Cu on the structural stabilities and electronic and mechanical properties using density functional theory (DFT) calculations. All calculated results are compared to those of AgGaTe2 and CuGaTe2. We found that Cu0.5Ag0.5GaTe2 is both dynamically and elastically stable. Mechanically, the bulk modulus (B) increases, while the shear (G) and Young’s (E) moduli decrease when subjected to Cu doping. The electronic band structure of Cu0.5Ag0.5GaTe2 processes the direct band gap (Eg) of 1.20 eV, which aligns between those of AgGaTe2 (1.17 eV) and CuGaTe2 (1.29 eV). The increase in band gap and the trend of mechanical behavior with higher Cu content are attributed to the stronger covalent bonding between Cu–Ag and Cu–Ga pairs compared to Ag–Ag and Ag–Ga pairs. Additionally, the high-pressure phase transitions of this material are explored up to 100 GPa. The first structural phase transition occurs at approximately 6 GPa, from the ambient phase (I4̅) to a tetragonal (P4/mmm) phase. This transition pressure is also intermediate between AgGaTe2 (4 GPa) and CuGaTe2 (8 GPa). Further high-pressure phase transitions, including those to the orthorhombic (Pmm2) and monoclinic (Pm) phases, are observed at about 36 and 55 GPa, respectively. These findings demonstrate that Cu0.5Ag0.5GaTe2 exhibits consistent trends in the electronic band gap, mechanical properties, and high-pressure phase transitions relative to those of AgGaTe2 and CuGaTe2. This study provides valuable insights into material engineering applications and enhances the understanding of substitutionally doped materials under high-pressure conditions.