First-principles investigation of structural, elastic, mechanical, thermal, electronic, optical, and thermoelectric properties of narrow indirect band-gap bromide double perovskites A ₂ AuBiBr

Abstract

This study employs DFT first-principles calculations to investigate the structural, elastic, mechanical, thermal, electronic, optical, and thermoelectric properties of the bromide double perovskites A ₂ AuBiBr ₆ (A = K, Rb, Cs). The computed cubic (

) structural parameters and stability are in good agreement with the previously reported theoretical results, confirming the structural stability of these A ₂ AuBiBr ₆ compounds. The calculated elastic constants satisfy the Born stability criteria, while the negative formation energies indicate thermodynamic stability. Mechanical analysis reveals ductile behavior that is enhanced by A-site substitution. Electronic-structure calculations performed using both the GGA-PBE and TB-mBJ approaches show that all A ₂ AuBiBr ₆ compounds are narrow indirect band-gap semiconductors with promising optoelectronic properties. Thermoelastic analysis predicts favorable melting temperatures, Debye temperatures, and average sound velocities, indicating good thermodynamic stability. The TB-mBJ approach further improves the description of the electronic structure and related physical properties. Thermoelectric transport calculations performed using the BoltzTraP package reveal high figure-of-merit values of ZT  = 0.911, 0.868, 0.732 for A ₂ AuBiBr ₆ (A = K, Rb, Cs), respectively, suggesting promising thermoelectric performance. Overall, the investigated A ₂ AuBiBr ₆ double perovskites exhibit attractive narrow-gap semiconducting, optical, and thermoelectric characteristics, indicating their potential for thermoelectrics energy-harvesting, infrared optoelectronic, and other green-energy applications.