Comprehensive computational investigation for exploring physical properties of mechanically stable layered...

The Ruddlesden-Popper (RP) chalcogenides are considered to be excellent materials for use in efficient photovoltaic and optoelectronic devices. In this manuscript, the structural, mechanical, electronic, bond stiffness, and optical and vibrational properties of X₂CdSe₄ (X = Ga, Tl) are examined. The structural optimization and occurrence of the negative cohesive energy ensure their structural stability. The positive elastic constants confirm their mechanical stability from 0 to 10 GPa. These chalcogenides are noticed to be semiconductor because of the significant direct energy bandgap of 2.73 eV and 2.13 eV using TB-mBJ and 2.67 eV and 1.99 eV using TB-mBJ + SOC for Ga₂CdSe₄ and Tl₂CdSe₄, respectively. PDOS unveils major contribution of Ga-s, Tl s/d, Cd-d, and Se-p orbitals towards improvement of their electronic performance. Comparatively, Tl₂CdSe₄ exhibits the lowest effective mass presenting greater carrier mobility. As their relative stiffness is greater than $1/2$ , so these materials may be opted for ceramics with low fracture toughness and damage tolerance. Optical parameters entailing the refractive index, optical conductivity, dielectric function, and absorption coefficient are explored. Phonons modes are noticed at some imaginary frequencies in a specific direction of the tetragonal crystal structure. It is the first computational effort entrusted for X₂CdSe₄ (X = Ga, Tl) that may assist the future researchers for synthesizing.

In this ab initio study, all calculations are made by the full potential linearly augmented plane wave (FP-LAPAW) technique in purview of the density functional theory (DFT)-based WIEN2K simulation code. The band structure is calculated while using the TB-mBJ and TB-mBJ + SOC functional. Voigt-Reuss-Hill approximation is utilized to seek mechanical stability where elastic constants obey Born's criterion. Kramers-Kronig relations have been used to explore optical properties. The density functional perturbation theory (DFPT) is employed to determine the vibrational behavior of the lattice atoms.