A theoretical study of perovskite CsXCl3 (X=Pb, Cd) within first principles calculations

The structural, elastic, electronic, optical acoustic and thermodynamic properties of the cubic perovskite CsPbCl3 and CsCdCl3 unit cell, were studied using an ultra-soft pseudopotential plane wave, the Trouiller-Martins-Functional was utilized to perform these calculations. The study was implemented within both the Local Density Approximation (LDA) and the Generalized Gradient Approximation (GGA). the Generalized Gradient Approximation (GGA) scheme proposed by van Leeuwen-Baerends which is the same as the Perdew-Wang 92 functional have been carried out to preform our calculations. As for the Local Density Approximation (LDA) the Teter-Pade parametrization (4/93) was implemented which is the same as Perdew-Wang that in its turn reproduces the Ceperley-Alder-Functional. The computed GGA/LDA-lattice parameter for both CsCdCl3 and CsPbCl3 is in an exquisite agreement with the experimental and theoretical results. The energy band structure shows that CsCdCl3 is Γ–R indirect band gap insulator, while CsPbCl3 is an insulator with a direct band gap Γ–Γ separating the valence bands from the conduction bands, which shows metallic nature after pressure 30 GPa. A hybridization exists between Pb-p states and Cl-p states for CsPbCl3, and Cd-p states and Cs-p states for the CsCdCl3 in the valence bonding region. Optimization of both cell shape (geometry) volume were investigated as pressure of 0–20 GPa and 0–40 GPa for the CsCdCl3 and CsPbCl3 respectively. The Pressure dependence of cubic perovskite elastic constants, Young modulus, bulk and shear moduli, Lame’s constants, elastic anisotropy factor, elastic wave velocities, phonon dispersion, Debye temperature and the density of states of CsXCl3 (X=Pb, Cd) were theoretically calculated and compared with the other available theoretical results. The above elastic constants reveal the fact that both compounds are stable and show nature of ductility. For the optical properties, both the static refractive index and dielectric constant are found to be related proportionally to the indirect band gap of CsCdCl3. The refractive index, extinction coefficient, complex dielectric function, energy loss function, optical conductivity, reflectivity and absorption coefficient for 0–25 eV incident photon energies have been predicted. The phonon properties were investigated using response functions to predict the phonon lattice dispersion and the density of states. The thermal effect on the heat capacities, entropy, enthalpy and Free energy were predicted and compared using both the quasi-harmonic Debye model and response functions, the latter provided far better results. To the best of the authors' knowledge, most of the studied properties have not been experimentally reported so far. Generally, the computed results for both CsCdCl3 and CsPbCl3 are very satisfactory and show good agreement with other calculations.