First-principles calculations to investigate structural, electronic, phonon, magnetic and thermal properties of stable halide perovskite semiconductors Cs2GeMnI6 and Cs2GeNiI6

Since, the systematic and extensive study on halide double perovskite's (HDP's) have triggered a considerable interest, thus became a new thrust to the scientific research. In this organised report, we present a new promising halide double perovskite structures Cs2GeMnI6 and Cs2GeNiI6 to understand, more in-depth their intrinsic characteristics by means of Density Functional Theory (DFT). Initially, we reported the structural stability of both these crystal structures by evaluating their total ground state energy in stable configuration and cohesive energy at the behest of Brich Murnaghan equation of state. Afterwards, the structural stability is moreover extended to see their corresponding tolerance factor (τ) values and second order elastic constants (SOEC's). Subsequently, the density functional perturbation theory (DFPT) has been inducted to predict the dynamical context of these ordered systems. Later on, the quantum mechanical treatment of defining their electronic structures by the calibration of mix of two distinct spin-polarised approximation schemes like Perdew-Burke-Ernzerhof Generalised gradient approximation (PBE-GGA) followed by Tran-Blaha modified Becke-Johnson (TB-mBJ). Meanwhile, the unsymmetrical behaviour of electronic structures from their resulting band structures demonstrates the occurrence of spin-magnetic moment of 5 µB and 2 µB/formula unit respectively having the maximum contribution solely coming from the Mn+2 and Ni2+ 3d-transition elements. The evaluation of phonon dependent Grüneisen parameter (γ) defines the possible thermal stability against specific temperature regimes. And finally transport properties as a function of chemical potential (µ-EF) at distinct temperatures has been keenly explored. Therefore, the overall tendency of these designed materials can be envisaged to descript vast implications in various extending applications for conventional spin-based and thermoelectric technologies.