Paper Title
Comparative DFT Study of the Structural and Electronic Properties of the Phases of Ternary Halide Perovskite CsGeBr3

Halide perovskite compounds can have rich structural phases under different pressure and temperature conditions. On the other hand, CsGeX3 rhombohedral ternary halogenides (CGX, X = Cl, Br, I) have recently become a new class of non-linear optical materials (NLOs), which possess a potential application from the visible to the infrared. In this work, the three cubic, rhombohedral and monoclinic structures were explored using the generalized gradient approximation (GGA) and parameterized with the Perdew-Burke-Ernzerhof functional (PBE). We studied the structural, electronic and optical properties of CsGeBr3. The results obtained agree well with those determined experimentally and theoretically and the bulk modulus found in the three phases is low. The total density of states of CsGeBr3 revealed the semiconducor nature of this material. The partial densities of states in the different atomic species of the material CsGeBr3 show that the valence band is characterized by a strong presence of Br_p and Ge_p states. On the other hand, the conduction band is dominated by Ge_p. In the cubic structure of this material, we have found a transition of electrons from the valence band Br_4p5 towards the conduction band Ge_4p2, which corresponds to the energy gap Eg = 0.65 eV. Keywords - Ternary Halide Perovskites, Phase Stability, Electronic Properties, Density Functional Theory (DFT) I. INTRODUCTION Perovskites, in general, are widely used in modern electronics [1,2], for their ferroelectricity [3,4], their semi-conductivities [5], their catalytic activities [6] and their thermoelectricity [7, 8]. These properties have many technological applications including high temperature oxygen probes, dynamic selective access memories, piezoelectric materials, triggers. These highly promotive materials also find applications in transparent ceramics, PLZT (Lead-Lanthanum- Zirconate-Titanate) non-polluting dyes, photovoltaic cells and where, in particular, the cost is also reduced in fuelcells. In this work, we are interested in CsGeBr3 material. The latter belongs to the family of perovskite ternary halide materials which have been discovered to have second-order nonlinear optical properties (NLO) such as ABX3 (A = Cs, Rb, B = Ge, Cd, X = Cl , Br, I) [9], electronic properties, optoelectronic because of their semi-conductivities, their strong dielectric permittivities .These properties have several applications including optical waveguides, surface acoustic wave devices, frequency doublers,and high-kcapacitors[10,11]. Perovskite ternary halide materials in the field of optoelectronics, find use as a dielectric for the manufacture of multilayer capacitors, thermistors, transducers, as integrated capacitance on CMOS (Complementary Metal Oxide Semi Conductor)circuit. In subsequent