Síntese e caracterização estrutural dos óxidos de alta entropia La(Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)O3 e média entropia La(Cr0.25Mn0.25Fe0.25Ni0.25)O3

Abstract

High-entropy La(Co₀.₂Cr₀.₂Fe₀.₂Mn₀.₂Ni₀.₂)O₃ and medium-entropy La(Cr₀.₂₅Mn₀.₂₅Fe₀.₂₅Ni₀.₂₅)O₃ perovskites were synthesized for the first time using the Mechanical Alloying technique. Experimental characterization was performed through X-ray diffraction (XRD), thermal treatment (TT), and UV-Vis spectroscopy. Due to the structural complexity and computational cost of simulating 160 atoms (required to represent all atoms in the unit cell), a theoretical simplification was adopted. Medium- entropy structures with only four atoms in the 4a site were calculated, totaling 20 atoms per arrangement. Each atom was represented by a letter: A=Cr, B=Mn, C=Fe, D=Co, and E=Ni. Five main arrangements (ABCD, ABCE, ABED, EBCD, and AECD) were considered, yielding 120 possible permutations. Based on Density Functional Theory (DFT), five structures (ABCD, ABCE, CEDA, EBCD, and ABDE) were selected to describe the electronic structure and calculate energy bands and the projected density of states (PDOS). During the synthesis process of high-entropy perovskite, five parameters were adjusted to determine the optimal synthesis route: (1) use of metal oxide precursors with La₂O₃, (2) use of metallic precursors with La₂O₃, (3) increasing the ball- to-powder ratio (BPR), (4) addition of a lubricant (graphite), and (5) replacing the tungsten carbide milling jar with an iron jar. The most efficient route involved milling the metallic precursors (La₂O₃, Co, Cr, Mn, Fe, and Ni) for 20 hours in an iron jar, followed by thermal treatment at 400°C for 1 hour. The medium-entropy structure was synthesized using La₂O₃, Cr, Mn, Fe, and Ni through mechanical milling for 15 hours in an iron jar. Both perovskite structures exhibited orthorhombic symmetry with the Pnma space group. UV-Vis experimental characterization of the high-entropy structure revealed two bandgap values: 0.5 eV and 2.72 eV. Theoretical calculations for the medium-entropy structures showed that ABCD and ABCE exhibited gaps of 0.98 eV (spin-up) and 0.52 eV (spin-down), respectively. Meanwhile, EBCD, CEDA, and BDAE displayed metallic behavior for both spin-up and spin-down states.