Structural Effects on Redox Behavior in Mn-Based Perovskite Oxides for Solar Thermochemical Water Splitting

A novel series of high entropy Mn-based perovskite oxides, (Ln_2/3A_1/3)MnO₃(Ln = La, Pr, Nd, Gd; A = Ba, Sr), which possess great potential for eco-friendly and low-cost solar thermochemical hydrogen (STCH) production, was studied using Density Functional Theory (DFT).<br/>Starting with the single component LnMnO₃ and AMnO₃systems, it was found that the oxygen vacancy formation energy depends solely on the charge of the A-site ions, which is higher for Ln³⁺ ions than the A²⁺ ions. Alloy Theoretic Automated Toolkit (ATAT) was used for creating quasirandom (Ln_2/3A_1/3)MnO₃mixed structures with orthorhombic Pnma symmetry ((Gd_2/3Sr_1/3)MnO₃, (Pr_2/3Sr_1/3)MnO₃, (Nd_2/3Sr_1/3)MnO₃) and trigonal Rc symmetry ((La_2/3Sr_1/3)MnO₃, (Ln_2/3Ba_1/3)MnO₃).<br/>In mixed systems, the lattice parameters obtained from DFT calculations showed a similar trend for Sr and Ba mixed systems, increasing with the ionic radius, which agrees with experimental data for Ba mixed systems. However, for (Ln_2/3Sr_1/3)MnO₃systems, experimentally reported lattice parameters did not change with increasing ionic radius, indicating possible stronger 3d-4f spin-orbital coupling, requiring high level of DFT calculations.<br/>Oxygen vacancy formation energy distribution in mixed systems exhibited a clear dependence on the phase of the materials, with trigonal Rc structures having lower and more widespread formation energies compared to orthorhombic Pnma systems. Additionally, local structural distortion studies indicated that oxygen vacancy formation is more favorable in systems with trigonal symmetry.<br/>Thermogravimetric analysis (TGA) of the redox capability of all (Ln_2/3A_1/3)MnO₃systems showed that Ba-mixed systems with trigonal symmetry have a higher vacancy concentration change (Δδ) compared to those with orthorhombic symmetry. However, the TGA data for Ba-mixed systems do not correlate well with hydrogen production results, indicating kinetic also play an important role in H₂ production.