Evaluating transition metal oxides within DFT-SCAN and SCAN+U frameworks for solar thermochemical applications

Abstract

Using the strongly constrained and appropriately normed (SCAN) and SCAN+U approximations for describing electron exchange-correlation (XC) within density functional theory, we investigate the oxidation energetics, lattice constants, and electronic structure of binary Ce-, Mn-, and Fe-oxides, which are crucial ingredients for generating renewable fuels using two-step, oxide-based, solar thermochemical reactors. Unlike other common XC functionals, we find that SCAN does not over-bind the O2 molecule, based on direct calculations of its bond energy and robust agreement between calculated formation enthalpies of main group oxides versus experiments. However, in the case of transition-metal oxides (TMOs), SCAN systematically overestimates (i.e., yields too negative) oxidation enthalpies due to remaining self-interaction errors in the description of their ground-state electronic structure. Adding a Hubbard U term to the transition-metal centers, where the magnitude of U is determined from experimental oxidation enthalpies, significantly improves the qualitative agreement and marginally improves the quantitative agreement of SCAN+U-calculated electronic structure and lattice parameters, respectively, with experiments. Importantly, SCAN predicts the wrong ground-state structure for a few oxides, namely, Ce2O3, Mn2O3, and Fe3O4, while SCAN+U predicts the right polymorph for all systems considered in this work. Hence, the SCAN+U framework, with an appropriately determined U, will be required to accurately describe ground-state properties and yield qualitatively consistent electronic properties for most transitionmetal and rare earth oxides.

A First‐Principles‐Based Sub‐Lattice Formalism for Predicting Off‐Stoichiometry in Materials for Solar Thermochemical Applications: The Example of Ceria

“…Note that we prefer to use the SCAN+U framework instead of the previously employed [113,114] local density approximation (LDA [115] )+U or a generalized gradient approximation (GGA [116] )+U for two reasons: i) SCAN is a more accurate and more well-founded functional than LDA/GGA since it satisfies all 17 known constraints of a XC functional [112] and ii) the absolute U correction required with SCAN is normally lower compared to LDA/GGA, consistent with SCAN containing more of the correct XC physics. [92] All possible symmetry-distinct oxygen-vacancy configurations were enumerated within the conventional CeO 2 unit cell to evaluate the energy of CeO F 1.5 and we used the lowest energy obtained. For estimating E O 2 (g), an isolated oxygen molecule was placed in an asymmetric 15 Å × 16 Å × 17 Å cell to obtain the triplet electronic ground state of O 2 .…”

“…Similar to undoped-CeO x , we evaluate all of the input parameters in Equations (23), (26), and (29) and the energies of all binary and ternary oxides within the Ce-Zr-O system to obtain the lowest Gibbs energy as a function of the oxygen composition (and x eq as a result) at different Ce:Zr ratios in Zr-doped CeO x . Figure 5a plots the comparison between experimental (green, data from Zinkevich et al [67] ) and theoretical (red) O as a While errors in predicted (versus experimental) O for x < 1.5 (not relevant to STCH) are due to mixing DFT+U [92,97] (for Ce-oxides) and DFT (for Ce-metal) energies (Experimental Section), the differences in the range of O for CeO 2 and Ce 7 O 12 arise from the specific U value employed [92] and from comparing 0 K energies to experimental enthalpies at 298 K.…”

Structural, electronic, and magnetic properties of bulk and epitaxial LaCoO3 through diffusion Monte Carlo

“…This contrasts with LDA and PBE, where tuning the +U parameter to smaller values allows for studying the LS state as the ground state of LCO [106]. Various examples in the literature suggest a reduced self-interaction error and an improvement in performance with SCAN over GGA [58,[146][147][148]. This is also observed in our calculations such that significantly lower U values are required with SCAN to reproduce equivalent results with GGA+U , indicating a possible improved description of the exchange interactions is provided with SCAN.…”

Mott gapping in 3dABO3 perovskites without Mott-Hubbard interelectronic repulsion energy U

“…24 ) the recently developed SCAN meta-GGA functional 26 , without Mott-Hubbard interelectronic U. The use of the SCAN functional for transition metal and transition metal oxides is reported in many papers [27][28][29][30][31][32] .…”