Theoretical Study of Polar Spinel Surfaces: Effect of Termination and Cation Inversion on Structure and Stability

Abstract

The effect of termination and cation distribution on the stability of low-index MgAl2O4 and ZnFe2O4 surfaces is investigated theoretically at the generalized gradient approximation and self-consistent hybrid density functional theory level. MgAl2O4 is the prototype AB2O4 spinel, and ZnFe2O4 belongs to the spinel-type ferrites that have been proposed as photocatalysts for water splitting (Taffa, D. H.; Dillert, R.; Ulpe, A. C.; Bauerfeind, K. C. L.; Bredow, T.; Bahnemann, D. W.; Wark, M. J. Photonics Energy. 2016, 7, 012009). Because the catalytic activity of different surfaces of the same material can vary significantly, it is of utmost importance to determine the most relevant surface terminations. Spinels can easily undergo an interchange of cations on A and B sites, the so-called inversion. We therefore studied the low-index surfaces (100), (110), and (111) of both normal and fully inverse MgAl2O4 and ZnFe2O4. For each surface, the surface energy of several possible terminations was calculated with symmetric and stoichiometric slab models. It is found that the surfaces of inverse spinels are less stable than the corresponding normal surfaces. This indicates that full inversion is not facilitated in nanoparticles with a large surface-to-bulk ratio, which are used, for example, in photocatalysis. The Wulff theorem is applied to determine the equilibrium shapes of MgAl2O4 and ZnFe2O4 crystallites. For all compounds, {100} facets dominate the single-crystal surface.

Effect of Surface Energetics on Phase Stability of CaMnO3

“…This is in line with the experimental observation for SrMnO 3 that a polymorph with SG 20 is favored for nanoparticles. [23] A polymorph with SG 20 has not yet been experimentally reported for CaMnO 3 , possibly due to kinetic hindrance, but based on our present results we propose its thermodynamic stability for small particle sizes.…”

“…There are two ways how to deal with such models. One strategy is to insert defects to make the model stoichiometric, as successfully done by Bauerfeind et al [ 20 ] In this case, one Ca atom and one O atom are removed on each side. The other possibility is to compute a nonstoichiometric model, as suggested by Bottin et al [ 19 ] In this case, the surface energy is dependent on two chemical potentials μ Ca and μ O .…”

“…Some surface cutting schemes do not automatically lead to stoichiometric slab models. The stoichiometry of the slab models can be achieved by incorporating Schottky defects, as discussed by Bauerfeind et al [ 20 ] In the present study, we follow their suggestions by constructing symmetric and stoichiometric surface models. Also, we check the relevance of nonstoichiometric models and reconstructions as described by Bottin et al [ 19 ]…”

Theoretical Study of the Propene Combustion Catalysis of Chromite Spinels: Reaction Mechanism and Relation between the Activity and Electronic Structure of Spinels

“…For ZnCr 2 O 4 , the antiferromagnetic state was calculated to be the ground state, but the ferromagnetic state was evaluated to be only 0.025 eV above the antiferromagnetic state . Considering that the energy difference is tiny and reaction temperature is much higher than room temperature, the ferromagnetic state was employed in previous theoretical studies of catalytic reactions by ZnCr 2 O 4 for easier calculation. ,, In accordance with those works, ,, the ferromagnetic state was employed in this work.…”

“…49 Considering that the energy difference is tiny and reaction temperature is much higher than room temperature, the ferromagnetic state was employed in previous theoretical studies of catalytic reactions by ZnCr 2 O 4 for easier calculation. 48,49,67 In accordance with those works, 48,49,67 the ferromagnetic state was employed in this work.…”