Density Functional Theory Study of Monoclinic FeNbO₄: Bulk Properties and Water Dissociation at the (010), (011), (110), and (111) Surfaces

Abstract

Monoclinic and orthorhombic FeNbO4-based materials have been developed for many applications, including hydrogen sensors and solid oxide electrolysis cell (SOEC) electrodes. Here, we have employed density functional theory (DFT) calculations to investigate the bulk and surface properties of the monoclinic FeNbO4 structure, as well as water adsorption and dissociation on its pristine surfaces. Our calculations show that the high-spin state Fe3+ cations have a relatively smaller Bader charge than the Nb5+ cations, which accounts for Nb–O bonds that are stronger than Fe–O bonds. The analysis of the density of states (DOS) shows that the O 2p orbital occupies most of the valence band, including its maximum (VBM), with negligible contributions from the 4d and 3d orbitals of Nb and Fe cations, respectively. We found that the 3d orbitals of Fe occupy the conduction band minimum (CBM), which explains that electrons are conducted via the Fe–O–Fe framework. The calculation of the elastic constants demonstrates that pure monoclinic FeNbO4 is mechanically stable. We have also considered the thermodynamic stability and structures of the seven low-Miller-index surfaces and found that the (010) facet has the lowest surface energy and expresses the largest area in the Wulff crystal shape of the particle. Finally, we have simulated the interaction of water with the Fe3+ and Nb5+ sites of the four most stable surfaces and found that the dissociative adsorption of water takes place only on the (110) surface, which has important implications for the use of this material as a SOEC electrode.

Antiferromagnet–Ferromagnet Transition in Fe1−xCuxNbO4

“…Figure 5 shows an illustrative scheme that helps elucidate the change in magnetic ordering and the contribution of each interaction. Figure 5 a shows a net AFM configuration in the structure as already reported in the literature for m-FeNbO 4 [ 10 ]. This configuration contains FM interactions associated with Fe-O-Fe interactions and AFM interactions associated with Fe chains mediated by Nb, resulting in a liquid AFM order.…”

“…FeNbO 4 is a polymorphic compound that crystallizes in three different crystalline phases depending on the annealing temperature [ 1 ].Considered an n-type semiconductor, it has a narrow bandgap of 1.81–2.25 eV [ 2 , 3 ], making it attractive for multipleapplications, e.g., as a photocatalyst [ 4 , 5 ], due to its excellent visible light activity attributed to the higher energy levels of the Nb 4d orbital;in gas sensors [ 3 ], capacitors [ 6 ], and lithium-ion batteries [ 7 , 8 ]; and as an anode material in solid oxide fuel cells (SOFCs) [ 9 ].The most stable phase, under ambient conditions, has monoclinic symmetry (m-FeNbO 4 , space group P2/c) and is obtained at temperatures below 1085 °C [ 10 ]. Its structure has ordered cations, with both Fe 3+ and Nb 5+ forming regular octahedra, coordinated by six oxygen ions, forming zig-zag chains of FeO 6 and NbO 6 (see Figure 1 a) [ 10 , 11 ]. A crystal structure with orthorhombic symmetry (o-FeNbO 4 , Pbcn space group) is formed in the temperature range of 1085 to 1380 °C [ 12 ].Unlike the m-FeNbO 4 phase, above 1100 °C, the distribution of cations in this structure becomes disordered.…”

“…Cross-chains of Fe-O-Nb-O-Fe result in antiferromagnetic (AFM) arrangements and, therefore, in net antiferromagnetism [ 15 ]. Recently, Wang et al, through theoretical calculations of functional density (DFT), hypothesized the existence of three distinct and stable magnetic ordering configurations, which can be explained as the possible formation of magnetic domains in the material [ 10 ].…”

Lattice distortion derived catalytic degradation in multi-oxide cathode catalyst for Li–oxygen batteries

“…It is obvious that (010) surface with Co-O terminated exhibits the lowest surface energy of 0.49 J m À2 , which is the thermodynamically favorable and prior exposing facet for catalytic reactions. 53,54 In this work, Co-O terminated (designated as 010Co) and W-O terminated (designated as 010W) 010 surfaces were applied to investigate the catalytic activity of CoWO 4 to identify the effect of distorted [WO 6 ] octahedral framework. The 010W surface exhibits a higher surface energy of 1.35 J m À2 compared to the 010Co surface.…”

Oxygen diffusion in the orthorhombic FeNbO4 material: a computational study

“…In previous work, 23 we have employed calculations based on the density functional theory (DFT) to study the bulk and surface properties of m -FeNbO 4 . Due to the disorder of the cations in the o -FeNbO 4 phase, it is more difficult to create a computational model to simulate its behaviours and properties, which is presumably the reason why few computational studies have been reported on the orthorhombic phase.…”