Charge localization and ordering in A2Mn8O16 hollandite group oxides: Impact of density functional theory approaches

Abstract

The phases of A2Mn8O16 hollandite group oxides emerge from the competition between ionic interactions, Jahn-Teller effects, charge ordering, and magnetic interactions. Their balanced treatment with feasible computational approaches can be challenging for commonly used approximations in Density Functional Theory. Three examples (A = Ag, Li and K) are studied with a sequence of different approximate exchange-correlation functionals. Starting from a generalized gradient approximation (GGA), an extension to include van der Waals interactions and a recently proposed meta-GGA are considered. Then local Coulomb interactions for the Mn 3d electrons are more explicitly considered with the DFT+U approach. Finally selected results from a hybrid functional approach provide a reference. Results for the binding energy of the A species in the parent oxide highlight the role of van der Waals interactions. Relatively accurate results for insertion energies can be achieved with a low U and a high U approach. In the low U case, the materials are described as band metals with a high symmetry, tetragonal crystal structure. In the high U case, the electrons donated by A result in formation of local Mn 3+ centers and corresponding Jahn-Teller distortions characterized by a local order parameter. The resulting degree of monoclinic distortion depends on charge ordering and magnetic interactions in the phase formed. The reference hybrid functional results show charge localization and ordering. Comparison to low temperature experiments of related compounds suggests that charge localization is the physically correct result for the hollandite group oxides studied here. Finally, while competing effects in the local magnetic coupling are subtle, the fully anisotropic implementation of DFT+U gives the best overall agreement with results from the hybrid functional.

Water-induced formation of an alkali-ion dimer in cryptomelane nanorods

“…The highly earth abundant and environmentally friendly manganese oxide compounds with a tunneled structure have drawn much attention in wide ranging applications in the realm of multiferroic materials, catalysts, super-capacitors, and batteries [1][2][3][4][5][6]. Owing to the large [2 × 2] tunnel sizes, α−Mn 8 O 16 , which possesses a hollandite structure, could accommodate and reversibly insert/retract large species such as Na + , K + , Ag + and H 2 O [7][8][9][10][11][12][13]. As shown in Fig.…”

Strain Induced Tunability of the Electronic Properties of SrTiO3 Interfaces

“…[19,20] SCAN often yields an accuracy comparable to or better than hybrid GGAs, almost at the cost of a GGA calculation, and without using a Hubbard U correction. [21][22][23] We note that although the electronic occupation is calcu-lated correctly using SCAN, the values of the band gap are underestimated. Despite this, trends and correlations are still valid.…”

Structural Refinement and Density Functional Theory Study of Synthetic Ge-Akaganéite (β-FeOOH)

“…The volume of Ge-akaganéite is V = 330.99 (5) Å 3 , which is smaller than that of natural akaganéite (V = 338.13 2 (Table 2). This octahedral distortion is a common phenomenon in this particular spin-polarized system originated from d-orbital t 2g energy differentiation of d xy (angular distortion in both Fe1 and Fe2), known as Jahn-Teller distortion [31][32][33] as described from other hollandite structures [25][26][27]. The extended X-ray absorption fine structure (EXAFS) study of the Ge local structures indicates a coordination number close to five ( Figure S2 and Table S2), which may be explained by the severe distortion of the octahedral Fe1 (Fe, Ge) site.…”

“…The differences in electron density can be minimized by positioning a cation with a higher atomic number than Fe, such as Ge, and so we considered that the octahedral Fe1 site could also be a possible site for Ge. Moreover, with consideration of suggested model structure from previous study [16] and other hollandite structure studies [25][26][27], we performed Rietveld refinement for the model with three possible Ge sites as follows: (1) Ge is found only at the Fe1 sites, (2) Ge is found at both the Fe1 and tunnel sites, and (3) Ge is found only at the tunnel sites, as proposed by Song et al (2011). The refinement results show that, convergence was only achieved in the case of (2), and the corresponding DELF map shows a distinct reduction in the differences at the Fe1 site ( Figure 1B).…”

“…All calculations have been performed based on spin-unrestricted calculation [22,23]. Geometrical optimization was performed using the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) [24] function to calculate the ground state enthalpy and to obtain the tunnel structure [25][26][27]. For van der Waals dispersion correction, the semi-empirical approach from Tkatchenko-Scheffler (TS) [28] was adopted.…”