Symmetry protected 1D chains in mixed-valence iron oxides

Abstract

During the last decade of high-pressure research a whole new series of iron oxides was discovered, like Fe 4 O 5 , Fe 5 O 6 , Fe 7 O 9 etc. [1-6], featuring closely related structures with arrays of one-dimensional (1D) chains of trigonal prisms embedded between slabs of octahedra. Here, we develop a unified approach to the series based on a specific crystallographic generation mechanism which predicts the structures of these oxides and naturally classifies them in terms of the slab cycle.When including magnetic interactions, we show that the 1D chains have a symmetry protection against magnetic perturbations from the iron ions in the slabs, and that the slab size determines the type of magnetic order, which is either ferromagnetic or antiferromagnetic. Dynamical meanfield theory calculations reveal the orbitally selective Mott state of the Fe ions and tendency of conductivity to low-dimensional behavior with particular enhancement along the 1D chains. Across the series, the decoupling of the chains increases, and so with the inherent charge ordering of the slabs, these structures have the potential to allow experimental realization of the model system of coupled 1D wires [7][8][9][10][11]. We point out the possibility to stabilize these compounds in the thin-film form that, together with a wide range of possible ionic substitutions and fact that these compounds are recoverable at ambient pressure, makes them a very promising platform to engineer physical systems with interesting magnetotransport phenomena, as corroborated by the recent discovery of quantum Hall effect in ZrTe 5 [12].

Electronic properties of single-crystalline Fe4O5

“…99 However, the above small difference in the energies of the lowermost d-orbitals of Fe1, Fe2, and Fe3 atoms in Fe 4 O 5 is challenging to trace both experimentally and by band structure calculations. 100 To date, band structure calculations have been inconsistent in the prediction of the electronic properties of Fe 4 O 5 , reporting it to be a metal, 49,101 a wide-gap semiconductor, 50 or to be a narrow-gap semiconductor with E g = 0.2 eV. 102 Our study resolves these discrepancies.…”

“…4b) well explains the non-zero density of states at the Fermi level. 49,101 Whereas, due to the low carrier mobility (Fig. 4c), its electrical conductivity indeed may look as semiconducting.…”

Orthogonal magnetic structures of Fe4O5: representation analysis and DFT calculations

“…Since the discovery of novel high-pressure iron oxides, their crystal chemistry and physical properties have been of great interest. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Based on the common packing motif, these compounds constitute a homologous series nFeO•mFe 2 O 3 , containing η-Fe 2 O 3 , HP-Fe 3 O 4 , Fe 4 O 5 , Fe 5 O 6 , Fe 5 O 7 , etc. 4 Their crystal structures consist of two main building blocks, namely the slabs of edge-shared Fe-O octahedra separated by arrays of one-dimensional (1D) chains of Fe ions coordinated by a trigonal prism (see Fig.…”

“…32 Recently, a unified approach to this series based on a specific crystallographic generation mechanism has been proposed, which naturally classifies these oxides in terms of a sequence of the slabs or, in other words, a slab cycle. 13 Depending on the type of the slab cycle, this series can be divided into two groups, an orthorhombic N-family and a monoclinic N,N + 1 family. The former is built by only one slab of N octahedra width, exemplified by η-Fe 2 O 3 , HP-Fe 3 O 4 , Fe 4 O 5 , and Fe 5 O 6 with N = 1, 2, 3, and 4, respectively.…”

“…13 The resulting magnetic structure will be either ferromagnetic or antiferromagnetic depending on whether N is odd or even, respectively. The remarkable feature here is that the 1D chains are symmetry protected from the magnetic perturbation of the surrounding slabs and order magnetically at much lower temperatures relative to the slabs, about 100 K versus 300 K. 13 These features, together with the fact that ab initio calculations show a strong dependence of the anisotropic conductivity on the magnetic state, 13 show the great potential of these compounds to host various magnetotransport phenomena and even to realize theoretical models of coupled 1D wires. 18,19 It is therefore important to carefully investigate this conjecture about magnetic structures in the N-family.…”