A correlated ferromagnetic polar metal by design

Polar metals have recently garnered increasing interest because of their promising functionalities. Here we report the experimental realization of an intrinsic coexisting ferromagnetism, polar distortion and metallicity in quasi-two-dimensional Ca3Co3O8. This material crystallizes with alternating stacking of oxygen tetrahedral CoO4 monolayers and octahedral CoO6 bilayers. The ferromagnetic metallic state is confined within the quasi-two-dimensional CoO6 layers, and the broken inversion symmetry arises simultaneously from the Co displacements. The breaking of both spatial-inversion and time-reversal symmetries, along with their strong coupling, gives rise to an intrinsic magnetochiral anisotropy with exotic magnetic field-free non-reciprocal electrical resistivity. An extraordinarily robust topological Hall effect persists over a broad temperature-magnetic field phase space, arising from dipole-induced Rashba spin-orbit coupling. Our work not only provides a rich platform to explore the coupling between polarity and magnetism in a metallic system, with extensive potential applications, but also defines a novel design strategy to access exotic correlated electronic states.

Metallicity and chemical bonding in anti-anatase Mo2N

Here we present a detailed analysis of the structure, bonding character, and electronic structure of anti-anatase β-Mo2N using density functional theory calculations. We analyze the crystal orbital Hamilton populations, phonon band structure, and electronic structure calculations to explain its low energy transport behavior. We further examine the electronic structures of (anti-)rutile and (anti-)anatase M3-nXn (X = N,O; n = 1,2) M = Ti and Mo nitrides and oxides to show that the atomic structure of anti-anatase leads to metallic behavior independent of the metal and ligand chemistry. Finally, we assess whether these anti-anatase compounds are viable electrides using electron density maps and electron localization functions. Our work shows anti-structures of known binary compounds can expand the phase space of available metallic ceramics beyond layered, hexagonal carbides and nitrides, e.g., Mn+1An (MAX) where n = 1-4.

Thermodynamic Descriptors to Predict Oxide Formation in Aqueous Solutions

We formulate the maximum driving force (MDF) parameter as a descriptor to capture the thermodynamic stability of aqueous surface scale creation over a range of environmental conditions. We use free energies of formation, Δ_fG's, sourced from high-throughput density functional theory (DFT) calculations and experimental databases to compute the maximum driving force for a range of materials, including oxides and hydroxides of varying compositions. We show how to use the MDF to describe trends in the aqueous corrosion of nickel thin films determined from experimental linear sweep voltammetry data. We also show how to account for subsurface oxidation behavior using depth-dependent effective chemical potentials. We anticipate this approach will increase the overall understanding of oxide formation on chemically complex multielement alloys, where competing oxide phases can form during transient aqueous corrosion.

Low Thermal Conductivity in Heteroanionic Materials with Layers of Homoleptic Polyhedra

Although BiAgOSe, an analogue of a well-studied thermoelectric material BiCuOSe, is thermodynamically stable, its synthesis is complicated by the low driving force of formation from the stable binary and ternary intermediates. Here we have developed a "subtraction strategy" to suppress byproducts and produce pure phase BiAgOSe using hydrothermal methods. Electronic structure calculations and optical characterization show that BiAgOSe is an indirect bandgap semiconductor with a bandgap of 0.95 eV. The prepared sample exhibits lower lattice thermal conductivities (0.61 W·m^-1·K^-1 at room temperature and 0.35 W·m^-1·K^-1 at 650 K) than BiCuOSe. Lattice dynamical simulations and variable temperature diffraction measurements demonstrate that the low lattice thermal conductivity arises from both the low sound velocity and high phonon-phonon scattering rates in BiAgOSe. These in turn result primarily from the soft Ag-Se bonds in the edge-sharing AgSe₄ tetrahedra and large sublattice mismatch between the quasi-two-dimensional [Bi₂O₂]²⁺ and [Ag₂Se₂]^2- layers. These results highlight the advantages of manipulating the chemistry of homoleptic polyhedra in heteroanionic compounds for electronic structure and phonon transport control.

Design of Heteroanionic MoON Exhibiting a Peierls Metal-Insulator Transition

Using a first-principles approach, we design the heteroanionic oxynitride MoON to exhibit a first-order isosymmetric thermally activated Peierls-type metal-insulator transition (MIT). We identify a ground state insulating phase (α-MoON) with monoclinic Pc symmetry and a metastable high temperature metallic phase (β-MoON) of equivalent symmetry. We find that ordered fac-MoO_{3}N_{3} octahedra with edge and corner connectivity stabilize the twisted Mo-Mo dimers present in the α phase, which activate the MIT through electron localization within the 4d a_{1g} manifold. By analyzing the temperature dependence of the soft zone-boundary instability driving the MIT, we estimate an ordering temperature T_{MIT}∼900  K. Our work shows that electronic transitions can be designed by exploiting multiple anions, and heteroanionic materials could offer new insights into the microscopic electron-lattice interactions governing unresolved transitions in homoanionic oxides.