Magnetic Structures of Orthorhombic Li2M(SO4)2 (M = Co, Fe) and LixFe(SO4)2 (x = 1, 1.5) Phases

Fe Site Order and Magnetic Properties of Fe1/4NbS2

Transition-metal dichalcogenides (TMDs) have long been attractive to researchers for their diverse properties and high degree of tunability. Most recently, interest in magnetically intercalated TMDs has resurged due to their potential applications in spintronic devices. While certain compositions featuring the absence of inversion symmetry such as Fe1/3NbS2 and Cr1/3NbS2 have garnered the most attention, the diverse compositional space afforded through the host matrix composition as well as intercalant identity and concentration is large and remains relatively underexplored. Here, we report the magnetic ground state of Fe1/4NbS2 that was determined from low-temperature neutron powder diffraction as an A-type antiferromagnet. Despite the presence of overall inversion symmetry, the pristine compound manifests spin polarization induced by the antiferromagnetic order at generic k points, based on density functional theory band-structure calculations. Furthermore, by combining synchrotron diffraction, pair distribution function, and magnetic susceptibility measurements, we find that the magnetic properties of Fe1/4NbS2 are sensitive to the Fe site order, which can be tuned via electrochemical lithiation and thermal history.

First principles investigation of anionic redox in bisulfate lithium battery cathodes

The search for an alternative high-voltage polyanionic cathode material for Li-ion batteries is vital to improve the energy densities beyond the state-of-the-art, where sulfate frameworks form an important class of high-voltage cathode materials due to the strong inductive effect of the S⁶⁺ ion. Here, we have investigated the mechanism of cationic and/or anionic redox in Li_xM(SO₄)₂ frameworks (M = Mn, Fe, Co, and Ni and 0 ≤ x ≤ 2) using density functional calculations. Specifically, we have used a combination of Hubbard U corrected strongly constrained and appropriately normed (SCAN+U) and generalized gradient approximation (GGA+U) functionals to explore the thermodynamic (polymorph stability), electrochemical (intercalation voltage), geometric (bond lengths), and electronic (band gaps, magnetic moments, charge populations, etc.) properties of the bisulfate frameworks considered. Importantly, we find that the anionic (cationic) redox process is dominant throughout delithiation in the Ni (Mn) bisulfate, as verified using our calculated projected density of states, bond lengths, and on-site magnetic moments. On the other hand, in Fe and Co bisulfates, cationic redox dominates the initial delithiation (1 ≤ x ≤ 2), while anionic redox dominates subsequent delithiation (0 ≤ x ≤ 2). In addition, evaluation of the crystal overlap Hamilton population reveals insignificant bonding between oxidized O atoms throughout the delithiation process in the Ni bisulfate, indicating robust battery performance that is resistant to irreversible oxygen evolution. Finally, we observe that both GGA+U and SCAN+U predictions are in qualitative agreement for the various properties predicted. Our work should open new avenues for exploring lattice oxygen redox in novel high voltage polyanionic cathodes, especially using the SCAN+U functional.

Multiple dimensionalities in A2M3(SO4)4 (A = Rb, Cs; M = Co, Ni) analogues

New representatives of the A₂M₃(SO₄)₄ (A = Rb and Cs, M = Co, Ni) family were found, inspired by the discovery and characterization of itelmenite, a mineral of composition Na₂CuMg₂(SO₄)₄. Four new compounds were obtained by high-temperature solid-state reactions in air. All new compounds were structurally characterized by single-crystal and powder X-ray diffraction. Rb₂Ni₃(SO₄)₄ and Rb₂Co₃(SO₄)₄ crystallize in the monoclinic space group P2₁/c, Cs₂Ni₃(SO₄)₄ in P2₁/n whereas Rb₂Co₃(SO₄)₄ crystallizes in the orthorhombic space group P2₁2₁2₁. In order to determine the temperature of crystallization of the new phases DTA and TG were performed for the mixtures of the precursors. Several synthesis strategies were tested and discussed. The investigation of the reactivity upon heating highlights the stability of the precursors before they collapse, explaining the difficulties to get pure powder samples.

Magnetism of NaFePO4 and related polyanionic compounds

Magnetic properties of maricite (m) and triphlyte (t) polymorphs of NaFePO4 are investigated by combining ab initio density functional theory with a model Hamiltonian approach, where a realistic Hubbard-type model for magnetic Fe 3d states in NaFePO4 is constructed entirely from first-principles calculations. For these purposes, we perform a comparative study based on the pseudopotential and linear muffin-tin orbital methods while tackling the problem of parasitic non-sphericity of the exchange-correlation potential. Upon calculating the model parameters, magnetic properties are studied by applying the mean-field Hartree-Fock approximation and the theory of superexchange interactions to extract the corresponding interatomic exchange parameters. Despite some differences, the two methods provide a consistent description of the magnetic properties of NaFePO4. On the one hand, our calculations reproduce the correct magnetic ordering for t-NaFePO4 allowing for magnetoelectric effect, and the theoretical values of Néel and Curie-Weiss temperatures are in fair agreement with reported experimental data. Furthermore, we investigate the effect of chemical pressure on magnetic properties by substituting Na with Li and, in turn, we explain how a noncollinear magnetic alignment induced by an external magnetic field leads to magnetoelectric effect in NaFePO4 and other transition-metal phosphates. However, the origin of a magnetic superstructure with q = (1/2, 0, 1/2) observed experimentally in m-NaFePO4 remains puzzling. Instead, we predict that competing exchange interactions can lead to the formation of magnetic superstructures along the shortest orthorhombic c axis of m-NaFePO4, similar to multiferroic manganites.

Stabilization of Lithium Transition Metal Silicates in the Olivine Structure

While olivine LiFePO₄ shows amongst the best electrochemical properties of Li-ion positive electrodes with respect to rate behavior owing to facile Li⁺ migration pathways in the framework, replacing the [PO₄]^3- polyanion with a silicate [SiO₄]^4- moiety in olivine is desirable. This could allow additional alkali content and hence electron transfer, and increase the capacity. Herein we explore the possibility of a strategy toward new cathode materials and demonstrate the first stabilization of a lithium transition metal silicate (as a pure silicate) in the olivine structure type. Using LiInSiO₄ and LiScSiO₄ as the parent materials, transition metal (Mn, Fe, Co) substitutions on the In/Sc site were investigated by computational modeling via atomic scale simulation. Transition metal substitution was found to be only favorable for Co, a finding confirmed by the successful solid state synthesis of olivine Li_xIn_yCo_2-x-ySiO₄. Stabilization of the structure was achieved by entropy provided by cation disorder.