Synthesis, structure, and magnetic and dielectric properties of magnetoelectric BaDyFeO4 ferrite

BaDyFeO4 was prepared by a conventional solid-state method in air at 1573 K. It crystallizes in space group Pnma (No. 62) with a = 13.16861(1) angstrom, b = 5.70950(1) angstrom, and c = 10.26783(1) angstrom, and it is isostructural with BaYFe0 4 . Three magnetic transitions were found in BaDyFeO4 at T-N3 = 9 K, T-N2 = 23 K, and T-N1 =47 K in zero magnetic field in comparison with two magnetic transitions observed in BaYFeO4. Magnetic-field-induced transitions were also detected in BaDyFeO4 at 18 and 28 kOe (at T= 1.8 K). Frequency-dependent broad dielectric peaks were observed in BaDyFeO4 spanning between T-N2 and T-N and centred at 35 K - this temperature does not coincide with any T-N. No dielectric anomalies were found at T-N1 and T-N3, while very weak frequency-independent dielectric anomalies were detected at T-N2. Positive and negative magnetodielectric effects were measured in BaDyFeO4 (within a range of -0.8 and + 0.4% up to 90 kOe) reflecting magnetic-field dependence of dielectric constant. Pyroelectric current measurements did not detect any ferroelectricity in BaDyFeO4 under measurement conditions used. No dielectric anomalies and no magnetodielectric effects were found in BaYFeO4. (C) 2019 Elsevier B.V. All rights reserved

Interplay between Fe(II) and Fe(III) and Its Impact on Thermoelectric Properties of Iron-Substituted Colusites Cu_{26−<i>x</i>}Fe<i>_x</i>V₂Sn₆S₃₂

Following the trend of finding better thermoelectric materials among synthetic analogs of copper–chalcogenide minerals, we have synthesized iron-bearing colusites of a general formula Cu26−xFexV2Sn6S32. They crystallize in the cubic space group P-43n with the unit cell parameter increasing linearly with the iron content. At a low iron concentration, the crystal structure features disorder manifested by an anti-site effect and a shift of a part of the tin atoms from their ideal positions, which is absent for higher iron contents. The magnetization and 57Fe/119Sn Mössbauer studies showed that, for x = 1, iron is present as Fe3+, whereas for x > 1, Fe2+ and Fe3+ coexist. Additionally, weak antiferromagnetic interactions between iron atoms and fast on the 57Fe Mössbauer time scale (107–109 s−1) electron transfer between adjacent Fe2+ and Fe3+ centers were revealed. Thermoelectric studies showed that iron-bearing colusites are p-type semiconductors with low thermal conductivity stemming from their complex crystal structure and structural disorder. The highest ZT of 0.78 at 700 K was found for the x = 1 iron content, where iron is present as Fe3+ only

61^{61}Ni Nuclear Forward Scattering Study of Magnetic Hyperfine Interactions in Double Perovskites A2NiMnO6A_{2}NiMnO_{6} (A = Sc, In, Tl)

The relationship between hyperfine interactions, local structure, and magnetism in ordered double perovskites A2NiMnO6 (A = Sc, In, and Tl) is investigated by nuclear forward scattering with the 61Ni nuclear transition. Special attention is given to a quantitative determination of on-site and transferred hyperfine fields on the 61Ni nuclei. The anomalous small value of the saturated hyperfine magnetic field Hhf(0) in antiferromagnetic Sc2NiMnO6 is explained by spin-orbital coupling and strong Ni-3d and O-2p hybridization. The temperature evolution of Hhf(T) is reproduced with Néel temperature TN1 ≈ 37 K, which is incompatible with the earlier assumption that magnetic ordering in the Ni sublattice takes place below TN2 ≈ 17 K. Significantly reduced fields Hhf(0) in In2NiMnO6 (∼21 kOe) with a cycloidal magnetic structure and Tl2NiMnO6 (∼18 kOe) with collinear ferromagnetic ordering are related to the supertransferred hyperfine field (HSTHF) induced by the nearest Mn4+ neighbors. Taking into account the angular dependence HSTHF(ϑ) on the Ni–O–Mn bond angle ϑ, we have shown that HSTHF in the A = In and Tl perovskites has the negative sign, thus drastically reducing the resulting Hhf value

“Hydrotriphylites” Li1-xFe1+x(PO4)1-y(OH)4y as Cathode Materials for Li-ion Batteries

Lithium iron phosphate LiFePO4 triphylite is now one of the core positive electrode (cathode) materials enabling the Li-ion battery technology for stationary energy storage applications, which are important for broad implementation of the renewable energy sources. Despite the apparent simplicity of its crystal structure and chemical composition, LiFePO4 is prone to off-stoichiometry and demonstrates rich defect chemistry owing to variations in the cation content and iron oxidation state, and to the redistribution of the cations and vacancies over two crystallographically distinct octahedral sites. The importance of the defects stems from their impact on the electrochemical performance, particularly on limiting the capacity and rate capability through blocking the Li ion diffusion along the channels of the olivine-type LiFePO4 structure. Up to now the polyanionic (i.e. phosphate) sublattice has been considered idle on this playground. Here, we demonstrate that under hydrothermal conditions up to 16% of the phosphate groups can be replaced with hydroxyl groups yielding the Li1-xFe1+x(PO4)1-y(OH)4y solid solutions, which we term “hydrotriphylites”. This substitution has tremendous effect on the chemical composition and crystal structure of the lithium iron phosphate causing abundant population of the Li-ion diffusion channels with the iron cations and off-center Li displacements due to their tighter bonding to oxygens. These perturbations trigger the formation of an acentric structure and increase the activation barriers for the Li-ion diffusion. The “hydrotriphylite”-type substitution also affects the magnetic properties by progressively lowering the Néel temperature. The off-stoichiometry caused by this substitution critically depends on the overall concentration of the precursors and reducing agent in the hydrothermal solutions, placing it among the most important parameters to control the chemical composition and defect concentration of the LiFePO4-based cathodes