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Spinel iron oxide nanoparticles of different mean sizes in the range 10–25 nm have been prepared by surfactant-free up-scalable near- and super-critical hydro­thermal synthesis pathways and characterized using a wide range of advanced structural characterization methods to provide a highly detailed structural description. The atomic structure is examined by combined Rietveld analysis of synchrotron powder X-ray diffraction (PXRD) data and time-of-flight neutron powder-diffraction (NPD) data. The local atomic ordering is further analysed by pair distribution function (PDF) analysis of both X-ray and neutron total-scattering data. It is observed that a non-stoichiometric structural model based on a tetragonal γ-Fe2O3 phase with vacancy ordering in the structure (space group P43212) yields the best fit to the PXRD and total-scattering data. Detailed peak-profile analysis reveals a shorter coherence length for the superstructure, which may be attributed to the vacancy-ordered domains being smaller than the size of the crystallites and/or the presence of anti-phase boundaries, faulting or other disorder effects. The intermediate stoichiometry between that of γ-Fe2O3 and Fe3O4 is confirmed by refinement of the Fe/O stoichiometry in the scattering data and quantitative analysis of Mössbauer spectra. The structural characterization is complemented by nano/micro-structural analysis using transmission electron microscopy (TEM), elemental mapping using scanning TEM, energy-dispersive X-ray spectroscopy and the measurement of macroscopic magnetic properties using vibrating sample magnetometry. Notably, no evidence is found of a Fe3O4/γ-Fe2O3 core-shell nanostructure being present, which had previously been suggested for non-stoichiometric spinel iron oxide nanoparticles. Finally, the study is concluded using the magnetic PDF (mPDF) method to model the neutron total-scattering data and determine the local magnetic ordering and magnetic domain sizes in the iron oxide nanoparticles. The mPDF data analysis reveals ferrimagnetic collinear ordering of the spins in the structure and the magnetic domain sizes to be ∼60–70% of the total nanoparticle sizes. The present study is the first in which mPDF analysis has been applied to magnetic nanoparticles, establishing a successful precedent for future studies of magnetic nanoparticles using this technique.

The magnetic order and the spin dynamics in the antiferromagnetic entropy-stabilized oxide (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O (MgO-ESO) have been studied using muon spin relaxation (μ SR) and inelastic neutron scattering. We find that antiferromagnetic order develops gradually in the sample volume as it is cooled below 140 K, becoming fully ordered around 100 K. The spin dynamics show a critical slowing down in the vicinity of the transition, and the magnetic order parameter grows continuously in the ordered state. These results indicate that the antiferromagnetic transition is continuous but proceeds with a Gaussian distribution of ordering temperatures. The magnetic contribution to the specific heat determined from inelastic neutron scattering likewise shows a broad feature centered around 120 K. High-resolution inelastic neutron scattering further reveals an initially gapped spectrum at low temperature which sees an increase in a quasielastic contribution upon heating until the ordering temperature.

We report experimental studies of a series of BaFe₂S_3−xSe_x (0≤x≤3) single crystals and powder specimens using x-ray diffraction, neutron-diffraction, muon-spin-relaxation, and electrical transport measurements. A structural transformation from Cmcm (BaFe₂S₃) to Pnma (BaFe₂Se₃) was identified around x=0.7−1. Neutron-diffraction measurements on the samples with x=0.2, 0.4, and 0.7 reveal that the Néel temperature of the stripe antiferromagnetic order is gradually suppressed from ∼120 to 85 K, while the magnitude of the ordered Fe²⁺ moments shows very little variation. Similarly, the block antiferromagnetic order in BaFe₂Se₃ remains robust for 1.5≤x≤3 with negligible variation in the ordered moment and a slight decrease of the Néel temperature from 250 K (x=3) to 225 K (x=1.5). The sample with x=1 near the Cmcm and Pnma border shows coexisting, two-dimensional, short-range stripe- and block-type antiferromagnetic correlations. The system remains insulating for all x, but the thermal activation gap shows an abrupt increase when traversing the boundary from the Cmcm stripe phase to the Pnma block phase. The results demonstrate that the crystal structure, magnetic order, and electronic properties are strongly coupled in the BaFe₂S_3−xSe_x system.

We report a comprehensive study of the spin ladder compound BaFe₂S_2.5Se_0.5 using neutron diffraction, inelastic neutron scattering, high pressure synchrotron diffraction, and high pressure transport techniques. We find that BaFe₂S_2.5Se_0.5 possesses the same C_mcm structure and stripe antiferromagnetic order as does BaFe₂S₃, but with a reduced Néel temperature of T_N=98 K compared to 120 K for the undoped system, and a slightly increased ordered moment of 1.40μB per iron. The low-energy spin excitations in BaFe₂S_2.5Se_0.5 are likewise similar to those observed in BaFe₂S₃. However, unlike the reports of superconductivity in BaFe₂S₃ below Tc∼14 K under pressures of 10 GPa or more, we observe no superconductivity in BaFe₂S_2.5Se_0.5 at any pressure up to 19.7 GPa. In contrast, the resistivity exhibits an upturn at low temperature under pressure. Furthermore, we show that additional high-quality samples of BaFe₂S₃ synthesized for this study likewise fail to become superconducting under pressure, instead displaying a similar upturn in resistivity at low temperature. These results demonstrate that microscopic, sample-specific details play an important role in determining the ultimate electronic ground state in this spin ladder system. We suggest that the upturn in resistivity at low temperature in both BaFe₂S₃ and BaFe₂S_2.5Se_0.5 may result from Anderson localization induced by S vacancies and random Se substitutions, enhanced by the quasi-one-dimensional ladder structure.

The structure of the molten salt (LiF)0.465(NaF)0.115(KF)0.42 (FLiNaK), a potential coolant for molten salt nuclear reactors, has been studied by ab initio molecular dynamics simulations and neutron total scattering experiments. We find that the salt retains well-defined short-range structural correlations out to approximately 9 Å at typical reactor operating temperatures. The experimentally determined pair distribution function can be described with quantitative accuracy by the molecular dynamics simulations. These results indicate that the essential ionic interactions are properly captured by the simulations, providing a launching point for future studies of FLiNaK and other molten salts for nuclear reactor applications.

We report neutron diffraction studies of FeS single crystals obtained from Rb_xFe₂−yS₂ single crystals via a hydrothermal method. While no √5×√5 iron vacancy order or block antiferromagnetic order typical of Rb_xFe_2−yS₂ is found in our samples, we observe C-type short range antiferromagnetic order with moments pointed along the c-axis hosted by a new phase of FeS with an expanded inter-layer spacing. The N'{e}el temperature for this magnetic order is determined to be 170±4 K. Our finding of a variant FeS structure hosting this C-type antiferromagnetic order demonstrates that the known FeS phase synthesized in this method is in the vicinity of a magnetically ordered ground state, providing insights into understanding a variety of phenomena observed in FeS and the related FeSe_1−xS_x iron chalcogenide system.