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Abstract: Defects are emerging as a key tool for fine-tuning the stimuli-responsive behavior of coordination polymers and metal–organic frameworks. Here, we study the ramifications of defects on the mechanical properties of the molecular perovskite [C(NH2)3]MnII(HCOO)3 and its defective analogue [C(NH2)3]Fe2/3III□1/3(HCOO)3, where □ = vacancy. Defects reduce the bulk modulus by 30% and give rise to a temperature-driven phase transition not observed in the nondefective system. The results highlight the opportunities that come with defect-engineering approaches to alter the mechanical properties and underlying thermodynamics, with important implications for the research on stimuli-responsive materials.

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Abstract: A melt-extracted HoErCo medium-entropy alloy (MEA) with large magnetic entropy change and excellent magnetic refrigerant capacity is successfully designed in this study. The microstructure evolution of these microwires near room temperature is studied by an in-situ high energy synchrotron X-ray diffraction (HEXRD). The wires show typical amorphous characteristics during room temperature to 250 K. For a field change of 5 T, the maximum magnetic entropy change (-ΔSMmax) of the microwires reaches a maximum value of 15.0 J•kg−1 K−1. Magnetization measurements revealed a paramagnetic to ferromagnetic phase transition at TC~16 K. The refrigerant capacity (RC) and relative cooling power (RCP) are 527 J•kg−1 and 600 J•kg−1, respectively. This investigation highlights the potential of HoErCo MEA microwires as promising magnetocaloric effect materials for cryogenic applications of magnetic cooling.

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Abstract: The mechanical behavior of different microstructural constituents in SAE 52100 bearing steel has been studied at room temperature in relation to quenching and partitioning (QP) and bainitization (B) process parameters, and compared to the standard quenched and tempered (QT) microstructure using high-energy synchrotron X-ray diffraction in situ during tensile loading. Owing to a larger degree of carbon entrapment in its body-centered cubic lattice and associated lattice distortion, martensite in the QT microstructure showed a larger lattice parameter and broadened diffraction peaks as compared to lower bainitic ferrite or partitioned martensite. A reduction in diffraction peak broadness in tempered martensite occurs at a true stress value of ∼1800 MPa, and preserves its peak broadness even after subsequent unloading. In contrast, an equivalent effect in peak broadness is detected at ∼1500 MPa in the lower bainitic ferrite or mixed bainitic/martensitic matrix characteristic of the B and QP microstructures. In all studied microstructures, the metastable austenite phase transforms when a critical stress is reached, the value of which increases with the bainitic ferrite/martensite fraction and with the carbon content in austenite, but remains lower in the QP and B microstructures compared to the standard QT steel. These results suggest that the carbon solid solution strengthening and associated lattice distortion in bainitic ferrite or martensite are key in determining the mechanical performance of the constituent phases in the steel, with the phase fraction and local carbon content playing an additional role on the austenite mechanical stability.

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Abstract: For magnesium ion batteries (MIBs) to be used commercially, new cathodes must be developed that show stable reversible Mg intercalation. VS4 is one such promising material, with vanadium and disulfide anions [S2]2– forming one-dimensional linear chains, with a large interchain spacing (5.83 Å) enabling reversible Mg insertion. However, little is known about the details of the redox processes and structural transformations that occur upon Mg intercalation and deintercalation. Here, employing a suite of local structure characterization methods including X-ray photoelectron spectroscopy (XPS), V and S X-ray absorption near-edge spectroscopy (XANES), and 51V Hahn echo and magic-angle turning with phase-adjusted sideband separation (MATPASS) NMR, we show that the reaction proceeds via internal electron transfer from V4+ to [S2]2–, resulting in the simultaneous and coupled oxidation of V4+ to V5+ and reduction of [S2]2– to S2–. We report the formation of a previously unknown intermediate in the Mg–V–S compositional space, Mg3V2S8, comprising [VS4]3– tetrahedral units, identified by using density functional theory coupled with an evolutionary structure-predicting algorithm. The structure is verified experimentally via X-ray pair distribution function analysis. The voltage associated with the competing conversion reaction to form MgS plus V metal directly is similar to that of intermediate formation, resulting in two competing reaction pathways. Partial reversibility is seen to re-form the V5+ and S2– containing intermediate on charging instead of VS4. This work showcases the possibility of developing a family of transition metal polychalcogenides functioning via coupled cationic–anionic redox processes as a potential way of achieving higher capacities for MIBs.

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Abstract: The spin state of the Prussian blue analogue FeIIPtIV(CN)6 is investigated in response to temperature, pressure, and X-ray irradiation. While cooling to 10 K maintains the high-spin state of FeII, compression at ambient temperature induces a first-order spin-crossover (SCO) transition with a small hysteresis loop (p↑ = 0.8 GPa, p↓ = 0.6 GPa). In addition, the high-spin to low-spin transition can be initiated at lower pressure through increased X-ray irradiation. Our study highlights a cooperative SCO with moderate pressure in a porous Prussian blue analogue.