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1. Interplay between core and shell in a RbCoFe@RbNiCo Prussian blue analogue spin transition heterostructure

   https://doi.org/10.1039/D1TC01514A  

   He, Wanhong; Cain, John M.; Meisel, Mark W.; Talham, Daniel R. (January 2021, Journal of Materials Chemistry C)

   null (Ed.)

   A series of core–shell heterostructures consisting of the spin transition Prussian blue analogue Rb a Co b [Fe(CN) 6 ] c · m H 2 O (RbCoFe–PBA) as core with different shell thicknesses of K j Ni k [Co(CN) 6 ] l · n H 2 O (KNiCo-PBA) has been prepared and studied as the cores undergo both thermal and light-induced phase changes. Synchrotron powder diffraction and SQUID magnetometry indicate the intersite cooperativity of the charge transfer coupled spin transition (CTCST) in the RbCoFe–PBA core decreases while the extent of lattice contraction is reduced relative to the uncoated particles. Isothermal relaxation measurements from the photo-induced high-spin (HS) state to the low-spin (LS) ground state of the RbCoFe–PBA core show that the energy barrier of the HS to LS transition dramatically decreases when adding the KNiCo-PBA shells, becoming smaller when the shell is thicker. The RbCoFe–PBA@KNiCo-PBA series is unique because the lattice parameter of KNiCo-PBA grown on the high-spin RbCoFe–PBA core particle is expanded relative to its equilibrium lattice parameter. As a result, the lattice mismatch is relieved during the spin transition. Analysis of the structural microstrain in both core and shell during the CTCST process reveals the different mechanisms by which the heterostructure accommodates the strain. 

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2. Characterizing the spatiotemporal evolution of paramagnetic colloids in time-varying magnetic fields with Minkowski functionals

   https://doi.org/10.1039/D0SM01100B  

   Hilou, Elaa; Joshi, Kedar; Biswal, Sibani Lisa (January 2020, Soft Matter)

   Phase separation processes are widely utilized to assemble complex fluids into novel materials. These separation processes can be thermodynamically driven due to changes in concentration, pressure, or temperature. Phase separation can also be induced with external stimuli, such as magnetic fields, resulting in novel nonequilibrium systems. However, how external stimuli influence the transition pathways between phases has not been explored in detail. Here, we describe the phase separation dynamics of superparamagnetic colloids in time-varying magnetic fields. An initially homogeneous colloidal suspension can transition from a continuous colloidal phase with voids to discrete colloidal clusters, through a bicontinuous phase formed via spinodal decomposition. The type of transition depends on the particle concentration and magnitude of the applied magnetic field. The spatiotemporal evolution of the microstructure during the nucleation and growth period is quantified by analyzing the morphology using Minkowski functionals. The characteristic length of the colloidal systems was determined to correlate with system variables such as magnetic field strength, particle concentration, and time in a power-law scaling relationship. Understanding the interplay between particle concentration and applied magnetic field allows for better control of the phases observed in these magnetically tunable colloidal systems. 

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3. Universally bistable shells with nonzero Gaussian curvature for two-way transition waves

   https://doi.org/10.1038/s41467-020-20698-9  

   Vasios, Nikolaos; Deng, Bolei; Gorissen, Benjamin; Bertoldi, Katia (January 2021, Nature Communications)

   Abstract Multi-welled energy landscapes arising in shells with nonzero Gaussian curvature typically fade away as their thickness becomes larger because of the increased bending energy required for inversion. Motivated by this limitation, we propose a strategy to realize doubly curved shells that are bistable for any thickness. We then study the nonlinear dynamic response of one-dimensional (1D) arrays of our universally bistable shells when coupled by compressible fluid cavities. We find that the system supports the propagation of bidirectional transition waves whose characteristics can be tuned by varying both geometric parameters as well as the amount of energy supplied to initiate the waves. However, since our bistable shells have equal energy minima, the distance traveled by such waves is limited by dissipation. To overcome this limitation, we identify a strategy to realize thick bistable shells with tunable energy landscape and show that their strategic placement within the 1D array can extend the propagation distance of the supported bidirectional transition waves. 

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4. Phase-change materials: The view from the liquid phase and the metallicity parameter

   https://doi.org/10.1557/mrs.2019.207  

   Wei, Shuai; Lucas, Pierre; Angell, C. Austen (September 2019, MRS Bulletin)

   While fast-switching rewritable nonvolatile memory units based on phase-change materials (PCMs) are already in production at major technology companies such as Intel (16–64 GB chips are currently available), an in-depth understanding of the physical factors that determine their success is still lacking. Recently, we have argued for a liquid-phase metal-to-semiconductor transition (M-SC), located not far below the melting point, T m , as essential. The M-SC is itself a consequence of atomic rearrangements that are involved in a fragile-to-strong viscosity transition that controls both the speed of crystallization and the stabilization of the semiconducting state. Here, we review past work and introduce a new parameter, the “metallicity” (inverse of the average Pauling electronegativity of a multicomponent alloy). When T m -scaled temperatures of known M-SCs of Group IV, V, and VI alloys are plotted against their metallicities, the curvilinear plot leads directly to the composition zone of all known PCMs and the temperature interval below T m , where the transition should occur. The metallicity concept could provide guidance for tailoring PCMs. 

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5. Disorder Dynamics in Battery Nanoparticles During Phase Transitions Revealed by Operando Single‐Particle Diffraction

   https://doi.org/10.1002/aenm.202103521  

   Huang, Jason_J; Weinstock, Daniel; Hirsh, Hayley; Bouck, Ryan; Zhang, Minghao; Gorobtsov, Oleg_Yu; Okamura, Malia; Harder, Ross; Cha, Wonsuk; Ruff, Jacob_P_C; et al (February 2022, Advanced Energy Materials)

   Abstract Structural and ion‐ordering phase transitions limit the viability of sodium‐ion intercalation materials in grid scale battery storage by reducing their lifetime. However, the combination of phenomena in nanoparticulate electrodes creates complex behavior that is difficult to investigate, especially on the single‐nanoparticle scale under operating conditions. In this work, operando single‐particle X‐ray diffraction (oSP‐XRD) is used to observe single‐particle rotation, interlayer spacing, and layer misorientation in a functional sodium‐ion battery. oSP‐XRD is applied to Na_2/3[Ni_1/3Mn_2/3]O₂, an archetypal P2‐type sodium‐ion‐positive electrode material with the notorious P2‐O2 phase transition induced by sodium (de)intercalation. It is found that during sodium extraction, the misorientation of crystalline layers inside individual particles increases before the layers suddenly align just prior to the P2‐O2 transition. The increase in the long‐range order coincides with an additional voltage plateau signifying a phase transition prior to the P2‐O2 transition. To explain the layer alignment, a model for the phase evolution is proposed that includes a transition from localized to correlated Jahn–Teller distortions. The model is anticipated to guide further characterization and engineering of sodium‐ion intercalation materials with P2‐O2 type transitions. oSP‐XRD, therefore, opens a powerful avenue for revealing complex phase behavior in heterogeneous nanoparticulate systems. 

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