Simon Cassidy

Abstract:

The contrasting crystal and magnetic structures of four related iron oxide chalcogenides are reported. Ae₂FeO₃CuCh (Ae = Ca, Sr; Ch = S, Se) all crystallize in the Sr₂GaO₃CuS structure with alkaline earth iron oxide layers containing double layers of linked FeO₅ square pyramids containing Fe³⁺ ions separated by antifluorite-type [Cu₂Ch₂]^2- layers. Structural distortions occur below room temperature when the small Ca²⁺ ions are present, and these involve cooperative tilting of the FeO₅ square pyramids. Magnetic reflections present in the diffraction patterns can be indexed using either √2a × √2a × c or √2a × √2a × 2c expansions of the nuclear cell with nearest-neighbor Fe³⁺ moments coupling antiferromagnetically and with temperature-dependent orientations relative to the crystallographic directions. The magnetic structures of these compounds are subtly different in detail, partly on account of the low directional preference of the high-spin d⁵ Fe³⁺ moments.

Abstract:

Electron exchange communication between nitroxide radical sites localized along polymer backbones creates a compelling platform for spin electronics, resistive memory, and optoelectronics. While radical site proximity, chain flexibility, and local ordering form the basis for this communication, how site-to-site spacer structure governs bulk redox charge transfer remains an open question. Herein, epoxide-cyclic anhydride ring-opening copolymerization produces TEMPO-functional radical polyesters, where strictly alternating enchainment installs a radical at every repeat unit while anhydride comonomer varies spacer structure from flexible aliphatic through alicyclic, bicyclic, and semiaromatic. SQUID magnetometry and EPR spectroscopy confirm radical contents of 86-98%; except for the thioether-containing polyester, where sulfur-specific quenching occurs. Density functional theory calculations reveal that rigid aromatic spacers position radical sites closer than flexible aliphatic ones of comparable through-bond atom counts. However, solid-state electrical conductivity measurements demonstrate that glass transition is the primary determinant of bulk charge transport, regardless of whether it is set through spacer flexibility, blending, or block copolymerization.

Abstract:

Hybrid materials, which combine inorganic and molecular components, often exhibit structural flexibility that enables unusual functional responses. Among them, Prussian blue analogs (PBAs) are a promising class for post-lithium battery technologies. Here, we show that nonequilibrium transformation processes govern the charge-storage mechanism of a PBA electrode, K₂Mn[Fe(CN)₆]. Ostensibly, this behavior mirrors that observed in high-rate cycling of conventional cathodes such as LiFePO₄ yet arises here for fundamentally different reasons-namely, low elastic moduli and cooperative distortions inherent to the hybrid framework. Using operando x-ray absorption spectroscopy with Metropolis matrix factorization and x-ray diffraction, we show that framework flexibility limits transport kinetics and promotes collective, metastable pathways. Our results not only highlight various directions for PBA cathode optimization but also suggest a broader relevance of nonequilibrium mechanisms for mass transport in hybrid materials beyond PBAs alone.

Abstract:

Iron-based superconductors have attracted much attention for their high superconducting temperatures and high upper critical fields, which make them promising candidates for application as well as fundamentally important for our understanding of superconductivity. One feature of these superconductors is their ability to intercalate and deintercalate species from between their iron-containing layers, something not available in cuprate high-temperature superconductors or niobium-based conventional superconductors used in technologies. This provides an opportunity for switchable changes in the superconducting properties as a function of chemical conditions, but the resulting structures are often hard to characterize due to loss of crystallinity and sometimes the formation of multiphase products. Here, we explore both the synthesis and decomposition of potassium and ammonia-intercalated iron selenide superconductors through in situ powder X-ray diffraction. We report a complete phase diagram including two new solution-stable ammonia-rich phases and several metastable forms. We give accurate characterization of the reported ammonia-poor forms using a combination of neutron and X-ray powder diffraction, using an innovative supercell approach to describe the phase breadth within the samples. These results give rare insight into stepwise changes occurring in solids along multiple reaction pathways, which demonstrate the importance of in situ diffraction techniques.

Abstract:

We use in situ synchrotron X-ray diffraction measurements to monitor the solvothermal crystallization mechanism of the aperiodic metal–organic framework TRUMOF-1. Following an initial incubation period, TRUMOF-1 forms as a metastable intermediate that subsequently transforms into an ordered product with triclinic crystal symmetry. We determine the structure of this ordered phase, which we call msw-TRUMOF-1, and show that it is related to TRUMOF-1 through topotactic reorganization of linker occupancies. Our results imply that the connectivity of TRUMOF-1 can be reorganized, as required for data storage and manipulation applications.