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Abstract: A variable temperature X-ray total-scattering study of K2IrCl6 reveals compelling evidence for local symmetry breaking in this material. While the average crystal structure remains cubic down to 11 K, consistent with earlier reports, large anisotropic chloride displacements suggest short-range distortions of the IrCl6 octahedra. Pair distribution function analysis confirms that the local structure is better described by a monoclinic P21/n model featuring a mix of in-phase and out-of-phase octahedral tilts. This behavior mirrors observations in related K2MX6 halides, where thermally driven cubic-to-monoclinic transitions occur. High-pressure synchrotron measurements further reveal two structural transitions: cubic Fm3̅m to tetragonal P4/mnc at 12.0 GPa, and tetragonal to monoclinic P21/n at 15.1 GPa. Both transitions are reversible on decompression. Lattice parameter refinements indicate anisotropic compression with the bulk modulus increasing dramatically from 23 GPa in the cubic phase to 121 GPa in the monoclinic structure. These results demonstrate that both temperature reduction and applied pressure drive K2IrCl6 toward lower-symmetry phases. Overall, this study provides the first direct local-structure evidence of symmetry breaking in K2IrCl6 and highlights the complex interplay among pressure, temperature, and local structure in vacancy-ordered double perovskites.

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Abstract: Self-assembly affords simpler synthetic routes to heterostructures compared with manual layer-by-layer stacking, yet controlling interlayer twist angles in a bulk solid remains an outstanding challenge. We report two new single-crystal heterostructures: (Sn2Cl2)(CYS)2SnCl4 (CYS = +NH3(CH2)2S–; Sn_CYS) and (Sn2Cl2)(SeCYS)2SnCl4 (SeCYS = +NH3(CH2)2Se–; Sn_SeCYS) synthesized in solution, with alternating perovskite and intergrowth layers. Notably, compared to the recently reported lead analog, (Pb2Cl2)(CYS)2PbCl4 (Pb_CYS), the tin heterostructures feature a twist between the perovskite and intergrowth layers. We trace this twist to local distortions at the Sn centers, which change the interfacial lattice-matching requirements compared to those of the Pb analog. Electronic band structure calculations show that the striking differences in the relative energies of perovskite- and intergrowth-derived bands in Sn_CYS and Pb_CYS arise from structural and not compositional differences. The structural anisotropy of Sn_CYS is also reflected in a large in-plane photoluminescence linear anisotropy ratio. Interfacial strain further affords differential incorporation of Pb into the perovskite and intergrowth layers of the Sn heterostructures, resulting in redshifted optical absorption onsets. Thus, we posit that local structural distortions may be exploited to manipulate the twist angle and interfacial strain in bulk heterostructures, providing a new handle for tuning the band alignments of bulk quantum-well electronic structures.

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Abstract: We employ a combined computational and experimental approach to systematically assess the hydrostatic properties of methanol–ethanol (MeOH–EtOH) mixtures of varying compositions, with the aim of evaluating their suitability as pressure-transmitting mediums (PTMs). PTMs are essential for enabling the characterization of materials properties at high pressure, perhaps most prominently in the context of diffraction measurements, to provide uniform compression and avoid strain on the sample. Molecular dynamics (MD) simulations indicate that the hydrostatic limit and several structural and dynamic properties of the widely used 4:1 MeOH–EtOH volume ratio do not exhibit any significant deviations from the monotonic trends observed as a function of MeOH content within the mixture. These findings are in agreement with X-ray pair distribution function measurements, which show no peculiar structural behaviour for the 4:1 composition. Experimental measurements of the hydrostatic limit confirm this result and demonstrate that, as previously reported, the role of ethanol is primarily to delay MeOH crystallization. However, we find that the same role can be fulfilled by other small molecules, such as propan-2-ol. Additional simulations of several MeOH–X binary mixtures suggest that these results might hold for a variety of similar mixtures. Thus, our findings indicate that the 4:1 ratio is neither peculiar nor optimal in terms of PTM performance; instead, its popularity seems to be mostly due to the influence of previous literature. Indeed, we find that the 9:1 MeOH–EtOH mixture is characterized by a hydrostatic limit which is superior (by nearly 1 GPa) to that observed for the 4:1 ratio. These findings offer a promising alternative PTM composition which is readily available, and pave the way towards future work aimed at the rational design of novel PTMs.

Open Access

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Abstract: Owing to their vast chemical and structural flexibility, layered double hydroxides (LDHs) are among some of the most promising materials for many catalytic applications. Thermal decomposition below 700 °C leads to the formation of a complex semiamorphous mixed metal oxide (LDO). In this study, the product of calcination of aqueous miscible organic solvent-treated AMO-[Mg0.70Al0.30(OH)2](CO3)0.15·yH2O·zEtOH at 600 °C (AMO-Mg2.33Al LDO) has been investigated using a synergistic combination of high-resolution synchrotron X-ray and neutron scattering techniques, as well as high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), solid-state NMR (ssNMR), and thermogravimetric analysis coupled with mass spectrometry (TGA-MS). The local and extended structure of AMO-Mg2.33Al LDO has been modeled by reciprocal and real space X-ray and neutron scattering analyses and is consistent with a modified rock salt structure consisting of octahedrally coordinated layers containing a small number of vacancies and the tetrahedrally coordinated Al3+ sites in contrast to previous reports.

Open Access

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Abstract: Layered double hydroxides (LDH) based on transition metals are highly flexible in tailoring their dimensionality, lattice, and electronic structures, making them promising candidates as multifunctional 2D materials for the development of clean energy technologies and boosting the use of hydrogen as an energy vector. In this paper, strategic anion substitution in cobalt LDH is an appealing strategy to produce a material with two-fold functionality, electrochemical and magnetocaloric response, offering a sustainable alternative to existing electrocatalysts and cryogenic refrigerants. It is unambiguously demonstrated that (poly)oxomolybdate-based specimens interleave in Co LDH nanosheets up to a Co:Mo = 1:0.4 ratio, leading to an interstratified material. This intercalation greatly benefits the kinetics of the oxygen evolution reaction for H2 production, boosting the catalytic sites due to the expansion of the interlayer space, induced by the bulky molybdates which also partially modify the Co oxidation state of αCo(OH)2 nanolayers, favoring charge transfer. In parallel, the interleaved Mo species strengthen superexchange interactions compared with pristine α-Co(OH)2, effectively adjusting the operating temperature toward the liquid hydrogen range (2030 K). This specific temperature range allows to fill a critical gap in magnetocaloric materials, as few systems can simultaneously achieve both large magnetic entropy changes and structural stability.