Explore the vast range of titles available.

Ammonia is a critical chemical in agriculture and industry that is produced on a massive scale via the Haber–Bosch process 1 . The environmental impact of this process, which uses methane as a fuel and feedstock for hydrogen, has motivated the need for more sustainable ammonia production 2 – 5 . However, many strategies that use renewable hydrogen are not compatible with existing methods for ammonia separation 6 – 9 . Given their high surface areas and structural and chemical versatility, metal–organic frameworks (MOFs) hold promise for ammonia separations, but most MOFs bind ammonia irreversibly or degrade on exposure to this corrosive gas 10 , 11 . Here we report a tunable three-dimensional framework that reversibly binds ammonia by cooperative insertion into its metal–carboxylate bonds to form a dense, one-dimensional coordination polymer. This unusual adsorption mechanism provides considerable intrinsic thermal management 12 , and, at high pressures and temperatures, cooperative ammonia uptake gives rise to large working capacities. The threshold pressure for ammonia adsorption can further be tuned by almost five orders of magnitude through simple synthetic modifications, pointing to a broader strategy for the development of energy-efficient ammonia adsorbents. A three-dimensional metal–organic framework reversibly binds ammonia by cooperative insertion into its metal–linker bonds to form a dense, one-dimensional coordination polymer, enabling high-capacity ammonia uptake with intrinsic thermal management.

The size-dependent and shape-dependent characteristics that distinguish nanoscale materials from bulk solids arise from constraining the dimensionality of an inorganic structure 1 – 3 . As a consequence, many studies have focused on rationally shaping these materials to influence and enhance their optical, electronic, magnetic and catalytic properties 4 – 6 . Although a select number of stable clusters can typically be synthesized within the nanoscale regime for a specific composition, isolating clusters of a predetermined size and shape remains a challenge, especially for those derived from two-dimensional materials. Here we realize a multidentate coordination environment in a metal–organic framework to stabilize discrete inorganic clusters within a porous crystalline support. We show confined growth of atomically defined nickel( ii ) bromide, nickel( ii ) chloride, cobalt( ii ) chloride and iron( ii ) chloride sheets through the peripheral coordination of six chelating bipyridine linkers. Notably, confinement within the framework defines the structure and composition of these sheets and facilitates their precise characterization by crystallography. Each metal( ii ) halide sheet represents a fragment excised from a single layer of the bulk solid structure, and structures obtained at different precursor loadings enable observation of successive stages of sheet assembly. Finally, the isolated sheets exhibit magnetic behaviours distinct from those of the bulk metal halides, including the isolation of ferromagnetically coupled large-spin ground states through the elimination of long-range, interlayer magnetic ordering. Overall, these results demonstrate that the pore environment of a metal–organic framework can be designed to afford precise control over the size, structure and spatial arrangement of inorganic clusters. The pore space in the metal–organic framework Zr 6 O 4 (OH) 4 (bpydc) 6 can be used as a scaffold to grow precisely defined atomically thick sheets of metal halide materials, taking advantage of multiple binding sites to direct complexation of the metal ions; these metal halide nanosheets fill the size gap between discrete molecular magnets and bulk magnetic materials, with potentially unusual magnetic properties arising from this size regime.

The layered square-planar nickelates, Nd n +1 Ni n O 2 n +2 , are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd 6 Ni 5 O 12 thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the n  = 3 Ruddlesden-Popper compound, Nd 4 Ni 3 O 10 , and subsequent reduction to the square-planar phase, Nd 4 Ni 3 O 8 . We synthesize our highest quality Nd 4 Ni 3 O 10 films under compressive strain on LaAlO 3 (001), while Nd 4 Ni 3 O 10 on NdGaO 3 (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in Nd 4 Ni 3 O 10 on SrTiO 3 (001). Films reduced on LaAlO 3 become insulating and form compressive strain-induced c -axis canting defects, while Nd 4 Ni 3 O 8 films on NdGaO 3 are metallic. This work provides a pathway to the synthesis of Nd n +1 Ni n O 2 n +2 thin films and sets limits on the ability to strain engineer these compounds via epitaxy. The discovery of superconductivity in the infinite-layer nickelates reignites an interest in the nickelates as cuprate analogues. Here, the authors investigate the role of epitaxial strain in the synthesis of the n=3 layered nickelate, Nd 4 Ni 3 O 8 .

In the Acknowledgements section of this article the grant number relating to NSF was incorrectly given as DMR 2045826 and should have been DMR-2045826. The original article has been corrected.

Nanoscale materials display emergent electronic and magnetic properties that are highly correlated to their size and shape. The work in this dissertation describes efforts to synthesize nanoscale materials including molecular clusters and layered solids with exquisite control over size and shape. Particular emphasis is paid to studying the magnetic properties of these materials and how these properties relate to those of their bulk counterparts. Chapter 1 introduces basic concepts of magnetism including interactions between spin centers. The nature of these interactions dictates many magnetic properties and is essential in understanding the behavior of the materials contained within this dissertation. Further emphasis is paid to the effects of quantum confinement on magnetic properties, and major advances in this field are provided. Finally, general strategies to purposefully control the size and shape of confined materials are highlighted along with illustrative examples. Chapter 2 describes the synthesis of atomically precise metal(II) halide clusters (M19X38, M = Fe, Co, Ni; X = Cl, Br) using the metal–organic framework Zr6O4(OH)4(bpydc)6 (bpydc2− = 2,2′-bipyridine-5,5′-dicarboxylate) as a template. Single-crystal X-ray diffraction techniques reveal that these clusters represent fragments excised from a single layer of the bulk, layered structure of the corresponding parent metal halide. Magnetometry and Mössbauer spectroscopy are then used the probe the magnetic behavior of these clusters. Remarkably, the intralayer ferromagnetic magnetic exchange pathways characteristic of the bulk materials are conserved in the clusters, leading to the isolation of high-spin magnetic ground states as well as superparamagnetism in the case of Fe19Cl38 clusters. While Chapter 2 focuses on the magnetic behavior of metal(II) halide clusters with predominantly ferromagnetic exchange coupling between spin centers, Chapter 3 focuses on the behavior of clusters with antiferromagnetic exchange coupling. In particular, the magnetic properties of a Mn19Br38 cluster are studied. In this cluster, spin centers are arranged on a triangular lattice. This topology combined with antiferromagnetic exchange correlations can be expected to lead to a geometrically frustrated magnetic ground state. Magnetometry as well as computational methods are used to probe the magnetic behavior of this cluster and confirm a highly frustrated ground state spin configuration. Chapter 4 focuses on the synthesis of a promising new metal–organic framework Cr(pz)2 (pz = pyrazine) that has exceptional magnetic properties. The framework is a layered material constructed of a square net of Cr(II) cations that are bridged by singly reduced pz radical anions. The framework is ferrimagnetic with an ordering temperature of 242 °C and a coercive field of 0.75 T at room temperature, extremely impressive parameters for a metal–organic system. Currently available synthetic methods, however, only yield a microcrystalline powder that is not amenable to in depth characterization techniques. This chapter therefore explores potential synthetic routes toward single-crystals or thin films of Cr(pz)2 using molecular precursors.

Ammonia is a critical chemical in agriculture and industry that is produced on a massive scale via the Haber-Bosch process . The environmental impact of this process, which uses methane as a fuel and feedstock for hydrogen, has motivated the need for more sustainable ammonia production . However, many strategies that use renewable hydrogen are not compatible with existing methods for ammonia separation . Given their high surface areas and structural and chemical versatility, metal-organic frameworks (MOFs) hold promise for ammonia separations, but most MOFs bind ammonia irreversibly or degrade on exposure to this corrosive gas . Here we report a tunable three-dimensional framework that reversibly binds ammonia by cooperative insertion into its metal-carboxylate bonds to form a dense, one-dimensional coordination polymer. This unusual adsorption mechanism provides considerable intrinsic thermal management , and, at high pressures and temperatures, cooperative ammonia uptake gives rise to large working capacities. The threshold pressure for ammonia adsorption can further be tuned by almost five orders of magnitude through simple synthetic modifications, pointing to a broader strategy for the development of energy-efficient ammonia adsorbents.

The layered square-planar nickelates, Nd\\(_{n+1}\\)Ni\\(_{n}\\)O\\(_{2n+2}\\), are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd\\(_{6}\\)Ni\\(_{5}\\)O\\(_{12}\\) thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the \\(n=3\\) Ruddlesden-Popper compound, Nd\\(_{4}\\)Ni\\(_{3}\\)O\\(_{10}\\), and subsequent reduction to the square-planar phase, Nd\\(_{4}\\)Ni\\(_{3}\\)O\\(_{8}\\). We synthesize our highest quality Nd\\(_{4}\\)Ni\\(_{3}\\)O\\(_{10}\\) films under compressive strain on LaAlO\\(_{3}\\) (001), while Nd\\(_{4}\\)Ni\\(_{3}\\)O\\(_{10}\\) on NdGaO\\(_{3}\\) (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in Nd\\(_{4}\\)Ni\\(_{3}\\)O\\(_{10}\\) on SrTiO\\(_{3}\\) (001). Films reduced on LaAlO\\(_{3}\\) become insulating and form compressive strain-induced \\(c\\)-axis canting defects, while Nd\\(_{4}\\)Ni\\(_{3}\\)O\\(_{8}\\) films on NdGaO\\(_{3}\\) are metallic. This work provides a pathway to the synthesis of Nd\\(_{n+1}\\)Ni\\(_{n}\\)O\\(_{2n+2}\\) thin films and sets limits on the ability to strain engineer these compounds via epitaxy.

The discovery of superconductivity in square-planar nickelates has offered a rich materials platform to explore the origins of cuprate-like superconductivity. Experimental investigations however have largely been limited to the infinite-layer \\(R\\)NiO\\(_2\\) (\\(R\\)=rare-earth) nickelates. Here, we construct a phase diagram of multi-layer square-planar Nd\\(_{n+1}\\)Ni\\(_n\\)O\\(_{2n+2}\\) compounds and discover signatures of superconductivity for \\(n\\) = 4 - 8. Upon decreasing the dimensionality \\(n\\), the superconducting anisotropy evolves due to 4\\(f\\) electron effects, and electronic structure characteristics approach cuprate-like behavior. Magnetic fluctuations persist from within the superconducting regime and into the over-doped, non-superconducting regime. Remarkably, the superconducting regime overlaps with that of chemically-doped infinite-layer nickelates, demonstrating underlying commonalities and distinct differences across varying structural realizations of square-planar nickelates. Our work establishes this layered template for creating new nickel-based superconductors.

Layered perovskites such as the Dion-Jacobson, Ruddlesden-Popper, and Aurivillius families host a wide range of correlated electron phenomena, from high-temperature superconductivity to multiferroicity. Here we report a new family of layered perovskites, realized through topotactic oxygen intercalation of La_{n+1}Ni_{n}O_{3n+1} (n=1-4) Ruddlesden-Popper nickelate thin films grown by ozone-assisted molecular-beam epitaxy. Post-growth ozone annealing induces a large c-axis expansion - 17.8% for La_{2}NiO_{4} (n=1) - that monotonically decreases with increasing n. Surface X-ray diffraction coupled with Coherent Bragg Rod Analysis reveals that this structural expansion arises from the intercalation of approximately 0.7 oxygen atoms per formula unit into interstitial sites within the rock salt spacer layers. The resulting structures exhibit a spacer layer composition intermediate between that of the Ruddlesden-Popper and Aurivillius phases, defining a new class of layered perovskites. Oxygen-intercalated nickelates exhibit metallicity and significantly enhanced nickel-oxygen hybridization, a feature linked to high-temperature superconductivity. Our work establishes topotactic oxidation as a powerful synthetic approach to accessing highly oxidized, metastable phases across a broad range of layered oxide systems, offering new platforms to tune properties via spacer-layer chemistry.

ObjectivesTo assess the long-term effects of arthroscopic partial meniscectomy (APM) on the development of radiographic knee osteoarthritis, and on knee symptoms and function, at 5 years follow-up.DesignMulticentre, randomised, participant- and outcome assessor-blinded, placebo-surgery controlled trial.SettingOrthopaedic departments in five public hospitals in Finland.Participants146 adults, mean age 52 years (range 35–65 years), with knee symptoms consistent with degenerative medial meniscus tear verified by MRI scan and arthroscopically, and no clinical signs of knee osteoarthritis were randomised.InterventionsAPM or placebo surgery (diagnostic knee arthroscopy).Main outcome measuresWe used two indices of radiographic knee osteoarthritis (increase in Kellgren and Lawrence grade ≥1, and increase in Osteoarthritis Research Society International (OARSI) atlas radiographic joint space narrowing and osteophyte sum score, respectively), and three validated patient-relevant measures of knee symptoms and function (Western Ontario Meniscal Evaluation Tool (WOMET), Lysholm, and knee pain after exercise using a numerical rating scale).ResultsThere was a consistent, slightly greater risk for progression of radiographic knee osteoarthritis in the APM group as compared with the placebo surgery group (adjusted absolute risk difference in increase in Kellgren-Lawrence grade ≥1 of 13%, 95% CI −2% to 28%; adjusted absolute mean difference in OARSI sum score 0.7, 95% CI 0.1 to 1.3). There were no relevant between-group differences in the three patient-reported outcomes: adjusted absolute mean differences (APM vs placebo surgery), −1.7 (95% CI −7.7 to 4.3) in WOMET, −2.1 (95% CI −6.8 to 2.6) in Lysholm knee score, and −0.04 (95% CI −0.81 to 0.72) in knee pain after exercise, respectively. The corresponding adjusted absolute risk difference in the presence of mechanical symptoms was 18% (95% CI 5% to 31%); there were more symptoms reported in the APM group. All other secondary outcomes comparisons were similar.ConclusionsAPM was associated with a slightly increased risk of developing radiographic knee osteoarthritis and no concomitant benefit in patient-relevant outcomes, at 5 years after surgery.Trial registrationClinicalTrials.gov (NCT01052233 and NCT00549172).