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The quantum spin liquid is a highly entangled magnetic state characterized by the absence of static magnetism in its ground state. Instead, the spins fluctuate in a highly correlated way down to the lowest temperatures. Quantum spin liquids are very rare and are confined to a few specific cases where the interactions between the magnetic ions cannot be simultaneously satisfied (known as frustration). Lattices with magnetic ions in triangular or tetrahedral arrangements, which interact via isotropic antiferromagnetic interactions, can generate such a frustration. Three-dimensional isotropic spin liquids have mostly been sought in materials where the magnetic ions form pyrochlore or hyperkagome lattices. Here we present a three-dimensional lattice called the hyper-hyperkagome that enables spin liquid behaviour and manifests in the compound PbCuTe 2 O 6 . Using a combination of experiment and theory, we show that this system exhibits signs of being a quantum spin liquid with no detectable static magnetism together with the presence of diffuse continua in the magnetic spectrum suggestive of fractional spinon excitations. Quantum spin liquids have magnetic moments that do not form magnetic order even as the temperature approaches zero, leading to the dominance of quantum fluctuations. Chillal et al. present evidence that the hyper-hyperkagome lattice of PbCuTe 2 O 6 hosts a three-dimensional quantum spin liquid.

Mixed-valent transition metal compounds display complex structural, electronic and magnetic properties which can often be exquisitely tuned. Here the charge-ordered state of stoichiometric CaFe 3 O 5 is probed using neutron powder diffraction, Monte Carlo simulation and symmetry analysis. Magnetic ordering is dominated by the formation of ferromagnetic Fe 3+ –Fe 2+ –Fe 3+ trimers which are evident above the magnetic ordering transition. Between T N  = 289 K and 281 K an incommensurate magnetically ordered phase develops due to magnetic frustration, but a spin Jahn-Teller distortion lifts the frustration and enables the magnetic ordering to lock in to a charge-ordered commensurate state at lower temperatures. Stoichiometric CaFe 3 O 5 exhibits single phase behaviour throughout and avoids the phase separation into two distinct crystallographic phases with different magnetic structures and Fe valence distributions reported recently, which likely occurs due to partial Fe 2+ for Ca 2+ substitution. This underlines the sensitivity of the magnetism and chemistry of these mixed-valent systems to composition. A significant challenge in understanding the behaviour of strongly correlated materials is their sensitivity to composition. Here the authors show that stoichiometric CaFe 3 O 5 avoids the electronic phase separation reported in a non-stoichiometric sample, and also exhibits an incommensurate magnetic phase.

Supramolecular interactions are fundamental to host–guest binding in many chemical and biological processes. Direct visualization of such supramolecular interactions within host–guest systems is extremely challenging, but crucial to understanding their function. We report a comprehensive study that combines neutron scattering, synchrotron X-ray and neutron diffraction, and computational modelling to define the detailed binding at a molecular level of acetylene, ethylene and ethane within the porous host NOTT-300. This study reveals simultaneous and cooperative hydrogen-bonding, π ··· π stacking interactions and intermolecular dipole interactions in the binding of acetylene and ethylene to give up to 12 individual weak supramolecular interactions aligned within the host to form an optimal geometry for the selective binding of hydrocarbons. We also report the cooperative binding of a mixture of acetylene and ethylene within the porous host, together with the corresponding breakthrough experiments and analysis of adsorption isotherms of gas mixtures. Gas sorption and separation in porous materials is dependent on the host–guest binding within any given system. Now, the molecular details of cooperative binding between small hydrocarbons and a metal–organic framework, NOTT-300, at multiple sites have been elucidated by complementary scattering and diffraction techniques. This material is also capable of separating C 1 and C 2 hydrocarbons under ambient conditions.

Altermagnets are a novel class of magnetic materials, where magnetic order is staggered both in coordinate and momentum space. The metallic rutile oxide RuO 2 , long believed to be a textbook Pauli paramagnet, recently emerged as a putative workhorse altermagnet when resonant X-ray and neutron scattering studies reported nonzero magnetic moments and long-range collinear order. While some experiments seem consistent with altermagnetism, magnetic order in RuO 2 remains controversial. We show that RuO 2 is nonmagnetic, both in bulk and thin film. Muon spectroscopy complemented by density-functional theory finds at most 1.14 × 10 −4   μ B /Ru in bulk and at most 7.5 × 10 −4   μ B /Ru in 11 nm epitaxial films, at our spectrometers’ detection limit, and dramatically smaller than previously reported neutron results that were used to rationalize altermagnetic behavior. Our own neutron diffraction measurements on RuO 2 single crystals identify multiple scattering as the source for the false signal in earlier studies.

Preparing materials which simultaneously exhibit spontaneous magnetic and electrical polarisations is challenging as the electronic features which are typically used to stabilise each of these two polarisations in materials are contradictory. Here we show that by performing low-temperature cation-exchange reactions on a hybrid improper ferroelectric material, Li 2 SrTa 2 O 7 , which adopts a polar structure due to a cooperative tilting of its constituent TaO 6 octahedra rather than an electronically driven atom displacement, a paramagnetic polar phase, MnSrTa 2 O 7 , can be prepared. On cooling below 43 K the Mn 2+ centres in MnSrTa 2 O 7 adopt a canted antiferromagnetic state, with a small spontaneous magnetic moment. On further cooling to 38 K there is a further transition in which the size of the ferromagnetic moment increases coincident with a decrease in magnitude of the polar distortion, consistent with a coupling between the two polarisations. Fabricating materials with simultaneously spontaneous magnetic and electrical polarisations is challenging due to contradictory electronic features. Here, the authors report a synthesis path toward a perovskite MnSrTa 2 O 7 by performing low-temperature cation-exchange reactions on Li 2 SrTa 2 O 7 .

(Ba,K)BiO 3 constitute an interesting class of superconductors, where the remarkably high superconducting transition temperature T c of 30 K arises in proximity to charge density wave order. However, the precise mechanism behind these phases remains unclear. Here, enabled by high-pressure synthesis, we report superconductivity in (Ba,K)SbO 3 with a positive oxygen–metal charge transfer energy in contrast to (Ba,K)BiO 3 . The parent compound BaSbO 3− δ shows a larger charge density wave gap compared to BaBiO 3 . As the charge density wave order is suppressed via potassium substitution up to 65%, superconductivity emerges, rising up to T c  = 15 K. This value is lower than the maximum T c of (Ba,K)BiO 3 , but higher by more than a factor of two at comparable potassium concentrations. The discovery of an enhanced charge density wave gap and superconductivity in (Ba,K)SbO 3 indicates that strong oxygen–metal covalency may be more essential than the sign of the charge transfer energy in the main-group perovskite superconductors. High-pressure synthesis is used to stabilize superconducting (Ba,K)SbO 3 , whose properties provide a fresh perspective on the origin of superconductivity in these types of materials.

Understanding the mechanism of gas-sorbent interactions is of fundamental importance for the design of improved gas storage materials. Here we report the binding domains of carbon dioxide and acetylene in a tetra-amide functionalized metal-organic framework, MFM-188, at crystallographic resolution. Although exhibiting moderate porosity, desolvated MFM-188a exhibits exceptionally high carbon dioxide and acetylene adsorption uptakes with the latter (232 cm 3  g −1 at 295 K and 1 bar) being the highest value observed for porous solids under these conditions to the best of our knowledge. Neutron diffraction and inelastic neutron scattering studies enable the direct observation of the role of amide groups in substrate binding, representing an example of probing gas-amide binding interactions by such experiments. This study reveals that the combination of polyamide groups, open metal sites, appropriate pore geometry and cooperative binding between guest molecules is responsible for the high uptakes of acetylene and carbon dioxide in MFM-188a. Understanding the mechanism of gas-sorbent interactions at a molecular level is important for the design of improved gas storage materials. Here, the authors study the binding domains of carbon dioxide and acetylene in a tetra-amide functionalized metal-organic framework at crystallographic resolution.

Transition metal nitrides are an important class of materials with applications as abrasives, semiconductors, superconductors, Li-ion conductors, and thermoelectrics. However, high oxidation states are difficult to attain as the oxidative potential of dinitrogen is limited by its high thermodynamic stability and chemical inertness. Here we present a versatile synthesis route using azide-mediated oxidation under pressure that is used to prepare the highly oxidised ternary nitride Ca 4 FeN 4 containing Fe 4+ ions. This nitridometallate features trigonal-planar [FeN 3 ] 5− anions with low-spin Fe 4+ and antiferromagnetic ordering below a Neel temperature of 25 K, which are characterised by neutron diffraction, 57 Fe-Mössbauer and magnetisation measurements. Azide-mediated high-pressure synthesis opens a way to the discovery of highly oxidised nitrides. High-valent metal nitrides are difficult to stabilise due to the high thermodynamic stability and chemical inertness of N 2 . Here, the authors employ a large volume press to prepare an iron(IV) nitridoferrate Ca 4 Fe IV N 4 from Fe 2 N and Ca 3 N 2 via azide-mediated oxidation under high pressure conditions.

The physics of spin-orbit entangled magnetic moments of 4 d and 5 d transition metal ions on a honeycomb lattice has been much explored in the search for unconventional magnetic orders or quantum spin liquids expected for compass spin models, where different bonds in the lattice favour different orientations for the magnetic moments. Realising such physics with rare-earth ions is a promising route to achieve exotic ground states in the extreme spin-orbit limit; however, this regime has remained experimentally largely unexplored due to major challenges in materials synthesis. Here we report the successful synthesis of powders and single crystals of β -Na 2 PrO 3 , with 4 f 1  Pr 4+   j eff  = 1/2 magnetic moments arranged on a hyperhoneycomb lattice with the same threefold coordination as the planar honeycomb. We find a strongly non-collinear magnetic order with highly dispersive gapped excitations that we argue arise from frustration between bond-dependent, anisotropic off-diagonal exchanges, a compass quantum spin model not explored experimentally so far. Our results show that rare-earth ions on threefold coordinated lattices offer a platform for the exploration of quantum compass spin models in the extreme spin-orbit regime, with qualitatively distinct physics from that of 4 d and 5 d Kitaev materials. Two-dimensional honeycomb lattices of magnetic ions with strong spin-orbit coupling can host a rich variety of magnetic phases, including Kitaev quantum spin liquids. Here, Okuma et al succeeded in synthesising powders and single crystals of β -Na2PrO3, with Praseodymium ions with very strong spin-orbit coupling arranged in a three-dimensional hyperhoneycomb lattice.

Double corundum-related polar magnets are promising materials for multiferroic and magnetoelectric applications in spintronics. However, their design and synthesis is a challenge, and magnetoelectric coupling has only been observed in Ni 3 TeO 6 among the known double corundum compounds to date. Here we address the high-pressure synthesis of a new polar and antiferromagnetic corundum derivative Mn 2 MnWO 6 , which adopts the Ni 3 TeO 6 -type structure with low temperature first-order field-induced metamagnetic phase transitions ( T N  = 58 K) and high spontaneous polarization (~ 63.3 μC·cm −2 ). The magnetostriction-polarization coupling in Mn 2 MnWO 6 is evidenced by second harmonic generation effect, and corroborated by magnetic-field-dependent pyroresponse behavior, which together with the magnetic-field-dependent polarization and dielectric measurements, qualitatively indicate magnetoelectric coupling. Piezoresponse force microscopy imaging and spectroscopy studies on Mn 2 MnWO 6 show switchable polarization, which motivates further exploration on magnetoelectric effect in single crystal/thin film specimens. Double corundum-related polar magnets are promising for multiferroic and magnetoelectric applications in spintronics, but are limited by the challenging design and synthesis. Here the authors report the synthesis of Mn 2 MnWO 6 as well as its appealing multiferroic and magnetoelectric properties.