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Metal–organic frameworks (MOFs) provide exceptional chemical tunability and have recently been demonstrated to exhibit electrical conductivity and related functional electronic properties. The kagomé lattice is a fruitful source of novel physical states of matter, including the quantum spin liquid (in insulators) and Dirac fermions (in metals). Small-bandgap kagomé materials have the potential to bridge quantum spin liquid states and exhibit phenomena such as superconductivity but remain exceptionally rare. Here we report a structural, thermodynamic, and transport study of the two-dimensional kagomé metal–organic frameworks Ni3(HIB)2and Cu3(HIB)2(HIB = hexaiminobenzene). Magnetization measurements yield Curie constants of 0.989 emu K (mol Ni)−1Oe–1and 0.371 emu K (mol Cu)−1Oe–1, respectively, close to the values expected for ideal S= 1 Ni2+and S= 1/2Cu2+. Weiss temperatures of −10.6 and −14.3 K indicate net weak mean field antiferromagnetic interactions between ions. Electrical transport measurements reveal that both materials are semiconducting, with gaps (Eg) of 22.2 and 103 meV, respectively. Specific heat measurements reveal a large T-linear contribution γ of 148(4) mJ mol-fu–1K–2in Ni3(HIB)2with only a gradual upturn below ∼5 K and no evidence of a phase transition to an ordered state down to 0.1 K. Cu3(HIB)2also lacks evidence of a phase transition above 0.1 K, with a substantial, field-dependent, magnetic contribution below ∼5 K. Despite them being superficially in agreement with the expectations of magnetic frustration and spin liquid physics, we ascribe these observations to the stacking faults found from a detailed analysis of synchrotron X-ray diffraction data. At the same time, our results demonstrate that these MOFs exhibit localized magnetism with simultaneous proximity to a metallic state, thus opening up opportunities to explore the connection between the insulating and metallic ground states of kagomé materials in a highly tunable chemical platform.

Kagomé lattice magnets have emerged as a versatile platform on which to discover and explore the underlying physics of quantum-spin liquids and related states of matter, although experimental examples of ideal kagomé lattices remain rare. Here we report that Na2Ti3Cl8is an ideal realization of an insulating S= 1 kagomé magnet. This material undergoes a discrete two-step trimerization upon cooling, transforming from a centrosymmetric, paramagnetic high-temperature (HT) R3mphase to noncentrosymmetric, polar, and trimerized intermediate- (IT) and low-temperature (LT) R3mphases via two successive first-order phase transitions. Symmetry mode decomposition analysis shows that trimerization requires activation of the proper polar order parameter Γ2–and that this mode becomes active at the HT → IT phase transition. The magnitude of this order parameter approximately doubles at the IT → LT transition, with possible activation of a second polar mode, corresponding to Na2and Ti3Cl8displacing layers toward each other, at the IT → LT transition. Specific heat measurements reveal comparable changes in entropy between the LT → IT transition, 18.6(1.0) J (mol of f.u.)−1K–1, and the IT → LT transition, 16.8(1.0) J (mol of f.u.)−1K–1, demonstrating loss of the magnetic degrees of freedom and constraining possible models for the magnetic and electronic structures of the IT and LT phases. Thus, Na2Ti3Cl8demonstrates a novel mechanism to obtain polar structures driven by geometrically frustrated lattices and metal–metal bonding and highlights the rich physics arising from kagomé lattice materials.

Three new salt inclusion compounds (CsX)Cu5O2(PO4)2(X= Cl, Br, I), phosphate analogues of the kagomé mineral averievite, are reported. Their crystal structures are composed of trigonal networks of corner-sharing OCu4anion-centered tetrahedra, forming capped-kagomé planes, which can also be regarded as two-dimensional slices along the [111] direction of a pyrochlore lattice. Magnetization and heat capacity measurements reveal strong geometric frustration of this network and complex magnetic behavior. X-ray and neutron diffraction studies show that all three compounds undergo a trigonal-to-monoclinic phase transition upon cooling, with a first-order phase transition seen in CsBr and CsI analogues. Along with the previously reported (CsCl)Cu5O2(VO4)2, these three new compounds belong to a large family of OCu4-based networks, which are a playground for studying frustrated quantum magnetism.

Among oxide compounds with direct metal–metal bonding, the Y5Mo2O12(A5B2O12) structural family of compounds has a particularly intriguing low-dimensional structure due to the presence of bioctahedral B2O10dimers arranged in one-dimensional edge-sharing chains along the direction of the metal–metal bonds. Furthermore, these compounds can have a local magnetic moment due to the noninteger oxidation state (+4.5) of the transition metal, in contrast to the conspicuous lack of a local moment that is commonly observed when oxide compounds with direct metal–metal bonding have integer oxidation states resulting from the lifting of orbital degeneracy typically induced by the metal–metal bonding. Although a monoclinic C2/mstructure has been previously proposed for Ln5Mo2O12(Ln= La–Lu and Y) members of this family based on prior single crystal diffraction data, it is found that this structural model misses many important structural features. On the basis of synchrotron powder diffraction data, it is shown that the C2/mmonoclinic unit cell represents a superstructure relative to a previously unrecognized orthorhombic Immmsubcell and that the superstructure derives from the ordering of interchangeable Mo2O10and LaO6building blocks. The superstructure for this reason is typically highly faulted, as evidenced by the increased breadth of superstructure diffraction peaks associated with a coherence length of 1–2 nm in the c* direction. Finally, it is shown that oxygen vacancies can occur when Ln= La, producing an oxygen deficient stoichiometry of La5Mo2O11.55and an approximately 10-fold reduction in the number of unpaired electrons due to the reduction of the average Mo valence from +4.5 to +4.05, a result confirmed by magnetic susceptibility measurements. This represents the first observation of oxygen vacancies in this family of compounds and provides an important means of continuously tuning the magnetic interactions within the one-dimensional octahedral chains of this system.

A new misfit layered compound with the stoichiometry (EuS)1+δ(NbSe2)2(δ ≈ 0.13) has been successfully synthesized. High-resolution transmission electron microscopy and powder X-ray diffraction confirm the misfit structure with (EuS)–(EuS) spacing of 18.30(1) Å. Magnetization, electrical resistivity, heat capacity, and thermal transport measurements show that the material is a heavily doped semiconductor or poor metal with a low thermal conductivity of ∼1 W/m K and an antiferromagnetic ordering transition at TN= 4.7 K. In contrast to the parent materials, the misfit is neither ferromagnetic nor superconducting down to T= 0.4 K. We find evidence of a field-driven transition to a ferromagnetic state due to reorientation of ferromagnetic EuS layers at μoH= 0.5 T at T= 2 K.

We have identified six new 4d4and 4d5compounds with isolated RuCl6octahedra, with formulas (HMA)4RuCl6·Cl (1; MA = methylamine), (HGly)4RuCl6·Cl (2; gly = glycine), (HGly)3RuCl6·2H2O (3), (NH4)2RuCl6(4), (HPy)2RuCl6(5; py = pyridine), and H2(4,4′-bpy)RuCl6(6; 4,4′-bpy = 4,4′-bipyridine). We find that the temperature-dependent magnetization is well described by single-ion physics in the presence of spin–orbit coupling and negligible superexchange interactions. Further, we find that many compounds in the literature are also well described by single-ion physics, and our results demonstrate the importance of considering single-ion physics when evaluating candidate geometric frustrated magnets in the presence of spin–orbit coupling.