John P. Sheckelton

Possible valence-bond condensation in the frustrated cluster magnet LiZn2Mo3O8

The emergence of complex electronic behaviour from simple ingredients has resulted in the discovery of numerous states of matter. Many examples are found in systems exhibiting geometric magnetic frustration, which prevents simultaneous satisfaction of all magnetic interactions. This frustration gives rise to complex magnetic properties such as chiral spin structures, orbitally driven magnetism, spin-ice behaviour exhibiting Dirac strings with magnetic monopoles, valence-bond solids and spin liquids. Here we report the synthesis and characterization of LiZn(2)Mo(3)O(8), a geometrically frustrated antiferromagnet in which the magnetic moments are localized on small transition-metal clusters rather than individual ions. By doing so, first-order Jahn-Teller instabilities and orbital ordering are prevented, allowing the strongly interacting magnetic clusters in LiZn(2)Mo(3)O(8) to probably give rise to an exotic condensed valence-bond ground state reminiscent of the proposed resonating valence-bond state. Our results also link magnetism on clusters to geometric magnetic frustration in extended solids, demonstrating a new approach for unparalleled chemical control and tunability in the search for collective, emergent electronic states of matter.

Molecular Quantum Magnetism in LiZn2Mo3O8

Inelastic neutron scattering at low temperatures T≤30  K from a powder of LiZn2Mo3O8 demonstrates this triangular-lattice antiferromagnet hosts collective magnetic excitations from spin-1/2 Mo3O13 molecules. Apparently gapless (Δ<0.2  meV) and extending at least up to 2.5 meV, the low-energy magnetic scattering cross section is surprisingly broad in momentum space and involves one-third of the spins present above 100 K. The data are compatible with the presence of valence bonds involving nearest-neighbor and next-nearest-neighbor spins forming a disordered or dynamic state.

Photoinitiated Reactivity of a Thiolate-Ligated, Spin-Crossover Nonheme {FeNO}7 Complex with Dioxygen

The nonheme iron complex, [Fe(NO)(N3PyS)]BF4, is a rare example of an {FeNO}(7) species that exhibits spin-crossover behavior. The comparison of X-ray crystallographic studies at low and high temperatures and variable-temperature magnetic susceptibility measurements show that a low-spin S = 1/2 ground state is populated at 0-150 K, while both low-spin S = 1/2 and high-spin S = 3/2 states are populated at T > 150 K. These results explain the observation of two N-O vibrational modes at 1737 and 1649 cm(-1) in CD3CN for [Fe(NO)(N3PyS)]BF4 at room temperature. This {FeNO}(7) complex reacts with dioxygen upon photoirradiation with visible light in acetonitrile to generate a thiolate-ligated, nonheme iron(III)-nitro complex, [Fe(III)(NO2)(N3PyS)](+), which was characterized by EPR, FTIR, UV-vis, and CSI-MS. Isotope labeling studies, coupled with FTIR and CSI-MS, show that one O atom from O2 is incorporated in the Fe(III)-NO2 product. The O2 reactivity of [Fe(NO)(N3PyS)]BF4 in methanol is dramatically different from CH3CN, leading exclusively to sulfur-based oxidation, as opposed to NO· oxidation. A mechanism is proposed for the NO· oxidation reaction that involves formation of both Fe(III)-superoxo and Fe(III)-peroxynitrite intermediates and takes into account the experimental observations. The stability of the Fe(III)-nitrite complex is limited, and decay of [Fe(III)(NO2)(N3PyS)](+) leads to {FeNO}(7) species and sulfur oxygenated products. This work demonstrates that a single mononuclear, thiolate-ligated nonheme {FeNO}(7) complex can exhibit reactivity related to both nitric oxide dioxygenase (NOD) and nitrite reductase (NiR) activity. The presence of the thiolate donor is critical to both pathways, and mechanistic insights into these biologically relevant processes are presented.

Dynamic charge disproportionation in the 1D chain material PdTeI

PdTeI features quasi-1D –Pd–Te–Pd–Te– chains with palladium formally in the 3+ oxidation state. Using pair distribution function analysis of X-ray and neutron total scattering data, we find that there is a local charge-density wave arising from the disproportionation of Pd3+ towards Pd2+ in pseudo-square planar, and Pd4+ in pseudo-octahedral, coordination. The magnitude and coherence length for this distortion is small, such that the average structure determined by Bragg diffraction techniques possesses higher symmetry than the local structure at all temperatures. Temperature-dependent resistivity measurements show a transport anomaly at TCDW = 50 K, corresponding to the reduced fluctuations of the charge separated Pd2+ and Pd4+ sites. At higher temperatures, the charge separation is dynamic, with local Pd2+/Pd4+ pairs persisting up to room temperature.

Electronic tunability of the frustrated triangular-lattice cluster magnet LiZn2−xMo3O8

LiZn2Mo3O8 is an electrically insulating geometrically frustrated antiferromagnet in which inorganic Mo3O13 clusters each behaves as a single S = 1/2 unit, with the clusters arranged on a two-dimensional triangular lattice. Prior results have shown that LiZn2Mo3O8 does not exhibit static magnetic order down to at least T = 0.05 K, and instead possesses a valence bond ground state. Here, we show that LiZn2Mo3O8 can be hole doped by oxidation with I2 and subsequent removal of Zn2+ cations to access the entire range of electron count, from one to zero unpaired electrons per site on the triangular lattice. Contrary to expectations, no metallic state is induced; instead, the primary effect is to suppress the number of sites contributing to the condensed valence-bond state. Further, diffraction and pair-distribution function analysis show no evidence for local Jahn–Teller distortions or other deviations from the parent trigonal symmetry as a function of doping or temperature. Taken together, the data and density functional theory calculations indicate that removal of electrons from the magnetic layers favors Anderson localization of the resulting hole and an increase in the electrical band-gap over the formation of a metallic and superconducting state. These results put strong constraints on the chemical conditions necessary to realize metallic states from parent insulating geometrically frustrated antiferromagnets.

Rearrangement of van der Waals stacking and formation of a singlet state at T = 90 K in a cluster magnet

Insulating Nb3Cl8 is a layered chloride consisting of two-dimensional triangular layers of Seff = 1/2 Nb3Cl13 clusters at room temperature. Magnetic susceptibility measurement show a sharp, hysteretic drop to a temperature independent value below T = 90 K. Specific heat measurements show that the transition is first order, with ΔS ≈ 5 J K−1 mol−1 f.u.−1, and a low temperature T-linear contribution originating from defect spins. Neutron and X-ray diffraction show a lowering of symmetry from trigonal Pm1 to monoclinic C2/m symmetry, with a change in layer stacking from –AB–AB– to –AB′–BC′–CA′– and no observed magnetic order. This lowering of symmetry and rearrangement of successive layers evades geometric magnetic frustration to form a singlet ground state. It is the lowest temperature at which a change in stacking sequence is known to occur in a van der Waals solid, occurs in the absence of orbital degeneracies, and suggests that designer 2-D heterostructures may be able to undergo similar phase transitions.

Direct assignment of molecular vibrations via normal mode analysis of the neutron dynamic pair distribution function technique

For over a century, vibrational spectroscopy has enhanced the study of materials. Yet, assignment of particular molecular motions to vibrational excitations has relied on indirect methods. Here, we demonstrate that applying group theoretical methods to the dynamic pair distribution function analysis of neutron scattering data provides direct access to the individual atomic displacements responsible for these excitations. Applied to the molecule-based frustrated magnet with a potential magnetic valence-bond state, LiZn2Mo3O8, this approach allows direct assignment of the constrained rotational mode of Mo3O13 clusters and internal modes of MoO6 polyhedra. We anticipate that coupling this well known data analysis technique with dynamic pair distribution function analysis will have broad application in connecting structural dynamics to physical properties in a wide range of molecular and solid state systems.

Scaling and data collapse from local moments in frustrated disordered quantum spin systems

Recently measurements on various spin–1/2 quantum magnets such as H3LiIr2O6, LiZn2Mo3O8, ZnCu3(OH)6Cl2 and 1T-TaS2—all described by magnetic frustration and quenched disorder but with no other common relation—nevertheless showed apparently universal scaling features at low temperature. In particular the heat capacity C[H, T] in temperature T and magnetic field H exhibits T/H data collapse reminiscent of scaling near a critical point. Here we propose a theory for this scaling collapse based on an emergent random-singlet regime extended to include spin-orbit coupling and antisymmetric Dzyaloshinskii-Moriya (DM) interactions. We derive the scaling C[H, T]/T ~ H−γFq[T/H] with Fq[x] = xq at small x, with q ∈ {0, 1, 2} an integer exponent whose value depends on spatial symmetries. The agreement with experiments indicates that a fraction of spins form random valence bonds and that these are surrounded by a quantum paramagnetic phase. We also discuss distinct scaling for magnetization with a q-dependent subdominant term enforced by Maxwell’s relations.There are many proposals for new forms of quantum matter in frustrated magnets but in practice disorder prevents the realisation of theoretically-tractable idealised models. Kimchi et al. show that recently observed scaling behavior common to several disordered quantum magnets can be understood as the emergence of a universal random-singlet regime.

Observation of Vacancies, Faults, and Superstructures in Ln5Mo2O12 (Ln = La, Y, and Lu) Compounds with Direct Mo–Mo Bonding

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 B2O10 dimers 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/m structure 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/m monoclinic unit cell represents a superstructure relative to a previously unrecognized orthorhombic Immm subcell and that the superstructure derives from the ordering of interchangeable Mo2O10 and LaO6 building 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 La5Mo2O11.55 and 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.