The emergence of electrically conductive metal-organic frameworks (MOFs) has been one of the most exciting, if somewhat paradoxical, developments in porous materials, and has already led to applications as varied as chemical sensing and electrical energy storage. The most conductive MOFs are those made from organic ligands and square-planar transition metal ions connected into two-dimensional (2D) sheets that stack in a similar manner to the graphene sheets in graphite. Their electrical properties are thought to depend critically on the covalency of the metal-ligand bond. Much less importance is given to charge transport normal to the 2D sheets, not least because there is little synthetic opportunity to control their stacking sequence or distance. Here, we report exquisite control over the stacking sequence and distance in a series of materials made from 2D sheets of organic ligands connected in the third dimension by infinite lanthanide-oxygen chains. Contrary to transition metal MOFs, efficient charge transport leading to conductivity values of up to 0.5 S/cm in the lanthanide materials occurs primarily normal to the 2D sheets. We further show that the smaller lanthanides Yb 3+ and Ho 3+ enforce a shorter stacking distance of only 3.002(6) Å and afford consistently higher conductivity than the larger lanthanides Nd 3+ and La 3+ , which distend the sheets up to 3.068(2) Å. This first systematic study of structure-function relationships in layered conductive MOFs is enabled by the high degree of crystallinity afforded by the relatively ionic lanthanide-ligand bonds. These results demonstrate that increasing the covalency of the metal-ligand bond is not the only viable path to achieve record conductivity in 2D MOFs, and that the interactions of the organic ligands alone can produce efficient charge transport pathways.

A series of Yb2+xTi2−xO 7−δ doped samples demonstrates the effects of off-stoichiometry on Yb2Ti2O7's structure, properties, and magnetic ground state via x-ray diffraction, specific heat, and magnetization measurements. A stoichiometric single crystal of Yb2Ti2O7 grown by the traveling solvent floating zone technique (solvent = 30 wt% rutile TiO2 and 70 wt% Yb2Ti2O7) is characterized and evaluated in light of this series. Our data shows that upon positive x doping, the cubic lattice parameter a increases and the Curie-Weiss temperature θCW decreases. Heat capacity measurements of stoichiometric Yb2Ti2O7 samples exhibit a sharp, first-order peak at T = 268(4) mK that is suppressed in magnitude and temperature in samples doped off ideal stoichiometry. The full entropy recovered per Yb ion is 5.7 J K −1 ≈ Rln2. Our work establishes the effects of doping on Yb2Ti2O7's physical properties, which provides further evidence indicating that previous crystals grown by the traditional floating zone method are doped off ideal stoichiometry. Additionally, we present how to grow high-quality colorless single crystals of Yb2Ti2O7 by the traveling solvent floating zone growth method.

Inelastic neutron scattering reveals a broad continuum of excitations in Pr2Zr2O7, the temperature and magnetic field dependence of which indicate a continuous distribution of quenched transverse fields (∆) acting on the non-Kramers Pr 3+ crystal field ground state doublets. Spin-ice correlations are apparent within 0.2 meV of the Zeeman energy. A random phase approximation provides an excellent account of the data with a transverse field distribution ρ(∆) ∝ (∆ 2 + Γ 2 ) −1 where Γ = 0.27(1) meV. Established during high temperature synthesis due to an underlying structural instability, it appears disorder in Pr2Zr2O7 actually induces a quantum spin liquid.Instead of a discrete set of states that satisfy all interactions, frustrated spin systems support a high density of low energy states from which novel collective phenomena may emerge at temperatures (T ) well below the bare interaction strengths.[1] A prominent example is quantum spin ice (QSI).[2] By introducing quantum spin fluctuations to classical spin ice through transverse inter-spin interactions, it has been proposed that a quantum spin liquid (QSL) phase with gapless photon-like excitations can be realized. [2] Several materials have been examined in the search for QSI including Yb 2 Ti 2 O 7 and Tb 2 Ti 2 O 7 , but so far there is no experimental evidence for salient features such as low energy electrodynamics.[2] Instead unanticipated features have been discovered including a very strong dependence of physical properties on sample quality. In a recent study of Tb 2+x Ti 2−x O 7+y it was found that a change in the Tb/Ti molar ratio as small as 0.005 can tune the samples between an ordered and a disordered phase.[3] While such sensitivity is a distinguishing feature of systems with a high density of low energy states, there is so far no clear understanding of the microscopic mechanisms involved. Can this be explained in terms of small changes of the exchange interactions in the pseudo-spin-1/2 Hamiltonian[2] for materials near phase boundaries or are there new pieces of the puzzle yet to be discovered? Further insight into these questions will not only help clarify the complicated magneto-structural properties of specific materials, but may guide the broader search for QSL materials.In this paper we show quenched structural disorder acts as a transverse field on the non-Kramers Pr 3+ ion in Pr 2 Zr 2 O 7 (PZO) and in competition with exchange interactions induces a spatially correlated and disordered singlet ground state. A previous neutron study of PZO revealed weak diffuse elastic scattering with pinch points indicative of spin-ice correlations.[4] Here we show magnetic excitations in PZO are composed of two parts: a lower energy regime that is driven by inter-spin correlations, and a momentum transfer (q) independent higher energy part driven by quenched transverse fields. A nearest neighbor spin ice model augmented by random transverse fields provides an excellent account of these observations and points to the realization of a newly proposed QSL [...

Although gas adsorption properties of extended three-dimensional metal−organic materials have been widely studied, they remain relatively unexplored in porous molecular systems. This is particularly the case for porous coordination cages for which surface areas are typically not reported. Herein, we report the synthesis, characterization, activation, and gas adsorption properties of a family of carbazole-based cages. The chromium analog displays a coordination cage record BET (Brunauer−Emmett−Teller) surface area of 1235 m2/g. With precise synthesis and activation procedures, two previously reported cages similarly display high surface areas. The materials exhibit high methane adsorption capacities at 65 bar with the chromium (II) cage displaying CH4 capacities of 194 cm3/g and 148 cm3/cm3. This high uptake is a result of optimal pore design, which was confirmed via powder neutron diffraction experiments.

Porous molecular solids are promising materials for gas storage and gas separation applications. However, given the relative dearth of structural information concerning these materials, additional studies are vital for further understanding their properties and developing design parameters for their optimization. Here, we examine a series of isostructural cuboctahedral, paddlewheel-based coordination cages, M24(tBu-bdc)24 (M = Cr, Mo, Ru; tBu-bdc2− = 5-tert-butylisophthalate), for high-pressure methane storage. As the decrease in crystallinity upon activation of these porous molecular materials precludes diffraction studies, we turn to a related class of pillared coordination cage-based metal-organic frameworks, M24(Me-bdc)24(dabco)6 (M = Fe, Co; Me-bdc2− = 5-methylisophthalate; dabco = 1,4-diazabicyclo[2.2.2]octane) for neutron diffraction studies. The five porous materials display BET surface areas from 1,057 – 1,937 m2/g and total methane uptake capacities of up to 143 cm3(STP)/cm3. Both the porous cages and cage-based frameworks display methane adsorption enthalpies of −15 to −22 kJ/mol. Also supported by molecular modeling, neutron diffraction studies indicate that the triangular windows of the cage are favorable methane adsorption sites with CD4-arene interactions between 3.7 and 4.1 Å. At both low and high loadings, two additional methane adsorption sites on the exterior surface of the cage are apparent for a total of 56 adsorption sites per cage. These results show that M24L24 cages are competent gas storage materials and further adsorption sites may be optimized by judicious ligand functionalization to control extra-cage pore space.

Pyrochlore Pr 3+ 2+x Zr 4+ 2-x O 7-x/2 samples in the form of both powders (-0.02≤x≤0.02) and bulk single crystals have been studied to elucidate the dependence of their magnetic, compositional and structural properties on synthesis and growth conditions. All samples were characterized using X-ray diffraction, specific heat, and DC magnetization measurements. The crystals were also studied using the X-ray Laue technique and scanning electron microscopy. Increasing the Pr content for the Pr 2+x Zr 2-x O 7-x/2 powders enlarged the lattice parameter, and resulted in systematic changes in magnetic susceptibility and specific heat.Stoichiometric and high quality single crystals of Pr 2 Zr 2 O 7 were grown using the optical floating zone technique under a high purity static argon atmosphere, to avoid inclusions of Pr 4+ and limit Pr vaporization. Increasing the growth speed was found to significantly reduce Pr vaporization for better control of stoichiometry. Scanning electron microscopy provided direct evidence of spinodal decomposition during growth that is controllable via rotation rate. An intermediate rotation rate of 3-6 rpm was found to produce the best microstructure. The magnetic susceptibility of crystals grown at rates from

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MCB 11 H 12 (M: Li, Na) dodecahydro-monocarba-closo-dodecaborate salt compounds are known to have stellar superionic Li + and Na + conductivities in their hightemperature disordered phases, making them potentially appealing electrolytes in all-solidstate batteries. Nonetheless, it is of keen interest to search for other related materials with similar conductivities while at the same time exhibiting even lower (more device-relevant) disordering temperatures, a key challenge for this class of materials. With this in mind, the unknown structural and dynamical properties of the heavier KCB 11 H 12 congener were investigated in detail by X-ray powder diffraction, differential scanning calorimetry, neutron vibrational spectroscopy, nuclear magnetic resonance, quasielastic neutron scattering, and AC impedance measurements. This salt indeed undergoes an entropy-driven, reversible, order−disorder transformation and with a lower onset temperature (348 K upon heating and 340 K upon cooling) in comparison to the lighter LiCB 11 H 12 and NaCB 11 H 12 analogues. The K + cations in both the low-T ordered monoclinic (P2 1 /c) and high-T disordered cubic (Fm3̅ m) structures occupy octahedral interstices formed by CB 11 H 12 − anions. In the low-T structure, the anions orient themselves so as to avoid close proximity between their highly electropositive C−H vertices and the neighboring K + cations. In the high-T structure, the anions are orientationally disordered, although to best avoid the K + cations, the anions likely orient themselves so that their C−H axes are aligned in one of eight possible directions along the body diagonals of the cubic unit cell. Across the transition, anion reorientational jump rates change from 6.2 × 10 6 s −1 in the low-T phase (332 K) to 2.6 × 10 10 s −1 in the high-T phase (341 K). In tandem, K + conductivity increases by about 30-fold across the transition, yielding a high-T phase value of 3.2 × 10 −4 S cm −1 at 361 K. However, this is still about 1 to 2 orders of magnitude lower than that observed for LiCB 11 H 12 and NaCB 11 H 12 , suggesting that the relatively larger K + cation is much more sterically hindered than Li + and Na + from diffusing through the anion lattice via the network of smaller interstitial sites.