Gapless spinons and a field-induced soliton gap in the hyper-honeycomb Cu oxalate framework compound [(C2_{2}H5_{5})3_{3}NH]2_{2}Cu2_{2}(C2_{2}O4_{4})3_{3}

We report a detailed study of the specific heat and magnetic susceptibility of single crystals of a spin liquid candidate: the hyper-honeycomb Cu oxalate framework compound [(C$_2$H$_5$)$_3$NH]$_2$Cu$_2$(C$_2$O$_4$)$_3$. The specific heat shows no anomaly associated with a magnetic transition at low temperatures down to $T\sim$ 180 mK in zero magnetic field. We observe a large linear-in-$T$ contribution to the specific heat $\gamma T$, $\gamma = 98(1)$ mK/mol K$^{2}$, at low temperatures, indicative of the presence of fermionic excitations despite the Mott insulating state. The low-$T$ specific heat is strongly suppressed by applied magnetic fields $H$, which induce an energy gap, $\Delta (H)$, in the spin-excitation spectrum. We use the four-component relativistic density-functional theory (DFT) to calculate the magnetic interactions, including the Dzyaloshinskii-Moriya antisymmetric exchange, which causes an effective staggered field acting on one copper sublattice. The magnitude and field dependence of the field-induced gap, $\Delta (H) \propto H^{2/3}$, are accurately predicted by the soliton mass calculated from the sine-Gordon model of weakly coupled antiferromagnetic Heisenberg chains with all parameters determined by our DFT calculations. Thus our experiment and calculations are entirely consistent with a model of [(C$_2$H$_5$)$_3$NH]$_2$Cu$_2$(C$_2$O$_4$)$_3$ in which anisotropic magnetic exchange interactions due to Jahn-Teller distortion cause one copper sublattice to dimerize, leaving a second sublattice of weakly coupled antiferromagnetic chains. We also show that this model quantitatively accounts for the measured temperature-dependent magnetic susceptibility. Thus [(C$_2$H$_5$)$_3$NH]$_2$Cu$_2$(C$_2$O$_4$)$_3$ is a canonical example of a one-dimensional spin-1/2 Heisenberg antiferromagnet and not a resonating-valence-bond quantum spin liquid, as previously proposed.Comment: 8 pages, 6 figure

All-in-one synthesis of mesoporous silicon nanosheets from natural clay and their applicability to hydrogen evolution

Silicon nanosheets have attracted much attention owing to their novel electronic and optical properties and compatibility with existing silicon technology. However, a cost-effective and scalable technique for synthesizing these nanosheets remains elusive. Here, we report a novel strategy for producing silicon nanosheets on a large scale through the simultaneous molten-salt-induced exfoliation and chemical reduction of natural clay. The silicon nanosheets thus synthesized have a high surface area, are ultrathin (similar to 5 nm) and contain mesoporous structures derived from the oxygen vacancies in the clay. These advantages make the nanosheets a highly suitable photocatalyst with an exceptionally high activity for the generation of hydrogen from a water-methanol mixture. Further, when the silicon nanosheets are combined with platinum as a cocatalyst, they exhibit high activity in KOH (15.83 mmol H-2 per s per mol Si) and excellent photocatalytic activity with respect to the evolution of hydrogen from a water-methanol mixture (723 mu mol H-2 per h per g Si).clos

Metal insertion in a microporous metal-organic framework lined with 2,2'-bipyridine

Reaction of AlCl(3)·6H(2)O with 2,2'-bipyridine-5,5'-dicarboxylic acid (H(2)bpydc) affords Al(OH)(bpydc) (1, MOF-253), the first metal-organic framework with open 2,2'-bipyridine (bpy) coordination sites. The material displays a BET surface area of 2160 m(2)/g and readily complexes metals to afford, for example, 1·xPdCl(2) (x = 0.08, 0.83) and 1·0.97Cu(BF(4))(2). EXAFS spectroscopy performed on 1·0.83PdCl(2) reveals the expected square planar coordination geometry, matching the structure of the model complex (bpy)PdCl(2). Significantly, the selectivity factor for binding CO(2) over N(2) under typical flue gas conditions is observed to increase from 2.8 in 1 to 12 in 1·0.97Cu(BF(4))(2).Eric D. Bloch, David Britt, Chain Lee, Christian J. Doonan, Fernando J. Uribe-Romo, Hiroyasu Furukawa, Jeffrey R. Long, and Omar M. Yagh

Slow spin dynamics in the hyperhoneycomb lattice [(C2H5)(3)NH](2)Cu-2(C2O4)(3) revealed by H-1 NMR studies

We report the results of magnetic susceptibility chi and H-1 nuclear magnetic resonance (NMR) measurements on a three-dimensional hyperhoneycomb lattice compound [(C2H5)(3)NH](2)Cu-2(C2O4)(3) (CCCO). The average value of the antiferromagnetic (AFM) exchange coupling between the Cu2+ (S = 1/2) spins was determined to be J similar to 50 K from the chi measurements. No long-range magnetic ordering has been observed down to T = 50 mK, although NMR lines become slightly broader at low temperatures below 1 K. The broadening of the NMR spectrum observed below 1 K reveals that the Cu spin moments remain at this temperature, suggesting a non-spin-singlet ground state. The temperature and magnetic field dependence of 1/T-1 at temperatures above 20 K is well explained by paramagnetic thermal spin fluctuations where the fluctuation frequency of Cu2+ spins is higher than the NMR frequency of the order of megahertz. However, a clear signature of the slowing down of the Cu2+ spin fluctuations was observed at low temperatures where 1/T-1 shows a thermally activated behavior. The magnetic field dependence of the magnitude of the spin excitation gap suggests that the magnetic behaviors of CCCO are characterized as an AFM chain at low temperatures.This article is published as Ding, Q-P., C. Dissanayake, Santanu Pakhira, W. J. Newsome, F. Uribe-Romo, D. C. Johnston, Y. Nakajima, and Y. Furukawa. "Slow spin dynamics in the hyperhoneycomb lattice [(C 2 H 5) 3 NH] 2 Cu 2 (C 2 O 4) 3 revealed by H 1 NMR studies." Physical Review B 105, no. 10 (2022): L100405. DOI: 10.1103/PhysRevB.105.L100405. Copyright 2022 American Physical Society. Posted with permission. DOE Contract Number(s): AC02-07CH11358; DMR-1944975

Exceptional chemical and thermal stability of zeolitic imidazolate frameworks

Twelve zeolitic imidazolate frameworks (ZIFs; termed ZIF-1 to -12) have been synthesized as crystals by copolymerization of either Zn(II) (ZIF-1 to -4, -6 to -8, and -10 to -11) or Co(II) (ZIF-9 and -12) with imidazolate-type links. The ZIF crystal structures are based on the nets of seven distinct aluminosilicate zeolites: tetrahedral Si(Al) and the bridging O are replaced with transition metal ion and imidazolate link, respectively. In addition, one example of mixed-coordination imidazolate of Zn(II) and In(III) (ZIF-5) based on the garnet net is reported. Study of the gas adsorption and thermal and chemical stability of two prototypical members, ZIF-8 and -11, demonstrated their permanent porosity (Langmuir surface area = 1,810 m(2)/g), high thermal stability (up to 550°C), and remarkable chemical resistance to boiling alkaline water and organic solvents