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Abstract During the search for transition metal‐free alkyne hydrogenation catalysts, two new ternary Ca−Ga−Ge phases, Ca2Ga4Ge6(Cmc21, a=4.1600(10) Å, b=23.283(5) Å, c=10.789(3) Å) and Ca3Ga4Ge6(C2/m, a=24.063(2) Å, b=4.1987(4) Å, c=10.9794(9) Å, β=91.409(4)°), were discovered. These compounds are isostructural to the previously established Yb2Ga4Ge6and Yb3Ga4Ge6analogues, and according to Zintl‐Klemm counting rules, consist of anionic [Ga4Ge6]4−and [Ga4Ge6]6−frameworks in which every Ga and Ge atom would have a formal octet with no Ga−Ga or Ga−Ge π‐bonding. These compounds are metallic, based on temperature dependent electrical resistivity and thermopower measurements for Ca3Ga4Ge6, along with density functional theory calculations for both phases. Unlike the highly active 13‐layer trigonal CaGaGe phase, these new compounds exhibit minimal activity in the semi/full alkyne hydrogenation of phenylacetylene, which is consistent with previous observations that the lack of a formal octet for framework atoms is essential for catalysis in these Zintl‐Klemm compounds.
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Statistical upscaling of ecosystem CO 2 fluxes across the terrestrial tundra and boreal domain: Regional patterns and uncertainties
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null (Ed.)Two new alkali vanadate carbonates with divalent transition metals have been synthesized as large single crystals via a high-temperature (600 °C) hydrothermal technique. Compound I , Rb 2 Mn 3 (VO 4 ) 2 CO 3 , crystallizes in the trigonal crystal system in the space group P 3̄1 c , and compound II , K 2 Co 3 (VO 4 ) 2 CO 3 , crystallizes in the hexagonal space group P 6 3 / m . Both structures contain honeycomb layers and triangular lattices made from edge-sharing MO 6 octahedra and MO 5 trigonal bipyramids, respectively. The honeycomb and triangular layers are connected along the c -axis through tetrahedral [VO 4 ] groups. The MO 5 units are connected with each other by carbonate groups in the ab -plane by forming a triangular magnetic lattice. The difference in space groups between I and II was also investigated with Density Functional Theory (DFT) calculations. Single crystal magnetic characterization of I indicates three magnetic transitions at 77 K, 2.3 K, and 1.5 K. The corresponding magnetic structures for each magnetic transition of I were determined using single crystal neutron diffraction. At 77 K the compound orders in the MnO 6 -honeycomb layer in a Néel-type antiferromagnetic orientation while the MnO 5 triangular lattice ordered below 2.3 K in a colinear ‘up–up–down’ fashion, followed by a planar ‘Y’ type magnetic structure. K 2 Co 3 (VO 4 ) 2 CO 3 ( II ) exhibits a canted antiferromagnetic ordering below T N = 8 K. The Curie–Weiss fit (200–350 K) gives a Curie–Weiss temperature of −42 K suggesting a dominant antiferromagnetic coupling in the Co 2+ magnetic sublattices.more » « less
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Abstract The starting point for this work was a set of crystal structures containing the motif of interaction between methyl groups in homodimers. Two structures were selected for which QTAIM, NCI and NBO analyses suggested an attractive interaction. However, the calculated interaction energy was negative for only one of these systems. The ability of methyl groups to interact with one another is then examined by DFT calculations. A series of (CH3PnHCH3)2homodimers were allowed to interact with each other for a range of Pn atoms N, P, As, and Sb. Interaction energies of these C⋅⋅⋅C tetrel‐bonded species were below 1 kcal/mol, but could be raised to nearly 3 kcal/mol if the C atom was changed to a heavier tetrel. A strengthening of the C⋅⋅⋅C intermethyl bonds can also be achieved by introducing an asymmetry via an electron‐withdrawing substituent on one unit and a donor on the other. The attractions between the methyl and related groups occur in spite of a coulombic repulsion between σ‐holes on the two groups. NBO, AIM, and NCI tools must be interpreted with caution as they can falsely suggest bonding when the potentials are repulsive.more » « less
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Abstract Spinel compounds are of great interest in both fundamental and application-oriented perspectives due to the geometric magnetic frustration inherent to their lattice and the resulting complex magnetic states. Here, we applied x-ray diffraction, magnetization, heat capacity and powder inelastic neutron scattering measurements, along with theoretical calculations, to study the exotic properties of chromite-spinel oxides CoCr2O4and MnCr2O4. The temperature dependence of the phonon spectra provides an insight into the correlation between oxygen motion and the magnetic order, as well as the magnetoelectric effect in the ground state of MnCr2O4. Moreover, spin-wave excitations in CoCr2O4and MnCr2O4are compared with Heisenberg model calculations. This approach enables the precise determination of exchange energies and offers a comprehensive understanding of the spin dynamics and relevant exchange interactions in complicated spiral spin ordering.more » « less
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- https://doi.org/10.1111/gcb.15659
