Rasmus D. Poulsen

Nonstoichiometry and chemical purity effects in thermoelectric Ba8Ga16Ge30 clathrate

Zone melting purification experiments have been carried out on the clathrate, Ba8Ga16Ge30. The impurities present have been identified and their approximate concentrations measured. Trace impurities were determined to be approximately 240 parts per million (ppm) in the most impure sample to 17 ppm in the most pure sample. The temperature-dependent Seebeck coefficient, thermal conductivity, and electrical conductivity are reported as a function of sample purity as well as the room-temperature Hall coefficient. Microprobe analysis suggests that the samples are nonstoichiometric with excess Ge relative to Ga, and there are indications of the presence of defects. Single-crystal x-ray investigations as well as synchrotron powder diffraction measurements support the presence of defects, but the x-ray data cannot accurately determine the relative amounts of Ga and Ge. Band-structure calculations in the generalized gradient approximation show that the measured Hall and Seebeck coefficients are consistent with a d...

Effects of Weak Intermolecular Interactions on the Molecular Isomerism of Tricobalt Metal Chains

Depending on the number of interstitial solvent molecules, n, crystals of the linear chain compound Co(3)(dipyridylamide)(4)Cl(2) x nCH(2)Cl(2) adopt either symmetrical or unsymmetrical metal chain structures. We explore here the possible reasons for such behavior using Hirshfeld surface analysis of intermolecular interactions as well as the charge density determined from 100(1) K X-ray diffraction data on the unsymmetrical complex Co(3)(dipyridylamide)(4)Cl(2) x 2.11 CH(2)Cl(2), u-1, and crystal structures of u-1 determined from single crystal synchrotron X-ray diffraction data at 20, 150, and 300 K. The new crystal structures are compared with previous structural results on a crystal with slightly different solvent content. This change in solvent content only affects the bond distances to atom Co(3), which are also strongly affected by temperature changes due to a spin crossover transition. Large differences in intermolecular interactions are revealed by the Hirshfeld surface analysis between symmetrical (s-1) and unsymmetrical (u-1) crystal solvates, suggesting that the molecular isomerism is strongly influenced by crystal packing effects. Topological analysis of the static electron density of u-1 suggests that there is direct metal-metal bonding for both the shorter Co(1)-Co(2) and the longer Co(2)-Co(3) contact. The approximate description of the system as a (Co(2))(2+)-dimer and an isolated Co(2+)-ion is reflected in the character of the metal-ligand interactions, which are more ionic for the isolated Co(3) atom, and the topological charges Co(1)(+0.50), Co(2)(+0.77), and Co(3)(+1.36). The two termini of u-1 are found to be very different, both in terms of structural surroundings as well as topology. The central Co(2) atom is similar to a cobalt atom in a tetragonally distorted octahedral environment resulting in preferred occupancy in the t(2g) orbitals. The Co(1) atom has significant deformation in the xz and yz planes (z along the chain axis, x and y toward ligands) reflecting covalent interactions with the terminal chlorine atom Cl(1). The Co(3) atom has a relatively high occupancy of the d(x(2)-y(2)) orbital and a relatively low occupancy of the d(xy) orbital confirming that these orbitals are involved in the spin crossover process and predominantly responsible for the observed variation in bond lengths with temperature.

Solvothermal synthesis, multi-temperature crystal structures and physical properties of isostructural coordination polymers, 2C4H12N+[M3(C8H4O4)4]2−·3C5H11NO, M = Co, Zn

Two isostructural metal organic framework (MOF) structures have been synthesized by solvothermal methods and examined by single-crystal X-ray diffraction. A microcrystal of 2C4H12N+[Co3(C8H4O4)4]2-.3C5H11NO (1) was investigated at T = 120 K using synchrotron radiation. 2C4H12N+[Zn3(C8H4O4)4]2-.3C5H11NO (2) was investigated at multiple temperatures (T = 30, 100, 200 and 300 K) on a conventional diffractometer. The thermal expansion of the structure of (2) is anisotropic and along the a axis, which corresponds to the metal chain direction. The structures contain anionic frameworks with cations and solvent molecules trapped in the voids. The magnetic susceptibility (chi) and heat capacity (C(p)) have been measured from 1.8 to 350 K. Compound (1) orders ferromagnetically with a broad phase transition observed in C(p) at approximately 6 K. The magnetic moment reaches a value of 3 micro(B) per Co at 2 K in a magnetic field of 9 T, and a Curie-Weiss fit to chi(T) gives an effective moment (mu(eff)) of 4.2 mu(B) and a Weiss temperature (theta) of 23 K. The exchange mechanism for the magnetic coupling is suggested to involve the Co-O-Co bridges in the individual three-metal-atom subchains. The three-dimensional magnetism presumably is due to super-exchange through two out of the three unique C8H4O4 linker molecules, which have the carboxylate and benzene pi systems well aligned.

The copper(II) complexes di-μ-bromo-bis{[2,6-bis­(pyrazol-1-yl)­pyridine]­per­chlorato­copper(II)} and [2,6-bis(pyra­zol-1-yl)­pyridine]­di­bromo­copper(II)

The two title compounds, di-mu-bromo-bis{[2,6-bis(pyrazol-1-yl-kappaN(2))pyridine-kappaN](perchlorato-kappaO)copper(II)}, [Cu(2)Br(2)(ClO(4))(2)(C(11)H(9)N(5))(2)], (I), and [2,6-bis(pyrazol-1-yl)pyridine]dibromocopper(II), [CuBr(2)(C(11)H(9)N(5))], (II), were synthesized by only slight modifications of the same reaction; compound (II) was formed by adding one molar equivalent of pyrazole (C(3)N(2)H(4)) to the reaction mixture of (I). Compound (I) is a bromo-bridged dinuclear copper(II) compound stabilized by weak interactions with the perchlorate anions (ClO(4)(-)), while (II) is a related mononuclear species, which has a distorted square-pyramidal geometry.