RESUMO
We report the hydrothermal synthesis, X-ray crystal structures, and thermal, optical, and magnetic properties of two isostructural layered coordination polymers, [M(II)(5)(OH)(2)(chtc)(2)(H(2)O)(10)]·2H(2)O where M = Co (1Co) or Ni (2Ni) and chtc = cis,cis,cis-1,2,4,5-cyclohexanetetracarboxylate. The structures consist of segregated layers where each layer is formed of M(3)(OH)(2) diamond chains, involving edge- and corner-sharing octahedra, bridged by M(chtc)(2). In the space created by the chains and the bridges exists channels, which house the coordinated and noncoordinated water molecules, all hydrogen bonded to form a dodecamer, with a central cyclohexane chair shape ring. Interestingly, the water molecules of 2Ni are more difficult to remove than those of 1Co, but pyrolysis of the latter takes place at higher temperature. The magnetic properties are characterized by Néel transition to fully compensated antiferromagnets at 5.8 K (1Co) and 3.0 K (2Ni). The moments are easily reversed in an applied field of 150 Oe (1Co) and 300 Oe (2Ni) to a ferrimagnet with one uncompensated moment followed by a nonlinear increase to a saturation corresponding to a ferrimagnet with three uncompensated moments. Further, point charge calculations to estimate the weak bonding energies between the various types of interaction are reported.
RESUMO
The structures (nuclear and magnetic), magnetic properties (2-300 K, 1-10(4) bar), and heat capacity of the layered ferromagnet Co5(OH)6(SO4)2(H2O)4 are reported. The crystal structure consists of brucite-like M(II)-OH layers of edge-sharing octahedra, but having two different Co sites, which are pillared by ...O3SO-Co(H2O)4-OSO3.... The absorption spectrum confirms the presence of divalent Co, and by comparison of the two isotopic materials, the assignment of the vibrational spectra is proposed. The magnetic properties are those of a ferromagnet with a Curie temperature of 14 K. Temperature and field dependence magnetization data taken on an aligned sample suggest an easy-plane magnet. The Curie temperature increases linearly with pressure at a rate of +0.12 K/kbar, suggesting small progressive and uniform modifications of the Co-Co exchange interactions. Rietveld refinement of the neutron powder diffraction data and consideration of a group analysis reveal the direction of the moments of the Co within the layer to be along the b-axis, with a maximum moment of 3.33 micro(B) per cobalt. Those of the pillars remain random. Estimation of the entropy from the heat capacity data accounts for the presence of four ordered moments of Co with spin 1/2 at the long-range ordering temperature, while the moment of the pillaring Co contributes only at lower temperature due to the increase of the internal field as the temperature is lowered. The purely 2D-magnetic ordering in an easy-plane magnet, evidenced by neutron diffraction and heat capacity, challenges the existing theories and is a rare example of a single-layer magnet.
RESUMO
The hydrothermal reactions of nickel(II) nitrate with a mixture of the geometric cis and trans isomers of 1,4-cyclohexanedicarboxylic acid (1,4-chdc or C6H10(COOH)2) and a base yield three structurally different complexes, [Ni3(mu3-OH)2(mu4-cis-1,4-chdc)2(H2O)4].2H2O (1), [Ni(3)(mu3-OH)2(mu4-trans-1,4-chdc)2(H2O)4].4H2O (2), and [Ni(H2O)4(mu2-trans-1,4-chdc)], depending on the reaction conditions. The single-crystal X-ray structure analyses of 1 and 2 reveal segregation of the isomers and formation of frameworks based on infinite Ni3(OH)2(H2O)4 chains, acting as secondary building units, connected by either cis- or trans-chdc for 1 and 2, respectively. The frameworks sustain channels that house two or four water molecules, respectively, according to the size and shape of the channels that depend on the particular isomer. The structure of 3 consists of chains of square-planar Ni(H2O)4 bridged by trans-chdc. Magnetic data as a function of temperature and field of the virgin samples for 1 indicate long-range ordering (LRO) to a ferrimagnetic ground state at 2.1 K that is reversibly transformed into a ferromagnet below 4.4 K upon partial dehydration and rehydration. Powder X-ray diffraction of 1, in its virgin state, after dehydration and after rehydration, confirms the stability of the framework. The magnetic data for 2 tend toward a LRO state to possibly a ferrimagnet below 2 K. The temperature dependence of the susceptibility of the two compounds is accounted for by the presence of both ferro- and antiferromagnetic exchanges within each chain via Ni-O-Ni and Ni-O-C-O-Ni pathways and weak coupling between neighboring chains via the 1,4-chdc unit. 3 is a uniform s = 1 antiferromagnetic chain (J/kB = 2.27(1) K).
