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Structural, Electronic, and Magnetic Properties of Quasi-1D Quantum Magnets [Ni(HF2)(pyz)2]X (pyz = pyrazine; X = PF6, SbF6) Exhibiting Ni-FHF-Ni and Ni-pyz-Ni Spin Interactions

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posted on 04.07.2011, 00:00 by Jamie L. Manson, Saul H. Lapidus, Peter W. Stephens, Peter K. Peterson, Kimberly E. Carreiro, Heather I. Southerland, Tom Lancaster, Stephen J. Blundell, Andrew J. Steele, Paul A. Goddard, Francis L. Pratt, John Singleton, Yoshimitsu Kohama, Ross D. McDonald, Rico E. Del Sesto, Nickolaus A. Smith, Jesper Bendix, Sergei A. Zvyagin, Jinhee Kang, Changhoon Lee, Myung-Hwan Whangbo, Vivien S. Zapf, Alex Plonczak
[Ni(HF2)(pyz)2]X {pyz = pyrazine; X = PF6 (1), SbF6 (2)} were structurally characterized by synchrotron X-ray powder diffraction and found to possess axially compressed NiN4F2 octahedra. At 298 K, 1 is monoclinic (C2/c) with unit cell parameters, a = 9.9481(3), b = 9.9421(3), c = 12.5953(4) Å, and β = 81.610(3)° while 2 is tetragonal (P4/nmm) with a = b = 9.9359(3) and c = 6.4471(2) Å and is isomorphic with the Cu-analogue. Infinite one-dimensional (1D) Ni-FHF-Ni chains propagate along the c-axis which are linked via μ-pyz bridges in the ab-plane to afford three-dimensional polymeric frameworks with PF6 and SbF6 counterions occupying the interior sites. A major difference between 1 and 2 is that the Ni–F–H bonds are bent (∼157°) in 1 but are linear in 2. Ligand field calculations (LFT) based on an angular overlap model (AOM), with comparison to the electronic absorption spectra, indicate greater π-donation of the HF2 ligand in 1 owing to the bent Ni–F–H bonds. Magnetic susceptibility data for 1 and 2 exhibit broad maxima at 7.4 and 15 K, respectively, and λ-like peaks in dχT/dT at 6.2 and 12.2 K that are ascribed to transitions to long-range antiferromagnetic order (TN). Muon-spin relaxation and specific heat studies confirm these TN’s. A comparative analysis of χ vs T to various 1D Heisenberg/Ising models suggests moderate antiferromagnetic interactions, with the primary interaction strength determined to be 3.05/3.42 K (1) and 5.65/6.37 K (2). However, high critical fields of 19 and 37.4 T obtained from low temperature pulsed-field magnetization data indicate that a single exchange constant (J1D) alone is insufficient to explain the data and that residual terms in the spin Hamiltonian, which could include interchain magnetic couplings (J), as mediated by Ni-pyz-Ni, and single-ion anisotropy (D), must be considered. While it is difficult to draw absolute conclusions regarding the magnitude (and sign) of J and D based solely on powder data, further support offered by related Ni(II)-pyz compounds and our LFT and density-functional theory (DFT) results lead us to a consistent quasi-1D magnetic description for 1 and 2.