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Title: 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

[Ni(HF{sub 2})(pyz){sub 2}]X {l_brace}pyz = pyrazine; X = PF{sub 6}{sup -} (1), SbF{sub 6}{sup -} (2){r_brace} were structurally characterized by synchrotron X-ray powder diffraction and found to possess axially compressed NiN{sub 4}F{sub 2} 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) {angstrom}, and {beta} = 81.610(3){sup o} while 2 is tetragonal (P4/nmm) with a = b = 9.9359(3) and c = 6.4471(2) {angstrom} and is isomorphic with the Cu-analogue. Infinite one-dimensional (1D) Ni-FHF-Ni chains propagate along the c-axis which are linked via {mu}-pyz bridges in the ab-plane to afford three-dimensional polymeric frameworks with PF{sub 6}{sup -} and SbF{sub 6}{sup -} counterions occupying the interior sites. A major difference between 1 and 2 is that the Ni-F-H bonds are bent (157{sup o}) 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 {pi}-donation of the HF{sub 2}{sup -} 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 {lambda}-like peaks in dxT/dT at 6.2 andmore » 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 x 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.« less
Authors:
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Publication Date:
OSTI Identifier:
1041979
Report Number(s):
BNL-97657-2012-JA
Journal ID: ISSN 0020-1669; INOCAJ; TRN: US201212%%390
DOE Contract Number:
DE-AC02-98CH10886
Resource Type:
Journal Article
Resource Relation:
Journal Name: Inorganic Chemistry; Journal Volume: 50; Journal Issue: 13
Research Org:
BROOKHAVEN NATIONAL LABORATORY (BNL)
Sponsoring Org:
USDOE SC OFFICE OF SCIENCE (SC)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ABSORPTION SPECTRA; ANISOTROPY; CRITICAL FIELD; DIFFRACTION; MAGNETIC PROPERTIES; MAGNETIC SUSCEPTIBILITY; MAGNETIZATION; MAGNETS; RELAXATION; SPECIFIC HEAT; SPIN; SYNCHROTRONS