Experimental and Theoretical Characterization of the Magnetic Properties of CuF2(H2O)2(pyz) (pyz = pyrazine): A Two-Dimensional Quantum Magnet Arising from Supersupere

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• 作者：Jamie L. Manson ; Marianne M. Conner ; John A. Schlueter ; Amber C. McConnell ; Heather I. Southerland ; Isabelle Malfant ; Tom Lancaster ; Stephen J. Blundell ; Michael L. Brooks ; Francis L. Pratt ; John Sing
• 刊名：Chemistry of Materials
• 出版年：2008
• 出版时间：December 23, 2008
• 年：2008
• 卷：20
• 期：24
• 页码：7408-7416
• 全文大小：353K
• 年卷期：v.20,no.24(December 23, 2008)
• ISSN：1520-5002

文摘

The structural, electronic, and magnetic properties of the new linear chain coordination polymer CuF₂(H₂O)₂(pyz) (pyz = pyrazine) were determined by single crystal X-ray diffraction at various temperatures, SQUID magnetometry, pulsed-field magnetization, ESR, muon-spin relaxation (μSR), and electronic structure calculations. Each Cu²⁺ ion of CuF₂(H₂O)₂(pyz) is located at a distorted CuF₂O₂N₂ octahedron with axial elongation along the Cu−N bonds. These octahedra are tethered together by strong F···H−O hydrogen bonds to yield two-dimensional (2D) square nets in the bc-plane that are linked along the a-direction by pyrazine linkages. Measurements of the g-factor by ESR along with first principles density functional theory electronic structure calculations show that the magnetic orbital of the Cu²⁺ ion lies in the CuF₂O₂ plane thus forming a 2D antiferromagnetic square lattice. A broad maximum observed in χ(T) at 10 K indicates a modest spin exchange interaction through the Cu−F···H−O−Cu supersuperexchange paths, and a theoretical fit of χ(T) to a 2D square model gives J_2D/k_B = −5.58(1) K (in the convention where J rather than 2J is used for spin exchange). At lower temperatures, χ(T) shows a sharp peak at 2.6 K, which signals a transition to a long-range magnetic ordering as confirmed by coherent precession of implanted muons. Isothermal M(B) measurements made at 0.5 K on an aligned single crystal reveal magnetic saturation, M_sat, at 28.8 T (Ba) while a higher field of 33.1 T is required to saturate the spins when B is applied perpendicular to the a-axis thus showing clear anisotropy in these orientations. The presence of strong hydrogen bonds in CuF₂(H₂O)₂(pyz) causes the chains to adopt a tilted packing arrangement, thus leading to a novel ground-state likely characterized by spin-canting within the 2D layers and a markedly increased critical temperature relative to the well-known Cu(NO₃)₂(pyz) polymer chain compound.