Antiferromagnetism in a family of S=1 square lattice coordination polymers NiX2(pyz)2 (X=Cl, Br, I, NCS; pyz=Pyrazine)

The crystal structures of NiX2(pyz)2 (X = Cl (1), Br (2), I (3), and NCS (4)) were determined by synchrotron X-ray powder diffraction. All four compounds consist of two-dimensional (2D) square arrays self-assembled from octahedral NiN4X2 units that are bridged by pyz ligands. The 2D layered motifs displayed by 1–4 are relevant to bifluoride-bridged [Ni(HF2)(pyz)2]EF6 (E = P, Sb), which also possess the same 2D layers. In contrast, terminal X ligands occupy axial positions in 1–4 and cause a staggered packing of adjacent layers. Long-range antiferromagnetic (AFM) order occurs below 1.5 (Cl), 1.9 (Br and NCS), and 2.5 K (I) as determined by heat capacity and muon-spin relaxation. The single-ion anisotropy and g factor of 2, 3, and 4 were measured by electron-spin resonance with no evidence for zero–field splitting (ZFS) being observed. The magnetism of 1–4 spans the spectrum from quasi-two-dimensional (2D) to three-dimensional (3D) antiferromagnetism. Nearly identical results and thermodynamic features were obtained for 2 and 4 as shown by pulsed-field magnetization, magnetic susceptibility, as well as their Néel temperatures. Magnetization curves for 2 and 4 calculated by quantum Monte Carlo simulation also show excellent agreement with the pulsed-field data. Compound 3 is characterized as a 3D AFM with the interlayer interaction (J⊥) being slightly stronger than the intralayer interaction along Ni–pyz–Ni segments (Jpyz) within the two-dimensional [Ni(pyz)2]2+ square planes. Regardless of X, Jpyz is similar for the four compounds and is roughly 1 K

Antiferromagnetism in a Family of S = 1 Square Lattice Coordination Polymers NiX2(pyz)2 (X = Cl, Br, I, NCS; pyz = Pyrazine)

NiX2(pyz)2 have been synthesized and characterized using several magnetic probes. Except for X = Cl, long-range AFM order was found below TN = 1.9 (X = Br and NCS) and 2.5 K (X = I). In addition, Jpyz ∼ 1 K regardless of X, and D = 8 K for NiCl2(pyz)2, whereas D ∼ 0 for X = Br, I, and NCS based on ESR and heat capacity

Antiferromagnetism in a Family of <i>S</i> = 1 Square Lattice Coordination Polymers NiX₂(pyz)₂ (X = Cl, Br, I, NCS; pyz = Pyrazine)

The crystal structures of Ni<i>X</i>₂(pyz)₂ (X = Cl (<b>1</b>), Br (<b>2</b>), I (<b>3</b>), and NCS (<b>4</b>)) were determined by synchrotron X-ray powder diffraction. All four compounds consist of two-dimensional (2D) square arrays self-assembled from octahedral NiN₄<i>X</i>₂ units that are bridged by pyz ligands. The 2D layered motifs displayed by <b>1</b>–<b>4</b> are relevant to bifluoride-bridged [Ni­(HF₂)(pyz)₂]­<i>E</i>F₆ (<i>E</i> = P, Sb), which also possess the same 2D layers. In contrast, terminal <i>X</i> ligands occupy axial positions in <b>1</b>–<b>4</b> and cause a staggered packing of adjacent layers. Long-range antiferromagnetic (AFM) order occurs below 1.5 (Cl), 1.9 (Br and NCS), and 2.5 K (I) as determined by heat capacity and muon-spin relaxation. The single-ion anisotropy and <i>g</i> factor of <b>2</b>, <b>3</b>, and <b>4</b> were measured by electron-spin resonance with no evidence for zero–field splitting (ZFS) being observed. The magnetism of <b>1</b>–<b>4</b> spans the spectrum from quasi-two-dimensional (2D) to three-dimensional (3D) antiferromagnetism. Nearly identical results and thermodynamic features were obtained for <b>2</b> and <b>4</b> as shown by pulsed-field magnetization, magnetic susceptibility, as well as their Néel temperatures. Magnetization curves for <b>2</b> and <b>4</b> calculated by quantum Monte Carlo simulation also show excellent agreement with the pulsed-field data. Compound <b>3</b> is characterized as a 3D AFM with the interlayer interaction (<i>J</i>_⊥) being slightly stronger than the intralayer interaction along Ni–pyz–Ni segments (<i>J</i>_pyz) within the two-dimensional [Ni­(pyz)₂]²⁺ square planes. Regardless of <i>X</i>, <i>J</i>_pyz is similar for the four compounds and is roughly 1 K