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

Abstract

Magnetism in a family of S = 1 square lattice antiferromagnets NiX 2 (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 at 298 K 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]ZF6 (Z = P, Sb) which also possess the same 2D layers. In contrast, terminal X ligands occupy axial positions in 1-4 and cause a staggering of adjacent layers. Long-range antiferromagnetic 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 are measured by electron spin resonance where no zero-field splitting was found. The magnetism of 1-4 crosses a spectrum from quasi-two-dimensional to three-dimensional antiferromagnetism. An excellent agreement was found between the pulsedfield magnetization, magnetic susceptibility and TN of 2 and 4. Magnetization curves for 2 and 4 calculated by quantum Monte Carlo simulation also show excellent agreement with the pulsed-field data. 3 is characterized as a three-dimensional antiferromagnet with the interlayer interaction (J ⊥ ) slightly stronger than the interaction within the two-dimensional [Ni(pyz)2] 2+ square planes (Jpyz).

Determining the anisotropy and exchange parameters of polycrystalline spin-1 magnets

“…The nearest neighbour Ni• • • Ni distance is 6.98 Å through the pyrazine molecules, and metal ions in adjacent planes are separated by 7.40 Å. Both pyrazine and H• • • F bonds have been shown to be mediators of antiferromagnetic exchange strengths of the order of 1-10 K in Ni(II) complexes [21,22]. It is not possible to tell from the structure alone which pathways will support significant exchange interactions, therefore we define the average nearest-neighbour exchange strength, J , as a sum of the exchange through pyrazine, J pyz , and water, J H 2 O , such that n J = 4J pyz + 2J H 2 O , where n is the total number of effective nearest-neighbour exchange pathways.…”

“…The fit yields the following characteristic amplitudes, A i (J K −1 mol −1 ), and temperatures, θ i (K), of the Debye (i = D) and Einstein (i = E) phonon modes: A D = 98 (13), θ D = 116(9), A E1 = 109(6), θ E1 = 208 (21), A E2 = 234 (10), θ E2 = 509 (20), A E3 = 247 (27) and θ E3 = 1287(128).…”

“…By contrast, because of the difficulty in making real examples of these systems, experimental work in this area moves more slowly and several predictions remain untested. While recent advances have been made with molecule-based magnets [15][16][17][18][19][20][21][22][23], difficulties in obtaining high-quality single crystals of the newest materials continue to hinder progress.…”

Combining microscopic and macroscopic probes to untangle the single-ion anisotropy and exchange energies in an S=1 quantum antiferromagnet

“…These F atoms belong to bridging HF • . The ordering of the pyz ligands in [Ni(HF 2 )(pyz) 2 ]SbF 6 contrasts the two-fold positional disorder encountered in the related quasi-two-dimensional (Q2D) layered coordination polymers NiZ 2 (pyz) 2 (Z = Cl, Br, I, NCO), which crystallize in the I4/mmm space group 31,32 [ Fig. 1; dashed lines].…”

“…2+ motifs 31,40 (see Table S7 [30]). From these five compounds, we glean an average J = 0.9(2) K. Applying this to [Ni(HF 2 )(pyz) 2 ]SbF 6 leads to J = 9.4(5) K. Therefore, the resultant D/J and J /J ratios are more than and less than one, respectively, consistent with a preliminary DFT study 27 used to calculate J .…”