Low-dimensional quantum magnets supported by non-covalent interactions*

At ambient pressure, [Ni(HF₂)(pyz)₂]SbF₆ contains S = 1 Ni(II) ions arranged in a tetragonal network consisting of 1D Ni-FHF-Ni chains (J) cross-linked by pyrazine (pyz) ligands (J’). Previous work determined D = 13.3 K, J = 10.4 K and J’ = 1.4 K [1]. Below T_N = 12.2 K, long-range magnetic order sets in and, as revealed by neutron diffraction, is of the XY-type with Ni(II) moments occupying any collinear orientation in the crystallographic ab-plane. Pressure-dependent X-ray powder diffraction data reveal a first-order structural phase transition at 3 GPa which leads to a distorted monoclinic network and kinked magnetic exchange pathways. In this high-pressure regime, the magnetic ground state switches to Ising-like with concomitant reduction in T_N. The related quasi-1D chain, NiI₂(3,5-lutidine)₄, also forms a tetragonal lattice but consiss of discrete molecular units. A chain-like motif affords close Ni-I...I-Ni contacts and strong magnetic coupling. Evidence suggests that this material is a rare isotropic Haldane chain with J = 17.5 K and D = 6.3 K [2]. Time permitting, structural and magnetic properties of each material will be presented.
[1] Brambleby, et al. PRB 95, 134435 (2017)
[2] Blackmore, et al. in prep.