Bespoke S = 1 quantum magnets based on planar, staggered or chiral architectures

We aim to control the spatial exchange and single-ion anisotropies (D) in bespoke S = 1 Ni(II) quantum magnets so that we may determine the Hamiltonian parameters to access novel regions of magnetic-field/temperature phase diagrams. Early work led to [Ni(HF₂)(pyz)₂]SbF₆ (pyz = pyrazine) which contains Ni-FHF-Ni chains (J_FHF) cross-linked by Ni-pyz-Ni segments (J_pyz) to form a tetragonal network [1]. The material undergoes long-range XY-AFM order below T_N = 12.2 K with collinear Ni(II) moments aligned in the [Ni(pyz)₂]²⁺ plane. High-field M(H) and inelastic neutron scattering (INS) revealed D ≈ J_FHF >> J_pyz. Exploiting differences in donor-atom hardness yields anisotropic lattice-strain by introducing isotropic Zn(II) ions in [Zn_0.8Ni_0.2(HF₂)(pyz)₂]SbF₆ [2]. Dimensional reduction of [Ni(HF₂)(pyz)₂]SbF₆ with monocoordinate 3,5-lutidine (lut) leads to neutral molecules of NiX₂(lut)₄ (X = HF₂, F‧H₂O, Cl, Br, I) that pack in X-Ni-X‧‧‧X-Ni-X columns [3]. The more diffuse electron density of iodide affords I‧‧‧I close contacts and Haldane physics with J_1D = 17.5 K and D = -1.2 K [4]. NiBr₂(lut)₄ displays weak intrachain coupling and D > 0 whilst X = HF₂, F‧H₂O and Cl demonstrate single-ion behavior [3]. Further chemical substitution of pyz for pyrimidine (pym) produces staggered or chiral topologies and enhanced magnetic interactions along Ni-pym-Ni relative to Ni-pyz-Ni. Along these lines, two new Q1D chains have been synthesized: staggered [Ni(pym)(H₂O)₂(NO₃)₂] [5], which is isostructural to the well-known S = 1/2 Cu(II) sine-Gordon system [6], and chiral Q1D [Ni(pym)(H₂O)₄]SO₄‧H₂O [5]. Both materials exhibit multiple field-induced phase transitions at low-temperatures. Lastly, we will describe a unique 3D chiral Ni(II) framework based on a diamond lattice with 90° alternately-rotated Ni(II) sites [5]. Time permitting, the structural and magnetic properties of these materials will be discussed.