A combination of organic and inorganic cations in the synthesis of transition metal nitrates: preparation and characterization of canted rectangular Ising antiferromagnet (PyH)CsCo2(NO3)6†

Pyridinium cesium cobalt nitrate, (PyH)CsCo₂(NO₃)₆, obtained from a nitric acid solution crystallizes in the orthorhombic space group Pnma with unit cell parameters a = 8.6905(14) Å, b = 11.9599(18) Å, c = 18.386(3) Å, V = 1911.0(5) ų, and Z = 4. It consists of [Co(NO₃)₃]⁻ layers, in which each Co²⁺ ion is connected with four monodentate bridging NO₃-groups and one bidentate terminal NO₃-group, forming a corrugated rectangular net. Magnetization and specific heat measurements show that (PyH)CsCo₂(NO₃)₆ undergoes a long-range canted antiferromagnetic ordering in two steps at T_C1 = 5.0 K and T_C2 = 2.6 K. The temperature dependence of the magnetic susceptibility and the field dependence of the magnetization measured for (PyH)CsCo₂(NO₃)₆ show that it is an Ising antiferromagnet. In support of these observations, our DFT + U + SOC calculations show that the Co²⁺ ions of (PyH)CsCo₂(NO₃)₆ have an easy-axis magnetic anisotropy with preferred spin orientation along the b-axis. To a first approximation, the spin lattice of (PyH)CsCo₂(NO₃)₆ is a weakly alternating Ising antiferromagnetic chain (J₁/J₂ ∼ 0.85), and these chains interact weakly (J₃/J₂ ∼ 0.07) to form a rectangular Ising antiferromagnetic lattice. In agreement with the prediction for a rectangular Ising antiferromagnet by Onsager, (PyH)CsCo₂(NO₃)₆ undergoes a long-range antiferromagnetic ordering.