A cascade of magnetic phase transitions and a 1/3-magnetization plateau in selenite-selenate Co 3 (SeO 3 )(SeO 4 )(OH) 2 with kagomé-like Co 2+ ion layer arrangements: the importance of identifying a correct spin lattice.

We prepared a new compound, Co(SeO)(SeO)(OH), having layers in a kagomé-like arrangement of Co (spin = 3/2) ions. This phase crystallizes in the orthorhombic space group (62) with unit cell parameters = 11.225(9) Å, = 6.466(7) Å and = 11.530(20) Å. Its layers, parallel to the -plane, are made up of Co1O square pyramids and Co2O and Co3O octahedra. As the temperature is lowered, Co(SeO)(SeO)(OH) undergoes three successive magnetic transitions at 27.5, 19.4 and 8.1 K, and the magnetization of Co(SeO)(SeO)(OH) measured at 2.4 K exhibits a 1/3-magnetization plateau between 7.8 and 19.9 T. The - magnetic phase diagram constructed for Co(SeO)(SeO)(OH) from ac and dc magnetic susceptibility, specific heat and magnetization measurements contains three magnetic phases I, II and III. Phase I is antiferromagnetic, while phases II and III are ferrimagnetic and responsible for the 1/3-magnetization plateau. To interpret these complex magnetic properties, we identified the correct spin lattice for Co(SeO)(SeO)(OH) by evaluating its intralayer and interlayer spin exchanges based on spin-polarized DFT+ calculations.