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 3 (SeO 3 )(SeO 4 )(OH) 2 , having layers in a kagomé-like arrangement of Co 2+ (spin S = 3/2) ions. This phase crystallizes in the orthorhombic space group Pnma (62) with unit cell parameters a = 11.225(9) Å, b = 6.466(7) Å and c = 11.530(20) Å. Its layers, parallel to the ab -plane, are made up of Co1O 5 square pyramids and Co2O 6 and Co3O 6 octahedra. As the temperature is lowered, Co 3 (SeO 3 )(SeO 4 )(OH) 2 undergoes three successive magnetic transitions at 27.5, 19.4 and 8.1 K, and the magnetization of Co 3 (SeO 3 )(SeO 4 )(OH) 2 measured at 2.4 K exhibits a 1/3-magnetization plateau between 7.8 and 19.9 T. The  H - T  magnetic phase diagram constructed for Co 3 (SeO 3 )(SeO 4 )(OH) 2  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 3 (SeO 3 )(SeO 4 )(OH) 2 by evaluating its intralayer and interlayer spin exchanges based on spin-polarized DFT+ U calculations.