Extended Anionic Network in Mixed-Valent Nitridocobaltates(I/II) LnCo₂N₂ (Ln = La, Pr, Nd)

Nitridometallates with extended anionic frameworks exhibit a variety of intriguing electronic properties, including band magnetism, metal-metal bonding, and superconductivity. Such extended framework materials are scarce with late transition metals due to the requirement of high metal oxidation states and nitrogen content. This study presents a family of nitridocobaltates LnCo₂N₂ (Ln = La, Pr, Nd) with an extended layered cobalt-nitrogen network related to the Kagome lattice that was synthesized at 8 GPa in a large volume press. The compounds crystallize in the trigonal space group R $\overline{3}$ with alternating layers of unusual trigonal planar CoN₃ and octahedral LnN₆ polyhedra. Magnetization and powder neutron diffraction studies indicate a metallic ground state and the suppression of magnetic ordering of the rare earth moments due to their arrangement on a trigonal lattice. The compounds have a surprising mixed-valent Co^+I/+II state, which is high for nitridocobaltates. This study demonstrates the structural diversity available in late nitridometallates via high-pressure synthesis, which can now be explored systematically.