High-pressure synthesis of Ruddlesden-Popper nitrides

Layered perovskites with Ruddlesden-Popper-type structures are fundamentally important for low-dimensional properties, for example, photovoltaic hybrid iodides and superconducting copper oxides. Many such halides and oxides are known, but analogous nitrides are difficult to stabilize due to the high cation oxidation states required to balance the anion charges. Here we report the high-pressure synthesis of three single-layer Ruddlesden-Popper (K₂NiF₄ type) nitrides-Pr₂ReN₄, Nd₂ReN₄ and Ce₂TaN₄-along with their structural characterization and properties. The R₂ReN₄ materials (R = Pr and Nd) are metallic, and Nd₂ReN₄ has a ferromagnetic Nd³⁺ spin order below 15 K. Thermal decomposition gives R₂ReN₃ with a Peierls-type distortion and chains of Re-Re multiply bonded dimers. Ce₂TaN₄ has a structural transition driven by octahedral tilting, with local distortions and canted magnetic Ce³⁺ order evidencing two-dimensional Ce³⁺/Ce⁴⁺ charge ordering correlations. Our work demonstrates that Ruddlesden-Popper nitrides with varied structural, electronic and magnetic properties can be prepared from high-pressure synthesis, opening the door to related layered nitride materials.