Synthesis of LaCN₃, TbCN₃, CeCN₅, and TbCN₅ Polycarbonitrides at Megabar Pressures

Inorganic ternary metal-C-N compounds with covalently bonded C-N anions encompass important classes of solids such as cyanides and carbodiimides, well known at ambient conditions and composed of [CN]^- and [CN₂]^2- anions, as well as the high-pressure formed guanidinates featuring [CN₃]^5- anion. At still higher pressures, carbon is expected to be 4-fold coordinated by nitrogen atoms, but hitherto, such CN₄-built anions are missing. In this study, four polycarbonitride compounds (LaCN₃, TbCN₃, CeCN₅, and TbCN₅) are synthesized in laser-heated diamond anvil cells at pressures between 90 and 111 GPa. Synchrotron single-crystal X-ray diffraction (SCXRD) reveals that their crystal structures are built of a previously unobserved anionic single-bonded carbon-nitrogen three-dimensional (3D) framework consisting of CN₄ tetrahedra connected via di- or oligo-nitrogen linkers. A crystal-chemical analysis demonstrates that these polycarbonitride compounds have similarities to lanthanide silicon phosphides. Decompression experiments reveal the existence of LaCN₃ and CeCN₅ compounds over a very large pressure range. Density functional theory (DFT) supports these discoveries and provides further insight into the stability and physical properties of the synthesized compounds.