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

Abstract

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 [CN 2 ] 2− anions, as well as the high-pressure formed guanidinates featuring [CN 3 ] 5− anion. At still higher pressures, carbon is expected to be 4-fold coordinated by nitrogen atoms, but hitherto, such CN 4 -built anions are missing. In this study, four polycarbonitride compounds (LaCN 3 , TbCN 3 , CeCN 5 , and TbCN 5 ) are synthesized in laser-heated diamond anvil cells at pressures between 90 and 111 GPa. Synchrotron singlecrystal X-ray diffraction (SCXRD) reveals that their crystal structures are built of a previously unobserved anionic singlebonded carbon−nitrogen three-dimensional (3D) framework consisting of CN 4 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 LaCN 3 and CeCN 5 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.