Changes in the bonding of carbon under high pressure leads to unusual crystal chemistry and can dramatically alter the properties of transition metal carbides. In this work, the new orthorhombic polymorph of yttrium carbide, $\gamma \text{−}{\mathrm{Y}}_4{\mathrm{C}}_5$, was synthesized from yttrium and paraffin oil in a laser-heated diamond anvil cell at $\sim 50\mathrm{GPa}$. The structure of $\gamma \text{−}{\mathrm{Y}}_4{\mathrm{C}}_5$ was solved and refined using in situ synchrotron single-crystal x-ray diffraction. It includes two carbon groups: [${\mathrm{C}}_2$] dimers and nonlinear [${\mathrm{C}}_3$] trimers. Crystal chemical analysis and density functional theory calculations revealed unusually high noninteger charges (${[{\mathrm{C}}_2]}^{5.2-}$ and ${[{\mathrm{C}}_3]}^{6.8-}$) and unique bond orders ($<1.5$). Our results extend the list of possible carbon states at extreme conditions.

• Received 19 May 2021
• Accepted 18 August 2021

DOI:https://doi.org/10.1103/PhysRevLett.127.135501