Tetrahedrally Bonded Densec2n3hwith a Defective Wurtzite Structure: X-ray Diffraction and Raman Scattering Results at High Pressure and Ambient Conditions

Synchrotron x-ray diffraction and Raman scattering data supported by ab initio calculations are reported for the dense tetrahedrally bonded phase $({\text{C}}_{2}{\text{N}}_{3}\text{H})$ with a defective wurtzite (dwur) structure synthesized by laser heating from dicyandiamide $({\text{C}}_{2}{\text{N}}_{4}{\text{H}}_{4})$ at high pressure in a diamond anvil cell. This work confirms the structure deduced in previous work from electron diffraction experiments. The phase $(\text{Cmc}{2}_{1})$ is recoverable to ambient conditions. The ambient pressure volume $({\text{V}}_{0}=137.9\text{ }{\text{\AA{}}}^{3})$ and bulk modulus $({\text{K}}_{0}=258\ifmmode\pm\else\textpm\fi{}21\text{ }\text{GPa})$ are in excellent agreement with density functional calculations $({\text{V}}_{0}=134.7\text{ }{\text{\AA{}}}^{3};\text{ }{\text{K}}_{0}=270\text{ }\text{GPa})$. The calculated Raman frequencies and pressure shifts are also in good agreement with experiment. Ammonia $(\text{P}{2}_{1}{2}_{1}{2}_{1})$ was identified among the reaction products as expected from the synthesis reaction.