Simple molecular solids become unstable at high pressures, typically transforming to dense framework and/or metallic structures. We report formation of an unusual ionic solid ${\mathrm{NO}}^{+}{\mathrm{NO}}_3^{-}$ (nitrosonium nitrate) from ${\mathrm{N}}_2\mathrm{O}$ at pressures above 20 GPa and temperatures above 1000 K. Synchrotron x-ray diffraction indicates that the compound crystallizes with a structure related to the aragonite form of ${\mathrm{CaCO}}_3$ and ${\mathrm{NaNO}}_3$. Raman and infrared spectroscopic data indicate that the structure is noncentrosymmetric and exhibits a strong pressure dependent charge transfer and orientational order.

• Received 6 March 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.135504