  We report experimental and theoretical evidence that solid molecular ammonia
becomes unstable at room temperature and high pressures and transforms into an
ionic crystalline form. This material has been characterised in both
hydrogenated NH3 and deuterated ND3 ammonia samples up to about 180 and 200
GPa, respectively, by infrared absorption, Raman spectroscopy and x-ray
diffraction. The presence of a new, strong IR absorption band centered at 2500
cm-1 in NH3 (1900 cm-1 in ND3) signals the ionization of ammonia molecules into
NH2- and NH4+ ions, in line with previous theoretical predictions. We find
experimental evidence for a coexistence of two crystalline ionic forms, which
our ab-initio structure searches predict to be the most stable at the relevant
pressures. The ionic crystalline form of ammonia is stable at low temperatures,
which contrasts with the behaviour of water in which no equivalent crystalline
ionic phase has been found.
