Is there a crystal structure of ammonia in ionic form?
According to the work of Ninet et al. published in 2014 [1], ammonia has a β-phase at 150 GPa, where it transitions into ionic crystalline form, forming $\ce{[NH4+][NH2-]}.$

References


Ninet, S.; Datchi, F.; Dumas, P.; Mezouar, M.; Garbarino, G.; Mafety, A.; Pickard, C. J.; Needs, R. J.; Saitta, A. M. Experimental and Theoretical Evidence for an Ionic Crystal of Ammonia at High Pressure. Phys. Rev. B 2014, 89 (17), 174103. DOI: 10.1103/PhysRevB.89.174103.