Does metallic ammonium exist?

THE ammonium ion NH₄⁺ behaves in many ways like an alkali metal ion. This led Ramsey¹ to propose that monovalent ammonium metal (NH₄⁺ ions immersed in an almost uniform, degenerate sea of electrons) is stable at pressures much lower than typical insulator-metal transition pressures² of megabars ~10¹¹ Pa (10⁵ Pa = 1 bar). Subsequently, it was estimated³ that the transition from a mixture of NH₃ and H₂ molecules to metallic ammonium, occurs at a pressure of less than 2.5×10¹⁰ Pa. If this estimate were correct, metallic ammonium would be of great interest to experimental high pressure physicists, and to planetary physicists. Indeed, metallic ammonium has been invoked in some^4,5 but not all models for the interiors of Uranus and Neptune; and its existence could be of importance⁶ for calculations of the interior magnetic fields of these planets.