Dissolved Intermediates in Ammonothermal Crystal Growth: Stepwise Condensation of [Ga(NH₂)₄]⁻ toward GaN

Reactions of Ga or GaN with RbNH₂ or CsNH₂ under ammonothermal conditions result in liquids representing intermediate compounds in the ammonobasic GaN crystal growth. These liquids are fully miscible with liquid ammonia at room temperature and under autogenous pressure, and may eventually solidify to amorphous solids. The Cs-containing liquid initially contains an equilibrium mixture of tetraamidogallate ions [Ga­(NH₂)₄]⁻ and a dinuclear complex [(H₂N)₃Ga­(μNH)­Ga­(NH₂)₃]^2–. Oxygen impurities induce the formation of a μ-O-bridged dinuclear complex [(H₂N)₃GaOGa­(NH₂)₃]^2–. Mononuclear tetraamidogallate ions are the dominating species directly after synthesis, but the equilibrium shifts gradually toward the imido complex when ammonia is removed, and eventually a second condensation step to [(H₂N)₂Ga­(μ-NH)₂Ga­(NH₂)₂]₂^– occurs. Addition of excess liquid ammonia under ambient conditions causes quantitative conversion of the dinuclear species to [Ga­(NH₂)₄]⁻. The [Ga­(NH₂)₄]⁻ ions are also the only observable gallium-containing species in saturated solutions of the solid intermediates Li­[Ga­(NH₂)₄] and Na₂[Ga­(NH₂)₄]­NH₂ in liquid ammonia at room temperature and autogenous pressure.