Effect of Oxygen on Ammonothermal Synthesis: Example of Na₂[Zn(NH₂)₄] ⋅ (NH₃)_x and Na₂[Zn(NH₂)₄] ⋅ (H₂O)_x

Abstract

Polycrystallinepowder and single crystals of Na 2 [Zn(NH 2 ) 4 ] • (NH 3 ) 0.24 and Na 2 [Zn(NH 2 ) 4 ] • (H 2 O) 0.37 were obtained under ammonobasic conditions at 823 K and 200 MPa. Upon substituting the constituent ammonia by water, the space group (C2/c) and crystal structure remain generally unchanged. Nevertheless, the presence of water during the ammonothermal synthesis has a fundamental impact on the transport direction, changing from exothermic for the pure ammonia system to an endothermic material transport for the hydrate formation under essentially the same experimental conditions. That indicates that even a small amount of oxygen can have a fundamental influence on the solubility and its temperature dependence of dissolved species during ammonobasic synthesis. Chemical analysis, Raman spectroscopy and thermal analysis consistently support the composition assignment. Solid-state NMR measurements back up the exchange of exclusively ammonia by water, rather than introduction of hydroxide ions. A minor enhancement of the thermal stability of the hydrate compared to the ammoniate is observed.