Effect of Oxygen on Ammonothermal Synthesis: Example of Na<sub>2</sub>[Zn(NH<sub>2</sub>)<sub>4</sub>] ⋅ (NH<sub>3</sub>)<sub><i>x</i></sub> and Na<sub>2</sub>[Zn(NH<sub>2</sub>)<sub>4</sub>] ⋅ (H<sub>2</sub>O)<sub><i>x</i></sub>
European journal of inorganic chemistry