First Direct Study of the Ammonolysis Reaction in the Most Common Alkaline and Alkaline Earth Metal Hydrides by in Situ SR-PXD

We report on the first in situ synchrotron radiation powder X-ray diffraction study (SR-PXD) of the ammonolysis reaction of selected alkaline and alkaline earth metal hydrides (i.e., LiH, NaH, KH, MgH 2, and CaH 2 ). The investigation was performed using an in situ SR-PXD pressure cell at an initial NH 3 pressure of 6.5 bar in a range of temperature between room temperature (RT) and 350 °C. The results of this work give new important insights into the formation of metal amides and imides starting from the corresponding metal hydrides. LiH was observed to react with NH 3 to form LiNH 2 already at RT, and then it decomposes into Li 2 NH at 310 °C through the formation of nonstoichiometric intermediates of the Li 1+x NH 2– x form. The formation of NaNH 2 takes place nearly at RT (28 °C), and it melts at 180 °C. As for LiH, KH reacts with NH 3 at RT to surprisingly form, what it seems to be, cubic KNH 2 . However, we believe this phase to be a solid solution of KH in KNH 2 . At high temperature, the possible formation of several solid solutions of K(NH 2 ) 1– y H y with defined composition is also observed. The formation of Mg(NH 2 ) 2 was observed to starts at around 220 °C, from the interaction γ-MgH 2 and NH 3 . At 350 °C, when all γ-MgH 2 is consumed, the formation of Mg(NH 2 ) 2 stops and MgNH is formed by the reaction between β-MgH 2 and NH 3. Our results indicate that the formation of the γ-MgH 2 is a key step in the synthesis of Mg(NH 2 ) 2 at low temperature (e.g., via ball milling technique). CaH 2 was observed to react with NH 3 at around 140 °C to form CaNH. At higher temperature the appearance of new reflections of possible Ca 1+x NH phases, with the same crystalline structure of CaNH but with a smaller cell parameter was observed.