Chemistry of Ammonothermal Synthesis

Abstract

Ammonothermal synthesis is a method for synthesis and crystal growth suitable for a large range of chemically different materials, such as nitrides (e.g., GaN, AlN), amides (e.g., LiNH₂, Zn(NH₂)₂), imides (e.g., Th(NH)₂), ammoniates (e.g., Ga(NH₃)₃F₃, [Al(NH₃)₆]I₃ · NH₃) and non-nitrogen compounds like hydroxides, hydrogen sulfides and polychalcogenides (e.g., NaOH, LiHS, CaS, Cs₂Te₅). In particular, large scale production of high quality crystals is possible, due to comparatively simple scalability of the experimental set-up. The ammonothermal method is defined as employing a heterogeneous reaction in ammonia as one homogenous fluid close to or in supercritical state. Three types of milieus may be applied during ammonothermal synthesis: ammonobasic, ammononeutral or ammonoacidic, evoked by the used starting materials and mineralizers, strongly influencing the obtained products. There is little known about the dissolution and materials transport processes or the deposition mechanisms during ammonothermal crystal growth. However, the initial results indicate the possible nature of different intermediate species present in the respective milieus.