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.
Highlights
Ammonothermal synthesis has gained increasing research interest over the last 20 years
The role of mineralizers in chemical reactions is not always known, they are used in ammonothermal synthesis [66,67] as well as in various further methods
The first binary nitride synthesized by the ammonothermal method was Be3N2, which was obtained as pure cubic phase in 1966 by Juza and Jacobs from beryllium in supercritical ammonia at 673 K and 20.3 MPa [2]
Summary
Ammonothermal synthesis has gained increasing research interest over the last 20 years. An abundance of further materials like hydroxides, chalcogenides and hydrogen sulfides can be obtained from supercritical ammonia. Both hydrothermal and ammonothermal synthesis are part of the large group of solvothermal methods. The characteristics of ammonothermal synthesis and its products are mainly influenced by high pressure and temperature and usually syntheses are carried out under supercritial conditions. Solvents exist as homogeneous supercritical fluids, where gas and liquid phase can no more be distinguished and the properties of the different phases converge. We define ammonothermal synthesis as a reaction in ammonia as one homogenous fluid next to or under supercritical conditions. Various solvents showing distinct different chemical properties are used for solvothermal syntheses. (Li1−xMn2O4−y [15]), magnetic and catalytic materials (La1−xCa/Sr/BaMnO3 [16]) and fine dielectric ceramics (BaTiO3 [17])
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