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Improvements to the growth of nitride crystals in ammonothermal growth environments can be achieved through improved autoclave designs, purity, and use of in-situ monitoring techniques. Given the limited data available on the stability of materials in supercritical ammonia solutions, this study intends to broaden the known dataset by experimentally investigating the mechanical and chemical stability of 35 bulk metals, 2 bulk metalloids, and 17 bulk ceramics while identifying suitable materials for future in-depth corrosion studies. This was performed by exposing each material to three different supercritical ammonia solutions in nickel-chromium superalloy autoclaves held at an external wall temperature of 575 °C for 4-12 days. The solutions were formed by initially filling the autoclave with pure ammonia (NH 3), ammonia and sodium (NH 3-Na), or ammonia and ammonium chloride (NH 3-Cl) to achieve total system pressures of 100-250 MPa. Zirconia, silicon carbide, tungsten carbide, molybdenum and its alloys, tungsten and its alloys, and a cobalt-tungsten-aluminum alloy

Due to the disparity between observed gallium nitride (GaN) growth under conditions for which literature reports normal solubility, GaN solubility in supercritical NH 3-Na containing solutions was re-evaluated. Isothermal gravimetric experiments on polycrystalline GaN were performed in the temperature range (T = 415-650 °C) for which retrograde growth of GaN routinely occurs (P 200 MPa, molar NH 3 : Na fill ratio = 20:1). Two previously-unreported error contributions to the gravimetric determination of GaN solubility were identified: Ga-alloying of exposed Nicontaining components, and the presence of a dense, Ga-absorbing Na-rich, second phase under these conditions. Due to the inability to measure Ga-alloying of the exposed autoclave wall for each experiment, considerable scatter was introduced in the refined GaN solubility curve. No clear dependence of GaN solubility on temperature was resolvable, while most solubility values were determined to be within a band of 0.03-0.10 mol. % GaN, normalized by fill NH 3 .

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