The use of amorphous high surface area silicon nitride is proposed as a raw material for crystallization experiments in supercritical ammonia. Compared with earlier studies, the use of highly dispersed solids results in the crystallization of inorganic nitrides under relatively mild conditions (673K). Mineralizers such as amides (LiNH₂, NaNH₂, KNH ₂) are found to be effective crystallization aids. The crystalline products, detected using powder X-ray diffraction, are either MSi ₂N₃ (M = Li, Na) or Si₂N₂NH. Si ₂N₂NH is also characterized using ²⁹Si MAS NMR. The spectrum shows a narrow line located at -44.7ppm, whereas for amorphous silicon nitride-based materials the line is broad. The ammonothermal reaction of NaAl(NH₂)₄ and high surface area silicon nitride at 673 K affords a new orthorhombic phase, isostructural with NaSi ₂N₃, but with extended lattice constants (a = 9.634, b = 5.643, c = 5.011Å). Effective crystallization is also achieved using fluoride mineralizers (KF, CsF) at 673 K. A new small scale autoclave, suitable for laboratory syntheses at temperatures up to 873 K, is presented that can be loaded under inert gas.