A bulk nitride mono-crystal according to claim 40, wherein the supercritical solvent contains NH3 and/or its derivatives, and includes ions of elements of Group I (IUPAC 1989)???at least potassium or sodium ions, the feedstock consists essentially of gallium-containing nitride and/or its precursors, selected from a group including azides, imides, amido-imides, amides, hydrides, gallium-containing