Formation of new crystalline phases upon high-temperature annealing of iron borate FeBo$_3$ in different gas media

Iron borate FeBO$_3$ crystals are annealed in neutral, oxidizing, and reducing gas media on a specially designed setup. The effect of the chemical medium and annealing modes on the surface morphology and phase composition of the samples is established. It is shown that annealing in a neutral or oxidizing medium leads to the recrystallization of FeBO$_3$ into the hematite $\alpha$-Fe$_2$O$_3$ phase, whereas annealing in a reducing medium results in the formation of the pyroborate Fe$_2$B$_2$O$_5$ phase and the metal $\alpha$-Fe iron phase. The difference in the nature of structural transformations is associated with the change in the valence state of iron ions upon annealing in the reducing atmosphere. This result is of interest as a method for the transformation of crystalline phases.