Peculiarities of phase formation in TbBO$_{3}$ during isothermal annealing

Detailed X-ray studies of changes in the structure of terbium orthoborate TbBO$_{3}$ in the course of successive high-temperature isothermal annealing of the initial mixture in the form of an amorphous precursor and in the form of a homogenized mixture of microcrystalline Tb$_{7}$O$_{12}$ and B$_2$O$_3$ powders have been carried out. It is shown that the formation of TbBO$_{3}$ crystals in both cases occurs through the formation of intermediate two-phase states. Particularly, the triclinic high-temperature $\nu$-TbBO$_{3}$ phase is formed already at the first crystallization stages (about 600$^{\circ}$C) almost simultaneously with the equilibrium vaterite modification of TbBO$_{3}$ (sp. gr. P 63/ mmc ) during low-temperature annealing of an amorphous precursor, which transforms into the vaterite modification at an annealing temperature of 850$^{\circ}$C. The phase of monoclinic terbium trioxoborate TbB$_3$O$_6$ is formed at the first stages of phase formation (about 800$^{\circ}$C) almost simultaneously with the vaterite phase when the feedstock is annealed in the form of a homogenized mixture of microcrystalline Tb$_{7}$O$_{12}$ and B$_{2}$O$_{3}$ powders. It also transforms into the vaterite modification at an annealing temperature of 950$^{\circ}$, which is preserved up to the highest annealing temperatures in the experiment (1200$^{\circ}$C). A hypothetical explanation of the formation of such two-phase states during low-temperature annealing of the feedstock and their disappearance at higher annealing temperatures is proposed.