X-ray analysis of the actual structure of yttrium orthoborate YBO$_{3}$

Detailed X-ray analysis of variations in the structure of yttrium orthoborate in the process of successive high-temperature isothermal anneals of an originally amorphous precursor state is performed. It is established that the diffraction reflex intensity distribution of YBO$_{3}$ measured at room temperature, obtained in the initial stages of crystallization, corresponds to the known low-temperature vaterite phase with the space group (sp. gr.) $P$6$_{3}/m$ and, after a series of high-temperature anneals, it transforms into a distribution known for the vaterite modification with the sp. gr. $P$6$_{3}/mmc$ and the same lattice parameters. This result is explained on the basis of the sphericity of X-ray waves and is connected with the transformation of the crystallites from a spherical shape upon low-temperature anneals to a dumbbell shape upon high-temperature anneals. As a result of in situ experiments conducted at 1250$^\circ$C, it was established that the initial low-temperature hexagonal vaterite cell transforms above 1000$^\circ$C into a monoclinic cell.