Thermal stability of the waylandite-structured nanocrystalline BiAl$_{3}$(PO$_{4}$)$_{2}$(OH)$_{6}$

A nanocrystalline powder of the waylandite-structured bismuth hydroaluminophosphate was obtained under hydrothermal conditions at 200$^{\circ}$C, 7 MPa and pH 7, and characterized by X-ray diffractometry, scanning electron microscopy (SEM), and energy dispersive microanalysis (EDAX). The simultaneous thermal analysis and high-temperature X-ray diffractometry have shown that the crystal-chemical formula of this compound can be represented as BiAl$_{3}$(PO$_{4}$)$_{2}$O(OH)$_{4}$ $\cdot$ (H$_{2}$O). This compound retains its structure and crystallite size ($\sim$65 nm) up to about 500$^{\circ}$C. It has been determined that the decomposition of this compound in the 540–800$^{\circ}$C range results in the formation of Bi$_{2}$O$_{3}$, Bi$_{2}$Al$_{4}$O$_{9}$ and AlPO$_{4}$ phases. At temperatures above 800$^{\circ}$C, a complete thermal decomposition of Bi$_{2}$Al$_{4}$O$_{9}$ and the formation of crystalline $\alpha$-Al$_{2}$O$_{3}$ occur in this system, while Bi$_{2}$O$_{3}$ keeps evaporating during the isothermal exposure.