The role of the reaction medium pH in the formation of nanocrystalline phases in the Bi$_2$O$_3$–P$_2$O$_5$–H$_2$O system

The work is devoted to studying the influence of pH values in an aqueous-salt medium on the formation of compounds in the Bi$_2$O$_3$–P$_2$O$_5$–H$_2$O (OH$^-$, H$^+$) oxide system. It has been shown that in an acidic environment (pH = 2) at a temperature of 298 K, hexagonal BiPO$_4$ forms, while at pH values of 8 and 12, X-ray amorphous substances are produced. After hydrothermal treatment at 473 K in an aqueous-salt environment, a monoclinic modification of bismuth phosphate forms from hexagonal bismuth phosphate in an acidic environment, and nanometer-sized particles of crystalline compounds Bi$_3$O(OH)(PO$_4$)$_2$ (with a crystallite size of about 62 nm) and Bi$_2$O$_3$ (with a crystallite size of about 70 nm) form in weakly alkaline and alkaline media. Using the method of thermodynamic calculation, the dependences of the equilibrium molar solubility of these crystalline compounds on the pH value of the aqueous-salt suspension were obtained. Thermodynamic calculations showed that the BiPO$_4$ compound is stable in the pH range from 0 to 5.8 at temperatures of 298 and 473 K. The pH range from 5.8 to 9.8 is characterized by the formation of the Bi$_3$O(OH)(PO$_4$)$_2$ compound at 298 K, and a further increase in the pH value leads to the precipitation of Bi$_2$O$_3$, BiOOH or Bi(OH)$_3$, which are similar in solubility, at 298 and 473 K. The data obtained from thermodynamic calculations are consistent with experimental data on the stability boundaries of BiPO$_4$, Bi$_3$O(OH)(PO$_4$)$_2$, and Bi$_2$O$_3$ compounds.