The role of pH of the reaction medium in the formation of nanocrystalline phases in the Bi₂O₃-P₂O₅-H₂O system

   The work is devoted to studying the influence of pH values in an aqueous-salt medium on the formation of compounds in the Bi₂O₃–P₂O₅–H₂O(OH−, H⁺) oxide system. It has been shown that in an acidic environment (pH = 2) at a temperature of 298 K, hexagonal BiPO₄ 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 Bi₃O(OH)(PO₄)₂ (with a crystallite size of about 62 nm) and Bi₂O₃ (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 BiPO₄ 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 Bi₃O(OH)(PO₄)₂ compound at 298 K, and a further increase in the pH value leads to the precipitation of Bi₂O₃, 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 BiPO₄, Bi₃O(OH)(PO₄)₂, and Bi₂O₃ compounds.

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