Pyrochlore Phase in the Bi2O3–Fe2O3–WO3–(H2O) System: its Formation by Hydrothermal-Microwave Synthesis and Optical Properties

Composites with the (Bi, Fe, ◻)2(Fe, W)2O6O'δ composition (pyrochlore, hereinafter BFWO)/amorphous phase were obtained by hydrothermal-microwave synthesis in the Bi2O3–Fe2O3–WO3–(H2O) system and characterized. The degree of the amorphous phase transformation into crystalline BFWO was found to increase from ~0.08 to ~0.82 with an increase in isothermal exposure duration at 180°C from 1 s to 5 min. The type of this dependence correlates well with the nature of the change in the BFWO phase crystallites average size (estimated by the Williamson–Hall graphical method), which increases from 270(20) to ~600 nm. It is shown that two morphological motifs are found in all samples, one of which is represented by particles whose average size correlates with the average size of the BFWO phase crystallites, and the other is represented by agglomerates sized at ~100 nm and composed of particles of the amorphous phase. Large particles have a core-shell type structure, in which BFWO crystallites form the core, and the shell is composed of the amorphous phase. The band gap (Eg) of the obtained composite materials for direct allowed electronic transitions is in the range of 2.26‒2.40 eV and decreases with an increase in the fraction of the BFWO crystal phase in the composite.