Abstract:
The effect of the hydrothermal fluid pH on the chemical and phase composition, as well as the size parameters and morphology of crystallites and particles of hydrothermal synthesis products formed in the Na$_2$O–Bi$_2$O$_3$–Fe$_2$O$_3$–MoO$_3$ system at $T$ = 170$^\circ$C and $P<$ 7 MPa has been studied. It has been established that in the acidic pH region, the bulk chemical composition of the hydrothermal synthesis products is depleted relative to the nominal composition specified for the synthesis in iron oxide, while in the alkaline pH region, it is depleted in molybdenum oxide and, to a lesser extent, in bismuth oxide, while the best correspondence between the nominal and bulk composition observed at pH = 2. It is shown that in the pH range from 2 to 6 new compounds of variable composition (Na$_{0.19-0.47}$Bi$_{0.42-0.85}$Fe$_{0.14-0.31}$MoO$_y$) with a scheelite-like structure (sp. gr. $I\bar4$, No. 82) are formed, which have not been previously described in the scientific literature. These compounds with the smallest mean crystallite size ($\sim$ 25 nm) were obtained at pH = 2, and it was shown that under these conditions polycrystalline plate-like particles (thickness $(h)\sim$ 50–150 nm) are formed, often having a curved shape, which grow together to form agglomerates with a “flower-like” morphology. It was found that fluorite-type solid solutions (Bi$_{3.65-4.30}$Fe$_{0.37-0.45}$MoO$_z$) are formed in alkaline media (isostructured to the oxide $\delta$-Bi$_2$O$_3$ (sp. gr. $Fm\bar3m$, No. 225)).
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