Phase composition and the structure of $(1-x)$KNO$_{3}$+$x$Al$_{2}$O$_{3}$ nanocomposites by X-ray diffraction

The phase composition and the structural properties of potassium nitrate KNO$_3$ and its heterogeneous composites with nanometer-sized powder of aluminum oxide Al$_{2}$O$_{3}$ have been studied by X-ray diffraction at various concentrations of an Al$_{2}$O$_{3}$ nanopowder. It is found that, in the $(1-x)$KNO$_{3}$+$x$Al$_{2}$O$_{3}$ nanocomposites, additional high-temperature rhombohedral phase of potassium nitrate (phase III) with lattice parameters $a$ = 5.4644 $\mathring{\mathrm{A}}$ and $c$ = 9.0842 $\mathring{\mathrm{A}}$. With increasing concentration of Al$_{2}$O$_{3}$ nanopowder, the content of the main potassium nitrate phase (phase II) is found to significantly decrease, and the relative fraction of the phase III in the total content of the nitrate in the composite composition increases. This phase is assumed to be “frozen” in the nanocomposite at the KNO$_{3}$–Al$_{2}$O$_{3}$ interface. The estimated size of KNO$_{3}$ crystallites in the phase III is more than 20 nm.