Mechanochemical synthesis: route to novel rock-salt-structured high-entropy oxides and oxyfluorides

A facile mechanochemical reaction at ambient temperature was successfully applied to synthesize novel single-phase rock-salt-structured high-entropy oxides, containing five, six and seven metal elements in equiatomic amounts. This synthesis approach overcomes the limitations of the commonly known synthesis procedures, which would result in multiple-phase compounds. Redox-sensitive elements, such as Fe$^{2+}$ and Mn$^{2+}$, can now be considered. The corresponding single-phase Li-containing high-entropy oxyfluorides were obtained by introducing LiF into the lattice using the same strategy. All materials show single-phase rock-salt structures with lattice parameters depending on the incorporated ion sizes. Solid solution states result in high configurational entropies, and all elements appear homogenously distributed over the whole cationic and anionic sublattice. The straightforward synthesis technique, combined with utilized simple binary oxide precursors, paves the way for a multitude of novel high-entropy oxide and oxyfluoride compounds. The compounds were studied by means of X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray spectroscopy and Mössbauer spectroscopy.