Formation of high-entropy octahedral crystals in multicomponent oxide systems

In the context of the project covering formation of high-entropy crystal phases with the magnetoplumbite structure the chemical composition, crystal structure, and formation conditions have been analyzed for multicomponent crystals with the spinel structure, obtained as by-products or experiment failures, including crystals with the composition not described in the literature previously. The list of chemical elements, which can be major components of phases with such structures, has been determined. Specifically, it has been found that the composition of the obtained spinel crystals can be expressed by the empirical formulas (Ni,Co,Mn2+)(Al,Cr,Fe,Ti,Ga,Mn3+)2O4 and (Co,Fe2+,Mn2+)(Al,Cr,Ti,V,Ga,Fe3+,Mn3+)2O4. At that Ba, Sr, Ca, K, Pb, La, and Bi in such phases can exist in the form of minor admixtures; their addition in all probability does not affect stabilization of the spinel-structured phase. Solubility of indium, In, judging from the obtained data, in solid solutions of the described kind can be limited. Important conclusions of great practical significance have been made, concerning the conditions to be created in the process of crystal growing from high-entropy melts. In particular, it has been shown that allowance must be made for additional oxidation during growing a monocrystal from a melt, compared to the level that can be ensured from the composition of the melt and the air atmosphere, in which the experiments were carried out. It is related to the fact that increasing synthesis temperature (specifically, from 1400 to 1600 °С) leads to reduction of the greater part of iron and manganese atoms to oxidation number +2, which favors formation of octahedral phase in great quantities and has a negative impact on formation of crystals with the magnetoplumbite structure.