Effects of synthesis conditions on the crystal and local structures of high-entropy oxides Ln₂M₂O₇ (Ln = La-Yb, Y; M = Ti, Zr, Ce)

The synthesis and detailed study of six series of high-entropy complex oxides containing lanthanides (Ln) and transition metals with the general formula Ln(2)M(2)O(7) (Ln = La-Yb, and Y; M = Ti, Zr, and Ce) with the number of different Ln cations not less than six in each case are reported. The influence of synthesis conditions (types of the Ln(3+) and M4+ cations, calcination temperature) used in the synthesis via either coprecipitation or sol-gel method on the crystal and local structures of target materials is comprehensively surveyed. The studies were carried out using a combination of long- (s-XRD), medium- (Raman, FT-IR, SEM-EDS) and short-range (XAFS) sensitive techniques, as well as AES-ICP and STA. It was established that the ratio of the cation radii gamma = (r) over bar (3+)(Ln)/(r) over bar (4+)(M) is the main factor that determines the type of initially formed crystal structure. In the boundary region (gamma similar to 1.42-1.47), the average radius of lanthanide cation ((r) over bar (3+)(Ln)), along with the (r) over bar (3+)(Ln)/(r) over bar (4+)(M) ratio, also plays a significant role in the type of the resulting crystal structure of the high-entropy lanthanide complex oxides. The presence of inhomogeneity in the distribution of elements in precursors significantly affects the phase composition of the resulting high-entropy oxides. An increase in the calcination temperature promotes not only the occurrence of subsequent phase transitions, but also an increase in the single-phase nature of the resulting high-entropy complex rare-earth oxides. At the same time, the cations included in the composition retain some independence, despite the fact that they occupy one crystallographic position in the resulting crystal structure.