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

Abstract

The synthesis and detailed study of six series of high-entropy complex oxides containing lanthanides (Ln) and transition metals with the general formula Ln2M2O7 (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 Ln3+ 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 γ = r̄Ln3+/r̄M4+ is the main factor that determines the type of initially formed crystal structure. In the boundary region (γ∼ 1.42–1.47), the average radius of lanthanide cation (r̄Ln3+), along with the r̄Ln3+/r̄M4+ 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.