Ultrahigh Degree of Cationic Disorder, Configurational Entropy in New Type of High-Entropy Pseudobrookite Phase

High-entropy ceramics exhibit various excellent properties owing to their high configurational entropy, which is caused by multi-principal elements sharing one lattice site. The configurational entropy will further increase significantly if multi-principal elements randomly share two different lattice sites. For this purpose, pseudobrookite phase containing two cationic lattice sites (A and B sites) is selected, and corresponding high-entropy pseudobrookite (M²⁺ _0.4M³⁺ _1.2)Ti_1.4O₅ is synthesized. Herein, the distribution of the 2-valent and 3-valent cations in the A and B sites are analysed in depth. The distance between the A and B sites in the crystal structure models which are constructed by the Rietveld analysis is calculated and defined as distance d. Meanwhile, the atomic column positions in the STEM images are quantified by a model-based mathematical algorithm, and the corresponding distance d are calculated. By comparing the distance d, it is determine that the 2-valent and 3-valent cations are jointly and disorderly distributed in the A and B sites in high-entropy (M²⁺ _0.4M³⁺ _1.2)Ti_1.4O₅. The density functional theory (DFT) simulations also demonstrate that this type of crystal structure is more thermodynamically stable. The higher degree of cationic disorder leads to a higher configurational entropy in high-entropy (M²⁺ _0.4M³⁺ _1.2)Ti_1.4O₅, and endows high-entropy (M²⁺ _0.4M³⁺ _1.2)Ti_1.4O₅ with very low thermal conductivity (1.187-1.249 W m^-1 K^-1).