Formation of Solid Solutions and Physicochemical Properties of the High-Entropy Ln(1−x)Sr(x)(Co,Cr,Fe,Mn,Ni)O(3−δ) (Ln = La, Pr, Nd, Sm or Gd) Perovskites

Phase composition, crystal structure, and selected physicochemical properties of the high entropy Ln(Co,Cr,Fe,Mn,Ni)O(3−δ) (Ln = La, Pr, Gd, Nd, Sm) perovskites, as well as the possibility of Sr doping in Ln(1−x)Sr(x)(Co,Cr,Fe,Mn,Ni)O(3−δ) series, are reported is this work. With the use of the Pechini method, all undoped compositions are successfully synthesized. The samples exhibit distorted, orthorhombic or rhombohedral crystal structure, and a linear correlation is observed between the ionic radius of Ln and the value of the quasi-cubic perovskite lattice constant. The oxides show moderate thermal expansion, with a lack of visible contribution from the chemical expansion effect. Temperature-dependent values of the total electrical conductivity are reported, and the observed behavior appears distinctive from that of non-high entropy transition metal-based perovskites, beyond the expectations based on the rule-of-mixtures. In terms of formation of solid solutions in Sr-doped Ln(1−x)Sr(x)(Co,Cr,Fe,Mn,Ni)O(3−δ) materials, the results indicate a strong influence of the Ln radius, and while for La-based series the Sr solubility limit is at the level of x(max) = 0.3, for the smaller Pr it is equal to just 0.1. In the case of Nd-, Sm- and Gd-based materials, even for the x(Sr) = 0.1, the formation of secondary phases is observed on the SEM + EDS images.