materials Article 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 Juliusz D ˛ abrowa 1, * , Klaudia Zieli ´ nska 2, *, Anna St ˛ epie ´ n 2 , Marek Zajusz 1 , Margarita Nowakowska 1 , Maciej Mo ´ zdzierz 2 , Katarzyna Berent 3 , Maria Szymczak 1 and Konrad ´ Swierczek 2,4   Citation: D˛ abrowa, J.; Zieli ´ nska, K.; St˛ epie ´ n, A.; Zajusz, M.; Nowakowska, M.; Mo ´ zdzierz, M.; Berent, K.; Szymczak, M.; ´ Swierczek, K. 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. Materials 2021, 14, 5264. https://doi.org/10.3390/ma14185264 Academic Editor: Joseph Poon Received: 21 August 2021 Accepted: 6 September 2021 Published: 13 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Faculty of Materials Science and Ceramics, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Krakow, Poland; zajuszm@agh.edu.pl (M.Z.); margarita@student.agh.edu.pl (M.N.); szymczak@student.agh.edu.pl (M.S.) 2 Faculty of Energy and Fuels, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Krakow, Poland; olszewska@agh.edu.pl (A.S.); mozdzier@agh.edu.pl (M.M.); xi@agh.edu.pl (K. ´ S.) 3 Academic Centre for Materials and Nanotechnology, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Krakow, Poland; kberent@agh.edu.pl 4 AGH Centre of Energy, AGH University of Science and Technology, ul. Czarnowiejska 36, 30-054 Krakow, Poland * Correspondence: dabrowa@agh.edu.pl (J.D.); zielinskak@student.agh.edu.pl (K.Z.); Tel.: +48-12-617-4641 (J.D.) Abstract: 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. Keywords: high entropy oxides; perovskites; structural properties; electrical properties 1. Introduction The development of high-entropy materials is widely considered to be a major break- through in the design of next-generation functional compounds, with the high-entropy oxides (HEOx) having special prominence in this regard. Since their initial development in 2015, when Rost et al. synthesized a single-phase, rocksalt-structured (Co,Cu,Mg,Ni,Zn)O solid solution [1], the high-entropy design principle has been successfully translated to a number of different crystallographic structures, such as transition metal-based high-entropy spinels [2–5], high-entropy perovskites [6,7], bixbyite- and fluorite-structured high-entropy oxides [8–11], high-entropy lanthanide sesquioxides [12], magnetoplumbite-structured high-entropy oxides [13], high-entropy pyrochlores [14], and high-entropy garnets [15]. By analogy to the conventional oxide materials, among all those listed above, one group in particular attracts much attention of the scientific community, namely, the high-entropy perovskites. The classical ABO 3 perovskites, where A and B denotes separate cation sublat- ticies, are widely considered to be among the most versatile functional materials, offering Materials 2021, 14, 5264. https://doi.org/10.3390/ma14185264 https://www.mdpi.com/journal/materials