Atomic Level Structural and Chemical Characterization of High-Entropy Oxides using Advanced (S)TEM Techniques

High entropy oxides (HEOs) exhibit unconventional properties, which have attracted intensive interest for a wide variety of applications such as electrochemical energy storage [1,2] and (electro) catalysis [3] as well as in fundamental studies on strongly correlated materials with competing magnetoelectric phases [4] giving rise to interesting properties such as colossal magnetoresistance (CMR) and metal-insulator transitions (MIT). The recent advances in (scanning) transmission electron microscopy (S)TEM have enabled a detailed structural analysis of HEOs providing atomic level chemical and oxidation state information together with local structural distortion and strain analysis as basis for a fundamental understanding of structure property correlations allowing to disentangle the complex high-entropy and ‘cocktail’ effects.<br/>In this presentation, I will illustrate some of these advanced TEM capabilities for a detailed analysis of HEOs. I will discuss how this information enabled deciphering the synergistic effects of the cations in a Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O electrode in lithium ion batteries [2] as well as the correlation between composition, oxidation state, structural distortions and magnetoelectric properties in strongly correlated high entropy manganates [4].<br/>The TEM investigation of a Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O electrode during various charging stages [3] revealed a nanoscale phase segregation during the initial discharging of the homogenous HEO into a metallic CoNiCu alloy phase, a Zn(Li) and a Mg oxide phase, which forms a highly defective, semi-coherent fcc composite structure. Although the material appears X-ray amorphous, 4D-STEM crystal orientation analysis shows that the composite structure extends throughout the originally single crystalline domains. Segregation of the metallic alloy at the grain boundaries gives rise to an interconnected conductive network throughout the composite particle. During recharging, Zn and partially Co are reoxidized (electrochemically active elements) forming a mixed fcc oxide with Mg (providing the structural stability) while the metallic NiCu(Co) phase (providing electrical conductivity) is maintained. This self-assembled nanostructure enables stable cycling of micron-sized particles and bypasses the need for nanoscale pre-modification required for conventional metal oxides. It demonstrates that the elemental diversity with a defined role of each element is key for optimizing these multi-cation electrode materials.<br/>In a series of single-phase orthorhombic (Gd_0.25La_0.25Nd_0.25Sm_0.25)_1-xSr_xMnO₃ (x=0-0.5) HE-manganites, which combines the high entropy (HE) concept with property control by Sr²⁺ (hole) doping, we extend the HE approach to design strongly-correlated systems. [4] Using atomic resolution STEM imaging, an orthorhombic distortion has been identified for x=0, which is reduced with increasing Sr concentration to pseudo-cubic for x=0.5. Atomically resolved elemental maps reveal a homogenous distribution of the rare-earth cations and Sr on the A-site and Mn on the B-site sub-lattice. The oxidation state change of Mn has been evaluated by EELS using the O K and Mn L-edge features, unambiguously confirming that the Sr²⁺ doping results in increased Mn⁴⁺. These electronic and structural changes correlate with a transition of the magnetoelectric properties from an insulting antiferromagnetic to a metallic ferromagnetic phase with good CMR values. This initial study signals the excellent potential to achieve complex magnetoelectric phase diagrams with unique temperature dependencies that stem from competing magneto-electronic interactions, which can be tuned by merging high entropy design with strongly correlated electron systems.<br/><br/><b>References</b><br/>[1] A. Sarkar <i>et al.,</i> <i>Nat. Commun.</i>, <b>2018</b>, <i>9</i><b>,</b> 3400.<br/>[2] K. Wang et al., <i>Nat. </i><i>Commun.</i>, <b>2023</b>, <i>14</i>, 1487.<br/>[3] L. Lin et al., <i>Small Structures, </i><b>2023</b>, 2300012<br/>[4] A. Sarkar et al., <i>Adv. </i><i>Mater.</i>, <b>2023</b>, 35, 2207436.