Ali Nasrallah1,Colin Freeman1,Ge Wang2,Derek Sinclair1
University of Sheffield1,The University of Manchester2
Ali Nasrallah1,Colin Freeman1,Ge Wang2,Derek Sinclair1
University of Sheffield1,The University of Manchester2
High-Entropy Oxides (HEOs) have gathered a lot of attention over the past few years particularly around their formation and properties<sup>1–4</sup>. The perovskite lattice (ABO<sub>3</sub>) is a particularly exciting opportunity as mixing can be carried out on both the A or B site presenting a wealth of enthalpic and entropic possibilities to achieve a single-phase solid solution with a favourable Gibbs free energy<sup>4</sup>.<br/>Using a combination of computational modelling and experimental studies, we have examined several 3-, 4-, and 5-B element oxides with combinations of (Ga, Y, In), Nb, (Ti, Zr, Sn). We have studied the influence of a range of factors including the ionic size of the sites, the tolerance factor and the ionic size variation which we can then link to the formation of single-phase materials.<br/>Our experiments on Ba-based perovskites demonstrate that single phases can be formed in a range of 4 and 5-B element perovskites with mixed valence charges (e.g. 3+, 4+ and 5+) but mixing was far more challenging with only 3-B element systems. By adjusting the A-site using Sr and Ca we were able to reduce the ionic size penalties and form further 3-B site single phase systems indicating that the size of the A cation also has an effect on the mixing process.<br/>We have coupled the experimental work with a mixture of classical and ab initio simulation work examining the enthalpy of mixing and comparing to the phase formation from experiment. Our DFT simulations have identified the different mixing and hybridisation characteristics of the ions which affects their ability to mix and can run counter to expected ion size behaviour. Our classical simulations have optimised and examined hundreds of different structures which allows us to comment on the ideality of the solid solutions formed and the competition between entropy and enthalpy in the stability of a phase. We are also able to examine different secondary structures and how their enthalpic stability compares. Moreover, using computational and experimental Raman spectroscopy we are able to comment on the polymorphs obtained experimentally for the various perovskites.<br/><br/>From our results, we offer thoughts on the differing rules governing the mixing processes in these perovskite phases.<br/><br/>References<br/>(1) Sarkar, A.; Wang, Q.; Schiele, A.; Chellali, M. R.; Bhattacharya, S. S.; Wang, D.; Brezesinski, T.; Hahn, H.; Velasco, L.; Breitung, B. High-Entropy Oxides: Fundamental Aspects and Electrochemical Properties. <i>Adv. Mater.</i> <b>2019</b>, <i>31</i> (26), 1806236. https://doi.org/10.1002/ADMA.201806236.<br/>(2) Sarkar, A.; Velasco, L.; Wang, D.; Wang, Q.; Talasila, G.; de Biasi, L.; Kübel, C.; Brezesinski, T.; Bhattacharya, S. S.; Hahn, H.; Breitung, B. High Entropy Oxides for Reversible Energy Storage. <i>Nat. Commun.</i> <b>2018</b>, <i>9</i> (1). https://doi.org/10.1038/S41467-018-05774-5.<br/>(3) Rost, C. M.; Sachet, E.; Borman, T.; Moballegh, A.; Dickey, E. C.; Hou, D.; Jones, J. L.; Curtarolo, S.; Maria, J. P. Entropy-Stabilized Oxides. <i>Nat. Commun. 2015 61</i> <b>2015</b>, <i>6</i> (1), 1–8. https://doi.org/10.1038/ncomms9485.<br/>(4) Banerjee, R.; Chatterjee, S.; Ranjan, M.; Bhattacharya, T.; Mukherjee, S.; Sourav Jana, S.; Dwivedi, A.; Maiti, T. High-Entropy Perovskites: An Emergent Class of Oxide Thermoelectrics with Ultralow Thermal Conductivity. <b>2022</b>, <i>8</i>, 10. https://doi.org/10.1021/acssuschemeng.0c03849.