Charge transport properties of a new family of entropy stabilized A6B2O17 oxides
JAWORSKI D. 1, ROSENBERG W. 2, MIRUSZEWSKI T. 1, MIELEWCZYK-GRYN A. 1, GAZDA M. 1, MCCORMACK S. 3
1 Institute of Nanotechnology and Materials Engineering, Gda?sk University of Technology, Gda?sk, Poland; 2 Department of Chemical Engineering, University of California, Davis, United States; 3 Department of Materials Science and Engineering, University of California, Davis, United States
High-entropy oxides have gained a lot of publicity in recent years due to the stabilization of new crystal structures where the formation entropy overcomes a positive formation enthalpy [1]. Entropy stabilization, and the associated disorder, can give rise to interesting new material properties. Configurational entropy is increased by mixing multiple components on a given sublattice. For any number of components, configurational entropy is maximized at equimolar stoichiometries. However, this is also possible in peculiar compounds that contain many different polyhedral positions that are occupied by fewer metal ions in the crystal structure [2].
In this work, studies of the electrical properties of a group of compounds with the formula A6B2O17 (where A = Zr, Hf; B = Nb, Ta) will be presented. The total conductivity of these materials was determined using electrochemical impedance spectroscopy (EIS) measurements. All samples were measured in a flux of synthetic air (pO2 ≈ 0.2 atm., pH2O ≈ 10-4 atm.) and argon (pO2 ≈ 10-6 atm., pH2O ≈ 10-4 atm.) over a wide temperature range.
The results show the thermally activated nature of conductivity with high activation energies (Ea > 1 eV) of the electrical charge carrier transport. Moreover, there are two conductivity regimes, high and low temperature, that differ in activation energy values in all samples. In some cases, it is also possible to separate the bulk and grain boundary conductivities. The total conductivity increases in the reducing atmosphere of argon, which might be associated with the formation of oxygen vacancies, increasing the oxygen ionic partial conductivity. The step changes in conductivity at high temperatures (around 1000 °C) may be connected to the entropy stability limit or generation of electronic carriers through the band gap.
References
[1] C. M. Rost, E. Sachet, T. Borman, A. Moballegh, E.C. Dickey, D. Hou, J.L. Jones, S. Curtarolo, J.P. Maria, Entropy-Stabilized Oxides, Nat. Commun. 2015, 6:8485
[2] A. A. Voskanyan, K. Lilova, S. J. McCormack, W. M. Kriven, A. Navrotsky, A new class of entropy stabilized oxides: Commensurately modulated A6B2O17 (A = Zr, Hf; B = Nb, Ta) structures, Scripta Materialia 203 (2021) 114139
Acknowledgements
S.J.M. acknowledges support from the National Science Foundation (NSF), Division of Materials Research (DMR), Ceramic (CER) program under Grant No. DMR 2047084.