Multicomponent Equiatomic Oxides
Recently, a new class of oxide materials was discovered, called high entropy oxides (HEOs). Similar to the high entropy alloys (HEA), the basic idea is to stabilize a single phase in these complex oxides, which consists of 5 or more cations in equiatomic amount. The first publication on HEOs was by the group of Rost et al. at North Carolina State University in 2016. While there are hundreds of publications on HEA, less than ten publications, published by three groups, have been published on HEO, until 2017. Therefore, one can state that the field of HEO is still in its infancy, but already the few publications demonstrate that the structure and properties of these materials are rather unique. As it is not always proven that all these oxide systems are entropy stabilized, we have named these oxide materials multicomponent equiatomic oxides (MEOs) as a more general term.
These new type of oxides deviate from the conventional well know binary or doped oxide systems. Our group has shown that in transition metal-based MEOs, the presence of multiple cations in equiatomic amounts promotes the formation of an entropy stabilized oxide (similar to the concept of high entropy alloys). However, we have demonstrated that in the case of rare earth MEOs, the presence of a specific element determines the stability of a single phase. Moreover, investigation of functional properties of MEOS is still in its early stages and the study of optical, transport and magnetic properties are part of our main focus. Therefore, our studies have contributed to explain the stabilization behavior of MEOs and stimulate new ideas to explore the vast composition space offered by MEOs.
The different oxide systems have been synthesized in our group using a variety of techniques, such as reverse co-precipitation, flame spray synthesis and nebulized spray pyrolysis (NSP). NSP has been shown to be the most versatile technique, in terms of flexibility in changing the composition and in obtaining well-crystalline nanostructured materials in the as-prepared state, i.e. without any post-synthesis annealing. We have concentrated on several transition metal oxide MEOs (TMO-MEO) as other groups in the field, and have synthesized, for the first time, rare earth metal oxide MEOs (REO-MEO) and combined transition and rare earth metal MEOs with perovskite structure. NSP allowed the synthesis of many different systems and thus a systematic investigation of the different systems.
Leonardo Velasco Estrada
Pavankumar Mannava (Masters Exchange Student)
Mukti Ranjan Mohapatra (Masters Exchange Student)
- A. Sarkar, C. Loho, L. Velasco, T. Thomas, S.S. Bhattacharya, H. Hahn, R.R. Djenadic, Multicomponent equiatomic rare earth oxides with narrow band gap and associated praseodymium multivalency, Dalt. Trans. 46, (2017), 12167-12176. doi:10.1039/C7DT02077E
- A. Sarkar, R. Djenadic, N.J. Usharani, K.P. Sanghvi, V.S.K. Chakravadhanula, A.S. Gandhi, H. Hahn, S.S. Bhattacharya, Nanocrystalline multicomponent entropy stabilised transition metal oxides, J. Eur. Ceram. Soc. 37 (2017) 747–754. doi:10.1016/j.jeurceramsoc.2016.09.018.
- R. Djenadic, A. Sarkar, O. Clemens, C. Loho, M. Botros, V.S.K. Chakravadhanula, C. Kübel, S.S. Bhattacharya, A.S. Gandhi, H. Hahn, Multicomponent equiatomic rare earth oxides, Mater. Res. Lett. 5 (2017) 102–109. doi:10.1080/21663831.2016.1220433.