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The electrical properties of the entropy stabilized oxides: Zr₆Nb₂O₁₇, Zr₆Ta₂O₁₇, Hf₆Nb₂O₁₇and Hf₆Ta₂O₁₇were characterized. The results and the electrical properties of the products (i.e. ZrO₂, HfO₂, Nb₂O₅and Ta₂O₅) led us to hypothesize the A₆B₂O₁₇family is a series of mixed ionic-electronic conductors. Conductivity measurements in varying oxygen partial pressure were performed on A₆Nb₂O₁₇and A₆Ta₂O_17.The results indicate that electrons are involved in conduction in A₆Nb₂O₁₇while holes play a role in conduction of A₆Ta₂O₁₇. Between 900 °C–950 °C, the charge transport in the A₆B₂O₁₇system increases in Ar atmosphere. A combination of DTA/DSC and in situ high temperature X-ray diffraction was performed to identify a potential mechanism for this increase. In-situ high temperature X-ray diffraction in Ar does not show any phase transformation. Based on this, it is hypothesized that a change in the oxygen sub-lattice is the cause for the shift in high temperature conduction above 900 °C–950 °C. This could be:(i)Nb(Ta)⁴⁺- oxygen vacancy associate formation/dissociation,(ii)formation of oxygen/oxygen vacancy complexes(iii)ordering/disordering of oxygen vacancies and/or(iv)oxygen-based superstructure commensurate or incommensurate transitions. In-situ high temperature neutron diffraction up to 1050 °C is required to help elucidate the origins of this large increase in conductivity. 

High entropy oxides are emerging as an exciting new avenue to design highly tailored functional behaviors that have no traditional counterparts. Study and application of these materials are bringing together scientists and engineers from physics, chemistry, and materials science. The diversity of each of these disciplines comes with perspectives and jargon that may be confusing to those outside of the individual fields, which can result in miscommunication of important aspects of research. In this Perspective, we provide examples of research and characterization taken from these different fields to provide a framework for classifying the differences between compositionally complex oxides, high entropy oxides, and entropy stabilized oxides, which is intended to bring a common language to this emerging area. We highlight the critical importance of understanding a material’s crystallinity, composition, and mixing length scales in determining its true definition.