Charge Transport in the A 6 B 2 O 17 (A = Zr, Hf; B = Nb, Ta) Superstructure Series

Abstract

The electrical properties of the entropy stabilized oxides: Zr 6 Nb 2 O 17 , Zr 6 Ta 2 O 17 , Hf 6 Nb 2 O 17 and Hf 6 Ta 2 O 17 were characterized. The results and the electrical properties of the products (i.e. ZrO 2 , HfO 2 , Nb 2 O 5 and Ta 2 O 5 ) led us to hypothesize the A 6 B 2 O 17 family is a series of mixed ionic-electronic conductors. Conductivity measurements in varying oxygen partial pressure were performed on A 6 Nb 2 O 17 and A 6 Ta 2 O 17. The results indicate that electrons are involved in conduction in A 6 Nb 2 O 17 while holes play a role in conduction of A 6 Ta 2 O 17 . Between 900 °C–950 °C, the charge transport in the A 6 B 2 O 17 system 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. Electrical properties of A 6 B 2 O 17 (A = Zr, Hf; B = Nb, Ta) has been studied; Hf 6 Nb 2 O 17 and Hf 6 Ta 2 O 17 conductivity dependence on oxygen partial pressure has been determined; A 6 B 2 O 17 (A = Zr, Hf; B = Nb, Ta) are mixed ionic-electronic conductors; Structural complexity of A 6 B 2 O 17 (A = Zr, Hf; B = Nb, Ta) influences their electrical properties; High temperature structural changes of Zr 6 Ta 2 O 17 has been investigated by XRD.