Anisotropic conductivity in brownmillerite-type Ca2Fe2O5 observed by measurements on oriented floating-zone grown single crystals

Abstract

Brownmillerite-type Ca₂Fe₂O₅ is a mixed ionic and electronic conductor with applications as an electrode material for solid oxide fuel cells and solid oxide electrolyser cells. Long-range oxide ion migration in Ca₂Fe₂O₅ has been computationally predicted to be predominantly two-dimensional, restricted to the (ac) crystallographic plane. We have used the floating zone method to grow large high-quality single crystals of Ca₂Fe₂O₅ and determine its conductivity on two differently oriented single crystal samples in order to directly probe the anisotropy of transport properties. Impedance spectroscopy measurements have shown the conductivity in the (ac)-plane to be up to one and a half order of magnitude higher than that parallel to the crystallographic b-axis. This degree of anisotropy of the conductivity is higher than that observed experimentally in most other oxide ion or mixed conductors belonging to the apatite and melilite structural family, highlighting the suitability of brownmillerite-type materials for applications in devices requiring components in oriented crystal or thin-film forms.