Conductivity and hydration trends in disordered fluorite and pyrochlore oxides: A study on lanthanum cerate–zirconate based compounds

In the present contribution we discuss the influence of order/disorder on the concentration and mobility of ionic charge carriers in undoped and acceptor (calcium) doped fluorite and pyrochlore structured lanthanum cerate–zirconate solid solutions: (La1 − yCay)2(Ce1 − xZrx)2O7 − δ (y = 0, 0.02, 0.10; x = 0, 0.50, 0.75). Characterization of the electrical conductivity as a function of temperature and oxygen partial pressure revealed contribution from electronic carriers in reducing atmosphere, but otherwise these materials are ionic conductors. Oxide ion conductivity dominates at high temperatures while protons become more dominating as charge carrier at temperatures below typically 500 °C under wet conditions. The hydration enthalpies were determined by simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC). The contribution from ionic conductivity increases and the hydration enthalpy becomes more exothermic with higher cerium content, i.e. with more disordered materials. The proton conductivity decreases upon acceptor substitution of La3 + with Ca2 + which is attributed to trapping of the charge carriers by the effectively negative acceptor.

► Effect of order/disorder on the hydration thermodynamics and ionic conductivity.
► ΔHHydr0 goes through a maximum across the (La1-yCay)2(Ce1-xZrx)2O7-δ series.
► Low stability of oxygen vacancies in disordered fluorites favours hydration.
► Predominant contribution of protons at intermediate and low temperatures.
► Partial trapping of the mobile ions by the Ca2+ acceptors.