Proton and Oxygen-Ion Conductivity of the Pure and Lanthanide-Doped Hafnates with Pyrochlore Structure

In this work, a high-density ceramics Ln2Hf2O7 (Ln = La, Nd, Sm, Eu, Gd) were synthesized by mechanical activation followed by high-temperature synthesis at 1600°C (3–10 h) and their transport properties were compared with those of Ln2.1Hf1.9O6.95 (Ln = La, Nd, Sm, Eu) doped solid solutions. The total conductivity of ceramics was studied using impedance spectroscopy and dc four-probe method; for Ln2Hf2O7 (Ln = Sm, Eu), by determining the total conductivity as a function of oxygen partial pressure. The maximum oxygen-ion conductivity was observed for Gd2Hf2O7 (~1 × 10–3 S/cm at 700°C); it was shown to approach the conductivity of Gd2Zr2O7 (~2 × 10–3 S/cm at 700°C) for the first time. Thus, the gadolinium hafnate can be a promising material for further doping in order to obtain highly conductive electrolytes. Among pure rare-earth hafnates, the proton conductivity was reliably observed for Nd2Hf2O7 only; however, ac measurements detected low-temperature proton conductivity in the Gd2Hf2O7 up to 450°С as well. With a decrease in the lanthanide ionic radius, the oxygen-ion conductivity increased in the Ln2Hf2O7 (Ln = La, Nd, Sm, Gd) series. Although the conductivity of samarium hafnate is an order of magnitude lower than that of Gd2Hf2O7, it has a wide range of oxygen-ion conductivity (~10–18–1 atm at 700, 800°C); there is no contribution from hole conductivity in air, in contrast to Eu2Hf2O7. Among doped Ln2.1Hf1.9O6.95 pyrochlore solid solutions (Ln = La, Nd, Sm, Eu), the proton conductivity of ~8 × 10−5 S/cm at 700°C was shown in Ln2.1Hf1.9O6.95 (Ln = La, Nd). With a decrease in the lanthanide ionic radius, the proton conductivity disappeared; the oxygen-ion one, increased.