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

• Published in: Russian journal of electrochemistry Vol. 59; no. 6; pp. 449 - 460
• Main Authors: Shlyakhtina, A. V., Lyskov, N. V., Kolbanev, I. V., Baldin, E. D., Kasyanova, A. V., Medvedev, D. A.
• Format: Journal Article
• Language: English
• Published: Moscow Pleiades Publishing 01.06.2023; Springer Nature B.V
• Subjects: Ceramics; Chemistry; Chemistry and Materials Science; Electrochemistry; Electrolytes; Gadolinium; High temperature; Hole conductivity; Lanthanum; Low temperature; Oxygen; Partial pressure; Physical Chemistry; Protons; Pyrochlores; Rare earth elements; Samarium; Solid solutions; Transport properties; proton conductivity; pyrochlore; oxygen-ion conductivity; solid-oxide fuel cell
• ISSN: 1023-1935; 1608-3342
• DOI: 10.1134/S1023193523060058

In this work, a high-density ceramics Ln 2 Hf 2 O 7 ( 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 Ln 2.1 Hf 1.9 O 6.95 ( Ln = La, Nd, Sm, Eu) doped solid solutions. The total conductivity of ceramics was studied using impedance spectroscopy and dc four-probe method; for Ln 2 Hf 2 O 7 ( Ln = Sm, Eu), by determining the total conductivity as a function of oxygen partial pressure. The maximum oxygen-ion conductivity was observed for Gd 2 Hf 2 O 7 (~1 × 10 –3 S/cm at 700°C); it was shown to approach the conductivity of Gd 2 Zr 2 O 7 (~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 Nd 2 Hf 2 O 7 only; however, ac measurements detected low-temperature proton conductivity in the Gd 2 Hf 2 O 7 up to 450°С as well. With a decrease in the lanthanide ionic radius, the oxygen-ion conductivity increased in the Ln 2 Hf 2 O 7 ( Ln = La, Nd, Sm, Gd) series. Although the conductivity of samarium hafnate is an order of magnitude lower than that of Gd 2 Hf 2 O 7 , 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 Eu 2 Hf 2 O 7 . Among doped Ln 2.1 Hf 1.9 O 6.95 pyrochlore solid solutions ( Ln = La, Nd, Sm, Eu), the proton conductivity of ~8 × 10 −5 S/cm at 700°C was shown in Ln 2.1 Hf 1.9 O 6.95 ( Ln = La, Nd). With a decrease in the lanthanide ionic radius, the proton conductivity disappeared; the oxygen-ion one, increased.