Preparation and electrical conductivity of binary rare earth metal fluoride oxides

Abstract

As new oxide ion conducting solid electrolytes, binary rare earth metal fluoride oxides, Ln a Ln′ b F c O d (Ln and Ln′ are different rare earth elements), were obtained by firing the powder mixtures of rare earth fluorides and oxides at a temperature from 1100–1400°C in an argon atmosphere. Two homogeneous phases were identified to be rhombohedral and tetragonal. The rhombohedral phase was formes as a stoichiometric compound, LnLn′ 2F 3O 3, from an equimolar mixture of LnF 3 and Ln′ 2O 3. The tetragonal one was found to have a wide homogeneity composition of Ln x Ln′ 2(1− x) F 3 x O 3(1− x) where x-value was from 0.58 to 0.78. All of 210 tetragonal compounds ofLn 2Ln′ 2F 6O 3 containing Pr or Nd or both gave a high electrical conductivity more than 10 −2 S cm −1 at 650°C under around 10 −7 atm oxygen and their crystal structures closely resembled the cubic. The charge carrying species of these binary rare earth metal fluoride oxides except Ce and Tb containing compounds was assumed to be the oxide ion. The electrical conductivity of the sintered Nd 2Y 2F 6O 3 sample obtained by hot-pressing at 1250°C under 300 kg cm −2 was measured to be 8.0 × 10 −2 S cm −1 at 650°C, and its oxide ion transport number was calculated to be over 0.93.