Preparation and the electrical conductivity of the binary rare earth metal fluoride oxides

Abstract

As new oxide ion conducting ceramics, binary rare earth metal fluoride oxides (BMFO, LnLn′F xO y, Ln,Ln′; rare earth) were obtained by firing the powdered mixture of rare earth fluoeide (LnF 3) and oxide (Ln′ 2O 3) at a temperature of 1100–1400 °C in an argon atmosphere. In the solid phase reaction, the anion exchange reaction between LnF 3 and Ln′ 2O 3 took place in the temperature range from 200 to 600 °C to give two simple metal fluoride oxides (SMFO) of LnF xO y and Ln′F xO y, and then above 900 °C, both SMFOs began to react with each other to produce BMFO. Two homogeneous phases were identified to be the rhombohedral and the tetragonal. The rhombohedral phase was formed 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 xLn′ 2(1−x)F 3xO 3(1−x) where x-value was 0.58–0.78. The electrical conductivity of the tetragonal was at least 100 times higher than for the rhombohedral. All of 210 tetragonal Ln 2Ln′ 2F 6O 3 samples involving reciprocal systems have been investigated. The samples containing Pr or Nd or both gave a high electrical conductivity more than 10 −2 Scm −1 at 650 °C under around 10 −7atm oxygen and their crystal structures closely resembled the cubic. The charge carring species of these BMFOs except Ce and Tb-compounds was assumed to be oxide ion. The electrical conductivity of the sintered Y 2Nd 2F 6O 3 sample obtained by hot-pressing at 1250 °C under 300 kgcm −2 was measured to be 8.0 x 10 −2 Scm −1 at 750 °C, and its oxide ion transport number was calculated to be over 0.93.