Preparation and electrical conductivity of neodymium–europium oxide fluorides

Abstract

Neodymium–europium oxide fluoride, Nd-EuO_xF_y, was prepared by heating appropriate mixtures of Nd₂O₃ and EuF₃ at temperatures ranging from 1273 to 1673 K for 2 h in argon. Two monophases, rhombohedral and tetragonal were identified. The rhombohedral phase was obtained at an EuF₃ composition of 50–55 mol%, and the tetragonal one was obtained in the composition range between 65 and 75 mol%. Nd₂Eu₂O₃F₆ gave a high electrical conductivity of 5 × 10^–2 S cm^–1 at 923 K under an oxygen partial pressure of 0.4 Pa. The charge carrier was determined to be mainly the oxide ion. The transport numbers of the oxide ion and the electron were measured to be ca. 0.9 and < 0.05, respectively, at temperatures between 723 and 923 K. Resulting from XPS measurements, it was found that: (1) the valence of Nd in Nd₂Eu₂O₃F₆ was higher than in Nd₂O₃ and that of Eu was lower than in EuF₃, i.e. the variance of the valency state of Nd (+3-+ 4) and Eu(+2-+ 3) affects the oxide ion conducting structure; (2) the covalency in the bonds between metal ions and oxide ions in Nd₂Eu₂O₃F₆ was weaker than that in Eu₂O₃ or Nd₂O₃; (3) the bond between the fluoride ion and the rare-earth-metal ion in Nd₂Ln₂O₃F₆ was stronger than in the individual rare-earth-metal fluorides. These might result in increased oxide ion mobility.