Electric conductivity and crystal structure of neodymium-containing binary rare-earth metal oxide fluorides

Abstract

Nd 2Ln 2O 3F 6 compounds (Ln; Y-Lu) are prepared by firing the mixture of 1-mol Nd 2O 3 and 2-mol LnF 3 at 1373 K for 2 h in argon. The crystal system of Nd 2Ln 2O 3F 6 can be assigned into pseudo-tetragonal (monoclinic). Nd 2Ln 2O 3F 6 compounds, except Ln: Nd, Yb and Lu, gave a high oxide ion conductivity of more than 1.0 S m −1 at 923 k. Among them, Nd 2Eu 2O 3F 6 gave the highest conductivity of 5.0 S m −1 under an oxygen partial pressure of 0.4 Pa. The oxide ion transport number for Nd 2Ln 2O 3F 6 (Ln: Y, Ce, Sm, Gd) decreases as the temperature falls. On the other hand, Nd 2Eu 2O 3F 6 holds a number of more than 0.9 even at 723 K. As the result of X-ray diffraction Rietveld analysis, the crystal structure of Nd 2Eu 2O 3F 6 was found to be expressed by the monoclinic based on the defect double layered fluorite structure (monoclinic-D″LF). The lattice constants of the monoclinic D″LF structure were calculated to be a 0 = 0.3959 nm, b 0= 1.132 nm, c 0 = 0.5615 nm, β= 134.69 ° and the lattice volume was 0.180 nm 3. The molecular number, Z, was calculated to be 1 from the density data (measured; 6.98 kg m −3, calculated; 6.96 kg m −3). One molecule of Nd 2Eu 2O 3F 6 is in this unit cell. The lattice plane at y = 0.25 crossing vertically with b-axis in the monoclinic-D″LF structure consists of oxide ions and vacant sites formed by defecting of fluoride ion. The oxide ion conduction in Nd 2Eu 2O 3F 6 was supposed to be arising from migration of oxide ion through the defects in the y = 0.25 plane, because the space of the vacant site of fluoride ion is similar to the size of oxide ion.