Oxide ionic conduction anisotropy of apatite-type RE_9.33Si₆O₂₆ (RE = La–Ho) ceramics

Abstract

Rare earth-silicate RE_9.33Si₆O₂₆ ceramics [RE = La(Nd), Pr, Nd, Sm, Gd, Dy, Ho, Y, Er, and Yb] with an apatite-type c-axis-oriented crystal structure were prepared under a magnetic field using a neodymium magnet. RE = La(Nd)–Ho had an apatite structure, whereas RE = Y, Er, and Yb did not show an apatite structure. The lattice constants a and c and cell volume decreased monotonically with decreasing ionic radius of RE³⁺ in La(Nd)–Ho. The intragranular conductivity parallel to the c-axis direction (σ∥) of c-axis oriented RE_9.33Si₆O₂₆ [RE = La(Nd), Pr, Nd, Sm, Gd, Dy, and Ho] is more than twice as large as the intragranular conductivity perpendicular to the c-axis direction (σ⊥). Both σ∥ and σ⊥ decreased monotonically as the ionic radius of RE³⁺ decreased from La(Nd) to Gd. However, in the case of Dy and Ho, the intragranular conductivity increased more than that of Gd. Furthermore, the difference between σ∥ and σ⊥ tends to decrease with decreasing ionic radius of RE³⁺.