Synthesis, structure and electrical properties of rare-earth doped apatite-type lanthanum silicates

Abstract

Rare-earth doped apatite-type lanthanum silicates, La9ASi6O27 (A=Nd, Sm, Gd, Yb), are synthesized by the high-temperature solid state reaction process. The microstructure and electrical properties of La9ASi6O27 ceramics are investigated by X-ray diffraction, scanning electron microscopy and complex impedance analysis. La9ASi6O27 ceramics are composed of hexagonal apatite-type structure and a very small amount of second phase La2SiO5. The total electrical conductivity of rare-earth doped lanthanum silicates obeys the Arrhenius relation, and gradually increases with increasing temperature from 673K to 1173K. Lanthanum silicates doped with Nd or Yb cations exhibit a higher total electrical conductivity than undoped lanthanum silicate. The highest total electrical conductivity value obtained at 773K is 4.31×10−4Scm−1 for La9NdSi6O27. At 1073K, La9NdSi6O27 and La9YbSi6O27 have a total electrical conductivity of 8.20×10−3Scm−1 and 1.03×10−2Scm−1, respectively, which are distinctly higher than undoped La10Si6O27 (5.84×10−3Scm−1). However, the situation is reversed when Sm or Gd rare-earth cations are doped into lanthanum silicate.