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

Abstract Rare-earth doped apatite-type lanthanum silicates, La 9 A Si 6 O 27 ( A  = Nd, Sm, Gd, Yb), are synthesized by the high-temperature solid state reaction process. The microstructure and electrical properties of La 9 A Si 6 O 27 ceramics are investigated by X-ray diffraction, scanning electron microscopy and complex impedance analysis. La 9 A Si 6 O 27 ceramics are composed of hexagonal apatite-type structure and a very small amount of second phase La 2 SiO 5 . The total electrical conductivity of rare-earth doped lanthanum silicates obeys the Arrhenius relation, and gradually increases with increasing temperature from 673 K to 1173 K. 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 773 K is 4.31 × 10 −4  S cm −1 for La 9 NdSi 6 O 27 . At 1073 K, La 9 NdSi 6 O 27 and La 9 YbSi 6 O 27 have a total electrical conductivity of 8.20 × 10 −3  S cm −1 and 1.03 × 10 −2  S cm −1 , respectively, which are distinctly higher than undoped La 10 Si 6 O 27 (5.84 × 10 −3  S cm −1 ). However, the situation is reversed when Sm or Gd rare-earth cations are doped into lanthanum silicate.