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

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 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 La9NdSi6O27. At 1073 K, La9NdSi6O27 and La9YbSi6O27 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 La10Si6O27 (5.84 × 10−3 S cm−1). However, the situation is reversed when Sm or Gd rare-earth cations are doped into lanthanum silicate.

► La9ASi6O27 ceramics were prepared by pressureless-sintering at 1923 K. ► La9ASi6O27 ceramics have apatite structure. ► The electrical conductivity of La9ASi6O27 increases with temperature. ► The highest conductivity is 1.03 × 10−2 S cm−1 at 1073 K for La9YbSi6O27.