Influence of doping with various cations on electrical conductivity of apatite-type neodymium silicates

Abstract Apatite-type neodymium silicates doped with various cations at the Si site, Nd 10 Si 5 B O 27− δ ( B =Mg, Al, Fe, Si), were synthesized via the high-temperature solid state reaction process. X-ray diffraction and complex impedance analysis were used to investigate the microstructure and electrical properties of Nd 10 Si 5 B O 27− δ ceramics. All Nd 10 Si 5 B O 27− δ ceramics consist of a hexagonal apatite structure with a space group P 63/ m and a small amount of second phase Nd 2 SiO 5 . Neodymium silicates doped with Mg 2+ or Al 3+ cations at the Si site have an enhanced total conductivity as contrasted with undoped Nd 10 Si 6 O 27 ceramic at all temperature levels. However, doping with Fe 3+ cations at the Si site has a little effect on improving the total conductivity above 873 K. The enhanced oxide-ion conductivity in a hexagonal apatite-type structure depends upon the diffusion of interstitial oxide-ion through oxygen vacancies induced by the Mg 2+ or Al 3+ substitution to the Si 4+ site and through the channels between the SiO 4 tetrahedron and Nd 3+ cations. At 773 K, the highest total conductivity is 4.19×10 −5 S cm −1 for Nd 10 Si 5 MgO 26 ceramic. At 1073 K, Nd 10 Si 5 AlO 26.5 silicate has a total conductivity of 1.55×10 −3 S cm −1 , which is two orders of magnitude higher than that of undoped Nd 10 Si 6 O 27 .