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

Apatite-type neodymium silicates doped with various cations at the Si site, Nd10Si5BO27−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 Nd10Si5BO27−δ ceramics. All Nd10Si5BO27−δ ceramics consist of a hexagonal apatite structure with a space group P63/m and a small amount of second phase Nd2SiO5. Neodymium silicates doped with Mg2+ or Al3+ cations at the Si site have an enhanced total conductivity as contrasted with undoped Nd10Si6O27 ceramic at all temperature levels. However, doping with Fe3+ 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 Mg2+ or Al3+ substitution to the Si4+ site and through the channels between the SiO4 tetrahedron and Nd3+ cations. At 773 K, the highest total conductivity is 4.19×10−5 S cm−1 for Nd10Si5MgO26 ceramic. At 1073 K, Nd10Si5AlO26.5 silicate has a total conductivity of 1.55×10−3 S cm−1, which is two orders of magnitude higher than that of undoped Nd10Si6O27.