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, Nd₁₀Si₅BO_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 Nd₁₀Si₅BO _27-δ ceramics. All Nd₁₀Si₅BO _27-δ ceramics consist of a hexagonal apatite structure with a space group P63/m and a small amount of second phase Nd₂SiO ₅. 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, Nd₁₀Si₅AlO_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 Nd₁₀Si ₆O₂₇.