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

摘要

Apatite-type neodymium silicates doped with various cations at the Si site, Nd_(10)Si_5BO_(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_5BO_(27-δ) ceramics. All Nd_(10)Si_5BO_(27-δ) ceramics consist of a hexagonal apatite structure with a space group P63/m and a small amount of second phase Nd_2SiO_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_6O_(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_5MgO_(26) ceramic. At 1073 K, Nd_(10)Si_5AlO_(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_6O_(27).