Influence of pentavalent niobium doping on microstructure and electrical conductivity of oxy-apatite La10Si6O27 electrolytes

Oxy-apatite type La10Si6–xNbxO27+δ ceramics doped with different pentavalent niobium contents are synthesized via the high-temperature solid state reaction method. Crystal structure and electrical properties of La10Si6–xNbxO27+δ ceramics are investigated by X-ray diffraction, scanning electron microscope, transmission electron microscope, Raman spectroscopy and complex impedance measurements. La10Si6O27 ceramic sintered at 1923 K consists of hexagonal oxy-apatite phase and a small amount of La2SiO5. Pentavalent niobium can partially substitute for tetravalent silicon to form a Nb-doped oxy-apatite structure. However, La2SiO5 phase disappears with increasing the pentavalent niobium content. Some fine La3NbO7 particles are found at grain boundaries of oxy-apatite phase in La10Si6–xNbxO27+δ ceramics when the pentavalent niobium content is beyond 0.1. Raman spectra of La10Si6–xNbxO27+δ ceramics generates three new characteristic modes at 710 cm−1, 342 cm−1 and 136 cm−1 with the addition of different pentavalent niobium contents, which are significantly enhanced especially at x = 0.1 and x = 0.5. These new modes are from NbO bonds of La3NbO7 phase and obstruct the interstitial oxide-ion O(5) from conducting in dry atmosphere, thus the grain boundary impedance of La10Si6–xNbxO27+δ ceramics increases correspondingly. La10Si5.9Nb0.1O27.05 ceramic exhibits the highest total conductivity among all the compositions above 873 K.