Crystal Structure and Ionic Conductivity of La9.5Ge6-xNbxO26.25+0.5x Compounds with Apatite Structure

Solid electrolyte materials of apatite structure based on lanthanum germanate and doped with niobium (La9.5Ge6-xNbxO26.25+0.5x, x = 0, 0.1, 0.15, 0.2, 0.25, 0.5) were prepared through solid-state method, and the relationship... [ view full abstract ]

Solid electrolyte materials of apatite structure based on lanthanum germanate and doped with niobium (La_9.5Ge_6-xNb_xO_26.25+0.5x, x = 0, 0.1, 0.15, 0.2, 0.25, 0.5) were prepared through solid-state method, and the relationship between crystal structure and ionic conductivity was investigated. All La/Ge based powders were calcined at 1250⁰C for 3 hours and were characterized with x-ray powder diffractometry (XRD).  XRD patterns showed that a secondary phase occurred when x = 0.5, which indicated the solid solubility limit was at x = ~0.5.  Crystal structure analysis through the Rietveld method showed that the samples have hexagonal symmetry (P6₃/m).  The specimens were then sintered at 1350⁰C for 3 hours for conductivity measurement.  Among all compounds, La_9.5Ge_5.85Nb_0.15O_26.325 (x = 0.15) has the highest conductivity (0.0324 S/cm) in this series at 800⁰C.  The result shows that, with the substitution of larger Nb⁵⁺ for Ge⁴⁺ ions, the crystal structure has become more open, making interstitial oxygen migrate easier, and increases the ionic conductivity.  However, the conductivity decreases when the secondary phase occurred at x = 0.5.