Preparation and transport properties of new oxide ion conductors KNb_1-xMg_xO₃-δ by high temperature and pressure

A series of oxide ion conductors KNb_1-xMg_xO3-δ (x = 0.05-0.30) were prepared at a temperature of 870°C and a pressure of 4.0 GPa. All samples were thermodynamically stable at ambient pressure and crystallized in an orthorhombic perovskite structure. The lattice volume enlarged with increment of dopant level, which was associated with the ionic substitution, variation of the relative content of oxygen vacancy Vö, and defect associations {Mg_Nb″Vö}, as well as an increase of disorder in Mg²⁺/Nb⁵⁺ distribution at B-sites of perovskite lattice. At higher temperatures, KNb_1-xMg_xO₃-δ underwent phase transitions from orthorhombic to tetragonal, pseudocubic, and cubic in sequence, as confirmed by DTA and high-temperature Raman spectra. No thermal effects associated with the decomposition reactions were observed in KNb_1-xMg_xO₃-δ during the successive heating process up to 1000 °C. The high-temperature phase had a relatively high structural stability. Impedance spectra of KNb_1-xMg_xO₃-δ showed bulk and grain boundary conduction. The total conduction was determined to be predominately ionic, while the p-type electronic contribution was extremely small. KNb₀₉₀Mg_0.10O_2.85 was found to provide a highly conductive phase with a conductivity of σ700°C = 1.10 × 10^-3 S·cm^-1. Further, the ionic conductivity data for KNb_1-xMg_xO₃-δ were separated into two linear ranges, corresponding to the pseudocubic and cubic phases, respectively. The variations of conductivity and activation energy for both pseudocubic and cubic phases can be explained in terms of the relative content of the oxygen vacancy and defect associations, delocalization of partial oxygen vacancies, and an order-disorder transition.