| Abstract
| - A high-temperature and high-pressure approach allows single-step preparation of single-phase KNb1-xMgxO3-δ. The doping of Mg2+ in the KNbO3 lattice proceeded through high-temperature phase transitions of orthorhombic, tetragonal, pseudocubic, and finally to cubic and, most remarkably, led to activation of oxide ion conduction in the bulk and along the grain boundaries by effectively suppressing the contribution of the p-type electrons.
- A series of oxide ion conductors KNb1-xMgxO3-δ (x = 0.05−0.30) were prepared at atemperature of 870 °C and a pressure of 4.0 GPa. All samples were thermodynamically stableat ambient pressure and crystallized in an orthorhombic perovskite structure. The latticevolume enlarged with increment of dopant level, which was associated with the ionicsubstitution, variation of the relative content of oxygen vacancy Vö, and defect associations{MgNb‘ ‘‘Vö}, as well as an increase of disorder in Mg2+/Nb5+ distribution at B-sites ofperovskite lattice. At higher temperatures, KNb1-xMgxO3-δ underwent phase transitions fromorthorhombic to tetragonal, pseudocubic, and cubic in sequence, as confirmed by DTA andhigh-temperature Raman spectra. No thermal effects associated with the decompositionreactions were observed in KNb1-xMgxO3-δ during the successive heating process up to 1000°C. The high-temperature phase had a relatively high structural stability. Impedance spectraof KNb1-xMgxO3-δ showed bulk and grain boundary conduction. The total conduction wasdetermined to be predominately ionic, while the p-type electronic contribution was extremelysmall. KNb0.90Mg0.10O2.85 was found to provide a highly conductive phase with a conductivityof σ700°C = 1.10 × 10-3 S·cm-1. Further, the ionic conductivity data for KNb1-xMgxO3-δ wereseparated into two linear ranges, corresponding to the pseudocubic and cubic phases,respectively. The variations of conductivity and activation energy for both pseudocubic andcubic phases can be explained in terms of the relative content of the oxygen vacancy anddefect associations, delocalization of partial oxygen vacancies, and an order−disordertransition.
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