AM_1-xAl_xO_3-x(A = Na or K; M = Nb or Ta): new anion-deficient perovskite oxides exhibiting oxide ion conduction

Abstract

Anion-deficient perovskite oxides of the formula AM_1-xAl_xO_3-x (A = Na or K; M = Nb or Ta) have been prepared for 0 < x ≤ 0.5. Diffraction experiments reveal that while the potassium compounds adopt orthorhombic/cubic perovskite structures similar to the parent KNbO₃/KTaO₃, the sodium compound, NaNb_0.5Al_0.5O_2.5, possesses a brownmillerite/LaSrCuAlO₅-like superstructure. ²⁷Al NMR spectra show an exclusive tetrahedral oxygen coordination for Al(III) in NaNb_0.5Al_0.5O_2.5 (I) and both tetrahedral and octahedral coordination for Al(III) in KNb_0.5Al_0.5O_2.5 (II). The results suggest a long-range and short-range ordering of oxide ion vacancies in I and II respectively. Electrical conductivity measurements show a significant oxide ion conduction for KNb_1-xAl_xO_3-x, with the conductivity increasing with x up to x = 0.5. The differences in the Arrhenius plots of the ionic conductivity of I and II have been rationalized in terms of the long-range and short-range ordering of oxide ion vacancies in the anion-deficient perovskite oxides.