AM(1-X)AL(X)O(3-X) (A=NA OR K M=NB OR TA) - NEW ANION-DEFICIENT PEROVSKITE OXIDES EXHIBITING OXIDE-ION CONDUCTION

Anion-deficient perovskite oxides of the formula AM(1-x)Al(x)O(3-x) (A = Na or K; M = Nb or Ta) have been prepared for 0 < x less than or eq ual to 0.5. Diffraction experiments reveal that while the potassium co mpounds adopt orthorhombic/cubic perovskite structures similar to the parent KNbO3/KTaO3, the sodium compound, NaNb0.5Al0.5O2.5, possesses a brownmillerite/LaSr-CuAlO5-like superstructure. Al-27 NMR spectra sho w an exclusive tetrahedral oxygen coordination for AI(III) in Na-Nb0.5 Al0.5O2.5 (I) and both tetrahedral and octahedral coordination for Al( III) in KNb0.5Al0.5O2.5 (II). The results suggest a long-range and sho rt-range ordering of oxide ion vacancies in I and II respectively. Ele ctrical conductivity measurements show a significant oxide ion conduct ion for KNb1-xAlxO3-x, with the conductivity increasing with x up to x = 0.5. The differences in the Arrhenius plots of the ionic conductivi ty of I and II have been rationalized in terms of the long-range and s hort-range ordering of oxide ion vacancies in the anion-deficient pero vskite oxides.