LOCAL-STRUCTURE AND OXIDE-ION MOTION IN DEFECTIVE PEROVSKITES

Macroscopic thermodynamic and transport properties of disordered mater ials are determined largely by their local structure, which may differ substantially from long-range crystalline symmetry. In order to bette r understand local structure and ionic motion in highly disordered per ovskite oxides, we have investigated several cubic perovskites using h igh-temperature oxygen-17 NMR in conjunction with other experimental t echniques. Materials we have studied include Ba (In0.67Zr0.33)Oy, Ba(I n0.67Ce0.33)Oy (La0.5Ba0.5) (Co0.7Cu0.3Oy, and (La0.6Sr0.4) (Co0.8Cu0. 2)Oy. We show that despite having long-range cubic symmetry as determi ned by X-ray and neutron powder diffraction, these materials possess m icrodomains with layered structures on a short length scale (50-500 an gstrom). These microdomains are apparent in HRTEM images of these mate rials, and manifest themselves as unit cell doublings in the electron diffraction patterns. Neutron powder profile refinements and oxygen-17 DAS NMR both suggest that oxygen nuclei are displaced from sites of c ubic symmetry in a manner reminiscent of layered perovskite-related st ructures. As is the case with known layered materials, the high temper ature oxygen-17 spectra and relaxation measurements show that few oxyg en atoms are mobile below 800-degrees-C due to trapping of oxygen-ion vacancies in ordered layers. In the case of (La0.6Sr0.4)(Co0.8Cu0.2)Oy , estimates of the vacancy trapping energy and the vacancy migration e nergy, extracted from NMR, appear to rationalize macroscopic transport measurements.