In search of the cubic phase of the Li+ ion-conducting perovskite La2/3-xLi3xTiO3: structure and properties of quenched and in situ heated samples

摘要

in search of the cubic phase of the Li+ ion-conducting perovskite La2/3-xLi3xTiO3 (LLTO), several samples, quenched and heated at different temperatures, are examined by powder X-ray diffraction (XRD). The best structural model (for 0.06 < x < 0.14)-probably representing an average situation-is discussed in comparison with various other structural studies. It presents a tetragonal unit cell (space group P4/mmm; a approximate to a(p,) c = 2a(p)) in which La3+ ions and vacancies are always unequally distributed in the two different A sites, centers of the perovskite dodecahedral cages. The quenching experiments bring the evidence of a very fast disordering/ordering of La3+ ions between their two unequivalent positions, and the in situ thermodiffractometry experiments show that the La3+ ion thermal diffusion becomes noticeable above 800 degreesC and depends on the solid solution composition x. Although the thermal diffusion of the La3+ ions occurs above 800 degreesC, the disordering of these ions remains limited for low x values (with c/2a values slightly greater than 1) even at 1200 degreesC, while a nearly complete disordering is reached at 1200 degreesC for x=0.11 (with c/2a very close to 1). As reported by earlier authors, the kinetics of the fast La3+ ions disordering/ordering in the temperature range 900-1200 C has to be fully considered during the preparation of the samples, and the thermal history can noticeably change the La3+ ion partition and hence the electrical conductivity of the prepared phase. The equilibrated samples can be obtained by using proper heating time and temperature. (C) 2002 Elsevier Science B.V All rights reserved. [References: 39]