New A-deficient perovskites in the series LixLa2/3Ti1-xFexO3 (0.12 <= x <=0.33) and La(2+x)/3Ti1-xFexO3 (0.5 <= x <= 1.0)

Two new series of A-deficient perovskites have been synthesized and structu rally characterized from Rietveld treatment of their X-ray diffraction powd er patterns. The first one, LixLa2/3Ti1-xFexO3, for 0.12 less than or equal to x less than or equal to 0.33, results from the substitution mechanism T i4+ --> Fe3+ + Li+. The structure is closely related to that of La2/3-xLi3x TiO3 (a approximate to a(p), b approximate to a(p) and c approximate to 2 a (p)) with a symmetry evolution leading to the existence of two domains: for 0.12 less than or equal to x less than or equal to 0.19 the symmetry is or thorhombic (Pmmm), while for 0.20 less than or equal to x less than or equa l to 0.33 a tetragonal symmetry is obtained (P4/mmm), In both cases, the po pulation of La3+ ions, unequally distributed on two adjacent sites for smal ler x values, is directly affected by the increase of the Li content and te nds to be equalized with larger x values. The second series is found to res ult from another substitution mechanism Ti4+, Fe3+ + 1/3La(3+), leading to the formula La(2+x)/3Ti1-xFexO3 in the domain 0.5 less than or equal to x l ess than or equal to 1.0, The structure is then closely related to that of LaFeO3, where La3+ ions occupy now only one crystallographic site in the Pn ma space group, Transmission Electron Microscopy and Mossbauer spectrometry confirm the above models. (C) 2001 Elsevier Science Ltd, All rights reserv ed.