Studies on the effect of substitution of tetravalent ions for La3+ in LaMnO3

We have investigated whether size of the doped ions is the crucial factor controlling the formation and stability of single phase compounds with tetravalent ions substituted in the A-site of the rare earth perovskite manganite, LaMnO3. These studies are carried out based on the recent controversial reports in the literature on the substitution of smaller tetravalent ions Ce4+ and Sn4+ for the trivalent ion La3+. The tetravalent ion, Th4+ is selected for the studies because of its larger ionic size (1.05 Angstrom) when compared to that of Ce4+ (0.97 Angstrom) and Sn4+ (0.81 Angstrom). Moreover, the ionic size of Th4+ is comparable to that of Gd3+ (1.053 Angstrom) which is a typical A-site cation in the perovskite manganite and can also be substituted for La3+ (1.16 Angstrom). It has been observed that a nominal composition, La0.7Th0.3MnO3, synthesized by the usual procedure, contains different phases as La0.7MnO3, ThO2 and Mn3O4. From powder X-ray diffraction and magnetic measurements, and by comparing with the results on Ce4+ and Sn4+ substituted compositions in different reports, it is concluded that generally it is not possible to substitute the trivalent La3+ ion by a tetravalent ion, due to the ionic size constraints, valency of Mn and site preferences of different ions. (C) 2002 Elsevier Science B.V. All rights reserved.