Single phase, electrically insulating, multiferroic La-substituted BiFeO3 prepared by mechanosynthesis

Abstract

Single phase, electrically insulating samples of Bi1-xLaxFeO3 solid solutions have been prepared by mechanosynthesis over the whole compositional range for the first time. Lanthanum substitution influenced the kinetics of the mechanochemical reaction and crystallite size of the products. For 0 <= x <= 0.15, an increase in the La content produced a significant decrease in the weight-normalized cumulative kinetic energy required to obtain the final product and an increase in the resulting crystallite size. Larger La contents did not affect either the reactivity or the crystallite size. The effect of x on the structure has been identified. Samples in the ranges x <= 0.15 and x >= 0.45 gave single phase solid solutions with R3c and Pnma space groups, respectively, while for the intermediate range, a non-centrosymetric Pn2(1)a(00 gamma)s00 super structure was obtained. For 0 <= x <= 0.30, differential scanning calorimetry showed two endothermic effects corresponding to the Neel temperature (T-N, antiferromagnetic-paramagnetic transition) and the Curie temperature (T-C, ferroelectric-paraelectric transition), demonstrating their multiferroic character. Compositions with a larger La content only showed T-N. Dilatometric and permittivity measurements confirmed the results obtained by DSC for the ferroelectric-paraelectric transition. The composition dependence of T-N and T-C showed that, at low x, T-N < T-C, but a cross-over, or isoferroic transition occurred at x similar to 0.28, when T-N = T-C = 386 degrees C. Ceramics with 0 <= x <= 0.15 were highly insulating at room temperature with a resistivity, extrapolated from the Arrhenius plots, of similar to 7 x 10(16) to 8 x 10(14) Omega cm and an activation energy similar to 1.14-1.20 eV. Magnetization of the samples improved with La substitution.