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

Abstract

Single phase, electrically insulating samples of Bi_1−xLa_xFeO₃ 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 Pn2₁a(00γ)s00 super structure was obtained. For 0 ≤ x ≤ 0.30, differential scanning calorimetry showed two endothermic effects corresponding to the Néel 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 ∼0.28, when T_N = T_C = 386 °C. Ceramics with 0 ≤ x ≤ 0.15 were highly insulating at room temperature with a resistivity, extrapolated from the Arrhenius plots, of ∼7 × 10¹⁶ to 8 × 10¹⁴ Ω cm and an activation energy ∼1.14–1.20 eV. Magnetization of the samples improved with La substitution.