Electrical conductivity of La_(1-x)Ca_xFeO_(3-δ) solid solutions

摘要

La_(1-x)Ca_xFeO_(3-δ) solid solutions (x=0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) were investigated. The samples were prepared by the polymerizable complex route and characterized by X-ray diffraction and complex impedance spectroscopy techniques. Results reveal the formation of a single perovskite phase for the La_(1-x)Ca_xFeO_(3-δ) (0 ≤ x ≤ 0.5) compositions. However, the La_(0.4)Ca_(0.6)FeO_(3-δ) sample is a mixture of many phases: perovskite, calcium ferrite and iron oxide. The unsubstituted lanthanum ferrite oxide, as well as the substituted samples, exhibits an orthorhombic symmetry. The direct current conductivity analyses reveal a typical negative temperature coefficient of the resistance behaviour for all the samples. The incorporation of calcium into the lanthanum ferrite lattice results in a significant improvement of the direct current conductivity. In fact, La_(0.8)Ca_(0.2)FeO_(3-δ) oxide shows the optimal conduction value. For all the studied compositions, a change in the activation energy is highlighted around 440℃. This behaviour is attributed to the antiferromagnetic to paramagnetic transition of lanthanum ferrite. As for the alternating current conductivity, it obeys the Jonsher's power law. The correlated barrier hopping model is proposed to describe the transport mechanism in the studied matrix.