Electrical and dielectric properties of the Ca2MnO4−δ system

The materials Ca2MnO4−δ(δ=0.0–0.5) were prepared in two steps: the preparation of the parent compound Ca2MnO4 by the sol–gel method followed by the creation of oxygen vacancies δδ. The morphology has been studied by scanning electron microscopy (SEM) technique and has revealed that the free surface morphology becomes more densified when increasing the oxygen vacancies δδ. The electrical properties have been studied using dc conductivity and ac impedance spectroscopy techniques. From the dc   conductivity measurements it was found that, at high temperatures, the conduction mechanism is controlled by small polarons. The activation energy increases with δδ, indicating an increase in the height of the potential barriers. The dielectric properties were studied using the modulus formalism, showing a non-Debye relaxation process, which is thermally activated presenting activation energy independent of δδ.

► Ca2MnO4−δ crystallizes in the tetragonal structure for δ=0δ=0 and in orthorhombic structure for δ>0δ>0.
► The increase of δδ promotes a decrease of the quantity of superficial pores.
► The dc   Ea increases from δ=0.0δ=0.0 to δ=0.20δ=0.20 and decreases with the increase of δδ.
► The decentred semi-circles indicate the existence of a relaxation time distribution.