Impurity level substitution of Cr and Ni in CaBaCo4O7 – a dielectric study

The fascinating physical characteristics of the 114-cobaltate, CaBaCo4O7, like alternate stacking of two-dimensional layers of CoO4 tetrahedra in triangular and kagomé patterns, geometrical frustration, and magnetoelectric coupling have attracted several studies in the magneto-structural and dielectric sectors. But the study of electrical conduction dynamics remains practically unknown. Here we have presented dc conduction studies and ac dielectric spectroscopy intending to understand the charge conduction and charge carrier relaxation dynamics of CaBaCo4O7, along with two chemically substituted derivatives – CaBaCo3.96Cr0.04O7 and CaBaCo3.96Ni0.04O7, of impurity level (1%) substitutions. We observe that dc conduction at low temperatures is mediated by variable range polaron hopping, while the ac conduction study points out small-polaron-tunneling across strongly localized states. Electric modulus obeys the Havriliak-Negami equation, with parameters α and γ significantly less than unity, indicating strong distribution in relaxation times; and its non-exponential nature, pointing out the cooperative motion of charge carriers. Imaginary electric modulus is not scaled to a single master curve for the three samples, signifying differences in charge relaxation dynamics.