Conduction in Mn substituted Ni-Zn ferrites

The d.c. electrical resistivity 'rho' and thermoelectric power 'alpha' are studied as function of temperature for Mn substituted ferrites with general formula Zn0.3Ni0.7+xMnxFe2-2xO4. At lower Mn concentrations, the increase in d.c. resistivity is attributed to the hindering of Verwey mechanism Fe2+ reversible arrow Fe3+ due to stable bonds of Mn3+ + Fe2+ pair. The decrease in resistivity at higher Mn concentrations (i.e. when x > 0.15) is attributed to the formation of Mn3+ cluster and Ni2+ reversible arrow Ni3+. The activation energy values show one to one correspondence with resistivity values. The compositional variation of thermoelectric power shows n-type behaviour for the samples with x < 0.2 whereas p-type behaviour for the samples with x greater than or equal to 0.2 The p - n transition is attributed to the formation of Ni3+, Fe2+ + vacancies which act as p-type carriers. The temperature dependences of alpha, rho, and mobility clearly confirm the conduction mechanism to be due to polaron hopping.