Electrical conduction in Mn-substituted Ni-Cd ferrites

The d.c. electrical resistivity (rho) and thermoelectric power (alpha) as a function of temperature for Mn-substituted ferrites with general formula Cd0.5Ni0.5+xMnxFe2-2xO4 have been studied. At lower Mn concentrations (x less than or equal to 0.2) the increase in d.c. resistivity with x is attributed to hindering of the Fe(3+)reversible arrow Fe2+ transition. The hindering is caused by the formation of Mn3+Fe2+ stable pair at the octahedral site. The decrease in resistivity with higher concentration of Mn (x>0.2) is attributed to the formation of Ni3+ and to the Mn3+-Mn4+ transition. The observed high value of activation energy for the present system is related to Mn3+ Jahn-Teller trapping. The compositional variation of thermo-emf shows n-type conduction for samples with x less than or equal to 0.2, and p-type conduction for samples with x>0.2. The n- to p-type transition is explained by the formation of p-type carriers such as Ni3+ and Fe2+ cation vacancies. The electron hopping model of electrical conductivity is applicable for samples with x less than or equal to 0.2, whereas above x>0.2 conductivity is governed mainly by the Mn concentration at the octahedral site.