Mixed Ionic–Electronic Conduction and Magnetoelectric Coupling in Li_0.5Fe_2.5–xCr_xO₄ (x = 1.0, 1.1, 1.3, 1.5, and 1.6) Involving Magnetization Compensation Phenomenon

Abstract

The correlations between the crystal structural, magnetic, and electrical properties of Li_0.5Fe_2.5–xCr_xO₄ (x = 1.0, 1.1, 1.3, 1.5, and 1.6) have been investigated by neutron diffraction, DC magnetization, and impedance spectroscopy. Crystal structural analysis reveals mixed cation (Li, Fe, and Cr) distributions over both tetrahedral and octahedral crystallographic sites and an enhancement of the site disorder with increasing Cr concentration. The magnetization study yields ferrimagnetic ordering for all the compounds below the respective TC (640 K for x = 1.0 to 470 K for x = 1.6), and magnetization compensation at the respective T_Comp (540 K for x = 1.0 to 290 K for x = 1.6). The comprehensive analyses of the frequency and temperature-dependent impedance data in terms of DC and AC conductivities reveal the presence of mixed ionic and electronic conductions. The ionic conduction in the compounds is anticipated due to the presence of Li⁺ ions over the tetrahedral and octahedral crystallographic sites, whereas electron migration through a small polaron hopping mechanism is responsible for electronic conductivity. The relative contributions of electronic and ionic conductions are determined by charge transference number analysis. Almost equal fractions of the ionic and electronic conductivities for the composition x = 1.5 over the entire temperature range of 300 to 600 K reveal the character of a nearly ideal mixed conductor. We have shown highly tunable relative contributions of the ionic and electronic conductions with Cr substitution at room temperature. In addition, a strong coupling between magnetic and electric degrees of freedoms has been demonstrated by the changes in the slopes of the temperature-dependent DC and AC conductivities at both the ferrimagnetic ordering temperature TC and at the magnetization compensation temperature T_Comp. The role of cation distributions over the tetrahedral and octahedral sites on the conduction as well as magnetic properties has been discussed.