Conduction mechanism in cobalt rich Co3-xMnxO4 (0.1 ≤ x ≤ 1.0) spinel oxide ceramics

Abstract

We report the structural and electrical properties of the Co3-xMnxO4 (CMO) with x = 0.1 to 1.0 ceramics, synthesized by conventional solid state reaction technique. These ceramics have been characterized at room temperature by the powder X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. XRD and FTIR spectra reveal the formation of polycrystalline single-phase with cubic spinel structure. The observed room temperature dc conductivity increases with the substitution of Mn in CMO and revealed the semiconducting nature of these ceramics, due to the hopping between the cations with the different valence states at the octahedral sites. The ac conductivity was estimated from the observed dielectric data as a function of frequency at different temperature. The estimated ac conductivity was explained through the small polaron tunneling (SPT) and correlated barrier hopping (CBH) model. The dispersion of ac conductivity has been estimated in terms of temperature dependent frequency exponent s(T), which varies with temperature and explained using SPT and CBH model.

Effect of Off-Stoichiometry on Electrical Conductivity in Ni-Fe and Mn-Co Spinel Systems

“…Although the cationic distributions for Mn 3 O 4 ((Mn 2+ )[Mn 3+ 2 ]O 4 ) 24 and Co 3 O 4 ((Co 2+ )[Co 3+ 2 ]O 4 ) 55 are well established, the cationic distributions of intermediate Mn x Co 3-x O 4 spinels have been notoriously difficult to determine due to the multivalency of both cations, resulting in many different suggested models. 21,[23][24][25]56 Contrary to the endpoint spinels (x = 0 and 3) which exhibit insulating electrical behavior, intermediate compositions display semiconductor behavior with relatively high conductivities. This elevated conductivity has been thought to be the result of Verwey-type small polaron hopping between either Mn 3+ -Mn 4+ or Co 2+ -Co 3+ ion pairs.…”

“…57 More recent models seem to agree that conduction likely takes place due to hopping between both Mn 3+ -Mn 4+ and Co 2+ -Co III+ ion pairs (III indicating a low spin 3 + configuration) on the octahedral sites. 23,24,56 It follows that Mn 3+ 3-x−y Mn 4+ y ]O 4 , where 1 ⩽ x ⩽ 3 and 0 ⩽ y ⩽ 0.40. 24 Here, y is a measure of the number of Co ions which take the 2 + state when replacing Mn 3+ on the octahedral positions and was determined experimentally.…”

Review—(Mn,Co)3O4-Based Spinels for SOFC Interconnect Coating Application

“…Several studies have reported that electrical conductivity of properly sintered intermediate Mn x Co 3−x O 4 spinels increases monotonically with Co content until x = 1-1.2 (see Fig. 5a), 51,61,72,87,88 suggesting that the number of active Verwey pairs is maximized for compositions approaching MnCo 2 O 4 .…”

“…Instead, several studies have suggested that the exceptional conductivity of intermediate Mn x Co 3−x O 4 spinels (1 ⩽ x ⩽ 2) is due to so-called double-mixed valency, wherein conduction proceeds through both Co 2+ /Co III and Mn 3+ /Mn 4+ ion pairs in the octahedral positions. 72,87,88 These intermediate spinels typically exhibit conductivities 3∼4 orders of magnitude higher than Cr 2 O 3 (0.01 − 0.1 S•cm −1 ), 89,90 and 2∼3 orders higher than Cr-rich (Mn,Co,Cr) 3 O 4 (0.01 − 1.0 S•cm −1 ). 91,92 Noticeably, several groups have reported conflicting values for the electrical conductivity of MCO spinels, as shown in Tables I and III.…”