Cation distribution in spinel (Mn,Co,Cr)₃O₄at room temperature

Abstract

As part of a study of the long‐term operation of solid‐oxide fuel cells, three (Mn,Co,Cr)3O4 samples have been synthesized and characterized. X‐ray and neutron diffraction patterns from the powder samples at room temperature were measured and the data were co‐refined. The neutron data were indispensible in locating Mn, Co and Cr within the crystallographic unit cell with their respective atomic occupancies. Two of these samples have been identified as cubic Mn0.76Co0.58Cr1.66O4 and Mn1.28Co1.72O4. The third is a two‐phase sample containing cubic Mn1.66Co1.34O4 and tetragonal Mn2.05Co0.91O4 in a 59.1 (6):40.9 (6)% mass fraction ratio. Cr, which might be introduced from reaction with chromia during oxidation of interconnect materials, exhibits a preference for the octahedral site rather than the tetrahedral site. Without Cr, Mn dominates the octahedral site.

Electrical Properties, Cation Distributions, and Thermal Expansion of Manganese Cobalt Chromite Spinel Oxides

“…The crystal structure of MnCoCrO 4 at high temperature is also a single cubic phase with space group (227), and its lattice parameter expanded with increasing the temperature. As for the occupancy preference, our result of Cr is also consistent with that in Mn 0.4 Co 0.6 Cr 2 O 4 spinel, the composition of the outer reaction layer, in which Cr 3+ occupy the octahedral sites analyzed at room temperature . Similar to Mn 1.5 Co 1.5 O 4 , the occupancy of Co on the tetrahedral site was much greater than that of Mn in MnCoCrO 4 .…”

“…Consistent with Yang et al . 's results, at room temperature Mn 1.5 Co 1.5 O 4 consists of two phases: cubic Co‐rich Mn 1+α Co 2−α O 4 (Space group: ) and tetragonal Mn‐rich Mn 2−δ Co 1+δ O 4 (Space group: I 4 1 / amd ) . As Cr content increases, the amount of the tetragonal phases also decreases, and when the Cr content is greater than 1, Mn 1.5−0.5 x Co 1.5−0.5 x Cr x O 4 forms the single cubic phase.…”

“…Thus, the effect of Cr on the conductivity of MnCoCrO 4 could be attributed to the reduction in the number of available neighboring octahedral cations for hopping. Consequently, as more Cr cations occupy the octahedral sites with a fixed valence +3, the conductivity is reduced.…”

Comparison of iron and copper doped manganese cobalt spinel oxides as protective coatings for solid oxide fuel cell interconnects

“…Chromium is known to have the strongest octahedral site preference among the transition metal cations [60,61] and is consequently expected to occupy only octahedral sites, viz. (Co)[Mn,Cr] 3 O 4 [12,59]. The higher Co/Mn fraction in the reaction layer compared to the as-prepared coating suggests that outward diffusing Cr from the alloy primarily replaces Mn in the spinel, as expected from the site preference.…”

“…Overall, the Co/Mn fraction in the reaction layers is greater than the Co/Mn fraction in the as-prepared coating. Cation distribution studies of MnCo 2 O 4 have shown that Mn primarily occupies the octahedral sites in the spinel structure while the tetrahedral sites are mostly filled by Co [10,58,59]. The distribution may be written as (Co)[Mn,Co]O 4 , where the round brackets designate tetrahedral sites and the square brackets designate octahedral sites.…”