Structural and Thermal Properties of the Tetragonal Cobalt Manganese Spinels Mn_xCo_3-xO₄ (1.4 < x < 2.0)

Abstract

Phase transitions experienced by Mn x Co 3-x O 4 (1.4 < x < 2.0) materials synthesized at low temperature have been followed by X-ray powder diffraction and thermal analysis. A tetragonal single phase (space group I41/amd) has been isolated for the first time for samples quenched from T > 700 °C, with a distortion parameter ranging from 1.0263(1) for x ) 1.45 to 1.1353(1) for x ) 1.9. The existence at T > 725 °C of a nonisolable cubic spinel single phase (space group Fd3m) in the whole compositional range has been shown by hightemperature X-ray diffraction techniques. It readily transforms on quenching into the new tetragonal single phase. When the cubic high-temperature single phase is allowed to cool slowly, it decomposes into a mixture of tetragonal (x ≈ 2) and cubic (x ≈ 1.3) spinel-type phases. The composition x ) 1.4 is found to be the upper compositional limit of existence of the cubic Fd3m spinel, when it is synthesized by a low-temperature procedure.

X-ray Absorption Spectroscopy of Mn/Co/TiO2 Fischer−Tropsch Catalysts: Relationships between Preparation Method, Molecular Structure, and Catalyst Performance

“…26,27 Due to the greater atomic radius of Mn 3+ (1.37 Å) with respect to Co 3+ (1.25 Å), 28 substitution of Co 3+ by Mn 3+ in the O h sites results in a cell expansion, increasing steadily with increasing Mn content in the Co 3 O 4 structure. 22,29,30 This phenomenon is also reflected in the Fourier transforms for the HDP catalysts, as illustrated in Figure 2, which shows a decrease of the fourth Co-Co shell intensity as the Mn loading increases. This indicates the presence of a higher level of disorder in the structure of these catalysts that is very likely induced by the incorporation of Mn 3+ ions into the Co 3 O 4 lattice.…”

“…Thus, the Mn-O distances measured in the HDP catalysts are an average between those of MnO 2 (1.88 Å) and those of Co 3-x Mn x O 4 (1.92-1.96 Å). 30 Hence, the formation of Co 3-x Mn x O 4 solid solutions occurs not only in Hcop-CoMn but also in H-CoMn, as revealed by EXAFS giving in both catalysts Mn-O distances of 1.91 Å. Taking into account the higher Mn loading contained in Hcop-CoMn compared to H-CoMn and, in turn, the same Mn-O distances measured with EXAFS (1.91 Å), this catalyst is expected to contain also a fair fraction of manganese in the form of MnO 2 .…”

Optimizing the synthesis of cobalt-based catalysts for the selective growth of multiwalled carbon nanotubes under industrially relevant conditions

“…In the diffraction patterns of samples Co3Mn2, Co3Mn3, Co2Mn3, and Co1Mn3 additional reflections of tetragonal spinel emerge. It is known that this structural change occurs when a higher amount of Mn is incorporated into the oxide structure [33]. In addition, a CoMnO 3 phase of the rhombohedral ilmenitetype can be identified [34].…”

“…10). Concerning cobalt manganese spinel-type oxides Vila et al [33] described the reaction of the tetragonal and the cubic phases between 773 K and 973 K to form a new cubic phase, which was not present in the samples calcined at 673 K. When this high temperature phase is cooled to room temperature, it decomposes again into the tetragonal and the new cubic phase, in which the manganese content of the cubic phase is increased [33]. A similar redistribution of the cations occurs in the samples calcined between 773 and 973 K. This increase of the Mn 3+ content at the Jahn-Tellerdistorted octahedral B sites results in a larger unit cell parameter although the lattices remains cubic, which is indicated by the slight shift of the line positions to smaller 2 angles observed by XRD.…”

Optical and mechanical characterization of novel cobalt-based metal oxide thin films synthesized using sol–gel dip-coating method

“…Manganese-cobalt oxides (Mn x Co y O z ) are a group of widely investigated metal oxides with particular emphasis on the influence of the synthesis conditions on the oxidation states and cation distribution in the cubic and tetragonal phases as typically described in the Mn-Co-O system [14]. The physicochemical properties of manganese-cobalt oxide are greatly affected by the synthesis temperature, crystal structure, anion oxidation states and composition [15].…”