Manganese oxide–zirconium oxide solid solutions. An X-ray diffraction, Raman spectroscopy, thermogravimetry and magnetic study

Abstract

Manganese oxide-doped zirconium oxide samples, prepared by heating mixtures of coprecipitated hydroxides at 1073 K in a hydrogen stream (water content 0.2% by volume), were analysed to obtain information on the solid solution formation. The state and the thermal stability of the incorporated species were also investigated. The samples (manganese content up to 14.74 mass%), were studied ‘as-prepared’ and after subsequent thermal treatments in oxygen up to 753 K. The results of several techniques [X-ray diffraction (XRD), Raman spectroscopy, thermogravimetry (TG) and magnetic susceptibility measurements] show that in the ‘as-prepared’ samples (1073 K, H₂) a high fraction of manganese is incorporated in the zirconia structure, only a small fraction being present as an MnO separate phase. Most of the manganese in solid solution is present in the +2 oxidation state, the remainder as + 3 and +4. TG experiments and magnetic susceptibility measurements reveal that the Mn³⁺ and/or Mn⁴⁺ are formed both during the cooling in hydrogen by reaction with water present as an impurity in the gas phase, and during the exposure to the atmosphere. As the amount of manganese in solid solution increases, the volume of the zirconia unit cell slightly decreases. The solid-solution formation favours the tetragonal and the cubic modifications at the expense of the thermodynamically stable monoclinic phase. When the samples are heated up to 753 K in oxygen, the Mn²⁺ in solid solution is partially oxidized to Mn³⁺ and/or Mn⁴⁺. TG and XRD experiments show that the oxidation starts at low temperature and takes place in solid solution without appreciable manganese oxide segregation.