MANGANESE OXIDE ZIRCONIUM-OXIDE SOLID-SOLUTIONS - AN X-RAY-DIFFRACTION, RAMAN-SPECTROSCOPY, THERMOGRAVIMETRY AND MAGNETIC STUDY

Manganese oxide-doped zirconium-oxide samples, prepared by heating mix tures of coprecipitated hydroxides at 1073 K in a hydrogen stream (wat er 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 co ntent up to 14.74 mass%), were studied 'as-prepared' and after subsequ ent thermal treatments in oxygen up to 753 K. The results of several t echniques [X-ray diffraction (XRD), Raman spectroscopy, thermogravimet ry (TG) and magnetic susceptibility measurements] show that in the 'as -prepared' samples (1073 K, H-2) a high fraction of manganese is incor porated 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 expe riments and magnetic susceptibility measurements reveal that-the Mn3and/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 exp osure to the atmosphere. As the amount of manganese in solid solution increases, the volume of the zirconia unit cell slightly decreases. Th e solid-solution formation favours the tetragonal and the cubic modifi cations at the expense of the thermodynamically stable monoclinic phas e. When the samples are heated up to 753 K in oxygen, the Mn2+ in soli d solution is partially oxidized to Mn3+ and/or Mn4+. TG and XRD exper iments show that the oxidation starts at low temperature and takes pla ce in solid solution without appreciable manganese oxide segregation.