Characterization of the products of the thermal decomposition of Mm(OH)3 (mishcmetal hydroxide) by various TEM techniques

E. ZELAYA; M.R. ESQUIVEL; D. SCHRYVERS

Ciudad de Rosario-Santa Fe-Argentina

Conferencia; 10 th International Congress of Electron Microscopy 2009. CIASEM 2009; 2009

Asociación Argentina de Microscopía

The structures obtained from the thermal decomposition of pure and mixed-lanthanide hydroxides have not been extensively studied up till now . The phase stability depends on both temperature and lanthanides content. These features and the presence of intermediaries of reaction during thermal decomposition govern the final micro and nanostructures. Differential scanning calorimetry and X-ray diffractometry, two microstructural techniques, were used to characterize the products of the thermal decomposition in air of La0.25Ce0.52Nd0,17Pr0,06(OH)3 0.25Ce0.52Nd0,17Pr0,06(OH)3  ((Mm(OH)3). The exothermal events were associated to the total formation of  stoichiometric La0.25Ce0.52Nd0,17Pr0,06O2. This oxide is represented by an Fm3m space group with a lattice parameter a=5,479(3)Å obtained by X-ray Diffraction (XRD) using the Rietveld method. To clarify and assess the results observed in the microstructural measurements, the nanostructure of Mm(OH)3 samples heated at 1000 °C and quenched in air was characterized by various transmission electron microscopy (TEM) techniques. Crystalline domains between 20 and 50 nm were observed in the dark field (DF) images . Particles with smaller domains ranging between 5 and 20 nm are also found in the same sample as shown in the dark field image. The diffraction ring pattern of the structure shown in this image can be indexed according to a primitive monoclinic structure with cell parameters: a=3.3 Å, b=3.8 Å, c=6.0 Å and β= 112o. This second structure was not detected by XRD. As measured by EDX analysis, both particles have similar composition. Since there is no reported phase transition with increasing temperature in cubic-lanthanide-dioxides, both structures observed are probably formed after the thermal decomposition of Mm(OH)3. Like pure CeO2, La0.25Ce0.52Nd0,17Pr0,06O2 is stable at the evaluated temperatures [2]. Therefore, the minority phase found could be unstable MmO(OH).One other characteristic found in the sample was the segregation of La-rich (La*) particles. This assessment is confirmed by the different Z contrast observed using STEM images with a short camera length (L=50 mm) The compositional inhomogeneity is also observed by EDX measurements. La segregation can indeed be distinguished, confirming the formation of secondary phases during the thermal decomposition of Mm(OH)3. The thermal formation/decomposition of the minor phase should follow the reversible scheme: 2La*(OH)3 2La*OOH + H2O La*2O3 + H2O . The La*OOH in this scheme is unstable on heating/cooling. This structure is monoclinic and non-coherent with the surrounding La0.25Ce0.52Nd0,17Pr0,06O2 cubic phase. The reaction is achieved with a mass loss of water and the process should lead to the formation of voids within the matrix of the dioxide. These characteristics appear in the BF images and detected in the STEM images . The results obtained by TEM clarify the mechanisms of thermal decomposition of Mm(OH)3 which can not be detected using the aforementioned microstructural detection techniques.