Lattice defects characterisation

Alkaline earth oxides (AEO = MgO, CaO, and SrO) doped with 5 mol% Nd203 have been synthesised either by evaporation of nitrate solutions and decomposition, or by sol-gel method. The samples have been characterised by chemical analysis, specific surface area measurement, XRD, CO2-TPD, and FTIR spectroscopy. Their catalytic properties in propane oxidative dehydrogenation have been studied. According to detailed XRD analyses, solid solution formation took place, leading to structural defects which were agglomerated or dispersed, their relative amounts depending on the preparation procedure and on the alkaline-earth ion size match with Nd3+. Relationships between catalyst synthesis conditions, lattice defects, basicity of the solids and catalytic performance are discussed. 

In general, a Mossbauer spectrum shows different components if the probe atoms are located at lattice positions that are chemically or crystallographically nonequivalent. For instance, from the parameters that characterise a particular Mdssbauer subspectrum, it is possible to establish whether the corresponding probe atoms reside in sites that are not affected by structural lattice defects, or whether they are located at defect-correlated positions. Each compound or phase that contains iron has characteristic parameters in its Mossabauer spectrum. This means that the method is suitable for quantitative as well as qualitative analysis. Mossbauer spectroscopy is also non-destructive and requires only relatively small quantities of samples (- 100mg) [169-171,196-198]. 

Spinel notation [93]. It is characterised by a pressed lattice (a=0.816 nm instead of 0.824 nm for LiMn204) and has all manganese ions in 4+ oxidation state. The composition of mechanochemically prepared spinels, probably, can be described by a similar formula (Li,D )-[(Mn""Mn l2.,D,]04 or LiMn204+z- On the increase of calcination temperature, the process of reduction of manganese ions and annealing of defects occur. 

Crystalline solids contain different types of stractural defects. If the imperfection is limited to one stractural or lattice site and its immediate vicinity, the imperfection is termed a point defect. Vacancies and interstitial atoms are point defects. An impurity atom present in a crystal and that either occupies a lattice site or an interstitial site is also termed a point defect. But in addition to the point defects the stractural defects also comprise line and plane defects. The line defects are dislocations which are characterised by displacements in the stractirre in certain directions. The plane defects comprise stacking faults, grain boundaries, internal and external surfaces. 

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