PREPARATION AND CHARACTERIZATION OF SOME RARE EARTH (III) - IRON (III) MIXED OXIDES

Abstract

Modern solid-state chemistry is characterized by the interplay between the synthesis of materials and their structure-property relations. At present solid-state chemistry is mainly concerned with the development of new methods of synthesis, identifying and characterizing materials. It includes new strategies for synthesis with desirable, but controlled properties like electronic, magnetic, dielectric, optical, adsorptive or catalytic. The structure and chemical composition of solids determine the macroscopic properties. Research on structure and composition is leading to new classes of materials with novel physical properties. Examples are optical materials, semiconductor compounds, superconductors, solid electrolytes, magnetic materials and thin metallic films with unusual properties. In many solids, the important properties arise from defects and chemical impurities, and the study and control of these form a major area of solid-state chemistry. Examples are non-stoichiometry in optical materials and chemical impurities as luminescence centres in semiconductor compounds. A large number of parameters affect the magnetic properties of materials. By careful control of composition and fabrication procedures, it is possible to prepare materials with a desired set of properties. Rare earths are no longer rare now, since their production has gone up in a big way because of their applications in newer technologies. Rare earths as such, their alloys, pure and mixed compounds are extensively used in the production of magnetic materials, information storage devices, superconducting materials etc. Rare earth-iron mixed oxides e.g. orthoferrites and garnets which form multisublattice systems have provided magnetic materials of technological relevance. Orthoferrites have recently been used for bubble domains. Garnets have gained prominence for their microwave applications. The well known permanent magnetic materials and some recently developed high frequency core materials belong to the hexagonal class of ferrites. (ii) Alot of work has been done on rare earth-iron mixed oxides with ratios giving their orthoferrites and garnets (RFe03 and R3Fe5012 where Ris a rare earth), but very little is reported in literature on the system having cationic ratios different from these two. This dissertation is an effort to partially fill this gap. Therefore, some efforts have been made to throw some light on the variation of properties of rare earth-iron mixed oxides as a function of rare earth concentration, temperature of heat treatment and ionic radii of R3+. Their properties are not only a sensitive function of the ionic radius and valency state of their constituents but also dependent on the method of their formation. Mostly ceramic methods have been used for the synthesis of orthoferrites and garnets. This dissertation deals with the studies on the samples prepared by thermal treatment of coprecipitated oxyhydroxides having varying atomic ratios of R3+ and Fe:u . A high degree of homogenization together with small particle size and thereby speeding up the reaction rate, is achieved by using coprecipitation as solid-state precursor method. The products so obtained are multiphasic. The dispersion of microphases in host lattice depends upon the relative amounts of coprecipitates. In view of the nature of such systems, the state and bonding of iron ions are complicated by features such as chemical reactivity and phase transformation effects. The properties of some of the investigated non-stoichiometric systems are rather quite different from those of the anticipated products. Special emphasis is laid on magnetic behaviour, structure and morphology of the mixed oxides. For the sake of convenience the work embodied in this thesis has been presented in the following chapters : 1. General Introduction 2. Methodology and Techniques 3. Studies on Ytterbium-Iron Mixed Oxide Systems 4. Studies on Gadolinium-Iron Mixed Oxide Systems 5. Studies on Praseodymium-Iron Mixed Oxide Systems 6 Conclusions

The first chapter of the thesis highlights the relevance of the topic and describes the importance of various oxides of rare earths, iron and their mixed oxides including garnets. The comprehensive review of the related literature available on these systems and compounds has also been incorporated in this chapter. The second chapter concerns with the preparation of rare earth-iron oxyhydroxides and various analytical techniques used to probe the physico-chemical properties of these compounds. The solid-state precursor method was used to prepare the rare earth-iron mixed oxides. Initially the coprecipitation of R3+ (Pr , Gd3+ and Yb3+) and Fe3+ with different atomic ratios 1:9, 1:3, 1:2, 1:1 and 2:1 has been carried out by ammonical hydrolysis of metal ions in aqueous solutions at pH 7.0. The mixed oxides were obtained after the thermal treatment of mixed oxyhydroxide gels. These were heat treated to 800°C, 1000°C and 1200°C. These mixed oxides were characterized by various techniques like Mossbauer resonance spectroscopy, optical metallography, microhardness testing for phase identification, magnetometry for magnetic studies, X-ray diffractrometry and thermal analysis. A brief introduction of the principles of these techniques along with description and make of the instruments is also given. Chapters third, fourth and fifth deal with the details of observations made on the systems ytterbium-iron, gadolinium-iron and praseodymium-iron respectively, using the various techniques. Every system and every sample annealed to different temperatures were characterized and its magnetic susceptibility variation with temperature is reported. The evolution of hyperfine features in Mossbauer spectra on thermal treatment has been carried out. Sixth chapter contains a comparison and discussion of the observations of the investigated systems. Efforts have been made to correlate the properties with ionic radii and charge of rare earths. It is found that the heat treatment temperature of the sample not only markedly influences the morphology of the samples, but also their magnetic susceptibility. The relative contribution to the magnetic susceptibility of rare earths in the mixed oxide systems is also examined.