PHYSICO CHEMICAL STUDIES OF IRON - RARE EARTH MIXED OXIDES

Abstract

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. A lot of work has been done on mixed rare earths - iron compounds with ratios giving their orthoferrites and garnets (RFe03 and R3FesOi2 where R is a rare earth), but very little is reported in literature on the systems having their ratios varying from the above two. This dissertation is an effort to partially fill this gap. Mostly ceramic methods have been used for the synthesis of orthoferrites and garnets. In this dissertation some investigations on the samples prepared by thermal treatment in air of coprecipitated oxyhydroxide having varying atomic ratios of R3+ and Fe3+ ions are presented. The products so obtained are likely to be multiphasic or despersion of constitutents formed. The nature of constituents depends upon the relative amounts of forming their coprecipitates. In view of the nature of such systems the state and bonding of iron ions are complicated by features such as chemical reactivity, phase transformation effects. The properties of some of the investigated nonstoichiometric systems are rather quite different from those of the anticipated products. 11 Special emphasis is laid on magnetic behaviour, structure and morphology of the mixed oxides. The techniques used for examining the samples are Mossbauer spectroscopy, Magnetometry, X-ray diffraction, Electron optics and Thermal analysis. The samples examined are as follows:- 1. Y and Fe having atomic ratios- 1:9, 1:3, 1:2, 1:1, 2:1, 3:1 and 9:1, heat treated to 850, 1000 and 1250°C. Even though, Y does not belong to the rare earth series of elements, it is classified along with them because of the similarity of chemical properties. 2. La and Fe having atomic ratios 1:9, 1:3, 1:2, 1:1, 2:1, 3:1 and 9:1, heat treated from 100 to 1250°C. The first sample was examined in greater details because of its unusual Xg values. 3. Coprecipitates of Nd and Sm with Fe, with the above atomic ratios, but heat treated to 850, 1000 and 1250°C were examined . In view of the above, the thesis consists of the following chapters:- 1. General Introduction 2. Experimental methods 3. Studies of iron - yttrium systems 4. Studies of iron - lanthanum systems 5. Studies of iron - neodymium systems 6. Studies of iron - samarium systems 7. Conclusions Ill Chapter 1 highlights the relevance of the topic and describes importance of rare earths and their compounds. A brief account of the work done on related type of systems and compounds is also given. Naturally this chapter includes current literature survey relevant to the work. In chapter 2 the preparation of iron rare earth oxyhydroxides and various analytical techniques used to probe the physico-chemical properties of these compounds are given. Also given is a brief introduction of the principles and history of the techniques which include, thermal analysis, Mossbauer spectroscopy, x-ray diffraction, vibrating sample magnetometry and transmission electron microscopy, along with description and make of the instruments. Where necessary the standardization technique of the instrument is also geven. Chapers 3, 4, 5 & 6 deal with the details of observations made on the systems Yttrium - iron, Lanthanum - iron, Neodymium - iron and Samarium - iron respectively using the various techniques. Every system and every sample annealed to different temperatures was characterized and its magnetic susceptibility variation with temperature is reported. The evolution of hyperfine features in Mossbauer spectra on thermal treatment have been sketched. Chapter 7 contains a comparison and discussion of the observation 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 temperature of heat treatment of the IV sample not only markedly influences the morphology of the samples, but also their magnetic susceptibility. For some samples heat treated to 850"C magnetic susceptibility versus temperature plots (heating and cooling both ways) show remarkably anamolous behaviour. The relative contribution to the magnetic susceptibility of rare earths in the mixed oxide systems is also examined.