PHYSICO CHEMICAL STUDIES ON COPRECIPITATED IRON (Ul)-CHROMIUM (111) HYDROXIDES AND MIXED OXIDE SYSTEMS

Abstract

Physioochemical studies on coprecipitated iron(III) - chromium(lll) hydroxides and mixed oxide systems, aim at comprehensive examination of the oxide hydrate gels containing Fe(III) and Cr(lll) ions in different weight percent ratios. The intermediate products that are formed preceding the formation of mixed oxides 36 a result of annealing these gels in air have also been characterized. Since, a number of parameters affect precipitation processes, differences of opinion persist about the structure and properties of iron(lll) and chromium(III) oxide hydrate gels, Therefor e_, these two gels and the products formed on thermal treatment of the pure gels have been independently investigated. This has been done to better understand the mixed systems. For investigations, a number of physico-chemical methods of materials analysis, namely Infrared spectroscopy, Electron microscopy, X-ray and Electron diffraction, Magnetometer, Thermal analysis and MSssbauer resonance spectroscopy have been applied. Ferric oxide hydrate gel and its transformation to oc-Fe20_ The bulk composition of the gel is given by the formula Fe20,.2.2H20 and it has hexagonal subunit particles (of size o 70-100A) and their agglomerates. Theparticles in superparamag netic state have incompletely compensated antiferromagnetic character. The gel consists of a hexagonal close packed arrangement of oxygen sublattices similar to those in oc-FegOv, but with a little degree of order, iron ions being quite (ii) randomly distributed in the gel. These features support for the formation of protoferrihydrite phase in the initial gel. During annealing, the removal of water from the gel takes place in a gradual and notable sluggish manner. Between 150-200 C, dehydration takes place and a new phase 'ferrihydrite ' emerges. This phase has greater cation ordering in the oxygen framework and a higher iron oxygen ratio, Dehydroxylation and removal of water from ferrihydrite between 250-325°C leads to the formation of pseudohematite v/hich may be considered as a'relic' structure related to the original gel and as immediate precursor phase of a-Fe2°5. Pseudohematite starts necleating between 350-400°C into microcrystalline hematite (particles size 250-300A) and near 500 C, crystalline character improves as a result of rserystallio zation ana the crystallites attain^n 325A size. Annealing between 7OO~800°C, finally results in the formation of sintered o and densified hematite crystallites (1000A), and the structural defects and inhomogeneties are eliminated. Chromium oxide hydrate gel and its transformation to a-Cr?0„ The chromium oxide hydrate gel prepared by addition of SercUuLvn ammonium hydroxide to an aqueous solution of chromium(lll) chloride and^dried at 60°C was found to have a bulk composition given /by formula Cr203.4.4H20. The structure of the gel is inhomogeneous and consists of three types of microstructure regions: poorly crystalline Cr(OH) with water, rhombohedral (iii) HCrO fraction and some amount of microorystalline Or(OH)-. 3 The inhomogeneous character of the gel persists even on annealing the gel to 700°C and pure ce-OrgO, is formed at 800°C and above. The amount of HCr02 component increases in the gel samples annealed at 160 and 300°C due to its initial hydrothermal formation; but this component vanishes on annealing the gel at 410 C and above. Surface oxidation of the gel that starts on annealing at 160°C and above, results in the c- • 4+ 6+ formation of Or -Cr species trapped in and onto the surface of the annealed gel. However, the Cr4+-Cr6+ species are not observed in the gel annealed at 000°C. The magnetization behaviour and the magnetic suscepti bility values indicate that the gel and its annealed products which have been characterized are antiferromagnetic. The variation in particle size of the annealed products are also reported, Ir an (III)~chromlum( III) cogre pipitated hydroxide ^els and their annealerd pr oduc t s Depending on their respective compositions, the morphology and microstructure of initial Fe(lH)-Or(lII) coprecipitated hydroxides show a continuous variation. Coprecipitates with Fe:Cr =1:9 and 9:1 arc identifiable with Fo- substituted chromium oxide hydrate gel and Crsubstituted protoferrihydrite respectively. With intermediate compositions i.e. Fe: Ur = 3:7, 5:5 and 7:3 the coprecipitated (iv) hydr oxide s c ontain: i) Fe-substituted chromium oxide hydrate gel as dominant phase alongwith some Cr- substituted protoferrihydrite in samples with Fe:Cr = 3:7, ii) both these phases in nearly equal proportions for samples with Fe:Cr = 5:5 and iii) Cr-substituted protoferrihydrite as dominant phase with significant concentration of Fe- substitued chromium oxide hydrate in samples with Fe:Cr = 7:3. Due to different degrees of hydration and carbonation for coprecipitated gels, complexity and variability in the properties of initial coprecipitates is observed. Incorporation of chromium appears to cause a loss in uniformity and contributes towards increasing amorphous character with its increasing incorporation in the gel. However, in samples with Fe;Cr = 9:1 and 3:7 chromium ions appear to contribute towards increasing coherency of the gels and thereby strengthening and stabilizing the gel structure. Incorporation of iron, on the other hand, results in better definition of the gel particles and their size and crystallinity is being increased. As regards the role of Fe and Cr ions on the morphological characteristics of the different phases formed, these results support the obser vations made by Zolotovskii et al. and Gibert et al. (Refs. (13) and (16) respectively given in Chapter 5). Coprecipitated hydroxides are stabilized initially with removal of COp and water molecules and surface oxidation of (v) Cr ions to higher oxidation states onsets, on annealing the samples to 160 C. Formation of Y-Pe O-.nHpO type of phase (with Cr incorporation; is indicated for coprecipitated hydroxides with Fe:Cr = 3:7%annealed at 300°C. The exothermic effects exhibited around 300-400 C for coprecipitates represent the coincidence of precipitation of poorly ordered Cr?0„ phase and surface oxidation. The relative extent to which these oxidation and crystallization processes occur depends on the morphology and micro structure of the gels. Delayed crystallization of mixed gels to cc-Fe20,, C^Cz solid solutions observed in the temperature range 410- 520°C is ov/ing to the formation of protective adsorbed layers of one form on the surface of another resulting into quelling of Gr20^ and Fe20^ by each other when mixed together to give solid solutions. It is observed that maximum protective action against crystallization took place in samples with Fe:Cr = 9:1 and minimum in with Fe:Cr = 1:9. These differences in different degrees of protective action are found to be due to the differences in microstructures and coherency characteristics of the initial coprecipitates. Changes observed in MSssbauer parameters and lattice parameters follow the formation of cone spiral magnetic spin arrangement in the system and reveal the extent of solid solubility and involvement of surface interactions of host and impurity species. For some samples annealed at 460 and 800°C, the Vegrad's law for solid solutions is not followed; also the high value of internal magnetic field splitting is (vi) observed for some samples annealed at 800 0 due to cluster formation. The incomplete ordering, random packing of the 3+ 3+ Fe and Cr ions within layers of rhombohedral corrundum structure and single phase inhomogeneity are factors to be held responsible for the deviations in the Vegard's law for the variations in lattice parameters for samples annealed at 460 and 800 C. For samples annealed at 1280 C, perfection in structure and growth of crystallites occur with redistribution of Cr ions in hematite lattice. Critical comparison of the data obtained from X-ray diffraction, Mossbauer effect spectro scopy and other techniques reveal interesting points regarding mechanism and dynamics of thermal transformations leading to the structural evolution of mixed oxide systems resulting with different cry stallochemical and magnetic properties.