H. C. Verma

Cation distribution in nanosized Ni–Zn ferrites

Nanoparticles of Ni1−xZnxFe2O4 (x=0.0, 0.25, 0.50, 0.75, and 1.0) in the size range of 6–12 nm have been synthesized by chemical precipitation followed by hydrothermal treatment. A strong correlation between the particle size and the zinc concentration has been identified. Mossbauer studies on these systems show that the cation distribution not only depends on the particle size but also on the preparation route. There are indications that in the present nanophase samples Fe occupies more tetrahedral sites as compared to the normal occupancy in the spinel ferrite structure. The occupancy returns to normal values after heat treatment at 1000u200a°C. Low-temperature Mossbauer studies indicate a significant amount of deviation of cation distribution from their bulk preferences.

Positron lifetime spectroscopic studies of nanocrystalline ZnFe2O4

By carrying out positron lifetime measurements in zinc ferrite (ZnFe2O4) samples of various grain sizes down to 5 nm, the defect microstructures have been identified. In the bulk samples composed of grains of large sizes, positrons were trapped by monovacancies in the crystalline structure. Upon reduction of the grain sizes to nanometer dimensions, positrons get trapped selectively at either the diffused vacancies on the grain surfaces and the intergranular regions. Below about 9 nm, the grains undergo the transformation from the normal spinel structure to the inverse phase. A concomitant lattice contraction results in substantial reduction of the octahedral site volume, and hence, a fraction of the Zn2+ ions with larger ionic radius fails to occupy these sites. This leaves vacancies at the octahedral sites which then turn out to be the major trapping sites for positrons. ZnFe2O4 samples prepared through different routes were investigated, which showed similar qualitative features, although those synthesi...

Effect of preparation conditions on formation of nanophase Ni-Zn ferrites through hydrothermal technique

Abstract Nanophase Ni 1− x Zn x Fe 2 O 4 ( x = 0 , 0.5, 1.0) samples were prepared by chemically precipitating hydroxides followed by hydrothermal processing. The products were characterized by X-ray diffraction and Mossbauer spectroscopy. Ferrite formation was found to be quite sensitive to the procedures adopted for preparing the hydroxide slurry prior to hydrothermal treatment. Nickel ferrite could be prepared in pure phase by co-precipitation as well as by mixing separately precipitated hydroxides. However, Ni 0.5 Zn 0.5 Fe 2 O 4 could only be prepared by co-precipitation. Leaving small amount of sodium in the slurry seems to obstruct pure phase formation of Ni 0.5 Zn 0.5 Fe 2 O 4 and results in partial formation of α-Fe 2 O 3 . A similar effect is observed in zinc ferrite.

Mössbauer Spectroscopic Studies of an Oxidized Ordinary Chondrite Fallen at Itawa-Bhopji, India

Mössbauer studies of the Itawa-Bhopji meteorite fallen on May 30, 2000 in Rajasthan, India, show that the main iron minerals in it are Fe–Ni (kamacite/taenite), troilite, olivine and pyroxene. These provide characteristic signatures of an ordinary chondrite. Mössbauer absorption areas corresponding to different phases favour its classification as L/LL-type ordinary chondrite. The iron in the olivine is unusually high and the metallic iron is quite low, showing that it has faced oxidizing conditions prior to fall.

Exact solution of the Schrodinger equation for a particle in a tetrahedral box

The authors obtain the exact solution of the Schrodinger equation for a particle confined to (i) an equilateral triangle, (ii) a tetrahedral box with corners (- pi / square root 2,- pi / square root 2,- pi / square root 2), ( pi / square root 2,- pi / square root 2, pi /2); (- pi / square root 2, pi / square root 2, pi /2) and ( pi square root 2, pi / square root 2,- pi /2). The energies are: for (i) Enl=(8h(cross)2 pi 2/9mL2)(n2+l2-ln) where L is the side of the triangle and l, n are distinct non-zero integers and for (ii), Elmn=(h(cross)2/8m)*(3(l2+m2+n2)-2lm-2mn-2nl) where l, m and n are distinct non-zero integers. The wavefunctions have been classified according to the irreducible representation of the corresponding symmetry groups.

Aqueous reduction of crystalline goethite under ammoniacal conditions

Abstract Pure crystalline goethite was laboratory-prepared and its reduction under various experimental conditions was studied. Both goethite and the reduced products obtained by the action of ammonium sulphite and/or ferrous sulphate were characterized using several techniques such as XRD, TG-DTA, TEM and Mossbauer spectroscopy and these compounds were also chemically analyzed. During reduction of goethite with ammonium sulphite, parameters such as temperature, ammonium sulphate concentration and addition of Cu(II), Ni(II) and Co(II) ions were studied. No reduction of goethite was observed during this study up to a temperature of 403 K. At this temperature, addition of ferrous sulphate promotes reduction of goethite and formation of magnetite was observed. The presence of ammonium sulphate and/or ammonium sulphite hindered the reduction reaction. Best results for quantitative reduction of goethite to magnetite were obtained with the following conditions: NH 3 25 g L −1 , Fe(II) 45 g L −1 , reduction temperature 403 K and reduction time 2 h.

Anomalous change in electron density at nuclear sites in nanosize zinc ferrite

Nanosize samples of zinc ferrite with different particle sizes synthesized by citrate precursor route have been studied by x-ray diffraction, transmission electron microscopy and Mossbauer spectroscopy. The observations suggest that the electronic processes depend sensitively on size, and a big variation occurs in the pattern of temperature dependence of isomer shift on changing the size from 5nm to 6nm and from 6nm to 7nm. The electron density at the nuclear sites first increases, then decreases and again increases as the temperature of the nanosize samples is increased from 12K to 400K. The electric-field gradient varies considerably as the size is changed, but remains almost independent of temperature for a particular size. It is concluded that different aspects of hyperfine interactions in nanophase become significant at different sizes.

Structural, thermal stability and defect studies of Fe-Ag alloy prepared by electrodeposition technique

An alloy of the immiscible constituents iron and silver was prepared by simultaneous electrodeposition of these materials from a single bath. A study of the crystallographic structure, magnetic behavior and defect kinetics of this alloy is presented. The alloy is heterogeneous, stable up to 573 K and shows no magnetization. The XRD pattern shows that the alloy is formed in pure fcc structure. Also, the crystallites are well-grown in size as shown by very sharp X-ray diffraction peaks. Mossbauer spectra indicated that the Fe atoms are totally dispersed in the Ag matrix. Positron lifetime measurements gave evidence for the presence of large vacancy clusters created during the electrodeposition process.

Validity regions of charge shift model

The charge shift model has been used to calculate electric field gradients (efg) for various probe-host combinations. The comparison with the corresponding experimental values indicates that the validity of the model depends on sign and magnitude of the charge shift. The model agrees fairly well with experiments for the hosts having positive charge shift. The agreement is good for hosts having negative charge shift only if the magnitude is small (< 0·012).

Lattice sum of electric field gradients in tetragonal crystals

A new method to calculate the lattice contribution to electric field gradients at a nuclear site in tetragonal crystals is developed. The crystal is regarded as an assembly of positive ions at lattice points embedded in a uniform background of negative charge (point charge model). The method uses Euler-Maclaurin formula and makes the plane-wise summation in the direct crystal space unlike most of the previous methods utilising Fourier transform to reciprocal space. The numerical values obtained using the above approach agree well with previous results.