Usha Chandra

A Mössbauer study of the effects of heating biotite, phlogopite and vermiculite

Abstract A Mossbauer study of heat treated biotite, phlogopite and vermiculite micas is reported. An attempt is made to interpret the spectra in terms of the structural changes due to oxidation and dehydroxylation. Heat treatment of all three micas at about 1000°C yields magnetic hyperfine patterns.

57Fe Mössbauer spectroscopy and electrical resistivity studies on naturally occurring native iron under high pressures up to 9.1 GPa

Abstract We report the pressure dependence of the Mössbauer spectra and the electrical resistivity up to 9.1 GPa at room temperature for a native iron sample collected from the Precambrian Chaibasa shales, Singhbhum Craton, Eastern India. The Mössbauer spectroscopy of the sample at ambient conditions yields isomer shifts and magnetic hyperfine field values that confirm the presence of Fe0 oxidation state. Many theories have been put forward to explain the origin of this native iron including a Precambrian meteoritic impact. High-pressure Mössbauer spectroscopic measurements using diamond anvil cell (DAC) showed a constant isomer shift up to 5.6 GPa with a subtle variation of -1.07 × 10-3 mm/s/GPa followed by sharper change -4.3 × 10-3 mm/s/GPa above 6.3 GPa, a pressure much lower than the usual value reported for metallic iron. Further increase of pressure to 9.1 GPa results in the emergence of a tiny peak at ~0 isomer shift indicating the onset of the martensitic phase transition of iron from the body-centered-cubic (bcc) to hexagonal-close-packed (hcp) transition 4 GPa lower than the transition pressure normally observed for pure iron. This phase transition in the native iron is confirmed by high-pressure electrical resistivity study. Lowering of the transition pressure could be due to nucleation of hcp by stacking faults caused by shock metamorphism resulting from the Precambrian impact in the region.

A Mossbauer study of the effect of heat treatment on biotite micas

Addison et al. (1962) and Vedder and Wilkins (1969) have proposed two alternative mechanisms to explain the oxidation which occurs in layered silicates. A Mossbauer study of tri-octahedral micas (iron-rich biotites) after heat treatment from 400 degrees C-1200 degrees C under different conditions has been performed. An attempt was made to explain the results using the mechanism of Vedder and Wilkins. It was found that a large amount of oxygen in contact with the mica grains was necessary for the mica to decompose completely.

High-pressure electrical resistivity behaviour of nanocrystalline perovskite (La, Sr)(Mn, Fe)O3

We report here the electrical resistivity of nanocrystalline perovskite-structured La–Sr manganites as a function of pressures up to 8 GPa, at room temperature. The nanocrystalline perovskite manganites were prepared by the sol–gel technique and found to have crystallite sizes of 12–18 nm. The pressure dependence of the electrical resistivity shows a first-order phase transition at 0.66(2) GPa and a subtle phase transition between 3.5 and 3.8 GPa. The first-order transition at 0.66 GPa can be related to the transition from localized-electron to band magnetism.

High-pressure electrical resistivity and Mössbauer spectroscopic studies on narrow band Co0.8Fe0.2S2 nanoparticles up to 8 GPa

Pyrite-structured Co0.8Fe0.2S2 colloid nanoparticles, synthesized using solution method with particle distribution around 3–4 nm determined by transmission electron microscope, were studied under high pressure up to 8 GPa using 57Fe Mössbauer spectroscopy and electrical resistivity techniques. Drastic decrease in TC by magnetic measurements indicated nanosize of the particle. Higher quadrupole splitting (QS) at ambient condition was due to large lattice strain and electric field gradient generated by higher surface to volume ratio of these nanosized particles. Under pressure, Mössbauer parameters – isomer shift and QS – showed an expected trend up to ∼5.6 GPa. Above 6.4 GPa, the QS remained constant up to 8 GPa with decreased value. Even after decompression, the high-pressure phase is retained. The variation of pressure coefficient of electrical resistivity from −0.021/GPa to −0.151/GPa across 6.8 GPa suggested a second-order phase transition. The coincidence of observed value with that of the bulk suggested that the particle size does not impart much influence on transition pressure. This is the first report on nanoparticles of Co0.8Fe0.2S2 under pressure.

XANES study of intermediate valence in SmSe1-x Asx alloys

X-ray absorption near edge structure (XANES) measurements on LII edge of Sm are employed to study the intermediate valence in SmSe1-x Asx (0.0 ≤ x ≤ 1.0) solid solutions in which the substitution of trivalent anion As for Se2− drives Sm to a higher valence from divalent towards trivalent in the SmSe lattice. Two absorption peaks corresponding to two different valent states of Sm provide direct evidence for mixed valence of Sm in these alloys. The split white line at the LII edge of Sm is used to determine the Sm mean valence in the alloys. Average valence data deduced from Sm LII edge XANES shows a continuous valence transition with increasing As concentration upto 80 at.% of As. At 90 at.% of As the Sm is fully converted into trivalent state. Mean valence data for Sm shows fair agreement with those measured employing other techniques.

XANES and EXAFS study of some metal-metalloid glasses

XANES and EXAFS techniques are proving very popular in the study of local environment in disordered systems. Results of such studies in a large number of metal (Fe, Co, Ni, etc)-metalloid (B, Si, C, etc) glasses are reported. Experiments were done with synchrotron radiation as well as an x-ray tube. The values of bond lengths and co-ordination numbers computed from one-electron single scattering Fourier transform method turn out substantially smaller. The values of bondlength determined from the other EXAFS calculation method and the multiple-scattering computation scheme show good agreement. Importance of choice of suitable reference materials for analysis of data is emphasized.

X-ray absorption study of some iron-rich micas

The K-absorption spectrum has been studied in the cases of muscovite, biotite and phlogopite micas, iron metal, FeSO4 and Fe2O3 with a view to demonstrating the usefulness of this technique in the study of minerals and to study their complex chemistry. The investigation has been carried out both before and after subjecting the mica samples to heat treatment around 800 degrees C and the observations interpreted in terms of the consequent chemical and structural changes. This heat treatment was found to lead to almost total conversion of Fe2+ to Fe3+ and distortion of the octahedra in biotite and phlogopite. Muscovite, however, shows no change in either the Fe2+ and Fe3+ ratio or the shape of the spectrum on heat treatment of the sample. The results are compared and found to be in very good agreement with those available from other techniques.

High-pressure phase transition in 57 Fe 0.03 Cr 0.97 Sb 2 : A Mössbauer and resistivity study up to 8 GPa

We report Mössbauer and resistivity measurements at 2.3 and 5.6 GPa and at room temperature for the 57 Fe 0.03 Cr 0.97 Sb 2 system using a diamond anvil cell. The observed isomer shift and quadrupole splitting in Mössbauer data can be interpreted in terms of a first-order structural phase transition from marcasite-type orthorhombic to pyrite-type cubic at about 6 GPa. A transformation from the high-spin to the low-spin state of Fe 2+ is also observed at this pressure. Electrical resistivity measurements up to 8 GPa on the system confirmed a first-order pressure-induced structural transition.

XANES and EXAFS in Cu-Ti and Ni-Zr glasses

X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements have been done at the K-edge of Cu in Cu-Ti glasses and on the K-edges of Ni and Zr in Ni-Zr glasses using a synchrotron radiation source. The results are discussed in terms of the shape shift and intensity of the absorption edge as well as the principal absorption maximum. The values of bondlength calculated by the one-electron multiple scattering XANES theory as well as the graphical analysis EXAFS technique show good agreement.