A. Nath

Mössbauer Study of Oxygenated Iron-Phthalocyanines, a Precursor of Magnetic Storage Material

Abstract57Fe transmission Mössbauer spectroscopy and X-ray diffractometry were used to study α- and β-polymorphs of iron phthalocyanines (FePc) to get information about their oxygenation in connection with controlled pyrolysis of flat layered phthalocyanines initiated by radicals at relatively low temperatures, to form acicular carbide particles encaged in carbon. Mössbauer spectroscopy and XRD revealed that the oxygenation of both α- and β-polymorphs of FePc was successfully achieved. New Mössbauer spectral components (doublets) appeared upon the oxygenation in both α and β FePc. This can be attributed to iron microenvironments containing oxygen between the layers of iron phthalocyanines.

Emission Mössbauer study of CMR manganite La0.8Ca0.2MnO3. I. Anomalous ferromagnetism

Using 57Co emission Mossbauer technique, we present clear evidence that in Ca-doped manganite, the magnetic and paramagnetic phases coexist below TC, with the abundance of the latter increasing with temperature. In contrast with the regular ferromagnetic materials, the variation of the hyperfine internal magnetic field Hint with temperature deviates considerably from the Brillouin relation, and exhibits an abrupt drop at TC. These features characterize the magnetic transition as a first-order transition. The non-Brillouin behavior of Hint(T) and the temperature dependence of the shape of the magnetically split sextet indicate the presence of spin fluctuations in this material well below TC.

Emission Mössbauer study of oxygen deficient PrBa2Cu3(57Co)O6+x

Abstract The variation of Mossbauer parameters as a function of oxygen content in PrBa 2 Cu 3 ( 57 Co ) O 6+x has been studied and compared to the results of similar studies on YBa 2 Cu 3 ( 57 Co)O 6+x . It was found that the Mossbauer isomer shift of 57 Co( 57 Fe) dopant at Cu(1) site shows the same behaviour as for YBa 2 Cu 3 ( 57 Co ) O 6+x except that all isomer shifts are significantly higher than those for YBa 2 Cu 3 ( 57 Co)O 6+x and several other superconducting 1-2-3 compounds. These findings are explained by the tendency of Pr III to form Pr IV , which can be responsible for the absence of superconductivity, and charge transfer between the Cu(2)-O sheets and Cu(1)-O chains. It is also concluded that at high oxygen stoichiometries the Cu(1)-O chains have metallic character for both compounds.

Mössbauer Spectroscopy and Radiofrequency Absorption Investigations of Pr Substituted 1-2-3 Type Compounds

Abstract57Fe and 151Eu Mössbauer spectroscopy as well as RF susceptibility measurements were applied to study the effects of Pr substitution either into the rare earth or into the Ba site in Eu1−xPrxBa2Cu3O7−δ and EuBa2−xPrxCu3O7−δ, respectively. Site mixing of Pr between the rare earth and Ba sites could be excluded by the utilization of 57Fe Mössbauer spectroscopy. It was found that there exists a correlation between the 151Eu isomer shift and the onset temperature of the superconducting transition independent of the location of Pr. RF susceptibility measurements provide an evidence for a difference in the magnetic moment of Pr substituted for the Eu or Ba sites. The obtained results can be explained by hole filling as the dominant effect of Pr substitution.

Anomalous behavior of the Nd1.84Ce0.16CuO4 superconductor

The as-prepared electron-doped system Nd2−xCexCuO4 (where x≈0.16) is not superconducting. It becomes superconducting only after removal of a minuscule amount of extraneous oxygen (0.02 of O per unit formula). Mössbauer effect studies were carried out for oxygenated and deoxygenated Nd2−xCexCu(57Co)O4 with x=0.14, 0.16, and 0.18. The spectra show evidence of oxygen anions attaching to the probe57Co in apical positions, to form 5- and 6-coordinated species. A conventional procedure for deoxygenation brings about little change in the Mössbauer spectra both above and below the optimal superconducting concentration; however, for x=0.16, a dramatic change is observed—a major fraction of the magnetically split five-coordinated species manifests itself as a paramagnetically relaxed doublet upon deoxygenation, which costitutes a microscopic measure of the superconducting volume fraction. This apparently anomalous behavior at x≈0.16, where the extraneous oxygen is more readily desorbed, may be related to an electronic and/or local structural change in the CuO2 plane.

Anomalous magnetic and dynamic behavior in magnetoresistive compounds: origin of bulk colossal magnetoresistivity

We present the results of our extensive Mossbauer effect studies carried out on a wide variety of mixed valence manganites as well as other types of magnetoresistive materials, including pyrochlore Tl2Mn2O7 and the chalcospinels Fe0.5Cu0.5Cr2S4 and FeCr2S4 with absolutely different natures of the magnetism, in a search for similarities linked to their magnetoresistive behavior. The double exchange electron transfer and coupling between the electrons and Jahn–Teller lattice distortions invoked by most theories to explain the colossal magnetoresistivity and associated metal–insulator transition in manganites are not applicable to pyrochlore nor to chalcospinels. Nevertheless, we find intriguing similarities in the anomalous magnetic and dynamic behavior among these widely different systems at, above, and below the Curie temperature TC, which shed light on the origin of bulk magnetoresistivity in general. All these compounds share the following features. The long-range ferromagnetic order breaks down even be...

Emission and Transmission Mössbauer Spectroscopic (EMS and TMS) Study of Perovskite Oxides, (Ba, Ca)(Fe, Co)O3-δ, Absorbed with CO2

Perovskite oxides, (Ba0.95,Ca0.05) (Co1-x,Fex) O3-δ, are prominent materials for CO2 absorption at high temperature. The substitution of Ba sites with Ca ion, and the mixed valence states of Fe and Co ions due to the formation of oxygen vacancies at high temperatures are effective for CO2 absorption. Especially Co ions are more active for CO absorption than Fe ions. These oxides were analyzed by EMS and TMS before and after treatment in CO2 atmospheres. The difference between EMS and TMS spectra is found to be observed not only due to the oxygen affinity, but also to the different reactivity of Co and Fe ions in the B site towards CO2.

Lattice dynamics of Y0.9Pr0.1Ba2Cu3O7−δ

Abstract We find that the dynamics of the chain in both pristine YBCO as well in the Pr-doped material is determined by the oscillatio of chain oxygen atoms (namely O(4) atoms) between the two potential wells on either side of the chain. In addition, the oscillations of O(4) atoms are considerably modified at about 230 K, the temperature at which a phase transition has been reported in the literature. The substitution of Y by Pr does not affect the hole density along the chains as probed by the chemical shifts of the major species. This does not support the possibility of hole-filling by electron donation from Pr 3+ . However, even modest Pr substitution of 10% lowers considerably the enthalpy of interconversion of the two forms of the 5-coordinated species. These configurations around the minuscule amount of 57 Co are aquired with the help of an extraneous oxygen in the O(5) position.

Comparative Transmission and Emission Mössbauer Studies on Various Perovskite-Related Systems

A whole group of compounds which have the ABO3 stoichiometry and meet the appropriate conditions on the ratio of ionic sizes and metal-oxygen bond lengths is named after the well known mineral, perovskite (CaTiO3). The basic structure is cubic as depicted in Fig. 1. The stability of this structure depends on the cations occupying the A and B sites. Variation of the cations at these sites produces an astonishingly wide variety of magnetic and electric properties [1]. For example, the originally dielectric CaTiO3 becomes a ferroelectric material when Ca is substituted by Ba. Partial substitution of Ti by Zr, with Pb at the A site results in a piezoelectric substance. (Ba,La)TiO3 is a semiconductor, while a wide range of cuprates like YBa2Cu3O7-d shows high temperature superconductivity. One of the hottest topic in perovskite chemistry and physics today is the colossal magnetoresistance effect which is found for example in (La,Sr)MnO3, (La,Sr)(Fe,Co)O3 and other compounds (although the perovskite structure is not a precondition for the CMR effect).

57Fe Mössbauer Investigation of the Substitutional Effect of Praseodymium in 1-2-3 Compounds

Abstract57Fe Mössbauer spectroscopy measurements were performed on the perovskite compounds Eu0.7Pr0.3Ba2(Cu0.9957Fe0.01)3O7-δ, EuBa1.5Pr0.5(Cu0.9957Fe0.01)3O7-δ and EuBa1.3Pr0.7(Cu0.9957Fe0.01)3O7-δ. The observed 57Fe Mössbauer spectra provided an evidence for the correct site assignment of subspectra originating from 57Fe in different microenvironments. Apart from a minor component which was assigned to the 57Fe in the Cu(2) site of the copper oxide plane, all the subspectra could be attributed to the 57Fe in the Cu(1) copper oxide chain site with a fourfold (doublet D1), fivefold (doublet D2) or sixfold (doublet D3) oxygen coordination. In contrast, in the compound EuBa2(Cu0.9957Fe0.01)3O7-δ the 6-coordinated (D3) species has not been observed. The substitution of Pr for Eu or for Ba resulted in an increased occupancy of the O(5) antichain oxygen sites, which was explained by the charge neutrality criterion. Especially, the replacement of Ba2+ with Pr3+ led to an unusually high degree of occupancy of O(5) sites. In the 57Fe Mössbauer spectra the relative area of the 6-coordinated species (D3) increased, and that of the 4-coordinated one (D1) vanished completely in the case when Pr was substituted for Ba. Furthermore, the proportion of the 6-coordinated (D3) species increased at the expense of the 5-coordinated (D2) one with an increasing concentration of Pr at the Ba site. These experimental results are consistent with the variety of Mössbauer results reported so far.