V. Sedykh

Mössbauer and X-Ray studies of the dynamics of phase transformations and suppression of polymorphism in the LaMn1 − xFexO3 + δ compound (x = 0.015–0.500)

The effect of iron impurity on the structural transformations and the suppression of polymorphism of the LaMn1 − xFexO3 + δ compound (x = 0.015–0.500) was studied by X-ray diffraction and Mössbauer spectroscopy. It is shown that the iron impurity greatly influences the polymorphic transformations of the manganite. At low Fe impurity contents (up to 10 at %), the PnmaI phase is locally formed in the PnmaII phase. The mechanisms of suppression of the polymorphism are discussed.

Mössbauer study of modulated structure of Bi2Sr2CaFe2Oy phase

Abstract 57 Fe Mossbauer studies of the single crystals of the Bi 2 Sr 2 CaFe 2 O y compound with modulated structure show two quadruopole-splitting doublets (QS(1) = 1.03 mm/s and QS(2) = 0.45 mm/s). The compound is weak paramagnet in the temperature range 330 K−4.2 K. Analysis of the obtained data shows that the Fe atoms in this structure are separated into two groups, which can be attributed to small and strong distorted environments of the Fe atoms. They are in the Fe 3+ valence state. An attempt is made to relate the feature of the results with the structure modulation. From the full profile analysis of the X-ray diffraction data it follows that the modulation wave can be described by a spectral profile close to a “rectangular” one. In the obtained domain-like system the Fe atoms can be divided into two unequal groups having opposite directions of displacements in good agreement with Mossbauer data.

X-ray diffraction analysis of the modulated structure of Bi2Sr2CaFe2Oy

The modulated structure of the Bi2Sr2CaFe2Oy compound isomorphous with the Bi2Sr2CaCu2Oy phase was refined by the method of the full profile analysis on the basis of the powder X-ray diffraction data. The analysis was carried out in the superspace group ofN:Abmm: 1 – 11 using the parameters of the unit cell of the basic structurea = 5.466(3),b = 5.446(5) andc = 31.25(9) Å and the modulation vectorq = 0.220(5)b* +c*. The model of excess oxygen interstitials into the BiO plane was used by applying a saw-tooth-type function of the form:U = 2U0 [(X4 -X40/Δ)] [1]. The results point to a strong displacement of the atoms from their positions in the basic structure. An essential improvement of theR factors was obtained by taking into account the modulation waves of the higher harmonics.

Study of the modulated structure of Bi2Sr2CaFe2Ox

The ceramics and single crystals of the Bi2Sr2CaFe2Ox compound were synthesized. The X-ray diffraction data showed orthorhombic symmetry. The lattice parameters are equal toa=5.464 Å,b=5.453 Å,c=31.313 Å. The crystal structure is described by the primitive Bravais lattice with the possible space groups Pbmm. Pbm2 and Pb21m. The obtained X-ray patterns show the presence the incommensurate structural modulation with the following parameters: the modulation vector lies in the (100)-plane, the value of the wave-vector componentqb=0.22(3). The comparison of the obtained results with data for Fe-doped Bi2Sr2CaCu2Ox and Bi2Sr3Fe2Ox are presented. No magnetic peculiarities of the compound were found. Mőssbauer measurements of the ceramic samples indicate the presence mainly of Fe3+ oxidation state and Fe4+ (about 20%). The decrease of the quadrupole-splitting values for 2212Fe in comparison with Fe-doped 2212Cu was revealed that may be connected with higher symmetry of the local environment of Fe atoms in 2212Fe.

The structure modulation in the Bi2Sr4Fe2O10 compound

Abstract The influence of the extra Sr layer introduced into Bi2Sr3Fe2O9 (the obtained Bi2Sr4Fe2O10 compound) on the structure modulation has been investigated by X-ray diffraction analysis, electron microscopy and Mossbauer spectroscopy. It has been found that the obtained single crystals are characterized by the averaged orthorhombic cell with the lattice parameters a=5.4227(7) A, b=5.4248(8) A and c=18.600(3) A and two-component modulations of the structure with the vector (0,1/4.97,1/2). The real structure of the compound consists of domains tilted relative to each other by 90° around the c-axis.

57Fe Mössbauer study of YBa4Cu2.95Fe0.05O8.5+y

Abstract57Fe Mössbauer measurements were performed on the Fe-doped YBa4Cu3O8.5+y compound as a function of annealing temperature in vacuum. Mössbauer spectra were analyzed using three different quadrupole split doublets. The resistance measurements showed that the compound is not superconducting. The possible identification of the Cu sites with their nearest neighbor oxygen sites in this compound is suggested. Comparison of the results with data of the presence of the superconductor YBa2Cu3O7 allows us to suppose that the presence of the Cu-chain sites alone does not result in superconductivity.

About specific features of the structure modulation in the Bi-ferrate compounds isostructural with Bi2Sr2CaCu2O8

Abstract The single-crystalline specimens of the Fe-doped (5% 57 Fe) Bi 2 Sr 2 CaCu 2 O 8 (2212Cu) compound have been synthesized to compare the behavior of the structure modulation in Bi-cuprate and Bi-ferrate compounds. According to the Mossbauer data obtained, one can conclude that extra oxygen, related to the presence of Cu 3+ in 2212Cu, is most probably arranged in the Ca layer rather than in the Bi layer, as was supposed in the model of extra oxygen. Based on the analysis of the experimental data we propose the explanation of the structure modulation in Bi compounds. Due to the mismatch between the internal perovskite block and the external BiO layers the structure of the Bi compounds is distorted. The Sr layer being boundary between these blocks probably is most distorted and stressed. We suppose that the different changes related to the modulation can occur just in this layer. The Bi excess, which is always present in Bi compounds (especially in single crystals), can be just the necessary condition at which the modulation appears. Extra oxygen in principle can be present in modulated structures. However, in our opinion, it only complements the influence of other factors and is not the driving force in the structure modulation.

The effect of vacuum annealing on structure modulation in the Bi2Sr3Fe2Ox compound

Abstract The influence of vacuum annealing on the structure modulation has been studied in single crystalline samples of the Bi2Sr3Fe2Ox compound (232Fe) by electron microscopy and Mossbauer spectroscopy. It has been experimentally shown that the structure modulation does not change at vacuum annealing.

Structural transformations in La1 − xBaxMn0.9857Fe0.02O3 + δ (x = 0.05−0.20)

A systematic investigation of the structural transformations in La1 − xBaxMn0.9857Fe0.02O3 + δ (x = 0.05−0.20) at different Ba concentrations and under different heat treatment conditions (annealing in vacuum and in air) has been performed using the methods of Mössbauer spectroscopy and X-ray diffractometry. The samples have been synthesized by the sol-gel in air at a temperature of 1100°C. For the studied range of barium concentrations, the synthesis results in the formation of a rhombohedral structure with space group