Effect of La doping on the structure and cycloidal spin ordering in multiferroic BiFeO3.

A series of Bi1-xLaxFeO3 samples with 0.00 ≤ x ≤ 0.30 was synthesized by the sol-gel method. The effects of lanthanum concentration on the phase formation, microstructure and cycloidal spin ordering were studied using X-ray diffraction, scanning electron microscopy and Mössbauer spectroscopy. The crystal structure of the La-doped bismuth ferrite transformed from rhombohedral R3c (x ≤ 0.05) to a mixture of R3c and cubic Pm3m (0.07 ≤ x ≤ 0.15) and finally to a mixture of R3c, Pm3m and orthorhombic Pbam (0.20 ≤ x ≤ 0.30). The Pbam phase, with characteristic porous microstructure shown by microscopy images, was observed in Bi1-xLaxFeO3 compounds for the first time. Based on the Mössbauer spectroscopy, it was found that the cycloidal spin ordering started to disappear at x = 0.07. With increasing La concentration the share of the cycloid decreased from 100% at 0.00 ≤ x ≤ 0.05 to 0% at x = 0.30. At the beginning, for x ≤ 0.02, the anharmonicity parameter, m, of the cycloidal spin ordering was about 0.5, which is typical of a pure BiFeO3 compound. In the range 0.05 ≤ x ≤ 0.25, the m parameter was of the order of 0.1, which indicated the practically harmonic character of the cycloid. The structural transition at x = 0.07 was accompanied by a substantial increase in magnetization.

Synthesis and characterization of a novel composites derived from SBA-15 mesoporous silica and iron pentacarbonyl.

HYPOTHESIS: Fe(CO)5 was deposited on SBA-15 from gaseous phase and the adsorbed iron precursor was oxidized and hydrolyzed. We hypothesize that a novel method produces composites of unique and useful properties. The absence of sulfur and halides in the composites is of special importance with respect of their potential application as catalysts and catalyst supports. EXPERIMENTS: We obtained composites containing 1.3-157.5% of iron oxide with respect to silica (counted as 100%). The new materials were characterized by specific surface area, XRD (X-ray diffraction), Mossbauer spectroscopy, electron microscopy coupled with EDS (energy dispersive X-Ray spectroscopy), and microelectrophoresis. FINDINGS: In the composites low in iron (<57% of iron oxide) the Mossbauer spectra showed only doublets, which were interpreted as tiny nanoparticles of Fe(III) oxide in the pores of SBA-15. In the composites high in iron (>65%) the Mossbauer spectra showed doublets and sextets. The later were interpreted as larger nanocrystals of hematite, and the presence of such nanocrystals was confirmed by XRD. EDS confirmed that in the composites low in iron, iron oxide was evenly distributed in the entire volume of pores of SBA-15.

Two types of electrokinetic behavior of solid particles in the presence of anionic surfactants.

HYPOTHESIS: The electrokinetic curves of metal oxides obtained at moderate concentrations of sodium dodecylsulfate show a maximum at pH close to the pristine IEP of the oxide (IEP in the absence of surfactant). This behavior of SDS is very different from most other anions, in which the electrokinetic curves did not show a maximum. We hypothesize that the unusual behavior of SDS is due to its micellization in solution and on the surface. EXPERIMENTS: The ζ potentials of alumina, titania, and hematite containing sodium tetradecyl- and hexadecylsulfate at different concentrations have been studied. These surfactants do not form micelles in solution. FINDINGS: In dispersions of hematite, titania, and alumina stabilized with SDS, moderate concentrations of the surfactant induced a reversal of sign of the ζ potential at acidic pH (<4) while the effect of the surfactant at neutral and basic pH (>5) was less significant. Hematite dispersions stabilized with sodium tetradecyl- and hexadecylsulfate showed the electrokinetic behavior typical for most anions (no maximum in ζ potential) rather than that characteristic for SDS. The increase in the chain length from 12 to 16 C atoms induces depression and then disappearance of a maximum in the ζ potential vs. pH curves.