B. F. O. Costa

Rhombohedral-to-orthorhombic transition and multiferroic properties of Dy-substituted BiFeO3

Investigation of crystal structure, ferroelectric, and magnetic properties of polycrystalline Bi1−xDyxFeO3 (0.1≤x≤0.2) samples was carried out. X-ray diffraction study revealed composition-driven rhombohedral-to-orthorhombic R3c→Pnma phase transition at x∼0.15. Both structural phases were found to coexist in a broad concentration range. Piezoresponse force microscopy found suppression of the parent ferroelectric phase upon dysprosium substitution. Magnetometric study confirmed that the A-site doping induces appearance of a weak ferromagnetic behavior. Both the ferroelectric and magnetic properties were shown to correlate with a structural evolution.

Biofunctionalized magnetic hydrogel nanospheres of magnetite and κ-carrageenan

Magnetic hydrogel kappa-carrageenan nanospheres were successfully prepared via water-in-oil (w/o) microemulsions combined with thermally induced gelation of the polysaccharide. The size of the nanospheres (an average diameter of about 50 and 75 nm) was modulated by varying the concentration of surfactant. The nanospheres contained superparamagnetic magnetite nanoparticles (average diameter 8 nm), previously prepared by co-precipitation within the biopolymer. Carboxyl groups, at a concentration of about 4 mmol g(-1), were successfully grafted at the surface of these magnetic nanospheres via carboxymethylation of the kappa-carrageenan. The carboxylated nanospheres were shown to be thermo-sensitive in the 37-45 degrees C temperature range, indicating their potential as thermally controlled delivery systems for drugs and/or magnetic particles at physiological temperatures. Finally, preliminary results have been obtained for IgG antibody conjugation of the carboxylated nanospheres and the potential of these systems for bio-applications is discussed.

Infrared and Mössbauer studies of iron in aluminosilicate glasses

Abstract Samples of 10Fe2O3·10Al2O3·80SiO2 composition were prepared by the sol–gel method and heat-treated between 120 and 1300 °C under oxidising (air) conditions. These samples were studied by Fourier transformer infrared (FTIR) and Mossbauer spectroscopy. The results indicate that Fe3+ is in octahedral sites at 250 °C and in tetrahedral sites between 500 and 1300 °C. Goethite and hematite particles are present in the 250 and 500 °C samples and hematite particles in the samples heat-treated between 1000 and 1300 °C. The hematite particles, present in the samples, show a lower hyperfine field (47.91±0.08 T) than that usually seen for hematite (51.5 T). This is due to the fact of Al3+ ions take the place of Fe3+ ions in hematite particles.

Distinct microbial populations are tightly linked to the profile of dissolved iron in the methanic sediments of the Helgoland mud area, North Sea

Iron reduction in subseafloor sulfate-depleted and methane-rich marine sediments is currently a subject of interest in subsurface geomicrobiology. While iron reduction and microorganisms involved have been well studied in marine surface sediments, little is known about microorganisms responsible for iron reduction in deep methanic sediments. Here, we used quantitative PCR-based 16S rRNA gene copy numbers and pyrosequencing-based relative abundances of bacteria and archaea to investigate covariance between distinct microbial populations and specific geochemical profiles in the top 5 m of sediment cores from the Helgoland mud area, North Sea. We found that gene copy numbers of bacteria and archaea were specifically higher around the peak of dissolved iron in the methanic zone (250–350 cm). The higher copy numbers at these depths were also reflected by the relative sequence abundances of members of the candidate division JS1, methanogenic and Methanohalobium/ANME-3 related archaea. The distribution of these populations was strongly correlated to the profile of pore-water Fe2+ while that of Desulfobacteraceae corresponded to the pore-water sulfate profile. Furthermore, specific JS1 populations also strongly co-varied with the distribution of Methanosaetaceae in the methanic zone. Our data suggest that the interplay among JS1 bacteria, methanogenic archaea and Methanohalobium/ANME-3-related archaea may be important for iron reduction and methane cycling in deep methanic sediments of the Helgoland mud area and perhaps in other methane-rich depositional environments.

Effect of Fe-doping on the structure and magnetoelectric properties of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 synthesized by a chemical route

B-site Fe-doped (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 was synthesized by a facile chemical route to study the effect of doping on its physical properties. Detailed analysis of X-ray diffraction and Raman spectroscopy data revealed an increased lattice strain and thereby deviation from the morphotropic phase boundary with the progressive doping of Fe from 1 to 5 mol%. Such structural changes have resulted in the weakening of the energy band gap as well as deterioration of the ferroelectric polar nature which was evidenced by a shift of tetragonal to cubic transitions towards room temperature and hard doping effects in ferroelectric hysteresis. The doped samples exhibited room temperature ferromagnetism. Combined Mossbauer and X-ray photoelectron spectroscopic studies suggest that oxygen vacancies and defect complexes induced by Fe doping play a major role in magnetic properties. Local piezoresponse measurements illustrated imprint characteristics of ferroelectric domains in undoped and doped samples at the nanoscale. Room temperature magnetoelectric (ME) measurements revealed that 1 mol% Fe doped sample, having higher ferroelectric polarization and moderate magnetization, exhibits a strong ME response with a coefficient of 12.8 mV cm−1 Oe−1. The present study on Fe-doping effects on the structure and related ME properties of this important lead-free material is useful to tailor multiferroic applications in electronics.

Study of Alpha–Sigma Phase Transformation in Mechanically Alloyed Fe–Cr–Sn Alloys

(a)PhysicsDepartmentoftheUniversityofCoimbra,P-3000Coimbra,Portugal(b)LSG2M,UMRCNRS7584,EcoledesMines,F-54042NancyCedex,France(ReceivedApril3,2000;inrevisedformSeptember20,2000;acceptedSeptember21,2000)Subjectclassification:64.70.Nd;S1.1;S1.2The solubility of tin is significantly extended by mechanical alloying in near equiatomic Fe–Cralloys. The influences of Sn concentration and of grain size on the kinetics of formation of thes-phase have been studied using different techniques. The s-phase formation is much faster foras-milled alloys than it is for conventional alloys. The s-phase formation rate decreases with theincrease of Sn concentration in alloys with nanometer-sized grains as it does in coarse-grainedalloys.Themechanismswhichareresponsiblefortheslowing-downofthea–s transformationaredifferentinbothkindsofalloys.

Crystal structure of hydrated diphenylguanidinium hexafluoroferrate (III)

Abstract The crystal structure of 3C13H14N3+FeF63−·3.5H2O was determined. Both phenyl rings of the diphenylguanidine cation are oriented syn to the central CNH2 group. The anions and cations are held together by a three-dimensional network of hydrogen bonds. In the synthesis of this compound, a second phase (iron trifluoride trihydrate) was formed and identified by powder diffraction data. Mossbauer results show two different iron environments compatible with the two phases reported.

Debye temperature of disordered bcc-Fe?Cr alloys

The Debye temperature, Θ(D), of Fe(100-x)Cr(x) disordered alloys with 0 ≤ x ≤ 99.9 was determined from the temperature dependence of the centre shift of (57)Fe Mössbauer spectra recorded in the temperature range of 60-300 K. Its compositional dependence shows an interesting non-monotonous behaviour. For 0<x ≤ ∼ 45, as well as for ∼ 75 ≤ x ≤ ∼ 95, the Debye temperature is enhanced relative to its value of a metallic iron, and at x ≈ 3 there is a local maximum having a relative height of ∼ 12% compared to a pure iron. For ∼ 45 ≤ x ≤ ∼ 75 and for x ≥ ∼ 95 the Debye temperature is smaller than the one for the metallic iron, with a local minimum at x ≈ 55 at which the relative decrease of Θ(D) amounts to ∼ 12%. The first maximum coincides quite well with that found for the spin-waves stiffness coefficient, D(o), while the pretty steep decrease observed for x ≥ ∼ 95, which is indicative of a decoupling of the probe Fe atoms from the underlying chromium matrix, is likely related to the spin-density waves which constitute the magnetic structure of chromium in that interval of composition and show also anomalous dynamic behaviour. The harmonic force constant calculated from the effective Debye temperature of the least Fe-concentrated alloy (x ≥ 99.9) amounts to only 23% of the one characteristic of a pure chromium as determined from the heat capacity experiment.

Mössbauer studies of phase separation in nanocrystalline Fe0.55−xCr0.45Snx alloys prepared by mechanical alloying

Abstract Mossbauer spectrometry ( 57 Fe and 119 Sn) was used to investigate phase separation in coarse-grained Fe 0.55 Cr 0.45 and in mechanically-alloyed nanocrystalline Fe 0.55 Cr 0.45 , Fe 0.52 Cr 0.45 Sn 0.03 and Fe 0.49 Cr 0.45 Sn 0.06 alloys during isothermal annealing at 748 K. Phase separation occurs faster in nanocrystalline Fe–Cr than in cold-rolled coarse-grained alloys. The effect of the interconnected microstructure on room-temperature hyperfine magnetic field distributions of alloys aged for hundreds of hours is qualitatively discussed. Tin hinders grain growth of nanocrystalline alloys.

The Debye temperature of the Fe-Cr sigma-phase alloys

57Fe Mossbauer spectra were recorded for microcrystalline and nanocrystalline samples of the σ-phase of Fe100−xCrx alloys with x between 44.6 and 49.6 in the temperature range between 4.3 and 300 K. From the temperature dependence of the average centre shift derived from the spectra measured between 80 K and 300 K, Debye temperatures, θD, were determined. θD-values for the two types of sample show the same linear increase with x, namely ~15 K/at.%, which is faster than a similar trend observed for the α-phase Fe–Cr alloys.