International audienceThe nature of the physical mechanisms related to the gamma-Fe2O3 to alpha-Fe2O3 phase transition under laser irradiation and heat treatment has been investigated using in situ micro-Raman spectroscopy and X-ray powder diffraction (XRPD) analysis. Measurements were carried out on as-prepared gamma-Fe2O3 nanoparticles of about 4 nm in size as a function of laser power and on annealed gamma-Fe2O3 particles. Annealing temperature affects the relative fractions of the gamma-Fe2O3 and alpha-Fe2O3 phases, and at 450 degrees C, the phase transition into alpha-Fe2O3 becomes complete with apparent crystallite size < D > of about 30 nm. The hematite nanoparticles increase then up to more than 180 nm at 1400 degrees C. The excellent agreement between evolution of the wavenumbers and bandwidths confirms that the heat treatment and laser irradiation produces the same effects on nanoparticles. Correlations between structure modifications occurring at the nanometric scale during grain coalescence and the evolution of Raman vibrational spectra were quantitatively examined, and a physical mechanism for the gamma -> alpha-Fe2O3 phase transition was proposed

International audienceThe nature of the physical mechanisms responsible for the structural modification of the γ-Fe2O3 nanoparticles under laser irradiation has been investigated by Raman spectroscopy. In situ micro-Raman measurements were carried out on as-prepared γ-Fe2O3 nanoparticles about 4 nm in size as a function of laser power and on annealed γ-Fe2O3 particles. A baseline profile analysis clearly evidenced that the phase transition from maghemite into hematite is caused by local heating due to laser irradiation with an increase of grain size of nanoparticles. This increasing was clearly determined by X-ray diffraction from 4 nm in nanoparticles up to more than 177 nm beyond 900 °C in a polycrystalline state

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Niobium(V) oxy¯uoride, NbO 2 F, has a perovskite structure and presents the property of lithium intercalation by topotactic chemical reaction either with n-butyllithium dissolved in n-hexane or by electrochemical reaction. The intercalation leads to the reduction of the transition metal from the oxidation state Nb(V) to the oxidation state Nb(III). This allows a theoretical Li/NbO 2 F intercalation ratio of 2. In this paper we will show that this theoretical value can be approached by using micron the sized active material particles. Moreover, the electrical properties of the cathode studied by the galvanostatic intermittent titration technique and a.c. impedance spectroscopy are explained in terms of structural and grain size considerations. Results of cycling experiments are also described.

International audienceWe investigated the influence of the coating of maghemite nanoparticles (NPs) with oleic acid and oleylamine molecules on the thermal stability of maghemite and on the gamma -> alpha-Fe2O3 phase transformation. The uncoated maghemite NPs were synthesized by coprecipitation and the coated NPs by thermal decomposition of organometallic precursors. The morphology and size of the coated NPs were characterized by transmission electron microscopy and magnetic and structural properties by Fe-57 Mossbauer and Raman spectroscopies. The phase stability of coated maghemite NPs was examined under in situ laser irradiation by Raman spectroscopy. The results indicate that coated gamma-Fe2O3 NPs are thermally more stable than the uncoated NPs: the phase transformation of maghemite into hematite was observed at 15 mW for uncoated NPs of 4 nm, whereas it occurs at 120 mW for the coated NPs of similar size. The analysis of the Raman baseline profile reveals clearly that the surface coating of maghemite NPs results both in reducing the number of surface defects of nanoparticles and in delaying this phase transition

The comprehensive study of the Ni0.5Zn0.5Fe2O4 ferrite nanopowder crystallized in the inverse spinel structure and synthesized by co-precipitation method is presented. The distribution of Fe 3+ cations among tetrahedral and octahedral sites was confirmed. The microstructural investigations revealed the presence of ultrafine grained structure with an average crystallites size in the range of 14 ÷ 20 nm. Raman and Fourier-Transform Infrared (FTIR) spectroscopy studies confirmed typical spinel structure with tetrahedrally and octahedrally iron occupancy as well as indicate co-associated iron-oxide phases considered as factors responsible for the structural disorder. The magnetic properties revealed the superparamagnetic behavior at the room temperature with estimated critical size of single domain particles about 63 nm. The analysis of saturation magnetization pointed to the spin canting phenomenon in the surface layer. The valuation of exchange coupling parameters based on the mean field theory calculation strengthened the conclusion about opposite magnetization arrangement between tetrahedral and octahedral magnetic sublattices.

The effects of laser irradiation on γ-Fe2O3 4 ± 1 nm diameter maghemite nanocrystals synthesized by co-precipitation and dispersed into an amorphous silica matrix by sol-gel methods have been investigated as function of iron oxide mass fraction. The structural properties of γ-Fe2O3 phase were carefully examined by X-ray diffraction and transmission electron microscopy. It has been shown that γ-Fe2O3 nanocrystals are isolated from each other and uniformly dispersed in silica matrix. The phase stability of maghemite nanocrystals was examined in situ under laser irradiation by Raman spectroscopy and compared with that resulting from heat treatment by X-ray diffraction. It was concluded that ε-Fe2O3 is an intermediate phase between γ-Fe2O3 and α-Fe2O3 and a series of distinct Raman vibrational bands were identified with the ε-Fe2O3 phase. The structural transformation of γ-Fe2O3 into α-Fe2O3 occurs either directly or via ε-Fe2O3, depending on the rate of nanocrystal agglomeration, the concentration of iron oxide in the nanocomposite and the properties of silica matrix. A phase diagram is established as a function of laser power density and concentration.

A FePt-based hard-magnetic nanocomposite of exchange spring type was prepared by isothermal annealing of melt-spun Fe52Pt28Nb2B18 (atomic percent) ribbons. The relationship between microstructure and magnetic properties was investigated by qualitative and quantitative structural analysis based on the x-ray diffraction, transmission electron microscopy, and F57e Mössbauer spectrometry on one hand and the superconducting quantum interference device magnetometry on the other hand. The microstructure consists of L10-FePt hard-magnetic grains (15–45 nm in diameter) dispersed in a soft magnetic medium composed by A1 FePt, Fe2B, and boron-rich (FeB)PtNb remainder phase. The ribbons annealed at 700 °C for 1 h exhibit promising hard-magnetic properties at room temperature: Mr/Ms=0.69; Hc=820 kA/m and (BH)max=70 kJ/m3. Strong exchange coupling between hard and soft magnetic phases was demonstrated by a smooth demagnetizing curve and positive δM-peak in the Henkel plot. The magnetic properties measured from 5 to 750 K reveals that the hard characteristics remains rather stable up to 550 K, indicating a good prospect for the use of these permanent magnets in a wide temperature range.