Polymer Lamellae as Reaction Intermediates in the Formation of Copper Nanospheres as Evidenced by In Situ X‐ray Studies

The classical nucleation theory (CNT) is the most common theoretical framework used to explain particle formation. However, nucleation is a complex process with reaction pathways which are often not covered by the CNT. Herein, we study the formation mechanism of copper nanospheres using in situ X‐ray absorption and scattering measurements. We reveal that their nucleation involves coordination polymer lamellae as pre‐nucleation structures occupying a local minimum in the reaction energy landscape. Having learned this, we achieved a superior monodispersity for Cu nanospheres of different sizes. This report exemplifies the importance of developing a more realistic picture of the mechanism involved in the formation of inorganic nanoparticles to develop a rational approach to their synthesis.     

Spin-flop phase transitions in the one-dimensional antiferromagnets K2FeF5 and Rb2FeF5

The characteristics of the spin-flop transition in K₂FeF₅ and Rb₂FeF₅ are found by Mössbauer spectroscopy to be very different, and are discussed in relation to the ordered magnetic structures of the two systems.     

Synthesis and characterization of ultrafine lithium ferrite from a citrate precursor

Ultrafine particles of lithium ferrite have been prepared at the unusually low temperatures of 240 and 455°C by a citrate precursor method. The materials obtained after annealing at temperatures between 455 and 700°C consist of ordered lithium ferrite as shown by the characteristics infrared absorptions, the X-ray diffraction superstructure reflections, and the 4.2 K Mössbauer spectra. The values of magnetization obtained are somewhat low, due to the presence of small quantities of an -Fe₂O₃ phase, which can also be observed in the X-ray diffraction and the Mössbauer spectra.     

Uncertainty budget for determinations of mean isomer shift from Mössbauer spectra

The magnetite/maghemite content within iron oxide nanoparticles can be determined using the mean isomer shift (\(\overline {\delta }\)). However, accurate characterisation of the composition is limited by the uncertainty associated with \(\overline {\delta }\). We have identified four independent sources of uncertainty and developed a quantitative expression for the uncertainty budget. Sources of uncertainty are categorised as follows: that from the fitting of the Mössbauer spectrum (σ _fit), that of the calibration of the α-Fe reference spectrum (σ _cal), thermal corrections to the spectrum due to second order Doppler shift (SODS) (σ _Δδ ) and other experimental errors (σ _err). Each contribution is discussed in detail using ⁵⁷Fe Mössbauer spectra obtained from an iron oxide nanoparticle system at temperatures between 16 K and 295 K on different spectrometers in two different laboratories.     

Mössbauer study of NH4FeCl3: A quasi one-dimensional magnet

NH₄FeCl₃ is a quasi one-dimensional magnet. Mössbauer spectra of powder samples of NH₄FeCl₃ have been recorded in the temperature range 1.3 to 250 K. The structural phase change between 10 and 30 K was not accompanied by changes in the Mössbauer spectra. Apparently, the changes in the local environment of the Fe²⁺ ions are not sufficient to affect the quadrupole splitting. The Mössbauer spectrum at 1.3 K, belowT _N=1.7(2) K, is magnetically split, but cannot be fitted to just one iron site. It appears to be a superposition of at least two broadened spectra, one with an internal fieldB=4.7(1) T perpendicular to the principalz-axis of the electric field gradient, the other with zero field. The intensity for these sites was found to be 3.31. The fit is improved if slow relaxation is allowed between the two sites. Broadening of the lines starts to occur at 6 K, well aboveT _N. This has not been reported for the other members of the AFeX₃ group.     

Magnetic defect structure of iron-rich Fe x O

Mössbauer spectra were recorded at 80 and 4.2 K for Fe _x O (x> 0.95) samples that had been synthesized at 1000 C and pressures up to 10 GPa. The spectra, which consist predominantly of six broadened lines, were fit to three magnetic components: bulk Fe²⁺, defect cluster Fe²⁺ and defect cluster Fe³⁺. Mean hyperfine parameter values for bulk Fe²⁺ were calculated using second-order perturbation theory, and correlations between fluctuations were determined from the mean-square linewidths. Implications for the magnetic defect structure of Fe _x O are discussed.     

Applied field Mössbauer studies of the iron storage proteins ferritin and haemosiderin

Applied field Mössbauer spectra obtained from ferritin and haemosiderin have been fitted using a spin-Hamiltonian model, to determine the effect of the applied field on the direction of the iron magnetic moments. The results of these fits indicate that the ordering of magnetic moments within the ferrihydrite core of these proteins is antiferromagnetic. The values of the anisotropy field obtained from these fits correspond to much higher values of the superparamagnetic blocking temperatures than those actually observed. This anomaly has implications for our understanding of the superparamagnetic relaxation processes in these materials.     

Studies of oxides related to high temperature superconductors

Samples of the series of compounds YBa₂(Cu_1?x Fe _x )₃ O_6.5+p with 0<x≤0.2 have been characterised by X-ray diffraction, oxidation titration and Mössbauer spectroscopy yielding information on the oxygen content parameterp and the charge state ratios Cu³⁺/Cu²⁺ and Fe⁴⁺/Fe³⁺.