E. Sváb

The structure of liquid tetrachlorides CCl4,SiCl4,GeCl4,TiCl4,VCl4, and SnCl4

Neutron diffraction measurements have been carried out for determining the total structure factor of liquid CCl4, SiCl4, GeCl4, TiCl4, VCl4, and SnCl4. The data were interpreted using the reverse Monte Carlo method, where the procedure started from results of molecular dynamics calculations. It is demonstrated that simple repulsive interatomic potential models are suitable for describing the most important structural features qualitatively. Based on detailed analyses of particle configurations, it is shown that “corner-to-face” type near-neighbor arrangements, that have been promoted for the interpretation of these structures over the last 20 years, are actually very rare, their occurrence being around 5% only. Instead, the dominance of “corner-to-corner” type orientational correlations is found.

Short range order in amorphous in Ni62Nb38studied by isotopic neutron diffraction

Abstract The short range structure of amorphous Ni 62 Nb 38 has been investigated by high spatial resolution neutron diffraction. Four specimens of the same chemical composition but with different isotopic content of nickee were examined. The three partial atomic structure factors as well as the density-concentration structure factors were evaluated, and the corresponding pair correlation functions derived. The results indicate that this metal-metal type amorphous alloy is not a substitutional one and preferential correlation exists between unlike atoms.

Structure study of Ni62Nb38 metallic glass using reverse Monte Carlo simulation

Abstract Three-dimensional particle configurations of Ni 62 Nb 38 metallic glass were produced by reverse Monte Carlo simulation that are consistent with the three total coherent scattering functions obtained by isotopic substitution neutron diffraction measurement. From the configurations, local structure characteristics such as neighbour distribution functions and cosine distribution of bond angles were calculated. Significant differences were found between the microscopic environment of Ni and Nb atoms.

Partial correlations in NiNb amorphous alloys

Short range order in amorphous Ni 44 Nb 56 was investigated using time-of-flight neutron diffraction. The total structure factors were measured for three isotope substituted Ni 44 Nb 56 samples with 58 Ni, 60 Ni, and 0 Ni (a mixture of 62 Ni and 60 Ni isotopes with zero average coherent scattering amplitude). The partial structure factors, the atomic and the density-concentration correlation functions were determined. The characteristic parameters of the short range order are given, i.e., the neighbouring atomic pair distances, their distribution width and the coordination numbers. The results are compared with those given in the literature for amorphous Ni 62 Nb 38 , and the concentration dependence is analyzed in the amorphous Ni Nb system. It is established that chemical short range order is stronger in Ni-rich samples.

Nanosize effects on the microstructure of BaFe10.3Co0.85Ti0.85O19 hexaferrite

Nanosized and microcrystalline BaFe10.3Co0.85Ti0.85O19 samples were investigated by neutron diffraction. A strong anisotropy of the thermal expansion coefficient and its dependence on the grain size was established. The cation distributions and the magnetic moments were determined. The refined magnetic moments proved to be much lower than the theoretical spin only moments, especially for the 4e and 12k sites, indicating a local noncollinearity with short-range ordering.

Uranium surroundings in borosilicate glass from neutron and x-ray diffraction and RMC modelling

Neutron and high-energy x-ray diffraction measurements have been performed on multi-component 55SiO(2)·10B(2)O(3)·25Na(2)O·5BaO·ZrO(2) borosilicate host glass loaded with 30 wt% UO(3). Both the traditional Fourier transformation technique and the reverse Monte Carlo simulation of the experimental data have been applied to get structural information. It was established that the basic network structure consists of tetrahedral SiO(4) units and of mixed tetrahedral BO(4) and trigonal BO(3) units, similar to the corresponding host glass. Slight changes have been observed in the oxygen surroundings of the Na and Zr modifier cations; both the Na-O and Zr-O distances decrease and a more compact short-range structure has been obtained compared to the host glass. For the U-O correlations two distinct peaks were resolved at 1.84 and 2.24 Å, and for higher distances intermediate-range correlations were observed. Significant correlations have been revealed between U and the network former Si and B atoms. Uranium ions take part in the network forming, which may be the reason for the observed good glassy stability and hydrolytic properties.

Neutron diffraction studies on liquid CCl4 and C2Cl4

Abstract Neutron diffraction (ND) is arguably the most important tool of liquid structure determination. To be able to assess the information content of ND results one should see clearly the possible effects of experimental conditions on the outcome of ND experiments. For this purpose the structure factors of two simple molecular liquids, CCl 4 and C 2 Cl 4 have been measured at two different wavelengths and then analysed by reverse Monte Carlo simulation.

Mössbauer Spectroscopy of Spinels

Soon after the discovery of the Mössbauer effect, studies were performed on spinels containing various transition metal ions (mostly Fe2+). This method proved very useful for investigating the local symmetry at transition metal ions. In spite of the numerous results, the correct interpretation of the complex quadrupole split spectra is still not given for numerous spinel structures. Since spectra of different shapes were measured for FeAl2O4 and FeCr2O4 by different authors, we performed new measurements on these spinels. The results on FeAl2O4 showed that the statistical distribution of another kind of ions in the positions A may influence the electric field gradient at the Fe2+ ions in the tetrahedral interstices. In FeCr2O4 and in the mixed Fe0.5Mg0.5Cr2O4, the electric field gradient exists at the Fe2+ ions at room temperature indicating that the degeneracy of the orbital doublet of the Fe2+ is removed.

Magnetic cation distribution in a nanocrystalline Fe3O4 spinel

Neutron diffraction study of nanostructured magnetite was performed at room temperature. The actual grain-size was determined from evaluation of X-ray diffraction profiles by the modified Williamson-Hall plot. The atomic structure parameters and the sublattice magnetic moments were determined using the Rietveld refinement of the neutron diffraction spectra. The sublattice magnetization at the octahedral sites decreases with decreasing grain-size, while it remains practically constant at the tetrahedral sites.

Characterization of Y-type hexaferrite Ba 2 Mg 2 Fe 12 O 22 powders

Ba<inf>2</inf>Mg<inf>2</inf>Fe<inf>12</inf>O<inf>22</inf> is a Y-type hexagonal ferrite known for a relatively high temperature (∼200 K) of the magnetic transition to a spiral spin arrangement and an easy magnetization axis lying in a plane perpendicular to the c crystal axis. The multiferroicity exists in the absence of an external magnetic field H; a longitudinal-conical spin arrangement sets below 50 K; and the direction of the electric polarization P can be governed by relatively low magnetic fields (<0.02 T). The structural, magnetic and ferroelectric parameters have been mostly studied on monocrystals and, in spite of the attractive possibilities of designing promising composite multiferroic structures, the information on Ba<inf>2</inf>Mg<inf>2</inf>Fe<inf>12</inf>O<inf>22</inf>powders is scarce. We report on Ba<inf>2</inf>Mg<inf>2</inf>Fe<inf>12</inf>O<inf>22</inf>powders studied by X-ray and neutron powder diffraction, and magnetometry. The samples were prepared by two different techniques: sonochemical co-precipitation and sol-gel auto-combustion. The structures retained the rhombohedral crystal symmetry, but mixed occupancies of the 14 (Mg, Fe) cation sites was a new feature. The magnetizations at a magnetic field of 60 kOe at 300 K were 22.78 emu/g for the sample prepared by sol-gel auto-combustion, and 24.95 and 25.06 emu/g for the samples obtained by sonochemical co-precipitation and annealed at 1170 °C and 1200 °C, respectively.