Search

We present the study of the ferromagnetic behaviour of crystalline and amorphous samples with identical composition Fe 2 Zr. The crystalline sample was obtained by arc melting. The two amorphous samples were obtained by mechanical grinding from the intermetallic crystalline phase and by mechanical alloying starting from the elements, respectively. We investigated the magnetic properties of the alloy using Mossbauer spectroscopy and superconducting quantum interference device magnetometry. The values of the Mossbauer parameters are reported and discussed. We present the initial magnetization curves and hysteresis loops. The magnetization behaviour is marked by saturation at low field values with very small values of remanent magnetization and coercive field, indicating that the samples are soft ferromagnets; this feature is more sharpened in the crystal. The mechanically grinded sample is more similar to the crystalline compound than the mechanically alloyed sample.

Neutron diffraction experiments were performed on the liquid alloys NaGa, NaGa3, KGa3 and CsGa3. The structure factors of KGa3 and CsGa3 display prepeaks at small wavenumbers (0.64 and 0.61 (A) over circle -1, respectively). This may indicate the existence of extremely large aggregates of atoms in the liquid, which can not be associated with any of the crystal structures known for the solid counterparts of these alloys.

Molten alloys of Ga–Tl were investigated by neutron small- and wide-angle scattering around the miscibility gap which occurs in the liquid state of this binary system. For the alloy containing 40 at.% Tl (the critical composition) and these of 5 at.%Tl less and more than that of the critical composition, an increase of the small-angle scattering intensity was observed as the temperature is decreased and approaches the temperature at the binodal curve, whereas the wide-angle scattering data show no temperature-dependent behaviour in the considered temperature-range. The Ornstein–Zernike formalism was applied to the small-angle scattering data and values for the critical-point exponents ν and γ were estimated. The latter are in good agreement with theoretical values and with results published for other demixing systems.

The structure factor of the alkali metaphosphate glasses LiPO 3 and Na 0.5 Li 0.5 PO 3 has been measured at 300 K for both systems and up to 800 K for the latter. The first diffraction peak of alkali metaphosphate glasses merges at lower Q (around 1.1 A −1 ) than in other covalent glasses, with the highest intensity for Li-containing glasses. These points are discussed following the hypothesis of voids ordering in the structure and contrasts effects due to the negative neutron Li scattering length. The intensity of the prepeak is significantly growing with temperature, in case of Na 0.5 Li 0.5 PO 3 , in contrast to the other peaks of the structure factor. This point is discussed in relation with literature data from flow birefringence indicating structural changes in these glassy systems.

Liquid selenium undergoes a phase transition towards a metallic phase when sufficient pressure is applied. We performed an x-ray diffraction experiment at the ID30 beamline of the ESRF at the wavelength of 0.149 A to investigate the local order of the metallic liquid. The diffraction pattern of liquid Se has been recorded with the large-volume Paris-Edinburgh cell between (20 °C, 3 GPa) and (1650 °C, 4.1 GPa). The local order of the metallic liquid selenium is found to be close to that of liquid tellurium. In particular, the coordination number Z increases with temperature from 2.6 to 3.0.

Amorphous Fe2Zr samples were prepared by mechanical grinding from the intermetallic crystalline phase and by mechanical alloying from elemental powders. The structural changes during milling were monitored by X-ray diffraction. Mossbauer absorption permitted us to follow the transition from the magnetically ordered phase of iron to the paramagnetic state of the final amorphous products. X-ray and neutron diffraction, coupled with Reverse Monte Carlo simulation, were used to extract the partial structure factors for the two samples. The results for the two alloys are compared and discussed.

The crystalline structure of most stoichiometric IV–VI semiconductors can be viewed as a distortion of the NaCl structure; the distortion amplitude depends on the components. As a result, corrugated planes of covalently bonded atoms are linked by dispersion (Van der Waals) forces. Neutron scattering experiments were performed on the liquid phases of a series of IV–VI compounds (GeS, GeSe, GeTe, SnS, SnSe and SnTe). We show that the evolution of the local order is correlated with the conductivity measurements.

Neutron scattering measurements of the local order in SxTe1 − x mixtures were performed. Partial contributions to the first coordination shell were obtained by suitable modeling of the structure factors. Rapid changes in the local atomic environment are shown to be responsible for the closed-loop miscibility gap that occurs around x = 0.4 at high temperature.

Anomalous X-ray scattering (AXS) and extended X-ray absorption fine structure (EXAFS) spectroscopy have been used to investigate the structure of Sr metaphosphate glass. While the interpretation of standard neutron and X-ray diffraction data is complicated by the overlapping of peaks in the radial distribution functions, the combination of these two techniques allow a more precise interpretation of the structure. The resulting model is consistent with a description of the structure in terms of Sr2+ ions interposed between phosphate chains.

In order to clarify the reason of some discrepancies existing in literature, the zinc coordination in a Zn metaphosphate glass has been investigated by the complementary use of X-Ray Diffraction, Neutron Diffraction, Extended X-ray Absorption Fine Structure Spectroscopy and X-Ray Anoma-lous Scattering. All the techniques indicate a tetrahedral coordination of O atoms around Zn :+ ions, the Zn-O distance being 1.94 ± 0.01 Å. The simultaneous modelling of all the experimental data by Reverse Monte Carlo technique demonstrated that this coordination is consistent with an extended model in which metal ions are interposed between phosphate chains. The importance of describing the preparation conditions of the glasses is stressed when structural results have to be compared.

This paper deals with a neutron scattering structural study of binary GeS0 x glasses. Two compositions (GeS 6 and GeS 8.5 ) obtained from different quenching-rates qr (qr ≈ 100, 10 and 1°C s −1 ), were analyzed. The pair correlation function, g ( r ), obtained by Fourier transformation of the total structure factors, S ( q ), exhibit three peaks located around 2.2, 3.5 and 4.4 A. The first peak is due to the SGe and SS bonds while the second one is mainly constituted by the SGeS (3.6 A), SSS (3.3 A) and SSGe bridge-interactions. The third peak corresponds to the third- and fourth-neighbour S···S correlation of S 8 rings as observed in a neutron liquid sulphur study. This third peak has allowed us to describe the presence of S 8 rings in the glassy samples. The proportion of S 8 rings is small in the water-quenched GeS 6 glass (qr ≈ 100°C s −1 ) but increases when the quenching rate used to synthesize these glasses decreases and when the concentration is enriched in sulphur.

Two vitreous samples, one of composition Eu0.1Zn0.9(PO3)2.1 and the second comprising pure zinc metaphosphate, were examined by neutron diffraction. The coordination number of zinc in both samples was determined, as well as the structural modification induced in the vitreous metaphosphate matrix by the presence of the rare earth ion.

The reverse Monte Carlo simulation method has been applied to study the structure of B 2 O 3 and SiO 2 glasses. The standard procedure starting from a random distribution of atoms has not provided satisfactory results. In both cases, a considerable fraction of the network forming atoms has been found to be incorrectly coordinated. To overcome this fact, some constraints have been added in the refinement process. Advantages and disadvantages of this method are discussed. Different starting distributions of atoms have been used for simulating the structure of vitreous B 2 O 3 . No particular preference for the presence of boroxol rings has been found. The structure of vitreous SiO 2 has been well reproduced starting from the α-quartz crystalline form. A small amount of oxygen atoms has been allowed to be far from the first Si coordination shell.

In this paper we report a first application of the new idea of combining EXAFS and differential anomalous scattering techniques to obtain partial distribution functions for a binary amorphous sample. This method has been successfully employed in extracting Ni-Ni and Ni-Zr pair distribution functions for Ni2Zr prepared by mechanical alloying. A comparison with results from previous studies is also reported.

The reverse Monte Carlo simulation has been tested by modelling the partial distribution functions of amorphous Ni2Zr. Three runs of simulation involving experimental data from isotopic substitutions neutron scattering and X-ray anomalous scattering are presented

The results of an Anomalous X-ray Scattering experiment performed on Sr(PO3)2 and Eu0.1Sr0.9(PO3)2.1 glasses are presented. These are the first measurements carried out on GILDA (General purpose Italian beamLine for Diffraction and Absorption) diffractometer, located at the European Synchrotron Radiation Facility (ESRF), Grenoble. To obtain detailed information about the local order on pure and Eu-doped Sr-metaphosphate glasses both samples were investigated near the Sr K-edge (16.107 KeV), while the latter was also studied near the Eu K-edge (48.517 KeV).

We examine the local atomic order as well as some dynamic properties of the semiconducting liquid GeTe. We employ hot-neutron two-axis diffraction at three temperatures above the melting point and compare these results with ab initio molecular dynamics simulations. The simulations were based on interatomic forces derived from pseudopotentials constructed within density functional theory. At the melting temperature, the Peierls distortion responsible for the lower-temperature crystal phase is shown to manifest itself within the liquid structure. At higher temperatures in the liquid, increasing disorder in the Ge environment determines the eventual semiconductor-metal transition. The calculated kinematic viscosity of the liquid is found to agree with the experimental value and is shown to arise from the small diffusion coefficient of the Te atoms.

We analyze the structure and dynamics of semiconducting liquid GeSe using ab initio molecular-dynamics simulations. We show the local order of the liquid to be close to that of the low-temperature crystalline phase, \ensuremath{\alpha}-GeSe. In particular, we show that the Peierls distortion, which defines the \ensuremath{\alpha} phase and vanishes in the high-temperature \ensuremath{\beta} crystalline phase, reenters GeSe in the melt. Examining the distance histograms allows one to analyze the Ge environment as consisting of a ${\mathrm{GeSe}}_{3}$ unit and having one Ge-Ge defective bond. Evidence is presented that Peierls distortion is directly responsible for the semiconducting behavior of the melt. The calculated viscosity and electrical conductivity are in agreement with the experiment. An additional neutron-diffraction experiment indicates that this liquid structure is unmodified 200 K above the melting point.

Anomalous X-ray Scattering experiments have been performed to study the local order in an amorphous CuZr sample prepared by mechanical alloying. The three partial structure factors were extracted both using a regularization algorithm to solve an ill-conditioned system of linear equations and applying the reverse Monte Carlo technique to the experimental data. Then the short range atomic distributions of similar samples prepared by different methods and investigated by different techniques are compared.

Amorphous Fe 2 Zr samples were obtained by mechanical grinding from the intermetallic crystalline phase and by mechanical alloying starting from the elements. We used X-ray diffraction and 57 Fe Mossbauer transmission spectroscopy to monitor the amorphization process and to investigate the alloys prepared by the two methods. The Mossbauer data have been analysed accounting for the disorder proper to a glass structure. The probability distributions of the quadrupole splitting and isomer shift in the amorphous Fe 2 Zr samples are discussed.