Structure of alumina glass

The fabrication of novel oxide glass is a challenging topic in glass science. Alumina (Al2O3) glass cannot be fabricated by a conventional melt–quenching method, since Al2O3 is not a glass former. We found that amorphous Al2O3 synthesized by the electrochemical anodization of aluminum metal shows a glass transition. The neutron diffraction pattern of the glass exhibits an extremely sharp diffraction peak owing to the significantly dense packing of oxygen atoms. Structural modeling based on X-ray/neutron diffraction and NMR data suggests that the average Al–O coordination number is 4.66 and confirms the formation of OAl3 triclusters associated with the large contribution of edge-sharing Al–O polyhedra. The formation of edge-sharing AlO5 and AlO6 polyhedra is completely outside of the corner-sharing tetrahedra motif in Zachariasen’s conventional glass formation concept. We show that the electrochemical anodization method leads to a new path for fabricating novel single-component oxide glasses.

Neutron Diffraction Study on Ce-Doped Nickel Ferrite Nanoparticles

The NiCexFe2-xO4 polycrystalline spinel compound with composition x = 0.005, 0.010, 0.015, and 0.02 was synthesized by the solid reaction method using the high energy milling (HEM) apparatus. Measurement of the neutron diffraction patterns is carried out at room temperature using a high resolution powder neutron diffractometer (HRPD), wavelength λ = 1.8213 Å. The measured neutron diffraction data were analyzed by the Rietveld method utilizing the FullProf analysis code. The results of the neutron diffraction pattern refinement revealed that the sample has a cubic symmetry, the Fd-3m space group, mixed inverse spinel structure, A-site occupied by cation Fe3+, Ni2+, dan Ce3+. Whereas the B-site is only occupied by Fe3+ and Ni2+. The mole fraction of Fe3+ at the B-site is greater than the Fe3+ mole fraction at the A-site, whereas the mole fraction of Ni2+ at the B-site is smaller than the mole fraction of Ni2+ at the B-site. As a result of Ce3+ doping, oxygen position parameters increase, magnetic moments on B-site, and the net magnetic moments decrease. Ni2+ concentrations at the A-site and the B-site were not affected by the amount of Ce3+ substitutions. Without substitution and after Ce3+ substitution, the O-site occupancy factor is still oxygen deficient.

Neutron scattering length determination by means of total scattering

A new method for the measurement of bound coherent neutron scattering lengths is reported. It is shown that a relative measurement of the neutron scattering length, {\overline b}, of an element can be made by analysis of the neutron correlation function of a suitable oxide crystal powder. For this analysis, it is essential to take into account the average density contribution to the correlation function, as well as the contributions arising from distances between atoms in the crystal. The method is demonstrated and verified by analysis of the neutron correlation function for the corundum form of Al2O3, yielding a value {\overline b} = 3.44 (1) fm for Al, in good agreement with the literature. The method is then applied to the isotopes of iridium, for which the values of the scattering lengths were unknown, and which are difficult to investigate by other methods owing to the large cross sections for the absorption of neutrons. The neutron correlation function of a sample of Sr2IrO4 enriched in 193Ir is used to determine values {\overline b} = 9.71 (18) fm and {\overline b} = 12.1 (9) fm for 193Ir and 191Ir, respectively, and these are consistent with the tabulated scattering length and cross sections of natural Ir. These values are of potential application for obtaining improved neutron diffraction results on iridates by the use of samples enriched in 193Ir, so that the severe absorption problems associated with 191Ir are avoided. Rietveld refinement of the neutron diffraction pattern of isotopically enriched Sr2IrO4 is used to yield a similar result for Ir. However, in practice the Rietveld result is shown to be less reliable because of correlation between the parameters of the fit.

Structural analysis and thermoelectric properties of mechanically alloyed colusites

Structural analysis of colusite phases by neutron diffraction pattern refinement and high resolution transmission electron microscopy.

Magnetic and structural properties of Sc(Fe1−xSix)2 Laves phases studied by Mössbauer spectroscopy and neutron diffraction

The aim of the presented paper is to study an influence of replacement of Fe atoms by Si atoms in quasibinary Sc(Fe1−xSix)2 Laves phases on their structural and magnetic properties. Powder X-ray diffraction (XRD) and neutron diffraction (ND) measurements carried out at different temperatures from 4.3 K up to about 700 K revealed that samples were single phase with cubic C15 structure for Si concentration x from 0.05 to 0.20 and hexagonal C14 structure for higher concentration. The results of 57Fe Mössbauer measurements showed that the Sc(Fe1−xSix)2 compounds with x ≤ 0.30 are ferrimagnetic at 4.3 K. At temperature 80 K in the samples with x = 0.20 and 0.30, a magnetic cluster spin-glass state has been observed, as ferrimagnetic long-range order disappears. Such picture was supported by the results of ND measurements carried out at 8 K, which confirmed the lack of long-range order for x above 0.10 and an occurrence of hyperfine field distributions in the corresponding Mössbauer spectra. At room temperature, samples with x ≥ 0.20 became paramagnetic. A substitution of Si atoms for Fe ones leads to a decreasing of mean values of hyperfine magnetic fields in samples under investigation. From the neutron diffraction pattern analysis of Sc(Fe0.90Si0.10)2Fe magnetic moment was determined as to be equal to 1.5 μB at 8 K. Combining this result with a value of hyperfine magnetic field on 57Fe probes, the hyperfine coupling constant A in Sc(Fe0.90Cu0.10)2 phases is estimated at about 11.6 T/μB at 8 K.

Modulated order in ionic conductors: a fine line between helping and hindering

"Solid-state ionic conduction relies on essentially conflicting structural properties: long-range crystalline order, to provide structural stability (as fuel cell membranes, battery cathodes etc.); and short-range disorder, to provide smooth conduction pathways without deep local energy minima that could trap the conducting species. Materials that combine these features are generally metastable, and prone to ordering into complex modulated structured that can only be described in (3+n) dimensions using the superspace formalism. Such ordering would normally be expected to seriously compromise conduction properties. However, low-temperature modulated structures can be effective and stable precursors to high-temperature ionic conductors - and, in some cases, can coexist with regions of local disorder that actually enhance conduction. The relationship between modulated order and ionic conduction is relatively little studied, but some of our recent work points to its potential importance. This presentation will focus on two examples: the (3+3)-dimensional commensurately modulated proton conductor Ba4Nb2O9.1/3H2O; [1,2] and the (3+3)-dimensional incommensurately modulated oxide ion conductor ""Type II"" Bi2O3.xNb2O5 (for which a single-crystal neutron diffraction pattern and the refined structure are shown below). [3] The aim is to show how modulated structures can be designed and manipulated to optimise technological performance by striking a balance between stabilising the overall framework while destabilising the conduction pathways."