Oxide Crystals

Manganese Oxides adopt a large range of oxidation states and coordinations.




Hexagonal Perovskites





Understanding the vacancy ordering in hexagonal perovskites allows us to understand the complex stacking sequences they adopt.

http://www.chem.ox.ac.uk/icl/haywardgroup/haywardresearch.html



Oxide crystals have numerous applications. Progress in the understanding of the microscopic processes in oxides, and also the finding of new oxides with outstanding high Tc superconductivity have given new impetus to the studies. There are different groups of oxides but in all of them oxygen plays a very important role. Oxygen atom positions and concentration in the crystal lattice are the parameters that strongly influence the physical properties of oxides.

Very often oxides are accessible for study only in polycrystalline form. Therefore, very high resolutions are required in order to perform advanced structure studies by neutron or X-ray diffraction in polycrystalline materials. Some of the oxides have slightly distorted, highly symmetrical (e.g. perovskite) crystal lattices. This small distortion very often determines the physical properties of the oxide. Therefore, in order to obtain information on the crystal structure of oxides the natural width of diffraction maxima determined by the polycrystalline quality of the material has to be as small as possible.

Neutron and synchrotron radiation diffraction support high-resolution transmission electron microscopy (HRTEM) in the study of matter structure. ........Neutron diffraction is used for structure refinement rather than for ab initio structure analysis. The fluctuation dependence of the neutron scattering amplitude upon the atomic number contrasts with the monotonic dependence shown by X-rays. Neutrons are used to locate light atoms in the presence of very heavy atoms and to distinguish elements that are closely located in the periodic table. As the neutron scattering amplitude depends on the nucleus, deuterium and hydrogen have different
scattering amplitudes. Since neutrons also possess magnetic moments, additional scattering from materials containing paramagnetic ions occurs. In the case of
magnetically ordered compounds additional peaks usually appear in the neutron diffraction pattern.

Neutrons are also more suitable than X-rays for investigating crystal and magnetic phase transition.... and whether a cryostat or furnace is used has a minimal effect upon the results of the diffraction experiment (the neutron cross-sections of most elements are typically 104 times smaller than the X-ray ones.

.....In 1963 by using the new TOF method developed by Buras et al. the intensities of scattered neutrons as a function of their energies at a constant scattering angle were measured. It offers the best resolution at large interplanar spacing. This property of the TOF method is very important in magnetic structure determination and it was first applied for magnetic structure analysis of BiFeO3 at the Dubna Pulsed reactor IBR-1 by Sosnowska et al.

.......The next step is connected with the proton spallation neutron sources. The potential of this method was first recognized by Bartholomew in 1966, but in the 1970s the technology made the realization of this project possible as an alternative neutron source to the steady state reactor............ The most powerful
spallation source in the world today is the ISIS source at the Rutherford-Appleton Laboratory near Oxford.



References

1 Sosnowska I, (1999), Oxides: neutron and synchrotron X-ray diffraction studies, Journal of Electron Microscopy 48(6): 681-687, Oxford journals

2 Loong et al, (1990) Electron-phonon coupling, oxygen isotope effect, and superconductivity in Ba^KjiBiC^. In: Electron Phonon Interactions in Oxide Superconductors, ed. BaguaroR, pp. 122-145, (World Scientific, Singapore)

3 Smyth D M (1985) Defects and order in perovskite-related oxides. Ann Rev Mater Sci. 15: 329-57.



A perovskite-based compound of lithium lanthanum titanate (LLT) with a general formula La2/3−x Li3x TiO3 has been considered as one of the most promising candidates
for solid electrolytes in all-solid-state lithium batteries because of its very high ionic conductivity of 1.1 × 10−3 S cm−1 at room temperature occurring for x ∼ 0.1 and hightemperature stability......... However, the detailed mechanism of the ionic
conduction in LLT is yet to be clarified. The ionic conductivity of LLT is expected to be closely related to the crystal structures as well as the defect structures.


The crystal structures of LLT phase are, however, still controversial because of the difficulty in determining the ordering of cations and the tilting of TiO6 octahedra, both of which are frequently observed in the oxide materials based on the perovskite structure. This is due partly to the fact that in crystal structure determination, early studies employed only powder X-ray diffraction (XRD), with which determination
of the positions of light elements such as oxygen and lithium atoms is quite difficult. For this purpose, neutron diffraction is one of the most powerful tools because of its capability of detecting the positions of light elements.

Crystal and defect structures of La2/3−x Li3xTiO3 (x ∼ 0.1) produced by a melt process, 2007, WWW.OXFORDJOURNALS.ORG