The structural and magnetic properties of nanocrystalline manganese, cobalt, and nickel spinel ferrites dispersed in a highly porous SiO 2 aerogel matrix were studied. X-ray diffraction and high-resolution transmission electron microscopy indicate that single crystalline ferrite nanoparticles are well dispersed in the amorphous matrix. The cation distribution between the octahedral and tetrahedral sites of the spinel structure was investigated by X-ray absorption spectroscopy. The analysis of both the X-ray absorption near edge structure and the extended X-ray absorption fine structure indicates that the degree of inversion of the spinel structure increases in the series Mn, Co, and Ni spinel, in accordance with the values commonly found in the corresponding bulk spinels. In particular, fitting of the EXAFS data indicates that the degree of inversion in nanosized ferrites is 0.20 for MnFe 2 O 4 , 0.68 for CoFe 2 O 4 , and 1.00 for NiFe 2 O 4 . Magnetic characterization further supports these findings.

The intercalation of lithium in nanosized TiO2 has been studied for its application as an electrode for rechargeable Li-ion batteries. In this paper, we use a synthesis process in order to obtain a low-density anatase TiO2 presenting monocrystalline particles of 7−8 nm. These textural characteristics allow this material to host almost twice as much Li (300 mAh·g−1) than that of a micrometer sized anatase. In this study, in situ X-ray absorption spectroscopy is used to monitor and understand the structural changes that happen upon lithium insertion/removal, leading to a reversible two-phase transition process: TiO2 (I41/amd) → Li-titanate (Imma) → Li1TiO2 (I41/amd).

The local atomic environment of Ca in (CaO)x(SiO2)1-x glasses is of interest because of the role of Ca in soda-lime glass, the application of calcium silicate glasses as biomaterials, and the previous experimental measurement of the Ca-Ca correlation in CaSiO(3) glass. Molecular dynamics has been used to obtain models of (CaO)x(SiO2)1-x glasses with x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5, and with approximately 1000 atoms and size approximately 25 A. As expected, the models contain a tetrahedral silica network, the connectivity of which decreases as x increases. In the glass-forming region, i.e., x = 0.4 and 0.5, Ca has a mixture of 6- and 7-fold coordination. Bridging oxygen makes an important contribution to the coordination of Ca, with most bridging oxygens coordinated to 2 Si plus 1 Ca. The x = 0.5 model is in reasonable agreement with previous experimental studies, and does not substantiate the previous theory of cation ordering, which predicted Ca arranged in sheets. In the phase-separated region, i.e., x = 0.1 and 0.2, there is marked clustering of Ca.

Accurate measurements of the height and position of the preedge peak in Ti K-edge XANES (X-ray absorption near-edge structure) can distinguish 4-, 5-, and 6-fold coordination of Ti by O ([4]Ti, [5]Ti, and [6]Ti, respectively), and mixtures thereof. This approach has been applied to titania−silica xerogels, (TiO2) x (SiO2)1 - x , prepared from Si and Ti alkoxides using a prehydrolysis step and acid catalyst, with 0.18 < x < 0.75. As expected, samples with x = 0.75 contain phase-segregated, amorphous, pre-anatase [6]Ti, which is converted to anatase after heating at 500 °C. Samples with x = 0.18 contain a majority of isolated, distorted [6]Ti before heating, which is converted to [4]Ti substituted for Si after heating at 750 °C. This is the first time that the changes in the coordination of isolated Ti have been unambiguously described. Xerogels with 0.25 < x < 0.35 and heat treatment at 750 °C showed concentration limits of [4]Ti substitution for Si of 10−15mol %. Slightly higher values were obtained for xerogels prepared using acetylacetone, but the main difference was that the latter had a significantly larger content of distorted [6]Ti before heat treatment. There is no evidence of significant amounts of [5]Ti.

The version in the Kent Academic Repository may differ from the final published version. Users are advised to check http://kar.kent.ac.uk for the status of the paper. Users should always cite the published version of record.

The version in the Kent Academic Repository may differ from the final published version. Users are advised to check http://kar.kent.ac.uk for the status of the paper. Users should always cite the published version of record.

A neutron diffraction study of four rare-earth phosphate glasses of composition (R2O3)x(P2O5)1-x, where x = 0.197, 0.235, 0.187, 0.263 and R = Ce, Ce, Nd, Tb respectively is presented. The structures of these materials were investigated as a function of (a) rare-earth atomic number and (b) composition. The results show that samples containing the larger rare-earth ions (Ce3+ and Nd3+) are coordinated to seven oxygen atoms whereas the immediate environment of Tb3+ ions is six coordinate. This implies that rare-earth clustering must be present in the samples containing larger rare-earth ions although no R ... R correlations are directly observed. Terminal and bridging P-O correlations are resolved, existing in an approximately 1:1 ratio. Second-neighbour O(P)O separations are located with good accuracy and P(O)P correlations relating to the bridging chain are observed. There is also first evidence for the third neighbour correlation, P(OP)O, at ~2.8 Å. A residual feature in the neutron diffraction data, present at ~1.8 Å, is interpreted as Al-O correlations on the basis of 27Al MQMAS NMR experiments. This aluminium impurity originates from the use of aluminium oxide crucibles used in the glass synthesis and is shown to exist as a mixture of octahedral, tetrahedral and penta-coordinated Al-O species. No structural perturbations of the overall network were observed with varying sample composition.

A combination of X-ray techniques [diffraction and Zr K-edge absorption (EXAFS and XANES)] and multinuclear (1H, 13C, 17O) solid-state NMR spectroscopy is employed to follow in detail the structural development of nanocrystalline zirconia. 17O magic-angle spinning NMR spectroscopy of sol−gel produced undoped ZrO2 shows unequivocally that oxygen sites in the initial gel are monoclinic-like. This result is consistent with X-ray absorption measurements, which also suggest that the structures of the initial amorphous phases of doped and undoped samples produced by the hydroxide-precipitation and sol−gel methods are very similar. On crystallization, the local structure of the crystalline component is tetragonal, but a significant fraction of the sample remains disordered. Heating to higher temperatures results in conversion to monoclinic zirconia in undoped samples at room temperature. For sol−gel-produced ZrO2, 13C NMR shows that loss of all of the organic fragments occurs prior to crystallization. The 1H NMR experiments determined that the proton content remains significant until well above the crystallization temperature, so that the composition is not accurately described as ZrO2 until >500 °C.