High-Pressure X-Ray Diffraction Studies Of The Nanostructured Transparent Vitroceramic Medium K2O-Sio2-Ga2O3

Synchrotron-radiation-based, energy-dispersive x-ray -diffraction studies have been performed on a composite containing nanometer-size aggregates embedded in an amorphous matrix, in the pressure range from ambient up to 15 GPa. The optically transparent material containing β-Ga2O 3 nanocrystals was developed by the controlled crystallization of a silicon oxide-based amorphous precursor. Transmission electron microscopy and conventional x-ray-diffraction techniques allowed estimating the mean size of a single-crystalline phase to be 14.8±1.9 nm, distributed homogeneously in an amorphous medium. The pressure-driven evolution of x-ray-diffraction patterns indicated a progressive densification of the nanocrystalline phase. Astructural modification corresponding to a pressure-induced coordination change of the gallium atoms was evidenced by the appearance of new diffraction peaks. The overall changes of x-ray-diffraction patterns indicated a β-Ga2O3 to α-Ga2O3 phase transformation. The low- to high-density phase transition was initiated at around 6 GPa and not completed in the pressure range investigated. A Birch-Murnaghan fit of the unit-cell volume change as a function of pressure yielded a zero-pressure bulk modulus, K0, for the nanocrystalline phase of 191. ±4.9 GPa and its pressure derivative, K0′ =8.3±0.9

X-ray diffraction studies of rare-earth metaphosphate glasses

X-ray diffraction measurements have been performed at the Synchrotron Radiation Source, Daresbury, UK, in a structural study of the rare-earth metaphosphate glasses (Pr2O3)(0.216)(P2O5)(0.794), (Eu2O3)(0.252)(P2O5)(0.748) and (Tb2O3)(0.26)(P2O5)(0.74), whose compositions were determined by electron microprobe analysis. Such rare-earth metaphosphate glasses R(PO3)(3) containing high concentrations of rare-earth R(3+) ions are of growing interest in fundamental studies of magnetic glasses and in optical communications and laser technologies. The diffraction results prove to be consistent with a network model which is dominated by a phosphate glass skeleton having three-dimensional connectivity, constructed from PO4 tetrahedra linked to adjacent tetrahedra via bridging oxygen atoms. Results relating to rare-earth-oxygen correlations are consistent with a sixfold to eightfold coordination of the rare-earth atoms, with distances showing the trends expected from the lanthanide contraction: a reduction of the rare-earth ionic radii with increasing atomic number

An x-ray diffraction and 31P MAS NMR study of rare-earth phosphate glasses, (R2O3)x(P2O5)1-x, x = 0.175-0.263, R = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er

An X-ray diffraction and P-31 MAS NMR study of rare-earth phosphate glasses of composition, (R2O3)(x)P2O5)(1-x), where x = 0.175-0.263 and R = La-Er (except for Pm), is presented. The structures of these materials were investigated as a function of (a) rare-earth atomic number and (b) glass composition, The results show an increase in rare-earth coordination number from six to seven as the rare-earth ion increases in size. This effect is most evident for the rare earths, Ce, Pr and Nd, and appears to be independent of composition variation. The implications of sevenfold coordination in these glasses with respect to the possibilities of rare-earth clustering are discussed, as is the role of the incorporation of aluminium impurities in this regard. The increase in levels of cross-linking within the phosphate network, as a consequence of these small amounts of aluminium, is illustrated, as is the changing nature of the phosphate groups as a function of composition. The first reliable and quantitative parametrization of the second and third neighbour R-(O)-P and R-(OP)-O correlations is also given and the stability of the structures to strain when the glasses are drawn as fibres or exposed to different thermal conditions is described

A neutron diffraction and 27Al MQMAS NMR study of rare-earth phosphate glasses, (R2O3)x(P2O5)1-x, x = 0.187-0.263, R = Ce, Nd, Tb containing Al impurities

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 I:I 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 similar to 2.8 Angstrom. A residual feature in the neutron diffraction data. present at similar to 1.8 Angstrom, is interpreted as Al-O correlations on the basis of Al-27 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 rare-earth K-edge EXAFS study of rare-earth phosphate glasses, (R2O3)x(P2O5)1-x, x = 0.187-0.239, R = La, Nd, Sm, Eu, Gd, Dy, Er

A rare-earth K-edge extended X-ray absorption fine structure (EXAFS) study of rare-earth phosphate glasses, (R2O3)(x)(P2O5)(1-x), x = 0.187-0.239, R = La, Nd, Sm, Eu, Gd, Dy, Er, is presented. The structures of these materials were investigated as a function of (a) rare-earth atomic number and (b) temperature, and represent some of the first rare-earth K-edge EXAFS studies on a series of lanthanide-based materials. Results corroborate findings from complementary X-ray and neutron diffraction and magic-angle-spinning (NIAS) NMR experiments, and in addition, they provide a unique insight into the nature of the static disorder of the R-O correlations and of the neighbouring phosphate groups. The effects of multiple-scattering contributions are also discussed within this context. The variable temperature measurements illustrate the exceptionally high level of network rigidity present in these materials. The results are also compared to those obtained from an analogous rare-earth L-III-edge EXAFS (5.483-8.358 keV) study. Results show that the use of the much higher energies of the rare-earth K-edge (38.925-57.486 keV) enable one to avoid the double-electron excitation problems that are associated with the rare-earth L-III-edge EXAFS in the dynamic range of interest. EXAFS fitting and deconvolution simulations show that the large core hole lifetimes associated with the rare-earth K-edge do not significantly detract from the results. The deconvolution. studies also corroborate our findings that the level of fitting to our data cannot realistically be expanded beyond the first R-O shell. This limitation exists despite the exceptional counting statistics of the experiment and the highly uniform samples made possible by the ability to use much thicker samples at the higher energies compared to those used for the (higher absorption) rare-earth L-III-edge EXAFS studies