G. A. Saunders

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 31P MAS NMR study of rare-earth phosphate glasses of composition, (R2O3)xP2O5)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.

Pressure dependences of the elastic constants of PbTe, SnTe and Ge0.08Sn0.92Te

Measurements of the effects of hydrostatic and uniaxial pressures on ultrasonic wave velocities have been used to determine the six third-order elastic constants of PbTe. The hydrostatic pressure dependences of the elastic stiffness constants of SnTe and Ge0.08Sn0.92Te have also been obtained. Estimation of the third-order constants of PbTe on the basis of a Born-Mayer potential has established that nearest-neighbour repulsive forces are predominantly responsible for the large negative value of C111; however, this ionic model does not generally account well for the observed third-order elasticity of this mixed ionic-covalent compound. The Gruneisen mode gammas in the long-wavelength limit have been calculated for PbTe, SnTe and Ge0.09Sn0.92Te; the large anisotropy found has been explained in terms of the strong influence of the nearest-neighbour repulsive forces on C111. The elastic stiffness constants of SnTe have been measured as a function of pressure up to 15*108 Pa, quite close to the phase transition pressure of 18*108 Pa. C44 softens somewhat over about 9*108 Pa.

An ultrasonic study of superconducting and non-superconducting GdBa2Cu3O7-x

Detailed measurements are presented of ultrasonic wave velocity and attenuation of the orthorhombic superconducting and the tetragonal, non-superconducting forms of GdBa2Cu3O7-x. The orthorhombic state data display a sharp elastic transition at about 240 K which is absent from that of the tetragonal state; however, X-ray diffraction measurements show no evidence of a structural phase transition. The lattice thermal expansion is found to be fairly isotropic. The elastic response in the superconducting state is found to be dominated by conventional anharmonic and anelastic phenomena: no effects that can be specifically attributed to the formation of the superconducting state have been detected. The ultrasonic velocity and attenuation data are explained by the presence in the tetragonal state of one and in the orthorhombic state of two thermally activated anelastic relaxation mechanisms which are attributed to various oxygen vacancy hopping processes.

The second- and third-order elastic constants of amorphous arsenic

Abstract The hydrostatic and uniaxial pressure and the temperature dependences of the ultrasonic wave velocities in bulk amorphous arsenic (a-As) have been measured by the pulse echo overlap and pulse superposition techniques. The results provide the first complete set of components of the second- and third-order elastic stiffness tensors of an amorphous element. The ultrasound velocities, and elastic stiffnesses of the amorphous material are appreciably smaller than those of the crystalline, rhombohedral allotropic form. The bulk, shear and Youngs moduli and the pressure derivatives of the bulk and shear moduli of a-As conform well with trends noted previously for these properties in chalcogenide glasses in terms of the average number of covalent bonds per atom. In marked contrast to the anomalous behaviour of the insulating, silica-based glasses, the pressure derivatives of the bulk and shear moduli of a-As are found to be positive, while the temperature derivatives are negative. This more normal elast...

A rare earth L3-edge EXAFS and L1-edge XANES study of Ce, Nd and Eu phosphate glasses and crystals in the composition range from metaphosphate to ultraphosphate

Abstract Rare earth (R) phosphate glasses, (R 2 O 3 ) x (P 2 O 5 ) 1− x , can be prepared with compositions in the range from ultraphosphate, x =0.17 (RP 5 O 14 ), to metaphosphate, x =0.25 (RP 3 O 9 ), and it is important to know whether the R–O coordination changes significantly with composition. In rare earth phosphate crystals, the number of nearest neighbour oxygens changes from eight for ultraphosphate to six for metaphosphate. These R–O correlations are clearly distinguished in L 3 -edge extended X-ray absorption spectroscopy (EXAFS) of NdP 5 O 14 and EuP 3 O 9 crystals. Samples of Ce ( x =0.197 and 0.235), Nd ( x =0.187) and Eu ( x =0.218) phosphate glasses all show the same EXAFS and L 1 -edge X-ray absorption near edge structures (XANES) results. The results indicate an R–O coordination with six nearest neighbour oxygens, a similar level of static disorder to that in rare earth phosphate crystals and reduced centrosymmetry. There is no evidence for a change in R–O coordination with changing x .

An extended x-ray absorption fine structure study of rare-earth phosphate glasses near the metaphosphate composition

A variable-temperature (79, 145, and 293 K) extended x-ray absorption fine structure study, using rare-earth L-III absorption edges, is reported for phosphate glasses doped with rare-earth elements (R, where R = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) with compositions close to metaphosphate, R(PO3)(3). The results yield nearest-neighbor R-O distances that demonstrate the lanthanide contraction in a glassy matrix and an R-O coordination intermediate between 6 and 7 for ran-earth ions with smaller atomic number (Z) and 6 for rare-earth ions with larger Z, Thermal parameters show no significant changes in R-O distances or coordination numbers between 293 and 79 K. There is evidence of an R-P correlation between 3.3 and 3.6 Angstrom and the beginning of a second R-O correlation at approximately 4 Angstrom. No R-R correlations up to a distance of approximately 4 ii were observed.

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 (MAS) 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 LIII-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 LIII-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 LIII-edge EXAFS studies.

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 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 A. A residual feature in the neutron diffraction data, present at ~1.8 A, 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.

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 , and , whose compositions were determined by electron microprobe analysis. Such rare-earth metaphosphate glasses containing high concentrations of rare-earth 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 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.

A comparison of the ultrasonic evidence for mode softening in La1.8Sr0.2CuO4 and the electron-doped superconductor Nd1.85Ce0.15CuO4-y

Measurements of sound velocity show strong evidence for mode softening in La1.8Sr0.2CuO4 but not in Nd1.85Ce0.15CuO4-y. The results for La1.8Sr0.2CuO4 are in agreement with a new recently reported phase diagram.