Tessa Brennan

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.

Elastic and non-linear acoustic properties and thermal expansion of cerium metaphosphate glasses

Abstract To test predictions of the soft potential model (SPM) for the thermal and acoustic properties of glasses, the thermal expansion and the ultrasonic wave velocity and attenuation have been measured in cerium metaphosphate glasses with compositions in the vicinity of (Ce 2 O 3 ) 0.25 (P 2 O 5 ) 0.75 . The ultrasonic wave velocities have been determined as functions of temperature and hydrostatic pressure; the results provide the temperature dependences of the adiabatic elastic stiffnesses C 11 and C 44 and related elastic properties, and the hydrostatic-pressure derivatives (∂ C 11 /∂ P ) P =0 and (∂ C 44 /∂ P ) P =0 of the elastic stiffnesses and (∂ B S /∂ P ) P =0 of the bulk modulus. The longitudinal ultrasonic wave velocities increase under pressure. The hydrostatic pressure derivative (∂ B S /∂ P ) P =0 of the bulk modulus B S is positive: when compressed, the cerium metaphosphate glasses show a normal volume elastic response. However, the pressure derivative (∂ C 44 /∂ P ) P =0 of the shear modulus is negative but small, indicating weak softening of shear modes under pressure. The shear wave ultrasonic attenuation is characterised by a broad peak; the calculated relaxation parameters are consistent with phonon-assisted relaxation of two-level systems. The results found for C IJ and (∂ C IJ /∂ P ) P =0 are used to determine the long-wavelength acoustic-mode Gruneisen parameters, which quantify the vibrational anharmonicity and are needed to obtain the acoustic mode contribution to thermal expansion. The temperature dependence of the shear wave ultrasound velocity, after subtraction of the relaxation and anharmonic contributions, follows a linear law as predicted by the SPM for relaxation of soft harmonic oscillators. At low temperatures the excess low-energy vibrational states provide a negative contribution to thermal expansion, which can be understood on the basis of the SPM.

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.

Formation of CTAB-templated mesophase silicate films from acidic solutions

We have examined the effect of the different components in the spontaneous formation of surfactant-templated mesostructured silicate films grown at the air/solution interface. The rate of film formation shows differing dependencies on the concentrations of the silica precursor, tetramethoxysilane, the surfactant template, cetyltrimethylammonium bromide (CTAB), water and the solution pH. Further, we relate the nature of the film formation to the properties of the dried film using small angle scattering methods. From this data a general formation mechanism for the interaction between silica and surfactant micelles in this system is proposed. The polymerising silica can be considered as a cationic polyelectrolyte which interacts with the positively charged CTAB micelles through the bromine anion of the surfactant and via adsorption of the micelles as the polymer becomes more hydrophobic. This polyelectrolyte–surfactant system undergoes a liquid–liquid phase separation (coacervation) at a critical point dependant on the charge and molecular weight of the polymer and the charge on the surfactant micelle.

Spontaneous free-standing nanostructured film growth in polyelectrolyte-surfactant systems

Substitution of a polyelectrolyte for silica during formation of surfactant-templated films produces similar nano- and macroscale structures confirming that silica acts as a polyelectrolyte during thin film self-assembly.

A tale of two mechanisms: comparing mesostructure formation in cationic and non-ionic surfactant-templated silicas

Abstract The formation of surfactant templated silica films at the air-solution interface has been investigated in situ using a number of time-resolved surface-sensitive and bulk sensitive techniques. Results from templating solutions prepared using a non-ionic surfactant (C16EO8) are compared to those obtained when a cationic surfactant (CTAB) is used. In both cases specific solution phase interactions drive mesostructure organisation, in the precipitates formed in the subphase and in the thin films at the solution surface, however the templating interaction between silica and surfactant appears to occur differently in each case.

Formation of mesophase surfactant-templated silica thin films from acidic solutions

Abstract The formation of mesophase silica-surfactant thin films at the air/solution interface has been studied in situ using off-specular X-ray reflectivity, Brewster angle microscopy and small angle scattering. Results for cetyltrimethylammonium bromide-templated films suggest that the formation mechanism is strongly dependent on the silica:surfactant ratio, and this is confirmed by studies on the subphase solutions using time-resolved small angle X-ray and neutron scattering. Results of similar investigations for films templated with Pluronic® P123 triblock copolymer surfactant also show a strong dependence of mesostructure development on silica:surfactant ratio. A general formation mechanism for mesophase growth and thin film development is proposed.