Alex C. Hannon

The Polaris powder diffractometer at ISIS

Abstract The Polaris instrument at the ISIS spallation neutron source operates as a medium resolution powder diffractometer. The high incident neutron flux enables datasets to be collected with comparatively short counting times or from extremely small sample volumes. Examples of recent experiments performed on Polaris, which exploit the high count rate and the particular advantages offered by fixed geometry diffraction measurements performed on a pulsed neutron source, are presented.

Boroxol groups in vitreous boron oxide: new evidence from neutron diffraction and inelastic neutron scattering studies

Abstract Although the nuclear magnetic resonance (NMR) and neutron diffraction evidence for the majority of the boron atoms in vitreous boron oxide being involved in boroxol groups would seem to be irrefutable, nevertheless several recent papers have challenged this conclusion. The present paper, therefore, examines these claims in the light of neutron diffraction data, with much greater accuracy than those reported previously, and of neutron inelastic scattering results, optimised to give the highest possible energy transfer resolution in the region of the boroxol ring breathing mode. A fit to the interference function, Qi(Q) , indicates that the fraction, f, of boron atoms in boroxol groups is 0.80 ± 0.05, while the neutron weighted density of states exhibits a well resolved boroxol ring breathing mode of similar magnitude to the theoretical prediction for a structural model with f = 0.75. Both results confirm the conclusion of the earlier NMR and neutron studies.

Structures of Uncharacterised Polymorphs of Gallium Oxide from Total Neutron Diffraction

A structural investigation is reported of polymorphs of Ga(2)O(3) that, despite much interest in their properties, have hitherto remained uncharacterised due to structural disorder. The most crystalline sample yet reported of γ-Ga(2)O(3) was prepared by solvothermal oxidation of gallium metal in ethanolamine. Structure refinement using the Rietveld method reveals γ-Ga(2)O(3) has a defect Fd3m spinel structure, while pair distribution function analysis shows that the short-range structure is better modelled with local F43m symmetry. In further solvothermal oxidation reactions a novel gallium oxyhydroxide, Ga(5)O(7)(OH), is formed, the thermal decomposition of which reveals a new, transient gallium oxide polymorph, κ-Ga(2)O(3), before transformation into β-Ga(2)O(3). In contrast, the thermal decomposition of Ga(NO(3))(3)·9H(2)O first forms ε-Ga(2)O(3) and then β-Ga(2)O(3). Examination of in situ thermodiffraction data shows that ε-Ga(2)O(3) is always contaminated with β-Ga(2)O(3) and with this knowledge a model for its structure was deduced and refined--space group P6(3)mc with a ratio of tetrahedral/octahedral gallium of 2.2:1 in close-packed oxide layers. Importantly, thermodiffraction provides no evidence for the existence of the speculated bixbyite structured δ-Ga(2)O(3); at the early stages of thermal decomposition of Ga(NO(3))(3)·9H(2)O the first distinct phase formed is merely small particles of ε-Ga(2)O(3).

Structure of zinc and niobium tellurite glasses by neutron and x-ray diffraction

Neutron and x-ray diffraction experiments of high resolving power with neutrons from a spallation source and high-energy photons from a synchrotron have been performed on compositional series of binary Zn, Nb and on mixed Zn/Nb tellurite glasses. The Te?O, Zn?O and Nb?O coordination numbers are determined by Gaussian fitting of the first-neighbour peaks in the neutron and x-ray data simultaneously. The transition of TeO4 to TeO3 units with increasing fraction of a second component is indicated by decreasing total Te?O coordination numbers. This transition appears different for glasses with ZnO or Nb2O5 additions. Details of the Te?O peaks suggest there are two species of Te?O bonds with lengths of and ?nm. The change of their fractions shows excellent agreement with the existence of TeO4 trigonal bipyramids and TeO3 trigonal pyramids. All oxygen atoms from ZnO and Nb2O5 are used for rupture of Te?O?Te bridges, which is accompanied with a change of nearly all participating TeO4 to TeO3 groups. The tendency for a change decreases for glasses of higher second component content which is accompanied by the occurrence of TeO4 groups with non-bridging oxygens. The Nb tellurite glasses show transition to network-forming behaviour with the formation of Nb?O?Nb bridges. The fractions of TeO3 units of ternary Zn/Nb tellurite glasses agree with an additivity behaviour of the modifying effects of ZnO and Nb2O5 additions.

Floppy Modes in Crystalline and Amorphous Silicates

1Mineral Physics Group, Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom 2ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, United Kingdom 3Department of Engineering Materials, Sheffield University, Mappin Street, Sheffield, S1 3JD, United Kingdo 4Neutron and Condensed Matter Science Branch, AECL Research, Chalk River Laboratories, Chalk River, Ontario, K0J 1J0, Canada 5Cavendish Laboratories, University of Cambridge, Madingley Road, Cambridge, CB3 0HE, United Kingdom (Received 21 August 1996 )

The structure of aluminate glasses by neutron diffraction

Abstract Pulsed neutron diffraction with high real-space resolution has been used to study the structure of two melt-quenched calcium aluminate, CaO–Al 2 O 3 , glasses containing 30 and 38 mol% Al 2 O 3 , respectively. The data show that aluminium is tetrahedrally coordinated by oxygen in these glasses, with a bond length 1.76 A. There is no evidence for the presence of a fraction of aluminium atoms with a higher oxygen coordination number. The data also show that calcium is coordinated by four oxygen atoms at a well-defined interatomic distance of 2.34 A. However, bond-valence calculations show that there must be an additional Ca–O coordination of at least two which occurs at a distance of 2.45 A or greater.

Local distortions in the colossal magnetoresistive manganates La0.70Ca0.30MnO3, La0.80Ca0.20MnO3 and La0.70Sr0.30MnO3 revealed by total neutron diffraction

The manganates La0.80Ca0.20MnO3, La0.70Ca0.30MnO3 and La0.70Sr0.30MnO3 exhibit the colossal magnetoresistive (CMR) effect. Total neutron diffraction was employed to yield information on both the average and local atomic structure of these disordered crystalline materials as a function of temperature. The average structures were determined from Rietveld analysis of the Bragg scattering. Information on the local structures was obtained by Fourier transformation of the total diffraction pattern to yield the total correlation function, T(r). Particular attention is paid to changes in the Mn-O bond distances, which are widely believed to be important in the CMR effect. Jahn-Teller distortions of the MnO6 octahedra are absent at the lowest temperatures in the metallic phase. As the temperature is raised towards the paramagnetic-semiconducting to ferromagnetic-metallic transition at Tc, T(r) exhibits clear increases in the variance of Mn-O bond distances, which greatly exceed those expected from the increase in disorder due to atomic thermal motion. This is confirmed by comparing the behaviour of the three materials, which have different values for Tc. The advantage of studying the local structure directly by determining T(r) from total neutron scattering, rather than extrapolating from the average to local structure from Bragg scattering studies, is demonstrated. Comparisons are made with the results obtained for the ordered compound LaMnO3, which does not exhibit the CMR effect. The three CMR manganates studied here do not show a separation of the Mn-O distances into two well resolved sets above Tc as reported by other workers.

Intermediate range structure and low-energy dynamics of densified vitreous silica

Abstract The structure and low-energy dynamics of densified vitreous silica are studied with high accuracy by neutron scattering over a wide range of momenta and energy space. The structure factors (S(Q)) indicate that the change of the SiO4 tetrahedron (the short range order) by compaction is very small, but the intermediate range structure (IMRS) represented by the first sharp diffraction peak is strongly affected. This mainly attributed to a deformation of the sixfold ring structure. The low-energy dynamics the boson peak, BP also has a strong correlation with the change of IMRS. A reduction of the intensity of the boson peak by densification, however, cannot be explained by simple models such as phonon dumping or an eigenmode of IMRS. A shrinkage of the void space, surrounded by the sixfold ring structure, is related to the suppression of the low-energy dynamics. We consider that the origin of the boson peak can be attributed to a soft mode composed of an additional degree of freedom such as the void space.

The effect of water on the structure of silicate glasses : A neutron diffraction study

The first neutron diffraction study of water in silicate glasses is reported. Deuterated sodium tetrasilicate glasses, Na2Si4O9 · χD2O (χ = 5, 10 wt%), as well as a protonated glass, Na2Si4O9 · 10 wt%H2O, were prepared in rapid-quench high pressure autoclaves. Infrared (IR) measurements show, in accordance with results for other proton-containing glasses, the presence of both molecular H2O and OH species. Line shape analysis of the first sharp diffraction peak (FSDP) indicates a gradual decrease of the medium-range order with increasing total water (H2O + OH) content. A structural mechanism explaining the observed changes in the FSDP is proposed. Combined least-squares fits, using Gaussian functions, of the first SiO, NaO and OO peaks in the corresponding total pair correlation functions, T(r), indicate that the short-range order of the silicate network is not affected by the presence of water. However, the root mean square displacements of the Na atoms decrease, suggesting that the NaO coordination becomes more ordered in hydrous silicate glasses. The first-difference of the correlation functions, D(r), for the glasses with 10 wt% total water content is obtained. The OH contributions are identified and indicate, in accordance with the analysis of the FSDP, that the OH groups lead to significant changes in the silicate network on the 3–5 A scale. The average OH distance determined from the total pair correlation functions is 0.99 A. Three types of hydrous species (H2O and / or OH), participating in hydrogen-bonding, are observed in the IR spectra with average OH...O bond lenghts, equal to 3.0, 2.65 and 2.55 A, respectively.

The structure of alkali silicate glasses

Abstract Pulsed neutron diffraction has been used to study the structure of three alkali silicate glasses of nominal composition SiO 2 · (M 2 O) 0.5 , with M = K, K 0.46 Li 0.54 and Li. The data are quantitatively consistent with the modified random network model. Lithium is coordinated by two oxygen atoms with an interatomic distance of 1.97 A. The potassium-oxygen distance is similar to the tetrahedral oxygen-oxygen distance, ∼ 2.66 A, with potassium being coordinated by approximately four oxygen atoms.