Raymond Withers

An orthophosphate semiconductor with photooxidation properties under visible-light irradiation

The search for active semiconductor photocatalysts that directly split water under visible-light irradiation remains one of the most challenging tasks for solar-energy utilization. Over the past 30 years, the search for such materials has focused mainly on metal-ion substitution as in In(1-x)Ni(x)TaO(4) and (V-,Fe- or Mn-)TiO(2) (refs 7,8), non-metal-ion substitution as in TiO(2-x)N(x) and Sm(2)Ti(2)O(5)S(2) (refs 9,10) or solid-solution fabrication as in (Ga(1-x)Zn(x))(N(1-x)O(x)) and ZnS-CuInS(2)-AgInS(2) (refs 11,12). Here we report a new use of Ag(3)PO(4) semiconductor, which can harness visible light to oxidize water as well as decompose organic contaminants in aqueous solution. This suggests its potential as a photofunctional material for both water splitting and waste-water cleaning. More generally, it suggests the incorporation of p block elements and alkali or alkaline earth ions into a simple oxide of narrow bandgap as a strategy to design new photoelectrodes or photocatalysts.

The crystal chemistry underlying ferroelectricity in Bi4Ti3O12 Bi3TiNbO9, and Bi2WO6

Abstract The recent rerefinements of the crystal structures of Bi4Ti3O12, Bi3TiNbO9, and Bi2WO6 differ substantially from earlier reported structure refinements. A comparison of the major structural distortions present in each of these materials is presented and the main structural cause of ferroelectricity (in Bi4Ti3O12 and Bi3TiNbO9) is shown to be the a-axis displacement of Bi atoms in the perovskite A sites with respect to the chains of TiO6 octahedra and not the perovskite B atoms moving toward an octahedral edge. By using a modulated structure approach, the driving force for this displacement and other structural features can be simply understood in terms of the need to satisfy bond valence requirements.

Phase relations, structure and crystal chemistry of some aluminous silicate perovskites

Solid solution of ∼ 25 mole% Al2O3 expands the compositional stability field of Mg,Fe silicate perovskite well beyond the limits encountered in the simple ternary system MgOFeOSiO2. Aluminous perovskites synthesised in laser-heated diamond anvil cell experiments at 55–70 GPa from starting materials on the compositional join between Mg3Al2Si3O12 and Fe3Al2Si3O12 (pyrope and almandine) can contain as much as 90 mole% of the ferrous end member. However Fe0.75 Al0.50Si0.75O3 perovskite could not be synthesised. Predictions that garnet coexists with aluminous perovskite at these pressures are unsubstantiated. These new perovskites are approximately isochemical with garnet and accommodate the full complement of Al2O3 (25 mole%) even at ∼ 70 GPa. Some contain as much as 30 mole% Al2O3, and solid solution is probably facilitated by temperature. However, there is certainly no evidence to substantiate a recent proposal that the capacity of perovskite to accommodate Al2O3 in solid solution is progressively inhibited by pressure. Magnesian silicate perovskite should therefore have no difficulty in accommodating the mantle inventory of Al2O3 in solid solution throughout the entire lower mantle pressure regime. There is no reason to expect that a new aluminous phase would be stabilised at depth within the lower mantle. Nor would exsolution of an aluminous phase at core-mantle boundary pressures be a plausible explanation for the D″ layer. Aluminous perovskites are almost always rhombohedral R3c rather than orthorhombic Pbnm, and their unit cell volumes increase by about 3% as 75 mole% of ferrous iron replaces magnesium. These new perovskites are slightly non-stoichiometric, with modest amounts of an M2(Al,Si)O5.5(M =Mg,Fe) component in solid solution. Crystal chemistry fundamentals successfully predict the site occupancy of minor and trace elements in magnesian silicate perovskite.

An electron diffraction and lattice-dynamical study of the diffuse scattering in β-cristobalite, SiO2

Strong diffuse scattering has been observed on electron diffraction photographs of β-cristobalite. This is observed to be in the form of diffuse planes normal to the six (110) directions. A simple Born–von Karman force model including nearest-neighbour Si–O, O–O and Si–Si interactions has been used to calculate lattice-dynamical properties for β-cristobalite assuming the structure to be the simple Wyckoff [Z. Kristallogr. (1925), 62, 189–200] C9 structure. The force-model parameters used were obtained by fitting to the infrared and Raman frequencies of α-cristobalite and are very similar to those obtained by Barron, Huang & Pasternak [J. Phys. C (1976), 9, 3925–3940] for quartz. Calculated thermal diffuse scattering patterns obtained are in good agreement with the observed patterns, and may be interpreted in terms of low-lying relatively dispersionless branches of the phonon dispersion curve diagrams. These branches correspond to motions of the structure involving coupled rotations of the tetrahedra in columns along the (110) directions, the motion in neighbouring columns being virtually uncoupled.

The synthesis and structure of Ba2Cu3O5+δ

Ba2Cu3O5+δ has been prepared with ∼99% purity and possesses two related structural forms depending on δ. Both low-(δ = 0.6 to 1.0) and high- (δ ⋍ 0.5) Ba2Cu3O5+δ possess C-centered orthorhombic ‘average’ unit cells with ab ⋍ √3 and 1.91, respectively. Low-Ba2Cu3O5+δ also has a doubled c axis. Incommensurate satellite reflections are observed for both the low- and high-Ba2Cu3O5+δ structures. The modulation wave-vector varies with δ both in magnitude and direction. A structural model for low-Ba2Cu3O5+δ is proposed and the reason for the modulation discussed.

The Al-Mn decagonal phase 1. A re-evaluation of some diffraction effects 2. Relationship to crystalline phases

Abstract Abstract From a quenched, melt-spun ribbon of a specimen of Al76Mn24, bulk composition, the decagonal phase and a closely related ‘Al3Mn’ phase have been examined using analytical electron microscopic techniques. It is proposed that descriptions of the decagonal phase by analogy to an icosahedral axial system be abandoned and that a different six-axis system be adopted instead, and that the non-crystallographic space group previously proposed for the decagonal phase be changed to one containing glide planes. The ‘Al3Mn’ phase is seen to be an extremely close relative of the Al-Mn decagonal phase. However, the state of knowledge about the ‘Al3Mn’ structure is too poor to be of any use in improving the understanding of atomic arrangements in the Al-Mn decagonal phase.

Atmospheric degradation of the high-temperature superconductor, YBa2Cu3O7−x

The superconducting oxides YBa2Cu3O7−x and HoBa2Cu3O7−x degrade in the presence of atmospheric water. Planar defects appear in the crystal lattice normal to c, followed by gross distortion of the lattice, then complete breakdown to give a mixture of insulating products. Corrosion is enhanced by increasing the relative humidity and temperature.

The structure and microstructure of α-cristobalite and its relationship to β-cristobalite

Recent transmission electron microscopy (TEM) electron diffraction investigation of β-cristobalite has revealed a strong characteristic diffuse intensity distribution. In this work a similar, but less intense, diffuse scattering is reported for synthetic α-cristobalite. This material also displays both tetragonal twinning and characteristic striations correspond to planar boundaries which may exist on any {1, 0, l}tetragonal plane.

Colossal Dielectric Behavior of Ga+Nb Co-Doped Rutile TiO2

Stimulated by the excellent colossal permittivity (CP) behavior achieved in In+Nb co-doped rutile TiO2, in this work we investigate the CP behavior of Ga and Nb co-doped rutile TiO2, i.e., (Ga(0.5)Nb(0.5))(x)Ti(1-x)O2, where Ga(3+) is from the same group as In(3+) but with a much smaller ionic radius. Colossal permittivity of up to 10(4)-10(5) with an acceptably low dielectric loss (tan δ = 0.05-0.1) over broad frequency/temperature ranges is obtained at x = 0.5% after systematic synthesis optimizations. Systematic structural, defect, and dielectric characterizations suggest that multiple polarization mechanisms exist in this system: defect dipoles at low temperature (∼10-40 K), polaronlike electron hopping/transport at higher temperatures, and a surface barrier layer capacitor effect. Together these mechanisms contribute to the overall dielectric properties, especially apparent observed CP. We believe that this work provides comprehensive guidance for the design of new CP materials.

An optical transform and Monte Carlo study of the disorder in β-cristobalite SiO2

A previously reported lattice-dynamical model of β-cristobalite [Hua, Welberry, Withers & Thompson (1988). J. Appl. Cryst. 21, 458–465], which assumed the Wyckoff C9 structure but gave a satisfactory qualitative description of observed diffuse intensity in electron diffraction patterns, is shown to give atomic displacements which are much too small to be consistent with the 147° Si—O—Si angle observed in other forms of silica. Monte Carlo simulations of a modified model in which this angle is constrained to be close to 147° have been carried out and optical diffraction patterns obtained from the resulting lattice realisations. The diffraction patterns are very similar in appearance to those calculated for the simple C9 model, but the displacements are now consistent both with the expected geometry and also with mean-square atomic displacements derived from X-ray powder studies. In this simulation the oxygen atoms are distributed uniformly around an annulus which encircles the 16(c) sites of the Fd3m C9 structure. The hypothesis of Wright & Leadbetter [Philos. Mag. (1975). 31, 1391–1401], that the oxygen atoms preferentially occupy the six 96(h) sites which occur around this annulus, has been tested and it is concluded that if such ordering exists it is not very pronounced.