Ian P. Swainson

Neutron powder diffraction study of the åkermanite-gehlenite solid solution series

The crystal structures of members of the åkermanite-gehlenite solid solution series have been refined from high resolution neutron powder diffraction data. Synthetic samples of 0, 25, 50, 75 and 100% åkermanite composition were used. The results are consistent with the ordering scheme for the tetrahedrally-coordinated cations represented by the structural formula Ca2[MgxAl1-x]1[Si1+x Al1-x]2O7 where the superscripts denote inequivalent tetrahedral sites. No evidence was found for the existence of phase transitions in the samples with x ≤ 0.75, in contrast to the behaviour found in pure åkermanite. The relationship between the cell parameters of meilite and the tetrahedral conformations and the resulting non-ideality of the solid-solution are discussed.

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 )

Molecular dynamics simulation of alpha - and beta -cristobalite

A molecular dynamics simulation of the alpha - and beta -phases of cristobalite has been performed. The simulated beta -phase was found to be much more disordered than the alpha -phase, in agreement with evidence from many experimental techniques. The nature of the disorder was found to be dynamic, in agreement with results from many experimental techniques, but no distinct domains were detected. The results are in agreement with the rigid unit mode (RUM) model of framework structures, and recent inelastic measurements.

Damped soft phonons and diffuse scattering in 40 % Pb(Mg1/3Nb2/3)O3-60 % PbTiO3

(PT). We measure the structural properties of PMN-60PT to be identical topure PT, however, the lattice dynamics are exactly that previously found in relaxors PMN and PZN.PMN-60PT displays a well-defined macroscopic structural transition from a cubic to tetragonal unitcell at 550 K. The diffuse scattering is shown to be weak indicating that the structural distortionis long-range in PMN-60PT and short-range polar correlations (polar nanoregions) are not present.Even though polar nanoregions are absent, the soft optic mode is short-lived for wavevectors near thezone-centre. Therefore, PMN-60PT displays the same waterfall effect as prototypical relaxors PMNand PZN. We conclude that it is random fields resulting from the intrinsic chemical disorder whichis the reason for the broad transverse optic mode observed in PMN and PMN-60PT near the zonecentre and not due to the formation of short-ranged polar correlations. Through our comparisonof PMN, PMN-60PT, and pure PT, we interpret the dynamic and static properties of the PMN-xPT system in terms of a random field model in which the cubic anisotropy term dominates withincreasing doping of PbTiO

From soft harmonic phonons to fast relaxational dynamics in CH3NH3PbBr3

The lead-halide perovskites, including

The structure of monohydrocalcite and the phase composition of the beachrock deposits of Lake Butler and Lake Fellmongery, South Australia

{\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbBr}}_{3}

Ikaite, CaCO3·6H2O: Cold comfort for glendonites as paleothermometers

, are components in cost effective, highly efficient photovoltaics, where the interactions of the molecular cations with the inorganic framework are suggested to influence the electronic and ferroelectric properties.

Orientational ordering, tilting and lone-pair activity in the perovskite methylammonium tin bromide, CH3NH3SnBr3

{\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbBr}}_{3}

Magnetism of synthetic and natural annite mica: ground state and nature of excitations in an exchange-wise two-dimensional easy-plane ferromagnet with disorder

undergoes a series of structural transitions associated with orientational order of the

Transition to a new tetragonal phase of WO3 : crystal structure and distortion parameters

{\mathrm{CH}}_{3}{\mathrm{NH}}_{3}