CMMP TO END OF SEPTEMBER 1999

CONTENTS

1. NEW SCIENCE
2. PAPERS ACCEPTED/SUBMITTED FOR PUBLICATION
3. PAPERS PUBLISHED
4. TALKS GIVEN, CONFERENCES ATTENDED, VISITS MADE, MEETINGS RUN, SESSIONS CHAIRED
5. WORK FOR OUTSIDE BODIES
6. GRANT APPLICATIONS SUBMITTED
7. GRANTS AWARDED
8. EXPECTED VISITORS
9. OTHER ITEMS THAT MIGHT BE OF INTEREST

1. NEW SCIENCE

2. PAPERS ACCEPTED/SUBMITTED FOR PUBLICATION

Swollen stacks of finite-size disc-like Laponite clay platelets are investigated within a Wigner-Seitz cell model. Each cell is a cylinder containing a coaxial platelet at its centre, together with an overall charge-neutral distribution of microscopic co and counterions, within a "primitive model" description. The non-linear Poisson-Boltzmann (PB) equation for the electrostatic potential profile is solved numerically within a highly efficient Green's function formulation. Previous predictions of linearised Poisson-Boltzmann (LPB) theory are confirmed at a qualitative level, but large quantitative differences between PB and LPB theories are found at physically reLev Kantorovichant values of the charge carried by the platelets. A hybrid theory treating edge effect at the linearised level yields good potential profiles. The force between two coaxial platelets, calculated within PB theory, is an order of magnitude smaller than predicted by LPB theory.

We study a single cluster of Lennard-Jones atoms using a novel and physically transparent Monte Carlo simulation technique. We compute the canonical ensemble averages of the grand canonical growth and decay probabilities of the cluster, and identify the critical cluster, the size for which the growth and decay probabilities are equal. The size and internal energy of the critical cluster, for different values of the temperature and chemical potential, are used together with the nucleation theorems to predict the behaviour of the nucleation rate as a function of these parameters. Our results agree with those found in the literature, and roughly correspond to the predictions of classical theory. In contrast to most other simulation studies, we are able to concentrate on the properties of the clusters which are most important to the process of nucleation, namely those around the critical size. This makes our simulations computationally more efficient.

For ultrasound propagated along the a, b and c-axis of a high quality UPd₃ single crystal, we present both elastic constant and ultrasonic attenuation data. Besides the magnetic phase transition at T₂ = 4.4 ± 0.1 K and the quadrupolar transition at T₁ = 6.8 K we have also observed quadrupolar ordering at T₀ = 7.6 ± 0.1 K concomitant with a symmetry change from hexagonal to orthorhombic. Surprisingly, the quadrupolar phase transition at T₁ is split into a second order transition at T₊₁ = 6.9 ± 0.05 K and a first order transition at T_-1 = 6.7 ± 0.05 K. For the four phase transitions the quadrupole ordering components and the respective symmetry changes are specified.

Neutron diffraction measurements of the low temperature nuclear and magnetic structures in a series of polycrystalline and single crystal U_xY_1-xPd₃ pseudobinary compunds are reported, together with electrical resistivity, magnetisation and susceptibility, heat capacity and mSR measurements. The room temperature cubic crystal structures remain undistorted down to 4 K and long range magnetic order is present only in polycrystalline and single crystal U_0.45Y_0.55Pd₃ with ordered moments of 0.64 mB below T_N = 22.5 K. Calculations based upon the results of previous inelastic neutron scattering measurements are compared with the bulk property measurements and provide further confirmation of the ground state in these compounds. A crossover from a localised crystalline electric-field dominated response to a conduction electron dominated response is found in all measurements for uranium concentrations in the region of x = 0.28.

A comprehensive resistance study of erbium subjected to a hydrostatic pressure is presented. From the experimental results we derive a p-T phase diagram for the magnetic phases in erbium. In the zero-temperature limit, the conical structure is predicted to transform into the cycloidal one at a pressure of about 1.3 kbar. Experimentally, the transition is found to occur between 1 and 3 kbar at 4.5 K. The experimental results are analyzed in terms of a variational calculation of the resistivity using the model developed for erbium from previous experiments. The theory of Elliott and Wedgwood is utilized in the account of the superzone effects. The analysis indicates that the a-axis resistivity is slightly affected by the superzones. In the c-axis case the superzone effects do not simply scale with the magnetisation, but also reflect the 20% change of the ordering wave vector. This occurs between T_N and T_C at ambient pressure, and at 4.5K when the pressure is increased from 1 to 3 kbar. It is proposed that the cycloidal structure exists in Er, just above T_C at ambient pressure, and in the interval between 1.3 and 9 kbar at zero temperature.

We have examined the temperature-dependence of motions in a cryosolution of the enzyme glutamate dehydrogenase (GDH) and compared these with activity. Dynamic neutron scattering was performed with two instruments of different energy resolution, permitting the separate determination of the average dynamical mean-square displacements on the sub ~100 ps and sub ~5 ns timescales. The results demonstrate a marked dependence on timescale of the temperature profile of the mean-square displacement. The lowest temperature at which anharmonic motion is observed is heavily dependent on the time window of the instrument used to observe the dynamics. Several dynamical transitions (inflexions of the mean-squared displacement) are observed in the slower dynamics. Comparison with the temperature profile of the activity of the enzyme in the same solvent reveals dynamical transitions that have no effect on GDH function.

For the study of protein structure, dynamics, and function, at very low temperatures it is desirable to use cryosolvents that resist phase separation and crystallisation. We have examined these properties in a variety of cryosolvents. Using visual and x-ray diffraction criteria, methanol:ethanediol [70%:10%], methanol:glycerol [70%:10%], acetone:methoxyethanol:ethanediol [35%:35%:10%], dimethylformamide:ethanediol [70%:10%], dimethylformamide, [80%], methoxyethanol [80%], and methoxyethanol:ethanediol [70%:10%] were all found to be free of phase-changes down to at least -160 C. The least viscous of these, methanol:ethanediol [70%:10%], was miscible down to -130 C and showed no exo or endothermic transitions when examined using DSC. It is therefore potentially particularly suitable for very low temperature cryoenzymology.

Using neutron diffraction with isotopic substitution, the structures of aqueous solutions of tertiary butanol have been studied as a function of concentration. As the behaviour of this system is thought to be driven by hydrophobic interactions, particular attention has been paid to the hydration of the non-polar headgroups and the nature of the intermolecular contacts. As concentration is increased from 0.06 to 0.16 mole fraction tertiary butanol, there is evidence for the growth of small clusters of the alcohol molecules, with butanol-butanol co-ordination numbers of 2 to 3 even at the lowest concentration. Orientational pair correlation functions show that the dominant intermolecular contacts between alcohol molecules are between the non-polar head groups, as would be expected for a system driven by hydrophobic interactions. As concentration increases, however, there is evidence of mixed polar - nonpolar contacts. The alcohol group's hydrogen bonding requirements appear to be fully met by polar contacts with water molecules: there is no evidence for significant butanol-butanol hydrogen bonding.

Molecular dynamics simulations were used to model two stepped {104} surfaces of the calcium carbonate polymorph calcite. The acute monatomic steps were found to be more stable than the obtuse moatomic steps. The initial stages of dissolution from the steps were considered in vacuo and in water. In vacuo CaCO3 was shown to dissolve preferentially from the obtuse step. In aqueous environment both stepped surfaces are stabilised by the presence of water molecules although the relative stabilities remain similar. Using potential parameters which reproduce experimental enthalpies of the dissolution of calcite crystal, formation of the double kink on the obtuse step is shown to be less endothermic than dissolution from the acute step, probably due to the lower stability of the obtuse surface. Formation of the kink sites on the dissolving edge of the obtuse step is the rate determining step and this edge is predicted to dissolve preferentially, in agreement with experimental findings of calcite dissolution under aqueous conditions.

Nucleation and growth at defect sites is discussed in terms of rate equation models, which can be applied to metals deposited on oxide and halide surfaces. Strong trapping and rapid adatom diffusion is characterised by an extensive plateau, where the island density stays constant over a wide range of deposition parameters. Energies for defect trapping, adsorption, surface diffusion and pair binding are deduced for Pd deposited on Ar-cleaved MgO(001), which has recently been studied in-situ by variable temperature atomic force microscopy (AFM) Calculations of these energies for Pd and Ag metals on both NaCl and MgO(001) surfaces are presented, and compared with earlier data for Ag, Au and Pd on NaCl.

We review our recent applications of the Density Functional Theory (DFT) to studying perfect and defective MgO (001) surfaces. The calculations employ plane wave basis sets, the method of pseudopotentials and supercell geometry. The systems considered include perfect and irregular surfaces and surface defects such as a neutral F-centre and a peroxide ion. We discuss defect formation energies, adsorption energies and diffusion barriers for adsorbed species. Triplet excitation energies and luminescence energies are calculated for F centres and low coordinated oxygen sites at the surface. The theoretical results are compared with available experimental data including UPS and MIES. Finally, the limitations of the method are discussed.

We combine experimental studies using Ultraviolet Photoelectron Spectroscopy (UPS), Metastable Impact Electron Spectroscopy (MIES) and Temperature Programmed Desorption (TPD) with ab initio calculations of metal adsorption on the perfect MgO surface and at defect sites in order to elucidate the role of surface defects in the initial stages of nucleation and growth of metal clusters at oxide surfaces. MgO films (2 nm thick) grown on Mo and W substrates were used as a prototype system. The MIES and UPS (HeI) spectra were collected in situ, and growth of Mg clusters was observed by monitoring the dynamics of additional MIES peaks during Mg deposition. The TPD experiments were made in order to monitor the surface coverage by Mg clusters and to determine the Mg desorption energies. Interpretation of the results was made on the basis of theoretical modelling using Density Functional Theory (DFT) calculations in both periodic and embedded cluster models. The geometric and electronic structures of the surface terrace, F-centre, positively charged anion vacancy, and step edge at the MgO(001) surface were calculated, and their role in adsorption and clustering of Mg atoms on this surface was studied. The absolute position of the top of the surface valence band of MgO with respect to the vacuum was calculated and compared with the MIES results. The MIES spectra were modelled on the basis of surface density of states (SDOS) The calculated SDOS predicted the location of additional peaks in the band gap and their shift as a function of Mg concentration on the surface in agreement with the MIES data. The desorption energies of Mg atoms from small Mg clusters formed at step edges are found to be about 1.3 eV/atom. Comparison between the theoretical results and the experimental data suggests preferential initial adsorption of Mg atoms at steps and kinks, rather than at charged and neutral vacancies. At larger exposures these Mg atoms serve as the nucleation sites.

We re-examine the theory of Metastable Impact Electron Spectroscopy (MIES) in its application to insulating surfaces. This suggests a quantitative approach which takes advantage of recent developments of highly efficient many-electron computational techniques. It gives a basis to the interpretation of experimental MIES spectra for perfect and defective surfaces. Our method is based on a static approach to predicting Auger de-excitation (AD) rates of He* (1s2s) projectiles. A key quantity is the surface Density of States (DOS) projected on the 1s orbital of He* atom which is calculated along its trajectory. We use Density Functional Theory within both supercell geometry and embedded cluster models to calculate MIES spectra for the perfect MgO surface and for an MgO surface with different concentrations of adsorbed oxygen atoms. First we calculate the Auger de-excitation rates at various positions of the projectile above the surface. To predict MIES spectra, we integrate over projectile trajectories, with a subsequent weighted averaging with respect to various lateral positions of He* above the MgO surface unit cell. It is important to examine final state effects for a correct comparison between theory and experiment, especially when there are localised defect states.

(v) L. N. Kantorovich, A. S. Foster, A. L. Shluger, and A. M. Stoneham "Role of image forces in NC-SFM images of ionic surfaces" Surface Science (accepted)

We consider the effect of the image interaction on the force acting between tip and surface in non-contact Scanning Force Microscope (NC-SFM) experiments. This interaction is reLev Kantorovichant when a conducting tip interacts with either a polar bulk sample or with a thin film grown on a conducting substrate. We compare the atomistic contribution due to the interaction between the microscopic tip apex and the sample with the macroscopic van der Waals and image contributions to the force on the tip for several representative NaCl clusters adsorbed on a metal substrate. We show that the microscopic force dominates above the plain (001) terrace sites and is solely responsible for image contrast. However, the image force becomes comparable to the microscopic force above the surface di-vacancy and dominates the interaction above a charged step.

(w) L. N. Kantorovich "Elimination of the long-range dipole interaction in calculations with Periodic Boundary Conditions." Phys. Rev. B, 1999 (accepted)

In this paper we consider the effect of the long-range Coulomb interactions in calculations of aperiodic systems with non-vanishing dipole moment in the unit cell using Periodic Boundary Conditions (PBC) Specifically, the method of Makov and Payne (Phys. Rev. B51, 1995, p. 4014) for eliminating the effect of a net dipole moment in the cubic cells is re-examined and we show that its success for cubic cells is rather accidental. Instead we suggest another method which works well also in the case of simulation cells of arbitrary shape. Several examples are considered to illustrate the method.

(x) L. Kantorovich "Application of the group function theory to infinite systems." Int. J. Quant. Chem., 1999 (in press)

We consider application of the Group Function Theory to an arbitrary infinite system consisting of weakly overlapping structural elements which may be atoms, ions, molecules, bonds, etc. We demonstrate that the Arrow Diagram (AD) expansion developed previously is ill-defined for such a system resulting in divergences in any physical quantity associated with the entire system such as for example the energy and charge density. A ``linked-AD'' theorem is then formulated and proven which results in a diagrammatic expansion that scales correctly with the system size. Coulomb systems with long-range interactions between structure elements are also considered and the diagrammatic expansion is rearranged in such a way as to also give the correct (linear) scaling. We give an explicit expression for the total energy up to the 3rd order with respect to overlap. Finally, we shortly discuss the problem of choosing structure elements (SE) in a general insulating system and propose a variational method based on a Configuration Interaction (CI) type expansion within the Fock subspace associated with every SE.

(y) L. Fernández Barquín, M. V. Kuznetsov, Y. G. Morozov, Q. A. Pankhurst and I. P. Parkin Combustion synthesis of chromium substituted lithium ferrites Li_0.5Fe_2.5-xCr_xO₄ (x <= 2.0): Rietveld analysis and magnetic measurements J. Inorg. Mater., at press (1999)

3. PAPERS PUBLISHED

(authors, journal, abstract)

(a) H. Ness and A.J. Fisher Phys. Rev. Lett. 83, 452 (1999) "Quantum inelastic conductance through molecular wires"

We calculate non-perturbatively the inelastic effects on the conductance through a conjugated molecular wire-metal heterojunction, including realistic electron-phonon coupling. We show that at sub-band-gap energies the current is dominated by quantum coherent transport of virtual polarons through the molecule. In this regime, the tunneling current is strongly increased relative to the case of elastic scattering. It is essential to describe the full quantum coherence of the polaron formation and transport in order to obtain correct physics. Our results are generally applicable to one-dimensional atomic or molecular wires.

(b) D. Alfe, M. J. Gillan and G. D. Price 'The melting curve of iron at the pressures of the Earth's core from ab initio calculations' Nature 401 (1999) 462. [See item 9(b) also.]

(c) A M Stoneham Foreword to T C Choy's Effective Medium Theory (Oxford University Press 1999)

(d) H. Vehkamaki and I.J. Ford 'Nucleation theorems applied to the Ising model' Phys. Rev. E 59 (1999) 6483.

We use Monte Carlo simulations to study a single cluster of "up" spins in a sea of "down" spins in the three-dimensional Ising model. We evaluate the growth and decay rates for clusters of different sizes, identify the critical size for which these rates are equal, and obtain the internal energy of the critical size cluster. The results of the simulations at different temperatures and magnetic fields are used together with the first and second nucleation theorems to predict how the cluster nucleation rate changes when the external magnetic field and the temperature are changed. Our results are in agreement with literature values, but our method requires significantly less computational effort than the simulations reported earlier and avoids the difficult evaluation of free energies.

(e) J-G. Park, D.T. Adroja, K.A. McEwen, Y.J. Bi and J. Kulda 'Crystal field excitation of single crystal CeNiSn' Physica B 259-261 (1999) 288

We present recent inelastic neutron scattering results for single crystal CeNiSn. Apart from phonon related structures appearing mainly below 30 meV, we have observed clear indications of crystal field excitations centred around 40 meV. We discuss our current understanding of CeNiSn with reference to these new results.

(f) M.J. Bull, K.A. McEwen, R.S. Eccleston, K.N. Clausen and W. Kockelmann 'Antiferromagnetism, crystal fields and hybridisation in U_xY_1-xPd₃ studied by neutron scattering' Physica B 259-261 (1999) 254

We summarise our U_xY_1-xPd₃ inelastic neutron scattering experiments and present new neutron diffraction results for a single crystal of U_0.45Y_0.55Pd₃. Long-range antiferromagnetic order is unambiguously observed below T_N=22.5K. In contrast, no long-range order is found in polycrystalline samples of U_0.37Y_0.63Pd₃, U_0.28Y_0.72Pd₃ and U_0.20Th_0.25Y_0.55Pd₃ but peaks in the magnetic susceptibility indicate spin-glass ordering in these compositions.

(g) T.W. Roberts, S.W. Zochowski, F. Tasset and K.A. McEwen 'Magnetic moment directions in the 3-q phase of neodymium from neutron polarimetry with CRYOPAD' Physica B 267-268 (1999) 243

CRYOPAD has been used to study the moment directions in the zero field magnetic structures of the rare-earth metal neodymium. Results taken in the 3-q phase between ª 6K and ª 8K are presented. Here the directions of the moments giving rise to the modulation wave vectors q₁ and q₂ (relating primarily to the moments on the hexagonal sites of the DHCP nuclear structure) stay close to a* as one traverses the phase. This is not the case for the moment giving rise to the third modulation wave vector q₃ (relating primarily to the moments on the cubic sites) At the high temperature end of the 3-q phase this moment is in the a*-c* plane almost parallel to a*, but with a large c* axis component developing as the temperature is reduced. q₃ is split along c by t = 0.05t₀₀₁ near 8 K. The t+ satellite is due to a moment in the a*-c* plane with a positive c* axis component whereas the t- satellite is due to a moment in the a*-c* plane with a negative c* axis component equal in magnitude to that of t+. This result could not have been obtained without being able to measure the transverse component of polarisation.

(h) J. L. Finney. Review of "On Reflection" by Jonathan Miller. Acta Cryst. 55 (1999) 968-72.

(i) J.A. Venables, G. Haas, H. Brune and J.H. Harding "Epitaxial Growth - Principles and Applications" Chapter 42 Vol 570 (1999) pp57-60

Nucleation and growth of metal clusters at defect sites is discussed in terms of rate equation models, which are applied to the cases of Pd and Ag on MgO(100) and NaCl(001) surfaces. Pd/MgO has been studied experimentally by variable temperature atomic force spectroscopy (AFM) The island density of Pd on Ar-cleaved surfaces was determined in-situ by AFM for a wide range of deposition temperature and flux and stays constant over a remarkably wide range of parameters; for a particular flux, this plateau extends from 200 K < T < 600 K, but at higher temperatures the density decreases. The range of energies for defect trapping, adsorption, surface diffusion and pair binding are deduced, and compared with earlier data for Ag on NaCl, and with recent calculations for these metals on both NaCl and MgO

(j) J.H. Harding, D.J. Harris and S.C. Parker Phys Rev B60 (1999) 2720-2726

The structures and energies of a series of tilt grain boundaries and corresponding surfaces were calculated to obtain a general relationship between the grain boundary energy and the tilt angle for NiO and MgO. We show that the simple elastic expressions for the grain boundary give reasonable results for low-angle boundaries. We investigate the assumptions that lie behind thermal grooving experiments and show that the torque terms should be large, even when far from a major pole [the (100), (110) or (111) directions]. However, the measured angles agree better with the calculations when the torque terms are ignored. We discuss this effect in terms of faceting and oxidation of the surface.

(k) J.S. Braithwaite, C.R.A. Catlow, J.D. Gale and J.H. Harding Chemistry of Materials 11 (1999) 1990-1998

The atomic and electronic structures of three Li_xV₂O₅ phases have been investigated using plane wave calculations based on density functional theory. We show that reduction of the vanadium ions occurs during lithium intercalation. At low lithium concentrations this reduction is limited to specific ions. Changes to unit cell dimensions can be predicted correctly. Calculated cell potentials are in good agreement with experiment. Initial 'atomistic' calculated were successfully used to find the most stable sites for lithium in the V₂O₅ lattice. Calculated activation energies suggest that lithium is highly mobile along the 010 direction.

(l) A. L. Shluger, A. I. Livshits, A. S. Foster and C. R. A. Catlow "Models of Image Contrast in Scanning Force Microscopy on Insulators." J. Phys. : Condens. Matter 11, (1999) R295-R322

We review the results of theoretical modelling of Scanning Force Microscopy and discuss the possibility of obtaining atomic and chemical resolution in contact and non-contact mode SFM, and a related issue of a working model for interpretation of SFM images. As a prototype system we consider the interactions of hard tips with softer alkali halide surfaces in UHV. We briefly review experimental data, then discuss results of static atomistic simulations and molecular dynamical modelling of contact SFM to test some of the assumptions of intuitive SFM models. Then we illustrate the shortcomings of contact SFM by considering an image of a point defect. The mechanism of resolution in non-contact SFM and the effect of avalanche tip-surface adhesion are discussed next. We conclude by discussing the status of SFM with atomic resolution.

(m) L.N.Kantorovich and I.I.Tupitsyn "Coulomb potential inside big finite crystal" J. Phys.: Condens. Matter, v. 11, 1999, pp. 6159-6168.

We review long-range Coulomb interactions in systems consisting of periodically repeated replicas of identical unit cells containing a net dipole moment. Both cases of infinite and big finite sample of arbitrary shape are considered. The general result due to Smith (Proc. Roy. Soc. Lond. A 375, 1981, p. 475) for a big finite sample is considerably simplified. This allowed us to reveal further its physical meaning. We show that extra shape-dependent terms both in the electrostatic potential and energy have pure macroscopic origin. We also suggest an alternative derivation for the Coulomb potential of a big finite sample of arbitrary shape which is believed to be simpler than the original one due to Smith.

(n) L. Kantorovich, A. Shluger, J. Gunster, J. Stultz, S. Krischok, D. W. Goodman, P. Stracke, V. Kempter "Mg Clusters on MgO surfaces: Characterization by MIES and Electronic Structure ab initio Calculations." Nucl. Instr. Methods B, v. 157, 1999, pp. 162-166.

MgO films (2nm thick) were grown on Mo and W substrates while Metastable Impact Electron (MIES) and Ultraviolet Photoelectron Spectroscopy (HeI) spectra were collected in situ. Apart from the valence band (VB) emission no additional spectral features have been detected with electron spectroscopies. After exposing the oxide surface to Mg (substrate temperature between 100K and RT) an additional peak, not seen with UPS, located within the band gap, shows up in MIES. It is located at about 2 eV above the top of the VB with a FWHM of about 1eV at the lowest exposures. Electronic structure calculations based on the Density Functional Theory (DFT) are presented for the adsorption of up to 4 Mg atoms to the defective MgO surface. Based on these calculations we attribute the additional emission to the formation of small Mg clusters. The calculations suggest that the nucleation starts at extended defects, steps in particular.

(o) Warren Cross, Louise Affleck, Maxim V. Kuznetsov, Ivan P. Parkin and Quentin A. Pankhurst 'Self propagating high temperature synthesis of spinel ferrites MgFe₂O₄ (M = Mg, Co, Ni, Cu, Zn); reactions in an external magnetic field' J. Materials Chemistry 9 (1999) 2545-2552.

Spinel ferrites of the form MgFe₂O₄ (M = Mg, Co, Ni, Cu, Zn) and tetragonal BaFe₂O₄ were prepared by self-propagating high-temperature synthesis (SHS) reactions from iron, iron(III) oxide and metal(II) oxides or peroxides. The driving force for the reactions is the oxidation of iron powder. Reactions were carried out in air, under an oxygen atmosphere or with the addition of sodium perchlorate as an internal oxidising agent. Reactions were also carried out in the presence of an external magnetic field of 1.1T. Pre-organisation of the reaction mixture within the external magnetic field led to increases in the reaction velocity (from 2 to 5 mms^-1) and temperature (900 °C to 1050 °C) The ratio of tetragonal to cubic phase observed for the CuFe₂O₄ product was increased by carrying out the reaction in an external magnetic field. The inversion parameter observed for MgFe₂O₄ was influenced by the external applied field. All materials were characterised by X-ray powder diffraction (XRD), energy dispersive X-ray analysis (EDXA), scanning electron microscopy (SEM), Fourier-Transform Infrared spectroscopy (FTIR), Mössbauer spectroscopy and vibrating sample magnetometry (VSM)

(p) M. V. Kuznetsov, L. Fernández Barquín, Q. A. Pankhurst and I. P. Parkin 'Self-propagating high-temperature synthesis of barium-chromium ferrites BaFe_12-xCr_xO₄ (0 <= x <= 6.0)' J. Phys. D 32, 2590-2598 (1999)

Pure and chromium substituted barium ferrites BaFe_12-xCr_xO₁₉ (0 £ x £ 6.0) have been synthesised in air by self-propagating high-temperature synthesis (SHS), a combustion process involving reaction of barium peroxide, iron oxide, chromium oxide and iron metal powder. Two series of SHS samples were produced: series 1 - zero field SHS, and series 2 - SHS in a magnetic field of 1.1 T, both followed by sintering at 1200 °C for 2 hours. X-ray data showed that hexagonal ferrites were produced, and systematic changes in lattice parameters were seen as a function of Cr content. Scanning electron microscopy indicated crystallites of order 1 mm. EDAX showed that the samples were homogeneous with the expected Ba:Fe:Cr ratios. Mössbauer, x-ray and magnetic hysteresis data showed a progressive change in sublattice occupancy and magnetisation with Cr content. Some differences in magnetic parameters were observed between series 1 and 2, implying that the use of a magnetic field during SHS can influence product microstructure. In particular the coercive forces in the Cr doped ferrites showed maxima at x = 1.5 and x = 1.0 for series 1 and 2 respectively, with the series 2 coercivities being consistently 40-50% smaller than their series 1 counterparts. This indicates that applied field SHS provides an alternative route to reducing coercivity in hexagonal ferrites in lieu of conventional approaches such as cationic doping.'

(q) G. D. Forster, L. Fernández Barquín, R. L. Bilsborrow, Q. A. Pankhurst, I. P. Parkin and W. A. Steer 'Sodium borohydride reduction of aqueous iron-zirconium solutions: chemical routes to amorphous and nanocrystalline Fe-Zr-B alloys' J. Mat. Chem. 9, 2537-2544 (1999)

Fine particle Fe-Zr-B alloys have been prepared by sodium borohydride reduction of aqueous solutions of iron(II) and zirconium(IV) sulfate. A comprehensive survey of reaction conditions was completed, and optimum parameters found for the preparation of precipitated products. These included rapid stirring, controlled pH in the range 5-6, using degassed solvents, Fe:Zr ratios between 4:1 and 20:1, and sodium borohydride concentrations between 0.1 and 1.2M. Scanning electron microscopy showed that the products comprised spherical particles of 0.1 mm diameter, with metal compositions, determined by atomic absorption, electron microprobe and EDAX, which mirrored the initial starting solutions. The products fell into two categories: amorphous Fe-Zr-B alloys when the Fe:Zr ratio was near 9:1, and nanocrystalline alloys, comprising a small amount of nanometre scale a-Fe crystallites embedded in amorphous Fe-Zr-B alloy, when the Fe:Zr ratio was either higher or lower than 9:1. Minor impurity Fe(II) complex and Fe(III) oxide phases were also detected in the products. Full structural and magnetic characterisations were performed on selected samples, including X-ray diffraction, Mössbauer spectroscopy, differential scanning calorimetry, magnetometry and Zr-edge EXAFS measurements.

4. TALKS GIVEN, CONFERENCES ATTENDED, VISITS MADE, MEETINGS RUN, SESSIONS CHAIRED

(a) Herve Ness gave a seminar on 5 July 99 at "Centre de Recherche sur les Mecanismes de la Croissance Cristalline - CRMC2" - Marseille, on "Barrières à effet tunnel : effets du champ électrique sur le contraste STM, et transport inélastique." [Brings back fond memories - JLF spent three very enjoyable months there in 1976].

(b) Herve Ness also gave an oral presentation at the "Atomically Controlled Surfaces, Interfaces and Nanostructures" (ACSIN-5) conference in Aix-en-Provence (6-9 July 99) He talked on 'Quantum inelastic conductance though molecular wires'. He also crossed the Atlantic to gave a seminar on 'Quantum inelastic conductance through molecular wires' at the Lawrence Livermore National Laboratory (CA, USA), on 8 September 1999.

(c) Steve Shevlin and Louise Dash also attended the ACSIN-5 meeting. The conference involved a wide variety of people from all over the globe and was well attended. Steve gave a seminar on his work to that date (most of which he says is is now out of date) and also presented a paper (to be published in Thin Solid Films) As promised, he interrogated several of the experimentalists who were there and whose interpretations of experiment contradicted each other. The conclusion he came to was that both major groups were wrong and that there was a third interpretation which fitted all of the experimental data, the analysis of which is proving very fruitful. Also, he managed to get a minor suntan. [Mmmmm....echoes of 'The Third Way???}

(d) Raul gave an invited talk on 'Physics of Clays' at the Instituto de Fisica e Matematica, Universidade de Lisboa, Portugal on 23rd June 1999. He also presented a poster on 'Non-Linear Poisson-Boltzmann Theories for Swelling Clays' at the 4th Liquid Matter Conference of the EPS in Granada, Spain, 3th-7th July 1999.

(e) Louise Dash gave a talk on 'Electron electron interactions in atomic scale wires' to the basic metrology group at NPL on August 25th (an extension of the talk she gave at the graduate reviews.

(f) Michael Knott and Louise Dash attended the EPSRC/IOP Theory of Condensed Matter Summer School in Harlech, August 29th - September 10th, which as well as being very useful was great fun. Louise thoroughly recommends it to anyone thinking about going next year.

(g) Marshall Stoneham gave the opening plenary talk 'Atomic Processess in Passivation' at the 8th International Symposium on Passivity of Metals and Semiconductors in Canada in May.

(h) Marshall Stoneham also gave an invited talk 'Mesoscopic Modelling of Laser Ablation' at the 5th International Conference on Laser Ablation (COLA 99) in Göttingen.

(i) Ian Ford organised a one-day meeting at UCL on July 15th entitled 'Aerosol Research on the Climate'. There were 43 delegates from around the UK, who exchanged information about this important topic. Ian Ford also gave a talk at the meeting on his work with Charles Clement on bursts of nucleation in the atmosphere.

(j) Ian Ford also gave two posters at the European Aerosol Conference in Prague in September, and chaired an oral session on aerosol dynamics and a meeting of the Working Group on Fundamental Aerosol Properties.

(k) Keith McEwen participated in the Yamada Conference on Strongly Correlated Electron Systems (SCES'99) held in Nagano, Japan in August. Quadrupolar and orbital ordering and non-Fermi liquid behaviour were the hot topics. It is now clear that there are several mechanisms which can lead to the breakdown of normal Fermi liquid behaviour in metals. Keith McEwen presented the following 3 papers and also chaired the closing session. (i) Magnetic excitations and quadrupolar ordering in UNiSn (ii) Pressure dependent resistivity of U_0.45Y_0.55Pd₃ (iii) Neutron scattering study of CeTe₂.

(l) In September, Keith McEwen attended the European Conference on Neutron Scattering held in Budapest. He was Rapporteur for the Magnetism sessions, which involved attempting to review the highlights from the 200 posters presented in this field. He also chaired the EU-funded facilities session, and gave an overview of the new EU- Human Potential programme, which funds access of users to about 100 European research facilities.

(m) Also in September, Keith McEwen lectured on 'Neutron scattering from heavy fermion and non-Fermi liquid systems' at the 6th Nicolas Cabrera Summer School on Strongly Correlated Electron Systems, which took place near Madrid.

(n) John Finney gave an invited talk on the hydration of non-polar molecules in aqueous solution at an Adriatico Research Conference in Trieste, 28 June - 2 July 1999, and also chaired a session.

(o) In a very hot Granada, at the EPS 4th Liquid Matter conference which was attended by several of the group from 3 - 7 July, John Finney gave a talk on the structure of tertiary butanol in water. He also chaired a session on simple liquids and solutions. Shu Hayama and Jonathan Wasse presented a poster on the metal-amine systems, and also took the opportunity of reinforcing links with the NFL Studsvik neutron reactor source, Sweden, where they have been encouraged to perform neutron diffraction experiments on the SLAD instrument.

(p) Later in July, John Finney also gave a keynote lecture on Water, and also chaired a seesion on hydrogen bonding, at the IUPAC 26th International Conference on Solution Chemistry in a very wet Fukuoka, Japan.

(q) This year, the UK was host to the triennial congress of the International Union of Crystallography, which was attended by over 2,500 people in Glasgow in August. Alan Soper, Neal Skipper, and John Finney all gave invited talks on liquidy things, while Alan and John were co-organisers of sessions on Liquids and Disorder in Crystals respectively. John's collaborator Werner Kuhs from Goettingen gave a plenary lecture on ice, in which Colin Lobban's thesis work featured prominently.

(r) A very stimulating Bunsengesellschaft Discussion Meeting on Metastable Water in September was attended by John Finney In addition to chairing a session, he gave a talk on what happens to ices II and VI when pressurised at low temperature to above 50 kbar.

(s) John Harding was co-organiser of the CCP5 Annual General Meeting at which he also gave a talk. John also gave a talk at this meeting on mechanisms of dissolution in calcite. He is also an organiser of a workshop entitled 'Ions in Solids and Liquids and at Interfaces' to be held on Jan 5-6th 2000.

(t) Neal Skipper is chairing the organisation of a Liquids Network Workshop on Solvation, to be held in Liverpool, 5-7 January 2000. With the subject matter clearly overlapping with the CCP5 workshop mentioned just above, the date clash is not only to be regretted, but looks really embarrassing, especially as the organisers live in rooms only just down the corridor from each other. How can we avoid such clashes????

(u) Jonathan Allan and Eamonn Beirne attended the Oxford Summer School on Neutron Scattering in September 1999, with financial help from the Postgradute Training Committee (and the graduate school) The two weeks were packed full of techniques, problems, tutorials and talks, as well as a visit to ISIS, and were extremely useful for the both of us in different ways. We also had to give short talks. Eamonn spoke on 'Heavy-fermions and UCu₅', and Jon on 'X-ray diffraction studies of charge stripes', which was his 4th year project at Durham. The course was a great success, both academically and in meeting other users from all over the world and their work.

(v) On Andrew Fisher's Spetember trip to Australia he gave talks at Melbourne University, Monash University, and the AustralIan Ford National University. He also visited the University of New South Wales and Sydney University and gave a joint seminar for both. Andrew Fisher also went to Ottawa in June for a week (during the Quain chair talks - good timing!) to visit Bob Wolkow under their NRC/British Council project. He also went to a meeting at the RI in July on the future provision of supercomputing facilities in the UK.

(w) Martin Smalley gave a "scientific highlight" presentation at the Neutron and Muon User Meeting in Durham, on 9th Sept. The title was "Clay Colloids with Clothes". The topic was on the implications of the results for the theory of stability of lyophobic colloids.

(x) Jonathan Wasse attended and presented posters at both the 2nd European Conference on Neutron Scattering, Budapest, Hungary and the annual UK Neutron and Muon Users Meeting held in Durham in September. A main theme from the former was the news of new (proposed) neutron facilities that will be appearing in the next century. However, at Durham the infamous TICKET scheme raised its ugly head!! (see entries elsewhere....)

(y) Alex Shluger gave an invited talk at European Materials Research Society Spring Meeting in Strasbourg on "Simulating Scanning Force Microscopy on perfect and defective ionic surfaces", as well as an invited lecture at the International conference on "Radiation Effects in Insulators -10" July 18-23, Jena, Germany on "Radiation effects: What happens within first picoseconds and can we control that?"

(z) Adam Foster presented a paper on "Theoretical Modelling of Non-contact Atomic Force Microscopy" at the European Materials Research Society Spring Meeting in Strasbourg, and also one entitled "Role of Image Forces in Non-contact Atomic Force Microscopy" at the international conference on Non-contact Atomic Force Microscopy 99 in Pontresina (Switzerland)

(aa) Lev Kantorovich gave an invited talk on "Defects and metal clusters at oxide surfaces: theory and comparision with UPS and MIES" at the NATO Advanced Research Workshop, Jurmala, Latvia, August 23-25, 1999. He also presented a paper at the Faraday Discussion 114 "The surface science of metal oxides", Ambleside, UK, 1-3 September 1999 on "Mg clusters on MgO surfaces: Study of the nucleation mechanism with MIES and ab initio calculations".

(bb) Daniel Ucko visited Santander in June for ac susceptibility studies of his nanoscale granular alloys.

(cc) Quentin Pankhurst gave a presentation on a Mossbauer study of gold nanoparticles for use in automotive exhaust catalysts at the International Conference on the Applications of the Mossbauer Effect, in Garmisch-Partenkirchen, in early September 1999.

(dd) With Denise's expert help, Quentin Pankhurst organised and ran the 40th UK Mossbauer Discussion Group meeting at the University of Nottingham on September 13-14. They had 25 people there from the UK, US and continental Europe. Quentin Pankhurst gave a talk on chemical reduction synthesis of Fe-Zr-B (Andrew Fisher Steer's project), and showed posters on Fe-Cu-Ag granular alloys (Daniel Ucko's project), SHS ferrites (Louise Affleck's project) and gold nanoparticle catalysts.

5. WORK FOR OUTSIDE BODIES

(a) Marshall Stoneham has been appointed the UK member of the EU Committee to review the last 5 years of the EU Fusion Programme (which thus includes JET and also work done under EURATOM and the many Association projects in EU countries)

(b) Marshall Stoneham has been invited to be an Editorial Board member of COSMOS, the new science journal of the Singapore National Academy of Science.

(c) Marshall Stoneham has also been invited to become a member of the Warren Research Fund Joint Committee of the Royal Society.

(d) On June 14th, Ian Ford chaired a further meeting of the Programme Committee for the European Aerosol Conference, which is to be held in Dublin in September 2000. He has drafted the scientific sections of the second call for papers, due out in October.

(e) Keith McEwen was appointed by the EU's Joint Research Centre to be a member of the 4-person team to make a scientific audit of the Institute for Transuranium Elements, Karlsruhe, Germany. The audit was carried out in July and its purpose was to examine the Research Programme proposed by the Institute to be funded under the EU Framework V Programme, and to advise on its feasibility, scientific merit and resource implications.

(f) John Finney chaired - for the last time - the Liquids and Amorphous Materials Beam Selection Panel at ISIS in June. The last meeting of the Project Management Committee set up to oversee the construction of the new GEM instrument was held in September 1999.

(g) John Finney acted as External Examiner for the Chemical Physics Degree at the University of Kent at Canterbury.

(h) John Finney was elected to the Liquids Board of the European Physical Society. This means work in getting the next Liquid Matter Conference - to be held in Konstanz, Germany, in September 2002.

(i) Martin Smalley was external examiner for a master's thesis at King's College.

6. GRANT APPLICATIONS SUBMITTED

(a) Ian Ford and John Harding have submitted a grant application to PPARC entitled 'Models of the nucleation of cosmic dust'.

(b) Alex Shluger, together with Richard Catlow, have submitted a grant application on "Theory of electron and hole centres in crystalline and amorphous SiO₂ and Si₃N₄", to the EPSRC Physics Programme, requesting £145K over 3 years.

(c) Quentin Pankhurst applied to RAL for 3 days MuSR beamtime.

(d) Quentin Pankhurst submitted a proposal to the EPSRC Advanced Magnetics programme for MuSR studies of nanoscale granular alloys and thin films, asking for 18 days RAL tickets, 4 days DL tickets, a PhD studentship and a consumables and travel budget.

7. GRANTS AWARDED

(a) Marshall Stoneham and Andrew Fisher have won an EPSRC ROPA award (£85721) 'Theory of Quantum Coherence in Nanoscale Quantum Devices'.

(b) Tony Harker and Marshall Stoneham have won an EPSRC award in the Large Area Display Initiative. This gives them £117,000 for work on printable field emitter materials for displays.

(c) Marshall Stoneham and Ian Ford (with Juan Matthews, a UCL Honorary Fellow) have won a £20,000 grant from UKAEA Fusion for research into materials for fusion technology.

(d) Marshall Stoneham has been awarded £20,000 by Fujitsu European Centre for Information Technology, for a continuing programme on silicon oxidation studies.

(e) John Finney was awarded a travel grant of £1,500 to help fund a research visit to New Zealand, probably in October/November 2000. This is intended mainly to help develop further the collaboration with Roy Daniel at the University of Waikato on activity/dynamics relationships in enzymes.

(f) Funds were also offered to John Finney to enable him to spend 6 months at the ESRF, probably in late 2000 - early 2001.

(g) As mentioned in 1(e), John Finney was offered 'Director's Discretion' beam time at ISIS for measurements on t-butanol/salt solutions. He was also a co-applicant on a successful collaborative proposal with Wilson Poon in Edinburgh on water/methanol solutions.

(h) ILL beam on IN13 and IN16 was awarded to the enzyme dynamics collaboration involveng John Finney for further measurements on xylanase - the enzyme Valerie concentrated on in her time with us.

(i) Daniel Bowron and John Finney were awarded beam time at ESRF for (a) non-resonant Raman measurments on ices II and III (see 1(c) for initial results) and (b) Kr/water solutions to 1kbar.

(j) Tony Harker, with Nader Saffari (MEch Eng), has been awarded £190,000 by EPSRC for work on elastic waven propagation and scattering in composite materials.

(l) Alex Shluger has been awarded £53K over 2 years for "Laser-Materials Interaction: Theory and Experiment", by Pacific Northwest National Laboratory, USA.

(m) Quentin Pankhurst was awarded 3 days on MuSR at ISIS for muon studies of superparamagnetism in FeCuAg.

8. EXPECTED VISITORS

(a) Dr Juan Matthews has been made an Honorary Research Fellow (this is in relation to the contract with UKAEA Culham relating to materials for fusion reactors - see item 7(a) He will be working with Marshall Stoneham and Ian Ford.

(b) Prof. Toshio Yamaguchi from the University of Fukuoka, Japan, will spend six months in the group between about April and September 2000, working with John Finney

(c) Maxim Kuznetsov will be visiting us for six weeks from 1 November, continuing his work on combustion syntheses of ferrites. During this visit Maxim will be focussing on the garnet class of ferrites.

9. OTHER ITEMS THAT MIGHT BE OF INTEREST

(a) Andy Hemmings and Jonathan Wasse Allen join the group as EPSRC funded researfh students on 1 October 1999, working with Marshall Stoneham and Keith McEwen respectively.

(b) Raul was elected Member of the Institute of Physics (MInstP) and granted the title of Chartered Physicist (CPhys) in March 1999. Unfortunately for CMMP, he left the group towards the end of August 1999 to join the Asset Management Research group at Paribas in London on 6 September 1999.

(c) Channel 4 got hold of Mike Gillan and Dario's Nature paper on iron at the Earth's core, and did an item on it in the week it came out. More such publicity would be an excellent thing!

(d) At the European Aerosol Conference in Prague in September, Ian Ford received the Smoluchowski Prize for his recent work on developing and exploiting nucleation theorems. The award is given annually at this Conference to 'young' (under 40) researchers, by GAeF, the German Association for Aerosol Research. The prize is worth 3000DM, and Ian Ford was presented with a large cardboard cheque!

(e) The new ice XII phase continues to rumble around. Spies around Europe have reported pieces on the new phase on the Spanish El Pais web page in April 1999, and in Le Monde 9.4.99.

(f) Dr. James Titiloye will be joining us this October to do computer modelling of clay-fluid systems. He is taking over Raul's position in Neal Skipper's team.

(g) There has also been a lot of work getting the new HiPerSPACE facility up and running! It all seems to be working well now. Callum Wright started as the system manager for the new HiPerSPACE computer facility in August.

(h) Werner Hofer starts as a research fellow at the start of November, working on the electronic properties of molecules adsorbed in silicon with a view to possible device applications.

(i) Thornton Greenland will start as a research fellow 3 days a week with Marshall Stoneham and Andrew Fisher in December, working on their quantum coherence and computing project.

(j) In addition to the NUMBUM meeting in Durham, Martin Smalley also attended the ISIS muon user group meeting (14-Jun) and the ISIS large scale structures meeting (6-Jul) The impression he got from these three meetings was that ISIS faces only the winding-up operation unless the tickets system is soon abandoned.