David Keen

Magnetoresistance measurements on the magnetic semiconductor Nd0.5Pb0.5MnO3

Abstract We present magnetoresistance data up to 20 T and magnetisation measurements on the mixed-valence cubic perovskite semiconductor Nd 0.5 Pb 0.5 MnO 3 , which demonstrate that the material orders ferromagnetically below 184 K, and that around and above this temperature the conductivity is dominated by hopping of localized magnetic polarons, with an activation energy ∼95 meV.

Colossal Positive and Negative Thermal Expansion in the Framework Material Ag3[Co(CN)6]

We show that silver(I) hexacyanocobaltate(III), Ag3[Co(CN)6], exhibits positive and negative thermal expansion an order of magnitude greater than that seen in other crystalline materials. This framework material expands along one set of directions at a rate comparable to the most weakly bound solids known. By flexing like lattice fencing, the framework couples this to a contraction along a perpendicular direction. This gives negative thermal expansion that is 14 times larger than in ZrW2O8. Density functional theory calculations quantify both the low energy associated with this flexibility and the role of argentophilic (Ag+...Ag+) interactions. This study illustrates how the mechanical properties of a van der Waals solid might be engineered into a rigid, useable framework.

Large negative linear compressibility of Ag3[Co(CN)6]

Silver(I) hexacyanocobaltate(III), Ag3[Co(CN)6], shows a large negative linear compressibility (NLC, linear expansion under hydrostatic pressure) at ambient temperature at all pressures up to our experimental limit of 7.65(2) GPa. This behavior is qualitatively unaffected by a transition at 0.19 GPa to a new phase Ag3[Co(CN)6]-II, whose structure is reported here. The high-pressure phase also shows anisotropic thermal expansion with large uniaxial negative thermal expansion (NTE, expansion on cooling). In both phases, the NLC/NTE effect arises as the rapid compression/contraction of layers of silver atoms—weakly bound via argentophilic interactions—is translated via flexing of the covalent network lattice into an expansion along a perpendicular direction. It is proposed that framework materials that contract along a specific direction on heating while expanding macroscopically will, in general, also expand along the same direction under hydrostatic pressure while contracting macroscopically.

A rational kind of madness

The paper argues against the notion that wars, whether international or civil in nature, are essentially irrational and “imposed” on societies. Rather they should be regarded as emerging from political and economic processes taking place within such societies—particularly when “peace” involves violent processes as the corollary of economic development and political consolidation. The paper examines the rational nature of warfare, turning to historical examples before the modern period. This framework is applied to the civil wars in Sudan and Sierra Leone, leading to a general proposition as to the connection between “economic war” and the weak state. The paper concludes that it is essential to look at the functions of war as well as its costs in order to bring it to an end, and to design appropriate diplomatic and economic pressures to bring this about.

War and peace: What's the difference?

At one level, the question posed in the title of this contribution can be quickly dispensed with: war is violent and peace is, well, peaceful; in other words, peace is the antithesis of war. This is certainly the common-sense view, and it is one usually reinforced by the media. Journalists, after all, are interested in change: theirs is a world of news (what is new), of events, discontinuities and drama. What could be more dramatic than the change from one thing into its opposite? Historians, by contrast, are often interested in continuities, and it is this approach that informs this essay. What do war and peace have in common? Answering this question is particularly important if we hope to understand transitions: the transition from peace to war and the transition from war to peace. Perhaps we can also take a cue here from the natural sciences: how can one thing change into another - a bulb into a plant, a liquid into a gas - unless it has already begun to resemble it?

The high-temperature superionic behaviour of Ag2S

Powder neutron diffraction and molecular dynamics (MD) simulations have been used to investigate the structural behaviour of silver sulfide, Ag2S, at elevated temperatures. Above ~450 K Ag2S adopts the β phase in which the S2- possess a body-centred cubic arrangement. Analysis of the neutron diffraction is in good agreement with the previously proposed structural model in which the Ag+ predominantly reside within the tetrahedral interstices. At ~865 K Ag2S transforms to the α phase in which the anion sublattice adopts a face-centred cubic arrangement. Structural refinements of this phase indicate that the cations are distributed predominantly in the tetrahedral cavities but with a significant fraction in the octahedral holes. MD simulations, using established potentials for this compound, confirm the stability of the two high-temperature superionic phases and show good agreement with the measured Ag+ distribution within the unit cell.

Liberalization and Conflict

Externally encouraged policies of liberalization in Sierra Leone in the 1970s and 1980s fed into civil war in the 1990s; yet such policies are now being revived. This article analyzes the impact of liberalization on the war in Sierra Leone, suggesting that it affected the conflict in four ways: first, by encouraging inflation, extreme devaluation, and private oligopolies; second, by reducing key state services such as education and health; third, by fueling corruption as real state salaries were cut; and fourth, by taking attention away from soldiers’ abuses under the military government of 1992–96, a government that was praised and rewarded for its liberalization agenda.

Structural disorder and loss of piezoelectric properties in α-quartz at high temperature

The piezoelectric properties of α-quartz-based resonators, characterized by the mechanical quality factor, Q, are found to degrade beginning above 300 °C. This is well below the transition at 573 °C to the β phase, which in principle limits the piezoelectric response of this material. This gradual loss of piezoelectric response can be linked to the increase in structural disorder in α-quartz found in total neutron scattering measurements. Analysis of these data by reverse Monte Carlo modeling indicates that between 200 and 400 °C, the local disorder in the instantaneous structure of α-quartz becomes comparable to that of β-quartz.

Amorphization of faujasite at high pressure: an X-ray diffraction and Raman spectroscopy study

In situ X-ray diffraction and Raman spectroscopy measurements of synthetic powdered samples of faujasite 13X were carried out at high pressure using diamond anvil cells. Structural changes are detected, linked to a progressive reduction in crystallinity, before complete amorphization of the material. Three distinct compressibility regions are clearly observed, delimited by two discontinuities in the pressure dependence of the faujasite volume around 2 and 3.5 GPa. The transition from the crystal to the amorphous state is incomplete and partially reversible below pressures of between 8 and 12 GPa, depending on the pressure-transmitting medium used. This partial recovery of the initial structure, at least on a local level, could be related to the presence of hydrated sodium ions in the faujasite framework. In addition, the position of the first sharp diffraction peak in the X-ray diffraction pattern of a fully amorphous sample recovered from 24 GPa is consistent with the presence of 4-membered rings of tetrahedra and the persistence of a significant number of larger rings as compared to a dried amorphous faujasite precursor.

Local structure in Ag3[Co(CN)6]: colossal thermal expansion, rigid unit modes and argentophilic interactions

Local structure in the colossal thermal expansion material Ag3[Co(CN)6] is studied here using a combination of neutron total scattering and reverse Monte Carlo (RMC) analysis. We show that the large thermal variations in cell dimensions occur with minimal distortion of the [Co(CN)6 ] coordination polyhedra, but involve significant flexing of the Co–CN–Ag–NC–Co linkages. We find real-space evidence in our RMC configurations for the importance of low-energy rigid unit modes (RUMs), particularly at temperatures below 150 K. Using a reciprocal-space analysis we present the phonon density of states at 300 K and show that the lowest-frequency region is dominated by RUMs and related modes. We also show that thermal variation in the energies of Ag...Ag interactions is evident in both the Ag partial pair distribution function and in the Ag partial phonon density of states. These findings are discussed in relation to the thermodynamic properties of the material.