Antiferromagnetism in Cr3Al and relation to semiconducting behavior

Antiferromagnetism and chemical ordering have both been previously suggested as causes of the observed semiconductorlike behavior in Cr3Al. Two films of Cr3Al(001)/MgO(001) were grown under different conditions to achieve different types of chemical ordering and electronic properties: one X-phase structure (semiconducting) and one C11b structure (metallic). The films were investigated by x-ray and neutron diffraction. Both films show commensurate antiferromagnetic order, with a high Néel temperature greater than 578 K, showing that the antiferromagnetism in Cr3Al is quite robust. Density-functional theory calculations were performed and it was shown that the well-known antiferromagnetic pseudogap in the density of states occurs for all types of chemical ordering considered. The conclusion of these studies is that the antiferromagnetism causes a pseudogap in the density of states, which is a necessary condition for the semiconductorlike transport behavior; however, that antiferromagnetism is seen in both metallic and semiconducting Cr3Al samples shows that antiferromagnetism is not a sufficient condition for semiconducting behavior. Chemical ordering is equally important

Two Forms of Ice Identified in Mars-like Clay Using Neutron Spectroscopy

The capacity of clay minerals to store large amounts of water is utilized in a number of industrial and environmental applications on Earth, for example, as components in geosynthetic clay liners in landfills or ingredients in water-based drilling fluids, and could prove important on Mars to identify future human landing sites where water could be harvested. The subzero behavior of water interacting within the interlayer space of clay minerals is of particular interest in most applications but remains poorly understood. To better understand the hydrothermal mechanism by which water ice bonds and separates from clay interlayers, we have utilized neutron spectroscopy, spectral analysis, and phonon band assignment. The inelastic neutron scattering from sodium montmorillonite, hydrated at 24, 73, and 166% water content, as well as an oven-dried sample, were measured to assess the vibrational density of states. The water contents studied provide a range of pore dimensions within clay gels that have varying degrees of confinement. The type of ice formed from water held in larger intra- and interparticle pores differs substantially from that confined within the interlayer (pseudo-two-layer hydrate), and the differences vary with hydration level. Spectral subtraction over an energy transfer range 50 E –1 (8 E < 70 meV) produces clearly two different forms of ice: hexagonal and cubic in the two wetter samples. A form of interfacial ice, presumably of a lower density, is observed in the vibrational density of states spectrum of the sample hydrated to a pseudo-two-layer hydrate (ie 24% gravimetric water content (GWC), 10 H2O/Na+). No hexagonal or cubic ice is observed in this sample. The four vibrational modes within the translation band of hexagonal ice are apparent within the sample hydrated to 166% gravimetric water content, in which pores greater than 20 nm are largely water-filled. By considering hydrogen bonding of the water to the clay surface, our data indicate an increase in the strength of the H-bond due to a shorter distance to the hydroxyl. We attribute this decrease to the pores in the clay generating a localized negative pressure or “suction” effect, thus attracting the water