Computational and Experimental Investigations of Osmium-Rich Borides Hf2MOs5B2 (M = Mn, Fe, Co): From Spin Glass to Room-Temperature Magnetic Behaviors.

The metal borides, Hf2MOs5B2 (M = Mn, Fe, Co), which are the first Os-rich quaternary variants of the prolific Ti3Co5B2 structure type, were investigated computationally and experimentally. In their crystal structures, osmium builds a network of prisms, in which the other elements are located. The magnetic M elements are found in face-connected Os8 square prisms leading to M-chains with intra- and interchain distances of about 3.0 and 6.5 Å, respectively. Density functional theory (DFT) showed that magnetic ordering is hugely favored for M = Mn and Fe but only slightly favored for M = Co. Experimental investigations then confirmed and extended the DFT predictions as a metamagnetic behavior was found for the M = Mn and Fe phases, whereby the antiferromagnetic interactions (TN = 19 and 90 K) found at low magnetic fields change to ferromagnetic at higher fields. A very broad transition (TN = 45 K) is found for M = Co, suggesting spin-glass behavior for this phase. For M = Fe, a hard-magnet hysteresis at 5 K is found with a 40 kA/m coercivity, and even at room temperature, a significant hysteresis is found. This study paves the way for the discovery of Os-based magnets in this structure type and other intermetallics.

Pesticides and Related Toxicants in the Atmosphere.

Pesticides and other toxicants released into the environment can contaminate air, water, soil, and biota. This review focuses on sources, exposures, fate, analysis, and trends. The potential for exposures due to atmospheric transport and deposition of pesticides and related contaminants may pose risks to humans and wildlife. Emissions of chemicals to air are related to physicochemical properties (e.g., vapor pressure and chemical stability). Experimental design and computer-based modeling, as related to emissions and dispersion of pesticides along transects downwind from release sources, will be discussed using the example of pesticide volatilization and drift in California agriculture that results in the transport and deposition downwind to the Sierra Nevada mountains, where much work has been done to refine exposure data for use in risk assessment and management. Predictably, those chemicals found frequently in air are those used most extensively, have multiple emission sources, and resist degradation. Yet to be determined are definitive connections with adverse impacts to humans and wildlife, although the accumulating evidence suggests that endocrine disrupting chemicals, ChE inhibitors, and others warrant further attention. Steps that are being taken to limit emissions, such as in pest control and fuel combustion, offer promising opportunities for improving the quality of air and of the overall environment. Chemical degradation rates and products from trace organics in the air merit more attention, as do the potential for activation by photooxidation and bioaccumulation in food chains. The potential effect of climate change, on atmospheric processes affecting contaminant behavior, is an area ripe for further study.