The effects of human activity on the occupancy of Tamias striatus in the University of Michigan Biostation.

General EcologyThe occupancy of Tamias striatus, the eastern chipmunk, was measured in an area dominated by human structures as well as in a less-traversed deciduous forest at the University of Michigan Biostation on Douglas Lake. Habitat variables such as tree species composition, downed woody debris, above ground biomass, canopy coverage, leaf litter, and basal area were measured in fourteen locations at two chosen sites in order to test the assumption of no difference in forest conditions. It was found that chipmunk occupancy was higher in the area dominated by human structures while occupancy was lower in the less-traversed deciduous forest. However, as some aspects of the environment were significantly different, it was unclear whether the difference in occupancy was the result of physical environment or man-made structures.http://deepblue.lib.umich.edu/bitstream/2027.42/85747/1/Hacias_Miller_Park_2011.pd

Role of composition, bond covalency, and short-range order in the disordering of stannate pyrochlores by swift heavy ion irradiation

A2Sn2O7 (A=Nd,Sm,Gd,Er,Yb,and Y) materials with the pyrochlore structure were irradiated with 2.2 GeV Au ions to systematically investigate disordering of this system in response to dense electronic excitation. Structural modifications were characterized, over multiple length scales, by transmission electron microscopy, x-ray diffraction, and Raman spectroscopy. Transformations to amorphous and disordered phases were observed, with disordering dominating the structural response of materials with small A-site cation ionic radii. Both the disordered and amorphous phases were found to possess weberite-type local ordering, differing only in that the disordered phase exhibits a long-range, modulated arrangement of weberite-type structural units into an average defect-fluorite structure, while the amorphous phase remains fully aperiodic. Comparison with the behavior of titanate and zirconate pyrochlores showed minimal influence of the high covalency of the Sn-O bond on this phase behavior. An analytical model of damage accumulation was developed to account for simultaneous amorphization and recrystallization of the disordered phase during irradiation

Swift-heavy ion irradiation response and annealing behavior of A 2 TiO 5 (A = Nd, Gd, and Yb)

The structural responses of A2BO5 (A = Nd, Gd, and Yb; B = Ti) compositions irradiated by high-energy Au ions (2.2 GeV) were investigated using transmission electron microscopy, synchrotron X-ray diffraction and Raman spectroscopy. The extent of irradiation-induced amorphization depends on the size of the A-site cation, with smaller lanthanides having less susceptibility to the accumulation of radiation damage. In the track-overlapping regime, complete amorphization is observed in all three compounds, despite the ability of Yb2TiO5 to incorporate a great deal of structural disorder into its initial defect-fluorite structure (Fm-3m). This is attributed to the high cation radius ratio (A:B = 2:1), which reduces the stability of the structure upon ion irradiation. The fully-amorphized samples were subsequently isochronally heated at temperature intervals from 100 °C to 850 °C. X-ray diffraction analysis indicated a similar damage recovery process in Nd2TiO5 and Gd2TiO5, where both compositions recover their original structures (Pnma) at 850 °C. In contrast, Yb2TiO5 exhibited recrystallization of a metastable, non-equilibrium orthorhombic phase at ~ 550 °C, prior to a transformation to the stable defect-fluorite phase (Fm-3m) at 625 °C. These compositional variations in radiation tolerance and thermal recovery processes are described in terms of the energetics of disordering during the damage and recrystallization processes

Swift heavy ion irradiation-induced amorphization of La2Ti2O7La_2Ti_2O_7

Polycrystalline La2Ti2O7 powders have been irradiated with 2.0 GeV 181Ta ions up to a fluence of 1 × 1013 ions/cm2. Radiation-induced structural modifications were analyzed using synchrotron-based X-ray diffraction (XRD), small angle X-ray scattering (S

Mechanical and structural properties of radiation-damaged allanite-(Ce) and the effects of thermal annealing

The onset of thermally induced, heterogeneous structural reorganization of highly radiation-damaged allanite-(Ce) begins at temperatures below 700 K. Three strongly disordered allanite samples (S74 20414: ~ 0.55 wt% ThO2, 22.1 wt% REE oxides, and maximum radiation dose 3.5 × 1018 α-decay/g; LB-1: ~1.18 wt% ThO2, 19.4 wt% REE oxides, and maximum radiation dose 2.0 × 1019 α-decay/g; R1: ~ 1.6 wt% ThO2, 19.7 wt% REE oxides, and maximum radiation dose 2.6 × 1018 α-decay/g) were step-wise annealed to 1000 K in air. Using orientation-dependent nanoindentation, synchrotron single-crystal X-ray diffraction (synchrotron XRD), X-ray powder diffraction (powder XRD), differential scanning calorimetry and thermogravimetric analysis (DSC/TG), mass spectrometry (MS), 57Fe Mössbauer spectroscopy and high-resolution transmission electron microscopy (HRTEM), a comprehensive understanding of the structural processes involved in the annealing was obtained. As a result of the overall increasing structural order, a general increase of hardness (pristine samples: 8.2–9.3 GPa, after annealing at 1000 K: 10.2–12 GPa) and elastic modulus (pristine samples: 115–127 GPa, after annealing at 1000 K: 126–137 GPa) occurred. The initially heterogeneous recrystallization process is accompanied by oxidation of iron, the related loss of hydrogen and induced stress fields in the bulk material, which cause internal and surface cracking after stepwise annealing from 800 to 1000 K. HRTEM imaging of the pristine material shows preserved nanometer-sized crystalline domains embedded in the amorphous matrix, despite the high degree of structural damage. The results show that hardness and elastic modulus are sensitive indicators for the structural reorganization proces

Facile diamond synthesis from lower diamondoids

Carbon-based nanomaterials have exceptional properties that make them attractive for a variety of technological applications. Here, we report on the use of diamondoids (diamond-like, saturated hydrocarbons) as promising precursors for laser-induced high-pressure, high-temperature diamond synthesis. The lowest pressure and temperature (P-T) conditions that yielded diamond were 12 GPa (at ~2000 K) and 900 K (at ~20 GPa), respectively. This represents a substantially reduced transformation barrier compared with diamond synthesis from conventional (hydro)carbon allotropes, owing to the similarities in the structure and full sp3 hybridization of diamondoids and bulk diamond. At 20 GPa, diamondoid-to-diamond conversion occurs rapidly within <19 μs. Molecular dynamics simulations indicate that once dehydrogenated, the remaining diamondoid carbon cages reconstruct themselves into diamond-like structures at high P-T. This study is the first successful mapping of the P-T conditions and onset timing of the diamondoid-to-diamond conversion and elucidates the physical and chemical factors that facilitate diamond synthesis

A₂TiO₅ (A = Dy, Gd, Er, Yb) at High Pressure

The structural evolution of lanthanide A₂TiO₅ (A = Dy, Gd, Yb, Er) at high pressure is investigated using synchrotron X-ray diffraction. The effects of A-site cation size and of the initial structure are systematically examined by varying the composition of the isostructural lanthanide titanates and the structure of dysprosium titanate polymorphs (orthorhombic, hexagonal, and cubic), respectively. All samples undergo irreversible high-pressure phase transformations, but with different onset pressures depending on the initial structure. While each individual phase exhibits different phase transformation histories, all samples commonly experience a sluggish transformation to a defect cotunnite-like (<i>Pnma</i>) phase for a certain pressure range. Orthorhombic Dy₂TiO₅ and Gd₂TiO₅ form <i>P</i>2₁<i>am</i> at pressures below 9 GPa and <i>Pnma</i> above 13 GPa. Pyrochlore-type Dy₂TiO₅ and Er₂TiO₅ as well as defect-fluorite-type Yb₂TiO₅ form <i>Pnma</i> at ∼21 GPa, followed by <i>Im</i>3<i>̅m</i>. Hexagonal Dy₂TiO₅ forms <i>Pnma</i> directly, although a small amount of remnants of hexagonal Dy₂TiO₅ is observed even at the highest pressure (∼55 GPa) reached, indicating kinetic limitations in the hexagonal Dy₂TiO₅ phase transformations at high pressure. Decompression of these materials leads to different metastable phases. Most interestingly, a high-pressure cubic X-type phase (<i>Im</i>3<i>̅m</i>) is confirmed using high-resolution transmission electron microscopy on recovered pyrochlore-type Er₂TiO₅. The kinetic constraints on this metastable phase yield a mixture of both the X-type phase and amorphous domains upon pressure release. This is the first observation of an X-type phase for an A₂BO₅ composition at high pressure