199 research outputs found
Phase stability and structural temperature dependence in sodium niobate: A high resolution powder neutron diffraction study
We report investigation of structural phase transitions in technologically
important material sodium niobate as a function of temperature on heating over
300-1075 K. Our high resolution powder neutron diffraction data show variety of
structural phase transitions ranging from non-polar antiferrodistortive to
ferroelectric and antiferroelectric in nature. Discontinuous jump in lattice
parameters is found only at 633 K that indicates that the transition of
orthorhombic antiferroelectric P (space group Pbcm) to R (space group Pbnm)
phase is first order in nature, while other successive phase transitions are of
second order. New superlattice reflections appear at 680 K (R phase) and 770 K
(S phase) that could be indexed using an intermediate long-period modulated
orthorhombic structure whose lattice parameter along direction is 3 and 6
times that of the CaTiO3-like Pbnm structure respectively. The correlation of
superlattice reflections with the phonon instability is discussed. The critical
exponent ({\beta}) for the second order tetragonal to cubic phase transition at
950 K, corresponds to a value {\beta}, as obtained from the
temperature variation of order parameters (tilt angle and intensity of
superlattice reflections). It is argued that this exponent is due to a second
order phase transition close to a tricritical point. Based on our detailed
temperature dependent neutron diffraction studies, the phase diagram of sodium
niobate is presented that resolves existing ambiguities in the literature.Comment: 21 Pages, 8 Figure
Tillage impact on soil erosion by water: Discrepancies due to climate and soil characteristics
No-tillage (NT) is promoted for soil and water conservation, but research findings on overland flow and soil erosion are inconsistent across different ecosystems, with some studies showing no benefits of NT over conventional tillage (CT). A global literature review was conducted to quantify the impact of NT on water runoff, sediment concentration and soil losses. The objective was to identify the underlying causes of the variability in the performance of NT across different environments. Data from 282 paired NT and CT runoff plots from 41 research studies worldwide were analysed using meta-analysis and principal component analysis (PCA). Sediment concentration and soil losses were 56 and 60% lower under NT than CT, respectively. These tended to be greater under CT than NT on long plots (90% for sediment concentration and 94% for soil losses) and steepest slopes (79 and 77%, respectively). Greater differences in sediment concentration and soil losses between NT and CT were observed in low clay soils and under temperate climates. While on average there were no differences on runoff coefficient, NT decreased runoff coefficient by about 40% compared to CT in mulched soils, under cool climate (5 years. Overall, the results indicated that NT has greater potential to reduce runoff and soil losses in temperate regions where soils of peri-glacial influence are relatively young, moderately weathered and fragile compared to the heavily weathered clayey tropical soils that are well aggregated and less erodible. The results of this study are expected to inform scientists, practitioners and policy makers on the links between land management and soil functioning processes. Policy makers and development implementers will be able to make informed choices of land management techniques for effective NT implementation, for instance by having more mulch input under warm climates
The atomic structure of large-angle grain boundaries and in and their transport properties
We present the results of a computer simulation of the atomic structures of
large-angle symmetrical tilt grain boundaries (GBs) (misorientation
angles \q{36.87}{^{\circ}} and \q{53.13}{^{\circ}}),
(misorientation angles \q{22.62}{^{\circ}} and \q{67.38}{^{\circ}}). The
critical strain level criterion (phenomenological criterion)
of Chisholm and Pennycook is applied to the computer simulation data to
estimate the thickness of the nonsuperconducting layer enveloping
the grain boundaries. The is estimated also by a bond-valence-sum
analysis. We propose that the phenomenological criterion is caused by the
change of the bond lengths and valence of atoms in the GB structure on the
atomic level. The macro- and micro- approaches become consistent if the
is greater than in earlier papers. It is predicted that the
symmetrical tilt GB \theta = \q{53.13}{^{\circ}} should demonstrate
a largest critical current across the boundary.Comment: 10 pages, 2 figure
Synchrotron X-ray Diffraction Study of BaFe2As2 and CaFe2As2 at High Pressures up to 56 GPa: Ambient and Low-Temperatures Down to 33 K
We report high pressure powder synchrotron x-ray diffraction studies on
MFe2As2 (M=Ba, Ca) over a range of temperatures and pressures up to about 56
GPa using a membrane diamond anvil cell. A phase transition to a collapsed
tetragonal phase is observed in both compounds upon compression. However, at
300 (33) K in the Ba-compound the transition occurs at 26 (29) GPa, which is a
much higher pressure than 1.7 (0.3) GPa at 300 (40) K in the Ca-compound, due
to its larger volume. It is important to note that the transition in both
compounds occurs when they are compressed to almost the same value of the unit
cell volume and attain similar ct/at ratios. We also show that the FeAs4
tetrahedra are much less compressible and more distorted in the collapsed
tetragonal phase than their nearly regular shape in the ambient pressure phase.
We present a detailed analysis of the pressure dependence of the structures as
well as equation of states in these important BaFe2As2 and CaFe2As2 compounds.Comment: 26 pages, 12 figure
No-tillage lessens soil CO2 emissions the most under arid and sandy soil conditions: results from a meta-analysis
The management of agroecosystems plays a crucial
role in the global carbon cycle with soil tillage leading
to known organic carbon redistributions within soils and
changes in soil CO2 emissions. Yet, discrepancies exist on
the impact of tillage on soil CO2 emissions and on the
main soil and environmental controls. A meta-analysis was
conducted using 46 peer-reviewed publications totaling 174
paired observations comparing CO2 emissions over entire
seasons or years from tilled and untilled soils across different
climates, crop types and soil conditions with the objective
of quantifying tillage impact on CO2 emissions and assessing
the main controls. On average, tilled soils emitted 21%
more CO2 than untilled soils, which corresponded to a significant
difference at P<0:05. The difference increased to 29%
in sandy soils from arid climates with low soil organic carbon
content (SOCC < 1 %) and low soil moisture, but tillage
had no impact on CO2 fluxes in clayey soils with high background
SOCC (> 3 %). Finally, nitrogen fertilization and crop
residue management had little effect on the CO2 responses
of soils to no-tillage. These results suggest no-tillage is an
effective mitigation measure of carbon dioxide losses from
dry land soils. They emphasize the importance of including
information on soil factors such as texture, aggregate stability
and organic carbon content in global models of the carbon
cycle
Temperature dependence of the EPR linewidth of Yb3+ - ions in Y0.99Yb0.01Ba2Cu3OX compounds: Evidence for an anomaly near TC
Electron paramagnetic resonance experiments on doped Yb3+ ions in YBaCuO
compounds with different oxygen contents have been made. We have observed the
strong temperature dependence of the EPR linewidth in the all investigated
samples caused by the Raman processes of spin-lattice relaxation. The
spin-lattice relaxation rate anomaly revealed near TC in the superconducting
species can be assigned to the phonon density spectrum changesComment: 10 pages, 4 figures Renewed versio
GROWTH OF INVASIVE AQUATIC MACROPHYTES OVER TAPI RIVER
Aquatic macrophytes are important elements of freshwater ecosystems, fulfilling a pivotal role in the ecological functions of these environments and biogeochemical cycles. Although aquatic macrophytes are beneficial, some species can hinder human activity. They can clog reservoirs and reduce water availability for human needs. Surveys of macrophytes are hindered by logistic problems, and remote sensing represents a powerful alternative, allowing comprehensive assessment and monitoring. The objectives of this study was to map temporal changes in the macrophytes using time series multispectral dataset over Tapi River, Surat. The field trip was conducted over the Tapi River on 22nd June 2018, where in-situ spectral response dataset were acquired using ASD Spectroradiometer. Water samples were also collected over three locations, one before entering the city (Kamrej), second at the Sarthana water treatment plant and third at the outer end (causeway). The nutrient concentration was less before entering the city (Ammonical Nitrogen 0.056 mg/L and phosphate 0.0145 mg/l), while higher concentration (Ammonical Nitrogen 0.448 mg/l and phosphate 0.05 mg/l) was observed within the city. Maps of aquatic macrophytes fractional cover were produced using Resourcesat-2/2A (LISS-III) dataset covering a period of 2012–2018. Maximum extent was observed in February-March of every year. Although during monsoon, lot of agriculture run-off and nutrients will come into the river, but main flow of water will dilute its concentration. During summer, the same nutrient concentration will boost these macrophytes due to less availability of stream water. Within the area of 16 km2 between Kamrej and causeway, 3.35 % was covered by macrophytes during March 2013. This area coverage increase to 36.41 % in March 2018. Based on these maps, we discuss how remote sensing could support monitoring strategies and provide insight into spatial variability, and by identifying hotspot areas where invasive species could become a threat to ecosystem functioning
Pressure dependence of the low- temperature crystal structure and phase transition behaviour of CaFeAsF and SrFeAsF: A synchrotron x-ray diffraction study
We report systematic investigation of high pressure crystal structures and
structural phase transition upto 46 GPa in CaFeAsF and 40 GPa in SrFeAsF at 40
K using powder synchrotron x-ray diffraction experiments and Rietveld analysis
of the diffraction data. We find that CaFeAsF undergoes orthorhombic to
monoclinic phase transition at Pc = 13.7 GPa while increasing pressure. SrFeAsF
exhibits coexistence of orthorhombic and monoclinic phases over a large
pressure range from 9 to 39 GPa. The coexistence of the two phases indicates
that the transition is of first order in nature. Unlike in the 122 compounds
(BaFe2As2 & CaFe2As2) we do not find any collapse tetragonal transition. The
transition to a lower symmetry phase (orthorhombic to monoclinic) in 1111
compounds under pressure is in contrast with the transition to a high symmetry
phase (orthorhombic to tetragonal) in 122 type compounds. On heating from 40 K
at high pressure, CaFeAsF undergoes monoclinic to tetragonal phase transition
around 25 GPa and 200 K. Further, it does not show any post-tetragonal phase
transition and remains in the tetragonal phase upto 25 GPa at 300 K. The dPc/dT
is found to be positive for the CaFeAsF & CaFe2As2, however the same was not
found in case of BaFe2As2. We discuss observations of structural evolution in
the context of superconductivity in these and other Fe-based compounds. It
appears that the closeness of the Fe-As-Fe bond angle to its ideal tetrahedral
value of 109.470 might be associated with occurrence of superconductivity at
low temperature.Comment: 23 pages, 11 Figure
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