164 research outputs found
Spin-driven Phase Transitions in ZnCrSe and ZnCrS Probed by High Resolution Synchrotron X-ray and Neutron Powder Diffraction
The crystal and magnetic structures of the spinel compounds ZnCrS and
ZnCrSe were investigated by high resolution powder synchrotron and
neutron diffraction. ZnCrSe exhibits a first order phase transition at
K into an incommensurate helical magnetic structure. Magnetic
fluctuations above are coupled to the crystal lattice as manifested by
negative thermal expansion. Both, the complex magnetic structure and the
anomalous structural behavior can be related to magnetic frustration.
Application of an external magnetic field shifts the ordering temperature and
the regime of negative thermal expansion towards lower temperatures. Thereby,
the spin ordering changes into a conical structure. ZnCrS shows two
magnetic transitions at K and K that are accompanied by
structural phase transitions. The crystal structure transforms from the cubic
spinel-type (space group \={3}) at high temperatures in the paramagnetic
state, via a tetragonally distorted intermediate phase (space group /
) for into a low temperature orthorhombic phase
(space group ) for . The cooperative displacement of
sulfur ions by exchange striction is the origin of these structural phase
transitions. The low temperature structure of ZnCrS is identical to the
orthorhombic structure of magnetite below the Verwey transition. When applying
a magnetic field of 5 T the system shows an induced negative thermal expansion
in the intermediate magnetic phase as observed in ZnCrSe.Comment: 11 pages, 13 figures, to be published in PR
Recommended from our members
Effects of hydrogen absorption in TbNiAl and UNiAl
Although hydrides of intermetallic compounds are used extensively as hydrogen-storage media, little is known about the exact nature of metal-hydrogen interactions. However, this knowledge is of essential importance for the understanding of thermodynamics and other properties. Hydrides (deuterides) of TbNiAl and UNiAl have been widely studied because of drastic increase of magnetic ordering temperature under hydrogenation. Here the authors report neutron-diffraction results of the three deuterides, TbNiAlD{sub 1.28}, TbNiAlD{sub 0.8}a nd UNiAlD{sub 2.23}
Lattice anisotropy in uranium ternary compounds: UTX
Several U-based intermetallic compounds (UCoGe, UNiGe with the TiNiSi structure type and UNiAl with the ZrNiAl structure type) and their hydrides were studied from the point of view of compressibility and thermal expansion. Confronted with existing data for the compounds with the ZrNiAl structure type a common pattern emerges. The direction of the U-U bonds with participation of the 5f states is distinctly the "soft" crystallographic direction, exhibiting also the highest coefficient of linear thermal expansion. The finding leads to an apparent paradox: the closer the U atoms are together in a particular direction the better they can be additionally compressed together by applied hydrostatic pressure. (C) 2012 Elsevier B. V. All rights reserved
Glass Transition in the Polaron Dynamics of CMR Manganites
Neutron scattering measurements on a bilayer manganite near optimal doping
show that the short-range polarons correlations are completely dynamic at high
T, but then freeze upon cooling to a temperature T* 310 K. This glass
transition suggests that the paramagnetic/insulating state arises from an
inherent orbital frustration that inhibits the formation of a long range
orbital- and charge-ordered state. Upon further cooling into the
ferromagnetic-metallic state (Tc=114 K), where the polarons melt, the diffuse
scattering quickly develops into a propagating, transverse optic phonon.Comment: 4 pages, 4 figures. Physical Review Letters (in Press
Charge Ordering and Phase Competition in the Layered Perovskite Lasr2mn2o7
Charge-lattice fluctuations are observed in the layered perovskite manganite
LaSr2Mn2O7 by Raman spectroscopy as high as 340 K and with decreasing
temperature they become static and form a charge ordered (CO) phase below
TCO=210 K. In the static regime, superlattice reflections are observed through
neutron and x-ray diffraction with a propagation vector (h+1/4,k-1/4,l).
Crystallographic analysis of the CO state demonstrates that the degree of
charge and orbital ordering in this manganite is weaker than the charge
ordering in three dimensional perovskite manganites. A TN=170K a type-A
antiferromagnetism (AF) develops and competes with the charge ordering, that
eventually melts below T*=100K. High resolution diffraction measurements
suggest that that CO- and AF-states do not coincide within the same region in
the material but rather co-exist as separate phases. The transition to type-A
antiferromagnetism at lower temperatures is characterized by the competition
between these two phases.Comment: 9 pages, 6 figure
Cs2NaAl1-xCrxF6: A family of compounds presenting magnetocaloric effect
FUNDAÇÃO CARLOS CHAGAS FILHO DE AMPARO À PESQUISA DO ESTADO DO RIO DE JANEIRO - FAPERJFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFINANCIADORA DE ESTUDOS E PROJETOS - FINEPIn this paper we explore the magnetocaloric effect (MCE) of chromium-doped elpasolite Cs2NaAl1-x,CrxF6 (x = 0.01 and 0.62) single crystals. Magnetization and heat capacity data show the magnetocaloric potentials to be comparable to those of garnets, perovskites, and other fluorides, producing magnetic entropy changes of 0.5 J/kg K (x = 0.01) and 11 J/kg K (x = 0.62), and corresponding adiabatic temperature changes of 4 and 8 K, respectively. These values are for a magnetic field change of 50 kOe at a temperature around 3 K. A clear Schottky anomaly below 10 K, which becomes more apparent when an external magnetic field is applied, was observed and related to the splitting of the Cr3+ energy levels. These results hint at a new family of materials with potential wide use in cryorefrigeration.90616FUNDAÇÃO CARLOS CHAGAS FILHO DE AMPARO À PESQUISA DO ESTADO DO RIO DE JANEIRO - FAPERJFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFINANCIADORA DE ESTUDOS E PROJETOS - FINEPFUNDAÇÃO CARLOS CHAGAS FILHO DE AMPARO À PESQUISA DO ESTADO DO RIO DE JANEIRO - FAPERJFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFINANCIADORA DE ESTUDOS E PROJETOS - FINEPSem informaçãoSem informaçãoSem informaçãoSem informaçãoSem informaçãoWe thank Pedro von Ranke (UERJ, Brazil) and Walter Kalceff (UTS, Australia) for fruitful discussions. J.C.G.T.’s participation in this work was financed by the Science without Borders Program. Access to CICECO/Chemistry Department (Aveiro, Portugal), GPMR-UNICAMP (Campinas, Brazil), LMBT- UFF (Niter´oi, Brazil), LBT-UFRJ (Rio de Janeiro, Brazil), BERII facilities, and LaMMB MagLab (Berlin, Germany) are gratefully acknowledged by all authors. Financial support was provided by Proppi/UFF, FAPERJ, FAPESP, CAPES, CNPq, and FINEP
Effect of pulsed light on postharvest disease control-related metabolomic variation in melon (Cucumis melo) artificially inoculated with Fusarium pallidoroseum.
Pulsed light, as a postharvest technology, is an alternative to traditional fungicides, and can be used on a wide variety of fruit and vegetables for sanitization or pathogen control. In addition to these applications, other effects also are detected in vegetal cells, including changes in metabolism and secondary metabolite production, which directly affect disease control response mechanisms. This study aimed to evaluate pulsed ultraviolet light in controlling postharvest rot, caused by Fusarium pallidoroseum in 'Spanish' melon, in natura, and its implications in disease control as a function of metabolomic variation to fungicidal or fungistatic effects. The dose of pulsed light (PL) that inhibited F. pallidoroseum growth in melons (Cucumis melo var. Spanish) was 9 KJ m-2. Ultra-performance liquid chromatography (UPLC) coupled to a quadrupole-time-of-flight (QTOF) mass analyzer identified 12 compounds based on tandem mass spectrometry (MS/MS) fragmentation patterns. Chemometric analysis by Principal Components Analysis (PCA) and Orthogonal Partial Least Squared Discriminant Analysis (OPLS-DA) and corresponding S-Plot were used to evaluate the changes in fruit metabolism. PL technology provided protection against postharvest disease in melons, directly inhibiting the growth of F. pallidoroseum through the upregulation of specific fruit biomarkers such as pipecolic acid (11), saponarin (7), and orientin (3), which acted as major markers for the defense system against pathogens. PL can thus be proposed as a postharvest technology to prevent chemical fungicides and may be applied to reduce the decay of melon quality during its export and storage
Lattice Displacements Above Tc in the Layered Manganite La1.2Sr1.8Mn2O7
Neutron diffraction data presented in this paper demonstrates the relevance
of lattice displacement above TC, in our understanding of the evolution of the
crystal structure with temperature in the layered CMR manganite
La1.2Sr1.8Mn2O7. The anomalous temperature behavior of thermal diffuse
scattering (TDS) in La1.2Sr1.8Mn2O7 strongly suggests that it arises from
lattice displacements and correlates directly with anomalies in the
displacement parameters of the O- and Mn-atoms and Mn-O bond lengths. From our
measurements, the insulator - metal transition can be described as a transition
from a high temperature state with disordered Mn-O bond lengths to a low
temperature state with a more uniform distribution on Mn-O bonds. These
observations are in agreement with polaronic charge transport above TC in the
perovskite manganites; as electron hopping is responsible for bond disorder
above TC, below the transition where eg carries are delocalized, any lattice
displacements are uniformly averaged.Comment: 5 pages, 3 figures, submitted to Phys. Rev.
Busca por fontes de resistência a doenças de importância na cultura do cajueiro com base no comportamento de progênies.
bitstream/item/163513/1/BPD17020.pd
- …