687 research outputs found
High temperature behavior of Sr-doped layered cobaltites Y(Ba1-xSrx)Co2O5.5: phase stability and structural properties
In this article we present a neutron diffraction in-situ study of the thermal
evolution and high-temperature structure of layered cobaltites Y(Ba, Sr)Co2
O5+{\delta}. Neutron thermodiffractograms and magnetic susceptibility
measurements are reported in the temperature range 20 K <= T <= 570 K, as well
as high resolution neutron diffraction experiments at selected temperatures.
Starting from the as-synthesized samples with {\delta} ~ 0.5, we show that the
room temperature phases remain stable up to 550 K, where they start loosing
oxygen and transform to a vacancy-disordered "112" structure with tetragonal
symmetry. Our results also show how the so-called "122" structure can be
stabilized at high temperature (around 450 K) in a sample in which the addition
of Sr at the Ba site had suppressed its formation. In addition, we present the
structural and magnetic properties of the resulting samples with a new oxygen
content {\delta} ~ 0.25 in the temperature range 20 K <= T <= 300 K
Probing phase coexistence and stabilization of the spin-ordered ferrimagnetic state by Calcium addition in the YBa_{1-x}Ca_{x}Co_{2}O_{5.5} layered cobaltites using neutron diffraction
In this article we study the effects of a partial substitution of Ba with the
smaller cation Ca in the layered cobaltites YBaCo_2O_{5+\delta} for \delta
\approx 0.5. Neutron thermodiffractograms are reported for the compounds
YBa_{0.95}Ca_{0.05}Co_2O_{5.5} (x_{Ca}=0.05) and YBa_{0.90}Ca_{0.10}Co_2O_{5.5}
(x_{Ca}=0.10) in the temperature range 20 K \leq T \leq 300 K, as well as high
resolution neutron diffraction experiments at selected temperatures for the
samples x_{Ca}=0.05, x_{Ca}=0.10 and the parent compound x_{Ca}=0. We have
found the magnetic properties to be strongly affected by the cationic
substitution. Although the "122" perovskite structure seems unaffected by Ca
addition, the magnetic arrangements of Co ions are drastically modified: the
antiferromagnetic (AFM) long-range order is destroyed, and a ferrimagnetic
phase with spin state order is stabilized below T \sim 290 K. For the sample
with x_{Ca}=0.05 a fraction of AFM phase coexists with the ferrimagnetic one
below T \sim 190 K, whereas for x_{Ca}=0.10 the AFM order is completely lost.
The systematic refinement of the whole series has allowed for a better
understanding of the observed low-temperature diffraction patterns of the
parent compound, YBaCo_2O_{5.5}, which had not yet been clarified. A two-phase
scenario is proposed for the x_{Ca}=0 compound which is compatible with the
phase coexistence observed in the x_{Ca}=0.05 sample
New four-dimensional integrals by Mellin-Barnes transform
This paper is devoted to the calculation by Mellin-Barnes transform of a
especial class of integrals. It contains double integrals in the position space
in d = 4-2e dimensions, where e is parameter of dimensional regularization.
These integrals contribute to the effective action of the N = 4 supersymmetric
Yang-Mills theory. The integrand is a fraction in which the numerator is a
logarithm of ratio of spacetime intervals, and the denominator is the product
of powers of spacetime intervals. According to the method developed in the
previous papers, in order to make use of the uniqueness technique for one of
two integrations, we shift exponents in powers in the denominator of integrands
by some multiples of e. As the next step, the second integration in the
position space is done by Mellin-Barnes transform. For normalizing procedure,
we reproduce first the known result obtained earlier by Gegenbauer polynomial
technique. Then, we make another shift of exponents in powers in the
denominator to create the logarithm in the numerator as the derivative with
respect to the shift parameter delta. We show that the technique of work with
the contour of the integral modified in this way by using Mellin-Barnes
transform repeats the technique of work with the contour of the integral
without such a modification. In particular, all the operations with a shift of
contour of integration over complex variables of two-fold Mellin-Barnes
transform are the same as before the delta modification of indices, and even
the poles of residues coincide. This confirms the observation made in the
previous papers that in the position space all the Green function of N = 4
supersymmetric Yang-Mills theory can be expressed in terms of UD functions.Comment: Talk at El Congreso de Matematica Capricornio, COMCA 2009,
Antofagasta, Chile and at DMFA seminar, UCSC, Concepcion, Chile, 24 pages;
revised version, Introduction is modified, Conclusion is added, five
Appendices are added, Appendix E is ne
Neutron scattering at high temperature and levitation techniques
Studies of the liquid state present an obvious fundamental interest and are also important for technological applications since the molten state is an essential stage in various industrial processes (e.g. glass making, single crystal growing, iron and steel making). Most of the physical properties of a high-temperature liquid are related to its atomic structure. Thus it is important to develop devices to probe the local environment of the atoms in the sample. At very high temperature, it is difficult to use conventional furnaces, which present several problems. In particular, physical contact with the container can contaminate the sample and/or modify its structural properties. Such problems encouraged the development of containerless techniques, which are powerful tools to study high-temperature melts. By eliminating completely any contact between sample and container, it is possible to study the sample with a very high degree of control and to access very high temperatures. An additional advantage of levitation methods is that it is possible to supercool hot liquids down to several hundred of degrees below their equilibrium freezing point, since heterogeneous nucleation processes are suppressed
Comparison of short-range-order in liquid- and rotator-phase states of a simple molecular liquid: A reverse Monte Carlo and molecular dynamics analysis of neutron diffraction data
6 págs.; 6 figs.; 1 tab. ; PACS number s : 61.25.Em, 61.20. p, 61.43. j, 61.12. qThe short-range order (SRO) correlations in liquid- and rotator-phase states of carbon tetrachloride are revisited here. The correlation of some angular magnitudes is used to evaluate the positional and orientational correlations in the liquid as well as in the rotator phase. The results show significant similitudes in the relative position of the molecules surrounding a central one but striking differences in their relative orientations, which could explain the changes in SRO between the two phases and the puzzling behavior of the local density in the liquid and rotator phases. © 2007 The American Physical Society.We would also like to acknowledge “Generalitat de Catalunya” Project No. 2005SGR-
00535 and MEC Project No. FIS2005-00975 for partial support of this work.Peer Reviewe
Microscopic structure of Fe-Ni and Fe-Ni-S molten alloys of geophysical interest
9 págs.; 9 figs.; 2 tabs. ; PACS number(s): 61.25.Mv, 62.60.1v, 96.35.2jThe static S(Q) liquid structure factors for binary FexNi 1-x, x=0.90, 0.85, and ternary Fe0.85Ni 0.05S0.15 molten alloys are investigated by means of the concurrent use of neutron diffraction and reverse Monte Carlo simulations. The measured G(r) radial distributions reveal atomic orderings varying from that present in the Fe0.90Ni0.10 alloy, reminiscent of the ideal bcc structure of solid Fe, to a far more open structure found in Fe 0.85Ni0.05S0.15. From data at hand no clear evidence of the clustering of sulfur within the Fe-Ni matrix is found. In contrast, the addition of sulfur leads to significant changes in structural and some dynamical properties that can be inferred from knowledge of the static structure factors. Such dynamical changes seem to arise as a result of a strong decrease of the elastic moduli of the alloys resulting from interactions with a light element rather than from a density effect. ©2004 American Physical SocietyWe thank the Institut Laue Langevin for neutron beam
time.We are grateful to Pierre Palleau and to Dr. J. Campo of
ILL for his technical support during the diffraction experiments,
and Carlos Ayala of Centro Atómico Bariloche for the
sample preparationPeer Reviewe
Substitution site and effects on magnetism in Sr-for-Ca substituted CaBaCo4O7
Cationic substitutions in the novel magnetoelectric compound CaBaCo4O7 lead to profound changes in its magnetic and electric behaviors. In this work, we present a structural study of the isovalent substitution Sr-for-Ca in CaBaCo4O7. X-ray diffraction, as well as neutron powder diffraction experiments, are reported for a series of samples Ca1- xSrxBaCo4O7 with 0 = x = 0.10. Special emphasis is given to the identification of the substitution site, as Sr has also been reported to substitute for Ba in this crystal structure. The solubility limit for Sr at the Ca site is shown to be at x ¿ 0.08. The variation of lattice constants with Sr-doping firmly supports the Sr-for-Ca substitution. Rietveld refinements of the Sr-substituted samples are presented, and used as starting point to analyse the local structure around Sr by means of X-ray absorption spectroscopy at the Sr K-edge. Both the near-edge absorption and the extended absorption fine-structure confirm the substitution of Sr for Ca, giving definite support to the proposed nominal formula. In addition, macroscopic magnetization measurements are presented which reveal the striking effects of Sr-substitution over the magnetic landscape of this puzzling compound
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