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ABSTRACT: We report the structural and magnetic properties of a new class of cobaltates
with the chemical formula (BaSr)4-xLa2xCo4O15 (x = 0, 0.5 and 1). These
compounds crystallize in a hexagonal structure in which cobalt ions are
distributed among two distinct crystallographic sites with different oxygen
coordination. Three Co-O tetrahedra and one octahedron are linked by shared
oxygen atoms to form Co4O15 clusters, which are packed together into a
honeycomb-like network. Partial substitution of Sr and/or Ba atoms by La allows
one to adjust the degree of Co valence mixing, but all compositions remain
subject to a random distribution of charge. Magnetic susceptibility together
with neutron scattering measurements reveal that all studied specimens are
characterized by competing ferro- and antiferro-magnetic exchange interactions
that give rise to a three dimensional Heisenberg spin-glass state. Neutron
spectroscopy shows a clear trend of slowing down of spin-dynamics upon
increasing La concentration, suggesting a reduction in charge randomness in the
doped samples.
09/2012;
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ABSTRACT: We report measurements of the neutron diffuse scattering in a single crystal
of the relaxor ferroelectric material 95.5%Pb(Zn1/3Nb2/3)O3-4.5%PbTiO3
(PZN-4.5%PT). We show that the diffuse scattering at high temperatures has a
quasielastic component with energy width $\agt$ 0.1 meV. On cooling the total
diffuse scattering intensity increases, but the intensity and the energy width
of the quasielastic component gradually diminish. At 50 K the diffuse
scattering is completely static (i.e.the energy width lies within the limits of
our instrumental resolution). This suggests that the dynamics of the
short-range correlated atomic displacements associated with the diffuse
scattering freeze at low temperature. We find that this depends on the wave
vector q as the quasielastic diffuse scattering intensities associated with
<001> (T1-type) and <110> (T2-type) atomic displacements vary differently with
temperature and electric field.
03/2012;
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ABSTRACT: We studied the collective excitations in an aqueous solution of lithium chloride over the temperature range of 270-205 K using neutron and x-ray Brillouin scattering. Both neutron and x-ray experiments revealed the presence of low- and high-frequency excitations, similar to the low- and high-frequency excitations in pure water. These two excitations were detectable through the entire temperature range of the experiment, at all probed values of the scattering momentum transfer (0.2 Å(-1) < Q < 1.8 Å(-1)). A wider temperature range was investigated using elastic intensity neutron and x-ray scans. Clear evidence of the crossover in the dynamics of the water molecules in the solution was observed in the single-particle relaxational dynamics on the µeV (nanosecond) time scale, but not in the collective dynamics on the meV (picosecond) time scale.
Journal of Physics Condensed Matter 02/2012; 24(6):064102. · 2.55 Impact Factor
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ABSTRACT: The microscopic dynamics of hydration water exhibits some universal features that do not depend on the nature of the hydrated surface. We show that the hydration level dependence of the dynamic transition in the mean squared atomic displacements measured by means of elastic neutron scattering is qualitatively similar for hydration water in inorganic and organic hosts. The difference is that the former are 'rigid', whereas the dynamics of the latter can be enhanced by the motions of the hydration water. The overall hydration level appears to be the main parameter governing the magnitude of the mean squared atomic displacements in the hydration water, irrespective of the details of the hydrated host.
Journal of Physics Condensed Matter 02/2012; 24(6):064104. · 2.55 Impact Factor
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Epl. 01/2012; 97(6).
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Microporous and Mesoporous Materials 01/2012; 148(1):101-106. · 3.29 Impact Factor
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D J Wesolowski,
J O Sofo,
A V Bandura,
Z Zhang, E Mamontov,
M Predota,
N Kumar,
J D Kubicki,
P R C Kent,
L Vlcek,
M L Machesky,
P A Fenter,
P T Cummings,
L M Anovitz,
A A Skelton,
J Rosenqvist
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ABSTRACT: Liu and co-workers [Phys. Rev. B 82, 161415 (2010)] discussed the long-standing debate regarding whether H2O molecules on the defect-free (110) surface of rutile (alpha-TiO2) sorb associatively, or there is dissociation of some or all first-layer water to produce hydroxyl surface sites. They conducted static density functional theory (DFT) and DFT molecular dynamics (DFT-MD) investigations using a range of cell configurations and functionals. We have reproduced their static DFT calculations of the influence of crystal slab thickness on water sorption energies. However, we disagree with several assertions made by these authors: (a) that second-layer water structuring and hydrogen bonding to surface oxygens and adsorbed water molecules are "weak"; (b) that translational diffusion of water molecules in direct contact with the surface approaches that of bulk liquid water; and (c) that there is no dissociation of adsorbed water at this surface in contact with liquid water. These assertions directly contradict our published work, which compared synchrotron x-ray crystal truncation rod, second harmonic generation, quasielastic neutron scattering, surface charge titration, and classical MD simulations of rutile (110) single-crystal surfaces and (110)-dominated powders in contact with bulk water, and (110)-dominated rutile nanoparticles with several monolayers of adsorbed water.
Physical Review B. 01/2012; 85(16):5.
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ABSTRACT: We describe the design and current performance of the backscattering silicon spectrometer (BASIS), a time-of-flight backscattering spectrometer built at the spallation neutron source (SNS) of the Oak Ridge National Laboratory (ORNL). BASIS is the first silicon-based backscattering spectrometer installed at a spallation neutron source. In addition to high intensity, it offers a high-energy resolution of about 3.5 μeV and a large and variable energy transfer range. These ensure an excellent overlap with the dynamic ranges accessible at other inelastic spectrometers at the SNS.
The Review of scientific instruments 08/2011; 82(8):085109. · 1.52 Impact Factor
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ABSTRACT: Microscopic dynamics of water confined in nanometer and sub-nanometer pores of carbide-derived carbon (CDC) were investigated using quasielastic neutron scattering (QENS). The temperature dependence of the average relaxation time, τ, exhibits super-Arrhenius behavior that could be described by Vogel-Fulcher-Tammann (VFT) law in the range from 250 K to 190 K; below this temperature, τ follows Arrhenius temperature dependence. The temperature of the dynamic crossover between the two regimes in water confined in the CDC pores is similar to that observed for water in hydrophobic confinement of the larger size, such as 14 Å ordered mesoporous carbon (CMK) and 16 Å double-wall carbon nanotubes. Thus, the dynamical behavior of water remains qualitatively unchanged even in the very small hydrophobic pores.
EPL (Europhysics Letters) 07/2011; 95(5):56001. · 2.17 Impact Factor
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ABSTRACT: Aqueous solutions of lithium chloride are an excellent model system for studying the dynamics of water molecules down to low temperatures without freezing. The apparent dynamic crossover observed in an aqueous solution of LiCl at about 220 to 225 K [Mamontov, JPCB 2009, 113, 14073] is located practically at the same temperature as the crossover found for pure water confined in small hydrophilic pores. This finding suggests a strong similarity of water behavior in these two types of systems. At the same time, studies of solutions allow more effective explorations of the long-range diffusion dynamics, because the water molecules are not confined inside an impenetrable matrix. In contrast to the earlier incoherent quasielastic neutron scattering results obtained for the scattering momentum transfers of 0.3 Å(-1) ≤ Q ≤ 0.9 Å(-1), our present incoherent neutron spin-echo measurements at a lower Q of 0.1 Å(-1) exhibit no apparent crossover in the relaxation times down to 200 K. At the same time, our present nuclear magnetic resonance measurements of the diffusion coefficients clearly show a deviation at the lower temperatures from the non-Arrhenius law obtained at the higher temperatures. Our results are consistent with a scenario in which more than one relaxational component may exist below the temperature of the dynamic crossover in water.
The Journal of Physical Chemistry B 12/2010; 114(50):16737-43. · 3.70 Impact Factor
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ABSTRACT: The high flux and low background of the new backscattering spectrometer at the SNS combine to produce an excellent signal to noise ratio, allowing us to investigate a low lying weak excitation never seen before in the spin ice, Ho2Ti2O7. This non-dispersive excitation has been observed at E = 26.3 µeV below 100 K but is resolution limited only below ∼ 65 K. It is indifferent to magnetic fields below µ0H = 4.5 T, at 1.6 K. These characteristics help us to identify the excitation as due to the nuclear spin system.
J. Phys.: Conf. Ser. 01/2010; 25121949.
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Geochimica et Cosmochimica Acta 01/2010; 74(12):A200-A200. · 4.26 Impact Factor
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ABSTRACT: A single nondispersive excitation is observed by means of neutron backscattering, at E_{0}=26.3 microeV in the spin ice Ho2Ti2O7 but not in the isotopically enriched 162Dy2Ti2O7 analogue. The intensity of this excitation is rather small, less, similar0.2% of the elastic intensity. It is clearly observed below 80 K but resolution limited only below approximately 65 K. The application of a magnetic field up to micro_{0}H=4.5 T, at 1.6 K, has no measurable effect on the energy or intensity. This nuclear excitation is believed to perturb the electronic, Ising spin system resulting in the persistent spin dynamics observed in spin ice compounds.
Physical Review Letters 02/2009; 102(1):016405. · 7.37 Impact Factor
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G Ehlers, E Mamontov,
M Zamponi,
A Faraone,
Y Qiu,
A L Cornelius,
C H Booth,
K C Kam,
R Le Toquin,
A K Cheetham,
J S Gardner
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ABSTRACT: We have studied the evolution of the structural properties as well as the static and dynamic spin correlations of spin ice Ho(2)Ti(2)O(7), where Ho was partially replaced by non-magnetic La. The crystal structure of diluted samples Ho(2-x)La(x)Ti(2)O(7) was characterized by x-ray and neutron diffraction and by Ho L(III)-edge and Ti K-edge extended x-ray absorption fine structure (EXAFS) measurements. It is found that the pyrochlore structure remains intact until about x = 0.3, but a systematic increase in local disorder with increasing La concentration is observed in the EXAFS data, especially from the Ti K edge. Quasi-elastic neutron scattering and ac susceptibility measurements show that, in x≤0.4 samples at temperatures above macroscopic freezing, the spin-spin correlations are short ranged and dynamic in nature. The main difference with pure spin ice in the dynamics is the appearance of a second, faster, relaxation process.
Journal of Physics Condensed Matter 06/2008; 20(23):235206. · 2.55 Impact Factor
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G Ehlers, E Mamontov,
M Zamponi,
A Faraone,
Y Qiu,
A L Cornelius,
C H Booth,
K C Kam,
R Le Toquin,
A K Cheetham,
J S Gardner
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ABSTRACT: We have studied the evolution of the structural properties as well as the static and dynamic spin correlations of spin ice Ho2Ti2O7, where Ho was partially replaced by non-magnetic La. The crystal structure of diluted samples Ho2−xLaxTi2O7 was characterized by x-ray and neutron diffraction and by Ho LIII-edge and Ti K-edge extended x-ray absorption fine structure (EXAFS) measurements. It is found that the pyrochlore structure remains intact until about x = 0.3, but a systematic increase in local disorder with increasing La concentration is observed in the EXAFS data, especially from the Ti K edge. Quasi-elastic neutron scattering and ac susceptibility measurements show that, in x≤0.4 samples at temperatures above macroscopic freezing, the spin–spin correlations are short ranged and dynamic in nature. The main difference with pure spin ice in the dynamics is the appearance of a second, faster, relaxation process.
Journal of Physics Condensed Matter 04/2008; 20(23):235206. · 2.55 Impact Factor
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ABSTRACT: The high energy resolution, coupled with the wide dynamic range, of the new backscattering spectrometer (BASIS) at the Spallation Neutron Source, Oak Ridge National Laboratory, has made it possible to investigate the diffusion dynamics of hydration water on the surface of rutile (TiO(2)) nanopowder down to a temperature of 195 K. The dynamics measured on the BASIS on the time scale of tens of picoseconds to more than a nanosecond can be attributed to the mobility of the outer hydration water layers. The data obtained on the BASIS and in a previous study using the backscattering and disk-chopper spectrometers at the NIST Center for Neutron Research are coupled with molecular dynamics simulations extended to 50 ns. The results suggest that the scattering experiments probe several types of molecular motion in the surface layers, namely a very fast component that involves dynamics of water molecules with unsaturated hydrogen bonds, a somewhat slower component due to localized motions of all water molecules, and a much slower component related to the translational jumps of the fully hydrogen-bonded water molecules. The temperature dependence of the relaxation times associated with the localized dynamics remains Arrhenius down to at least 195 K, whereas the slow translational component shows non-Arrhenius behavior above about 205 K. Thus, an Arrhenius-type behavior of the faster localized dynamic component extends below the temperature of the dynamic transition in the slow translational component. We suggest that the qualitative difference in the character of the temperature dependence between these slow and fast components may be due to the fact that the latter involves motions that require breaking fewer hydrogen bonds.
Journal of Physical Chemistry C. 01/2008; 112(32):12334-12341.
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Journal of Physics Condensed Matter 01/2008; 20:502101. · 2.55 Impact Factor
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ABSTRACT: Quasielastic neutron scattering was used to probe the diffusion of water molecules in 2.3 molal CaCl(2) solution confined in 100% hydrated Vycor glass in the temperature range of 220 to 260 K. We observed a gradual transition from the restricted diffusion regime at lower temperatures to unrestricted diffusion regime at higher temperatures. The diffusion parameters were compared with the data on pure water confined in Vycor available in the literature. We found that the effect of dissolved ions onto the diffusion dynamics of the water molecules in the solution was amplified by confinement by at least an order of magnitude compared to bulk form, even though the dissolved ions were found to have little effect on the spatial characteristics of the restricted diffusion process of water molecules. At 260 K, the local diffusion coefficient of water molecules in the H(2)O-CaCl(2) confined in Vycor was only 6% of the value reported for pure water confined in Vycor.
Physical Chemistry Chemical Physics 12/2006; 8(42):4908-14. · 3.57 Impact Factor
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ABSTRACT: Using high-resolution quasielastic neutron scattering, we investigated the temperature dependence of single-particle dynamics of water confined in single- and double-wall carbon nanotubes with the inner diameters of 14+/-1 and 16+/-3 A, respectively. The temperature dependence of the alpha relaxation time for water in the 14 A nanotubes measured on cooling down from 260 to 190 K exhibits a crossover at 218 K from a Vogel-Fulcher-Tammann law behavior to an Arrhenius law behavior, indicating a fragile-to-strong dynamic transition in the confined water. This transition may be associated with a structural transition from a high-temperature, low-density (<1.02 gcm(3)) liquid to a low-temperature, high-density (>1.14 gcm(3)) liquid found in molecular dynamics simulation at about 200 K. However, no such dynamic transition in the investigated temperature range of 240-195 K was detected for water in the 16 A nanotubes. In the latter case, the dynamics of water simply follows a Vogel-Fulcher-Tammann law. This suggests that the fragile-to-strong crossover for water in the 16 A nanotubes may be shifted to a lower temperature.
The Journal of Chemical Physics 05/2006; 124(19):194703. · 3.33 Impact Factor
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E Mamontov
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ABSTRACT: A quasielastic neutron-scattering experiment carried out on a backscattering spectrometer with sub-micro eV resolution in the temperature range of 200-250 K has revealed the dynamics of surface water in cerium oxide on the time scale of hundreds of picoseconds. This slow dynamics is attributed to the translational mobility of the water molecules in contact with the surface hydroxyl groups. The relaxation function of this slow motion can be described by a slightly stretched exponential with the stretch factor exceeding 0.9, which indicates almost a Debye-type dynamics. Down to about 220 K, the temperature dependence of the residence time for water molecules follows a Vogel-Fulcher-Tamman law with the glass transition temperature of 181 K. At lower temperatures, the residence time behavior abruptly changes, indicating a fragile-to-strong liquid transition in surface water at about 215 K.
The Journal of Chemical Physics 12/2005; 123(17):171101. · 3.33 Impact Factor
