600 research outputs found
New high-pressure phase and equation of state of Ce2Zr2O8
In this paper we report a new high-pressure rhombohedral phase of Ce2Zr2O8
observed from high-pressure angle-dispersive x-ray diffraction and Raman
spectroscopy studies up to nearly 12 GPa. The ambient-pressure cubic phase of
Ce2Zr2O8 transforms to a rhombohedral structure beyond 5 GPa with a feeble
distortion in the lattice. Pressure evolution of unit-cell volume showed a
change in compressibility above 5 GPa. The unit-cell parameters of the
high-pressure rhombohedral phase at 12.1 GPa are ah = 14.6791(3) {\AA}, ch =
17.9421(5) {\AA}, V = 3348.1(1) {\AA}3. The structure relation between the
parent cubic (P2_13) and rhombohedral (P3_2) phases were obtained by
group-subgroup relations. All the Raman modes of the cubic phase showed linear
evolution with pressure with the hardest one at 197 cm-1. Some Raman modes of
the high-pressure phase have a non-linear evolution with pressure and softening
of one low-frequency mode with pressure is found. The compressibility, equation
of state, and pressure coefficients of Raman modes of Ce2Zr2O8 are also
reported.Comment: 33 pages, 8 figures, 6 table
Properties of the ferrimagnetic double-perovskite A_{2}FeReO_{6} (A=Ba and Ca)
Ceramics of A_{2}FeReO_{6} double-perovskite have been prepared and studied
for A=Ba and Ca. Ba_{2}FeReO_{6} has a cubic structure (Fm3m) with 8.0854(1) \AA whereas Ca_{2}FeReO_{6} has a distorted monoclinic symmetry with
and
. The barium compound is metallic from 5 K to 385
K, i.e. no metal-insulator transition has been seen up to 385 K, and the
calcium compound is semiconducting from 5 K to 385 K. Magnetization
measurements show a ferrimagnetic behavior for both materials, with T_{c}=315 K
for Ba_{2}FeReO_{6} and above 385 K for Ca_{2}FeReO_{6}. A specific heat
measurement on the barium compound gave an electron density of states at the
Fermi level, N(E_{F}) equal to 6.1. At 5 K, we
observed a negative magnetoresistance of 10 % in a magnetic field of 5 T, but
only for Ba_{2}FeReO_{6}. Electrical, thermal and magnetic properties are
discussed and compared to the analogous compounds Sr_{2}Fe(Mo,Re)O_{6}.Comment: 5 pages REVTeX, 7 figures included, submitted to PR
Metallic and nonmetallic double perovskites: A case study of AFeReO (A= Ca, Sr, Ba)
We have investigated the structure and electronic properties of ferrimagnetic
double perovskites, A2FeReO6 (A= Ca, Sr, Ba). The A=Ba phase is cubic (Fm3m)
and metallic, while the A=Ca phase is monoclinic (P21/n) and nonmetallic.
57Fe Mossbauer spectroscopy shows that iron is present mainly in the
high-spin (S=5/2) Fe3+ state in the Ca compound, while it occurs in an
intermediate state between high-spin Fe2+ and Fe3+ in the Ba compound. It is
argued that a direct Re t2g - Re t2g interaction is the main cause for the
metallic character of the Ba compound; the high covalency of Ca-O bonds and the
monoclinic distortion (which lifts the degeneracy of t2g states) seem to
disrupt the Re-Re interaction in the case of the Ca compound, making it
non-metallic for the same electron count.Comment: 1 eps fil
Superconducting to spin glass state transformation in {\ss}-pyrochlore KxOs2O6
{\ss}-pyrochore KOs2O6, which shows superconductivity below ~ 9.7K, has been
converted into KxOs2O6 (x < 2/3 - 1/2) electrochemically to show spin
glass-like behavior below ~ 6.1K. Room temperature sample surface potential
versus charge transfer scan indicates that there are at least two two-phase
regions for x between 1 and 0.5. Rattling model of superconductivity for the
title compound has been examined using electrochemical potassium
de-intercalation. The significant reduction of superconducting volume fraction
due to minor potassium reduction suggests the importance of defect and phase
coherence in the rattling model. Magnetic susceptibility, resistivity, and
specific heat measurement results have been compared between the
superconducting and spin glass-like samples.Comment: 8 pages, 7 figures, 1 tabl
Anomalous magnetic phase in an undistorted pyrochlore oxide Cd2Os2O7 induced by geometrical frustration
We report on the muon spin rotation/relaxation study of a pyrochlore oxide,
Cd2Os2O7, which exhibits a metal-insulator (MI) transition at T_{MI}~225 K
without structural phase transition. It reveals strong spin fluctuation
(>10^8/s) below the MI transition, suggesting a predominant role of geometrical
spin frustration amongst Os^{5+} ions. Meanwhile, upon further cooling, a
static spin density wave discontinuously develops below T_{SDW}~150 K. These
observations strongly suggest the occurrence of an anomalous magnetic
transition and associated change in the local spin dynamics in undistorted
pyrochlore antiferromagnet.Comment: 5 pages, 4 figure
Test and Analysis of a Buckling-Critical Large-Scale Sandwich Composite Cylinder
Structural stability is an important design consideration for launch-vehicle shell structures and it is well known that the buckling response of such shell structures can be very sensitive to small geometric imperfections. As part of an effort to develop new buckling design guidelines for sandwich composite cylindrical shells, an 8-ft-diameter honeycomb-core sandwich composite cylinder was tested under pure axial compression to failure. The results from this test are compared with finite-element-analysis predictions and overall agreement was very good. In particular, the predicted buckling load was within 1% of the test and the character of the response matched well. However, it was found that the agreement could be improved by including composite material nonlinearity in the analysis, and that the predicted buckling initiation site was sensitive to the addition of small bending loads to the primary axial load in analyses
Theory of Transition Temperature of Magnetic Double Perovskites
We formulate a theory of double perovskite coumpounds such as SrFeReO
and SrFeMoO which have attracted recent attention for their possible
uses as spin valves and sources of spin polarized electrons. We solve the
theory in the dynamical mean field approximation to find the magnetic
transition temperature . We find that is determined by a subtle
interplay between carrier density and the Fe-Mo/Re site energy difference, and
that the non-Fe same-sublattice hopping acts to reduce . Our results
suggest that presently existing materials do not optimize
Structural Order Parameter in the Pyrochlore Superconductor Cd2Re2O7
It is shown that both structural phase transitions in Cd2Re2O7, which occur
at T_{s1}=200 K and T_{s2}=120 K, are due to an instability of the Re
tetrahedral network with respect to the same doubly degenerate long-wavelength
phonon mode. The primary structural order parameter transforms according to the
irreducible representation E_u of the point group O_h. We argue that the
transition at T_{s1} may be of second order, in accordance with experimental
data. We obtain the phase diagram in the space of phenomenological parameters
and propose a thermodynamic path that Cd2Re2O7 follows upon cooling. Couplings
of the itinerant electronic system and localized spin states in pyrochlores and
spinels to atomic displacements are discussed.Comment: 5 pages. Submitted to J. Phys. Soc. Jpn. Best quality figures are
available at http://www.physics.mun.ca/~isergien/pubs.htm
Searching for the Slater Transition in the Pyrochlore CdOsO with Infrared Spectroscopy
Infrared reflectance measurements were made on the single crystal pyrochlore
CdOsO in order to examine the transformations of the
electronic structure and crystal lattice across the boundary of the metal
insulator transition at . All predicted IR active phonons are
observed in the conductivity over all temperatures and the oscillator strength
is found to be temperature independent. These results indicate that charge
ordering plays only a minor role in the MIT and that the transition is strictly
electronic in nature. The conductivity shows the clear opening of a gap with
. The gap opens continuously, with a temperature
dependence similar to that of BCS superconductors, and the gap edge having a
distinct dependence. All of these
observables support the suggestion of a Slater transition in CdOsO.Comment: 4 pages, 4 figure
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