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 $a\approx$8.0854(1) \AA whereas Ca_{2}FeReO_{6} has a distorted monoclinic symmetry with $a\approx 5.396(1) \AA, b\approx 5.522(1) \AA, c\approx 7.688(2) \AA$ and $\beta =90.4^{\circ} (P21/n)$. 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$\times 10^{24} eV^{-1}mole^{-1}$. 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

Buckling Imperfection Sensitivity of Conical Sandwich Composite Structures for Launch-Vehicles

Structural stability can be an important consideration in the design of large composite shell structures and therefore it is important to understand the buckling response of such structures. It is well known that geometric imperfections can significantly influence the buckling response of such structures by causing the buckling loads to be significantly lower than the theoretical buckling load of a geometrically perfect shell structure. Results are presented of an analytical study on the buckling imperfection sensitivity of large-scale conical sandwich structures for launch vehicles. In particular, representative structures from the Space Launch System launch-vehicle development activities will be considered. The study considered composite sandwich conical structures with multiple sandwich core thicknesses and facesheet layups consisting of tape and fabric composite layups. The results of this analytical study indicate that there is conservatism in the NASA current buckling knockdown factor of 0.33 for conical shell structures. Therefore, it is suggested that the buckling response of composite sandwich cones be further investigated through buckling tests and analytical predictions to potentially revise the buckling design recommendations for conical composite structures

ACE inhibitor use in patients with myocardial infarction. Summary ofevidence from clinical trials

Experimental evidence for the beneficial effects on heart failure of chronic treatment with ACE inhibitors accumulated from early 1980 in experimental models of LV dysfunction secondary to AMI. These studies demonstrated an improvement in hemodynamics, LV remodeling, and mortality with ACE inhibitor treatment. The effect of ACE inhibitors during the acute phase of AMI was less clear, although there was evidence of protection from ischemic damage, possibly mediated by an increase in collateral coronary blood flow

In-Vacuum Photogrammetry of a 10-Meter Solar Sail

In July 2004, a 10-meter solar sail structure developed by L Garde, Inc. was tested in vacuum at the NASA Glenn 30-meter Plum Brook Space Power Facility in Sandusky, Ohio. The three main objections of the test were to demonstrate unattended deployment from a stowed configuration, to measure the deployed shape of the sail at both ambient and cryogenic room temperatures, and to measure the deployed structural dynamic characteristics (vibration modes). This paper summarizes the work conducted to fulfill the second test objective. The deployed shape was measured photogrammetrically in vacuum conditions with four 2-megapixel digital video cameras contained in custom made pressurized canisters. The canisters included high-intensity LED ring lights to illuminate a grid of retroreflective targets distributed on the solar sail. The test results closely matched pre-test photogrammetry numerical simulations and compare well with ABAQUS finite-element model predictions

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

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

Metallic and nonmetallic double perovskites: A case study of A2_2FeReO6_6 (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

Metal-Insulator Transition and Magnetic Order in the Pyrochlore Oxide Hg2Ru2O7

We report results of NMR experiments on the ruthenium oxide Hg2Ru2O7 with the pyrochlore structure, which exhibits a metal-insulator transition at TMI = 107 K. In the metallic phase above TMI, the nuclear spin-lattice relaxation rate 1/T1 and the Knight shift at the Hg sites follow the Korringa relation, indicating the absence of substantial spatial spin correlation. At low temperatures in the insulating phase, 99,101Ru-NMR signals are observed at zero magnetic field, providing evidence for a commensurate antiferromagnetic order. The estimated ordered moment is about 1 muB per Ru, much smaller than 3 muB expected for the ionic (4d)3plus configuration of Ru5plus. Thus the localized spin models are not appropriate for the insulating phase of Hg2Ru2O7. We also discuss possible antiferromagnetic spin structures.Comment: 10 pages, 7 figure