Jahn-Teller, polarity and insulator-to-metal transition in BiMnO3 at high pressure

The interaction of coexisting structural instabilities in multiferroic materials gives rise to intriguing coupling phenomena and extraordinarily rich phase diagrams, both in bulk materials and strained thin films. Here we investigate the multiferroic BiMnO3 with its peculiar 6s2 electrons and four interacting mechanisms: electric polarity, octahedra tilts, magnetism, and cooperative Jahn-Teller distortion. We have probed structural transitions under high pressure by synchrotron x-ray diffraction and Raman spectroscopy up to 60 GPa. We show that BiMnO3 displays under pressure a rich sequence of five phases with a great variety of structures and properties, including a metallic phase above 53 GPa and, between 37 and 53 GPa, a strongly elongated monoclinic phase that allows ferroelectricity, which contradicts the traditional expectation that ferroelectricity vanishes under pressure. Between 7 and 37 GPa, the Pnma structure remains remarkably stable but shows a reduction of the Jahn-Teller distortion in a way that differs from the behavior observed in the archetypal orthorhombic Jahn-Teller distorted perovskite LaMnO3.Comment: 5 pages, 3 figures + supplemental material included (3 pages, 1 figure, 3 tables

Magnetic phase diagram of the S=1/2 triangular layered compound NaNiO2: a single crystal study

Using magnetic torque measurement on a NaNiO2 single crystal, we have established the magnetic phase diagram of this triangular compound. It presents 5 different phases depending on the temperature (4 K - 300 K) and magnetic field (0 - 22 T) revealing several spin reorientations coupled to different magnetic anisotropies

High pressure and high temperature in situ X-ray diffraction studies in the Paris-Edinburgh cell using a laboratory X-ray source

International audienceWe have developed a new laboratory experimental set-up to study in situ the pressure-temperature phase diagram of a given pure element or compound, its associated phase transitions, or the chemical reactions involved at high pressure and high temperature (HP-HT) between different solids and liquids. This new tool allows laboratory studies before conducting further detailed experiments using more brilliant synchrotron X-ray sources or before kinetic studies. This device uses the diffraction of X-rays produced by a quasi-monochromatic micro-beam source operating at the silver radiation (λ(Ag)Kα1,2 ≈ 0.56Å). The experimental set-up is based on a VX Paris-Edinburgh cell equipped with tungsten carbide or sintered diamond anvils and uses standard B-epoxy 5 or 7mm gaskets. The diffracted signal coming from the compressed (and heated) sample is collected on an image plate. The pressure and temperature calibrations were performed by diffraction, using conventional calibrants (BN, NaCl and MgO) for determination of the pressure, and by crossing isochores of BN, NaCl, Cu or Au for the determination of the temperature. The first examples of studies performed with this new laboratory set-up are presented in the article: determination of the melting point of germanium and magnesium under HP-HT, synthesis of MgB2 or C-diamond and partial study of the P, T phase diagram of MgH2

IV thrombolysis and renal function

OBJECTIVE To investigate the association of renal impairment on functional outcome and complications in stroke patients treated with IV thrombolysis (IVT). METHODS In this observational study, we compared the estimated glomerular filtration rate (GFR) with poor 3-month outcome (modified Rankin Scale scores 3-6), death, and symptomatic intracranial hemorrhage (sICH) based on the criteria of the European Cooperative Acute Stroke Study II trial. Unadjusted and adjusted odds ratios (ORs) with 95% confidence intervals (CIs) were calculated. Patients without IVT treatment served as a comparison group. RESULTS Among 4,780 IVT-treated patients, 1,217 (25.5%) had a low GFR (<60 mL/min/1.73 m(2)). A GFR decrease by 10 mL/min/1.73 m(2) increased the risk of poor outcome (OR [95% CI]): (ORunadjusted 1.20 [1.17-1.24]; ORadjusted 1.05 [1.01-1.09]), death (ORunadjusted 1.33 [1.28-1.38]; ORadjusted 1.18 [1.11-1.249]), and sICH (ORunadjusted 1.15 [1.01-1.22]; ORadjusted 1.11 [1.04-1.20]). Low GFR was independently associated with poor 3-month outcome (ORadjusted 1.32 [1.10-1.58]), death (ORadjusted 1.73 [1.39-2.14]), and sICH (ORadjusted 1.64 [1.21-2.23]) compared with normal GFR (60-120 mL/min/1.73 m(2)). Low GFR (ORadjusted 1.64 [1.21-2.23]) and stroke severity (ORadjusted 1.05 [1.03-1.07]) independently determined sICH. Compared with patients who did not receive IVT, treatment with IVT in patients with low GFR was associated with poor outcome (ORadjusted 1.79 [1.41-2.25]), and with favorable outcome in those with normal GFR (ORadjusted 0.77 [0.63-0.94]). CONCLUSION Renal function significantly modified outcome and complication rates in IVT-treated stroke patients. Lower GFR might be a better risk indicator for sICH than age. A decrease of GFR by 10 mL/min/1.73 m(2) seems to have a similar impact on the risk of death or sICH as a 1-point-higher NIH Stroke Scale score measuring stroke severity

Symmetry adapted analysis of the magnetic and structural phase diagram of Bi1−x_{1-x}Yx_xCrO3_3

11 pagesInternational audienceThe effect of the substitution of yttrium for bismuth in BiCrO3 distorted perovskite has been investigated by means of magnetic measurements and symmetry mode analysis of neutron and x-ray powder diffraction data. Bi1−xYxCrO3 compounds were synthesized under high pressure and high temperature. Magnetic and structural phase diagrams were established by means of magnetization measurements and x-ray and neutron powder diffraction. We found that the monoclinic distorted structure is replaced by an orthoferrite-type orthorhombic structure at x 0.05. Symmetry adapted analysis was performed, indicating the importance of the LD3 distortion mode for the stabilization of the monoclinic structure and its antiferroelectric arrangement. The interplay between crystal distortion modes and magnetic properties is discussed

High pressure synthesis of BiCrO3, a candidate for multiferroism

International audienceThe synthesis and the thermal stability of the high pressure and high temperature perovskite phase BiCrO3 have been studied. BiCrO3 can be synthesized in a wide range of pressure (1-6GPa) and temperature (690-865C). The thermal behavior of BiCrO3 depends on the atmosphere : it tends to be stable under inert gas whereas it decomposes at 650C under O2. The melting point of BiCrO3 is determined by DTA under argon and a value of 1040C is proposed. First SEM and TEM observations are also presented

Cu0.8Mg1.2Si2O6 : a copper-bearing silicate with the low-clinopyroxene structure

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SrMGe2O6 (M = Mn, Co): a family of pyroxene compounds displaying multiferroicity

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The structure of nano-twinned rhombohedral YCuO 2.66 solved by electron crystallography

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Magnetic and crystal structures of BiCrO3

International audiencePolycrystalline samples of BiCrO3 were synthesized by high pressure-high temperature solid state reaction in a Conac anvil-type apparatus at 2 GPa and 805 degrees C. Neutron powder diffractograms were collected on the D20 and D2B instruments of the ILL-Grenoble between 2 K and 470 K. Magnetic susceptibility measurements show the onset of magnetic order at 110 K followed by a large increase below 80 K. The neutron diffraction data indicate the appearance of G-type antiferromagnetic order at the 114 K transition temperature. with all spins aligned along one of the unit cell axes and antiparallel spins on different Cr sites. This is followed by a progressive spin reorientation between 80 K and 60 K. (C) 2008 Elsevier Masson SAS. All rights reserved