Ca(2.5)Sr(0.5)GaMn2O8: diamagnetic Ga in control of the structural and electronic properties of a bilayered manganate

The temperature dependence of the crystal structure and electronic properties of brownmillerite-like Ca(2.5)Sr(0.5)GaMn(2)O(8) has been studied by neutron powder diffraction and muSR spectroscopy. The results show that short-range 2D magnetic order begins to develop within the perovskite-like bilayers of MnO(6) octahedra approximately 50 K above the 3D Néel temperature of approximately 150 K. The bilayers show a structural response to the onset of magnetism throughout this temperature range whereas the GaO(4) layers that separate the bilayers only respond below the 3D ordering temperature. XANES spectroscopy shows that the sample contains Mn(3+) and Mn(4+) cations in a 1:1 ratio, and the behavior in the region of the Néel transition is interpreted as a local charge ordering. Electron diffraction and high-resolution electron microscopy have been used to show that the local microstructure is more complex than the average structure revealed by neutron diffraction, and that microdomains exist in which the GaO(4) tetrahedra show different orientations. It is argued that the bonding requirements of diamagnetic gallium control the electronic behavior within the perovskite-like bilayers.     

Structural and magnetic properties of the solid solution () YMn1−x(Cu3/4Mo1/4)xO3

Recently, the ferroelectromagnet YMnO₃ has been the focus of interest because it exhibits both antiferromagnetism (Néel temperature 80 K) and ferroelectricity (Curie temperature 914 K). There have been no reports of complete YMn_1−xM_xO₃ solid solutions in which substitution of the foreign M cation preserves the hexagonal P6₃cm structure. In contrast there exist several homeotypic phases with the general formula, Ln_1+nCu_nMO_3+3n (n=1 (M=Ti), 2 (M=V) and 3 (M=Mo); Ln: lanthanide). Several YMn_1−x(Cu_3/4Mo_1/4)_xO₃ compounds have been synthesized. The solid solution, from YMnO₃ (x=0) to YCu_3/4Mo_1/4O₃ (x=1) has been characterized by X-ray diffraction and transmission electron microscopy study. For 0<x<0.9, the compounds are found to crystallize in the non-centrosymmetric structure, space group P6₃cm, of YMnO₃. The Mn-free end member, x=1, crystallizes in a complex multiple cell, the superstructure being associated to Cu³⁺/Mo⁶⁺ cationic ordering. Dilution of the Mn³⁺ magnetic array by the paramagnetic (Cu²⁺) and diamagnetic (Mo⁶⁺) cations is found to decrease the antiferromagnetic ordering temperature and it becomes undetectable for x0.5 compositions.     

Determination des taux de deuteriation de molecules organiques par resonance magnetique nucleaire

The accuracy of several methods for deuterium content determination in organic molecules are tabulated. The sample-substitution (using one or two NMR sample tubes) and internal standard (inter- or intra-molecular) processes are discussed and compared. These methods are applied to some examples and the NMR results are confirmed by mass spectrometry.     

Non-equivalence Magnetique en Resonance Nucleaire—IV. Influence de Plusieurs Types de Dissymetries sur les Deplacements Chimiques et les Couplages Relatifs a des Systemes ?O?CH2?CH3 et ?S?CH2?CH3

Magnetic non-equivalence of methylenic protons in A? CH₂? CH₃ groups were studied for several types of compounds. Results are compared with respect (1) to the nature of heteroatom A:A = O and A = S and (2) to the structure of the local dissymmetry element: asymmetric carbon, ketal group and allenic system. Influence of substitution is discussed in relation to conformational problems. Furthermore a variation of the methylene geminal coupling constant with substituents of carbon α to OEt is observed.     

Cycles with a chord are graceful

In this paper, we prove that every cycle plus a chord is graceful, thus answering a conjecture of R. Bodendiek, H. Schumacher, and H. Wegner.     

On the use of some numerical damping methods of spurious oscillations in the case of elastic wave propagation

The use of finite element and finite difference methods of spatial and temporal discretization for solving structural dynamics problems gives rise to purely numerical errors. Among the many numerical methods used to damp out the spurious oscillations occurring in the high frequency domain, it is proposed here to analyse and compare the well-known Bulk Viscosity method, which modifies the stresses calculations and a method recently presented by Tchamwa and Wielgosz, which is based on a modification of an explicit time integration algorithm. The efficiency of both methods is evaluated in a 2-D axisymmetric compressive test.     

Contribution a l'etude des metallocenes : XXVIII. Reduction de cetones cycliques en series du ferrocene et du cymantrene. Etude de la stereospecificite. Assignation des configurations endo ou exo des alcools derives

The reduction of various 2,3-ferroceno-1-indanones and 1,2-(α-oxotetramethylene)-methylcymantrenes by metal hydrides or Grignard reagents is stereo-specific, except for LiAlH₄, the observed stereoselectivity being in all uses more than 95%. The endo configuration of the dominant alcohol is established from TLC, IR, RMN and mass data. These criteria for the assignment of the structure of these alcohols are shown to be general.     

Formation of pentagonal tunnels bordered with P2O7 groups in diphosphate tungsten bronzes (P2O4)2(WO3)2m: A HREM investigation

During the investigation of the phosphate bronzes (PO₂)₄(WO₃)_2m [MPTB_P] and K_x(P₂O₄)₂WO₃)_2m [DPTB_H] crystals of a new type were observed. HREM images of these crystals showed twinned ReO₃-type slabs the junction of which was parallel to the (102)_ReO3 plane. The proposed model identified the twin boundary as built from P₂O₇ groups involving the formation of pentagonal tunnels. The structure of this new type of extended defects is quite original: it corresponds to a new structural type named “diphosphate tungsten bronzes with pentagonal tunnels” [DPTB_P], for which no regular member could be synthesized. Image calculations were performed to confirm the junction model. Apart from the disordered stacking of the ReO₃-type slabs, very few defects were observed and shear planes were only obtained in reduced samples. This new structural type takes its place in the large family of phosphate tungsten bronzes where all members (DPTB_H, MPTB_H, MPTB_P) are very closely related.     

Praseodymium magnetic contribution in the low temperature structure of Pr0.8Ca0.2MnO3

Magnetic and crystal structures of the manganite Pr_0.8Ca_0.2MnO₃ have been studied by neutron powder and single-crystal X-ray diffraction. Structure refinements using single crystal data [orthorhombic system, Pnma, (No. 62), a_RT=5.5534(3) Å, b_RT=7.6548(8) Å, c_RT=5.4400(5) Å, D_x=6.422 g cm⁻³, R^RT=0.029, R_w^RT=0.038] are consistent with a single domain sample. Structure and atomic displacement parameters exclude any electronic localization, even in a disordered way at 300 and 100 K. Low temperature electron diffraction observations do not show any trace of charge ordering.A Pr contribution to the magnetic structure has been shown with a maximum moment of 0.79 μ_B and spins alignments roughly along [101] orientations, at a lower temperature than the ferromagnetic transition observed at 130 K, due to Mn spins ordering.