Phase transitions and oxygen ordering in La2CoO4+δ and (T, T')-La2CuO4 : single crystal growth and structural studies using synchrotron and neutron diffraction methods

Abstract : This thesis relates about structural studies of crystal oxides with variable oxygen stoichiometry. The synthesis procedure for large single crystal growth is detailed. These materials have a layered atomic structure in which extra-oxygen ions can be intercalated in a topotactic way by different methods, by electrochemistry already at ambient temperature or by thermal treatments under controlled atmosphere composition and pressure. The interstitial oxygen atoms are not statistically distributed within layers but are ordered at long-range, which provokes structural distortions of the crystal lattice. The atomic structure is then modulated by the occupation of interstitial sites and by the displacements of the surrounding atoms. The real structure of the oxygen-rich compounds has been studies by neutron and synchrotron X-ray single crystal diffraction. Reciprocal space plane reconstructions allowed us to measure precisely the positions and intensity of satellite reflections, and applying the maximum entropy method allowed us to visualize the nuclear densities with good precision, both favoring the study of short atomic displacements, and thus yielding better knowledge of the real crystal structure. The phase transition have also been studied with temperature by diffraction and thermogravimetry, showing that the oxygen content varies spontaneously and that the crystal phases of oxygen rich compounds astonishingly remain modulated until high temperature. The oxygen diffusion mechanism in these crystals could involve specific soft-phonon modes implying the rigid tilt of octahedra composing the crystal that amplify the mobility of interstitial oxygen atoms.
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Theses
Chemical Sciences. Université Rennes 1, 2011. English. <NNT : 2011REN1S072>


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Loïc Le Dréau. Phase transitions and oxygen ordering in La2CoO4+δ and (T, T')-La2CuO4 : single crystal growth and structural studies using synchrotron and neutron diffraction methods. Chemical Sciences. Université Rennes 1, 2011. English. <NNT : 2011REN1S072>. <tel-00634848>

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