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Besson R.,CNRS Materials and Physical Engineering Laboratory
Acta Materialia | Year: 2010

The aim of this work is to give the independent-point-defect thermodynamics of ordered compounds a sufficiently general flavour, adapted to and working for multicomponent alloys. Generalizing previous approaches, we first show that an appropriate description for a crystal with point defects allows treatment of the practically important pressure and defect volume parameters in the grand canonical framework, the equivalence of which is explicited with the closer to experiments isothermal-isobaric conditions. Since industrial applications often involve multialloyed compounds, we then derive an operational tool for atomic-scale investigations of long-range order alloys with complex crystallographies and multiple additions. © 2009 Acta Materialia Inc. Source

Le Gallic M.,CNRS Materials and Physical Engineering Laboratory | Roussel H.,CNRS Materials and Physical Engineering Laboratory
Journal of Solid State Chemistry | Year: 2013

A phase of trigonal structure has been reported in previous articles to be piezoelectric with a very high longitudinal d33 coefficient of several thousand pm/V (J. Solid State Chem. 184 (2011) 2023, 2033). It was observed in Ta2O5 thin films and assumed to be the result of a reversible monoclinic-trigonal phase transition. However, new investigations are reported because the cell parameters of this phase are actually very close to those of the natrotantite mineral Na2Ta 4O11. From analyses by X-ray energy dispersive spectroscopy (XEDS), the chemical composition of this trigonal phase corresponds to (NaxK1-x)2 Ta4O11 with x≈0.93. The origin of sodium, potassium, iron and oxygen atoms is found to be due to a pollution coming from alumina crucibles used for thermal treatments. Knowing both atomic structures Ta2O5 and Na 2Ta4O11 and their structural relationships, observed by high resolution transmission electron microscopy (HRTEM), the mechanism of the reaction of transformation is re-examined and discussed. This mechanism implies that Ta5+ vacancies likely exist in monoclinic structures of tantalum oxide and that electrical neutrality could be due to incorporated proton H+ instead of O2- vacancies or Ta 5+ interstitials. © 2013 Elsevier Inc. Source

Thuinet L.,CNRS Materials and Physical Engineering Laboratory | Legris A.,CNRS Materials and Physical Engineering Laboratory
Acta Materialia | Year: 2010

The influence of a crystallographic symmetry break on the morphology of precipitates during the coherent precipitation of a trigonal phase in a close-packed hexagonal matrix is analyzed. It is pointed out that in spite of the isotropy of the stress-free strain of the precipitate in the basal plane, the existence of an extra elastic constant in the precipitate (associated to the loss of symmetry) induces a morphological evolution from a shape having a symmetry of revolution around the threefold axis to a needle-like one oriented along the compact directions in the basal plane. These general considerations are applied to the case of zirconium hydrides, the crystallography of which has recently been identified to be coherent with that of the αZr matrix. © 2009 Acta Materialia Inc. Source

Ouisse T.,CNRS Materials and Physical Engineering Laboratory | Chaussende D.,CNRS Materials and Physical Engineering Laboratory
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We show that some important features characterizing the M n +1AX n phases, a family of hexagonal-structure ternary carbides and nitrides (X) including a transition metal (M) and an A-group element (A), can be reproduced by modifying the spin model known as the axial next-nearest-neighbor Ising model into a form where pseudospin inversion changes the system energy and requires the inclusion of single- and three-spin products. We describe the various MAX phases in terms of M-A or M-X bilayer stacking along the c axis. We discuss the dependence of the cohesive energy and phase stability on coupling parameters which characterize the first- and second-neighbor interactions between those bilayers. We also address the case of "hybrid" MAX phases. © 2012 American Physical Society. Source

Chaix-Pluchery O.,CNRS Materials and Physical Engineering Laboratory | Kreisel J.,CNRS Materials and Physical Engineering Laboratory
Phase Transitions | Year: 2011

We report an investigation of perovskite-type SmScO3 and NdScO3 single crystals by Raman scattering in various scattering configurations and at different wavelengths. The reported Raman spectra, together with the phonon mode assignment, set the basis for the use of Raman scattering for the structural investigation of rare earth (RE) scandates. Further to the phonon signature, a fluorescence signal is observed for both scandates and is particularly intense for NdScO3 when using a 488 or 514 nm excitation line. A comparison of Raman spectra of RE-scandates with literature Raman data on orthorhombic perovskites shows that the frequency of particular modes scales with the orthorhombic distortion in terms of the rotation (or tilt) angle of the ScO6 octahedra. © 2011 Taylor and Francis. Source

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