title: Phase Transitions in the Ruddlesden-Popper Phase Li2CaTa2O7: X-ray and Neutron Powder Thermodiffraction, TEM, Raman, and SHG Experiments.
abstract: The structure of the Ruddlesden-Popper layered perovskite Li2CaTa2O7, known for its high photocatalytic water activity since its discovery in 2008, is reinvestigated. This oxide has been characterized by powder X-ray and neutron thermodiffraction, TEM, second harmonic generation (SHG), and Raman experiments on powders and single crystals. It is shown that it undergoes two structural phase transitions (i) around 220 °C, mainly characterized by the progressive emergence of SHG signal at low temperatures, and (ii) at 660 °C, mainly characterized by changes of the temperature behavior of lattice parameters and by the emergence of Raman signals that linearly increase on decreasing temperature. It is shown by powder neutron diffraction profile refinements at RT, 400, and 800 °C that the space groups of the successive phases of Li2CaTa2O7 are the acentric Pna21 (RT ≤ T ≤ 220 °C), Pnma (220 °C ≤ T ≤ 660 °C), and Cmcm (T ≥ 660 °C). A soft mode associated with the transition to the highest symmetry for this structural arrangement (I4/mmm) is also found in the Raman spectra. All these transitions appear continuous: the high temperature ones can be attributed to progressive vanishings of the octahedra tiltings (displacives) while the transition in the vicinity of 220 °C from Pna21 to Pnma exhibits order-disorder character.
authors.affiliation: LUNAM Université du Maine, Institut des Molécules et Matériaux du Mans (IMMM), UMR CNRS 6283 , Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France.
authors.forename: Cyrille
authors.initials: C
authors.lastname: Galven
authors.affiliation: LUNAM Université du Maine, Institut des Molécules et Matériaux du Mans (IMMM), UMR CNRS 6283 , Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France.|Ecole Nationale Supérieure d'Ingénieurs du Mans (ENSIM), Université du Maine , Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France.
authors.forename: Denis
authors.initials: D
authors.lastname: Mounier
authors.affiliation: LUNAM Université du Maine, Institut des Molécules et Matériaux du Mans (IMMM), UMR CNRS 6283 , Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France.
authors.forename: Boris
authors.initials: B
authors.lastname: Bouchevreau
authors.affiliation: Institut Laue-Langevin , 6 rue G. Horowitz, 38042 Grenoble Cedex 9, France.
authors.forename: Emmanuelle
authors.initials: E
authors.lastname: Suard
authors.affiliation: LUNAM Université du Maine, Institut des Molécules et Matériaux du Mans (IMMM), UMR CNRS 6283 , Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France.
authors.forename: Alain
authors.initials: A
authors.lastname: Bulou
authors.affiliation: LUNAM Université du Maine, Institut des Molécules et Matériaux du Mans (IMMM), UMR CNRS 6283 , Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France.
authors.forename: Marie-Pierre
authors.initials: MP
authors.lastname: Crosnier-Lopez
authors.affiliation: LUNAM Université du Maine, Institut des Molécules et Matériaux du Mans (IMMM), UMR CNRS 6283 , Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France.
authors.forename: Françoise
authors.initials: F
authors.lastname: Le Berre