D. Mercurio

Superspace description of the hexagonal perovskites in the system Ba5Nb4O15-BaTiO3 as modulated layered structures

Abstract A general structural model of B -site deficient hexagonal layer perovskites AB 1− x X 3 having no tilting of anion octahedra in consecutive layers is presented. They are described as modulated layered structures using a (3+1)-dimensional superspace approach. The presence or not of a given atom along the stacking direction is described in superspace through step-like (crenel) occupational modulation functions. Being essentially composition independent, the model is applicable to any compounds related to this structural type. The component γ of the primary modulation vector q =γ c * is connected to the chemical composition by the relation γ =(1+ x )/3. The model is confirmed and the efficiency of the superspace approach is demonstrated by the structural analysis of some of the hexagonal perovskite found in the system Ba 5 Nb 4 O 15 –BaTiO 3 , where long-period intergrowth compounds have been recently identified.

Structural Behaviour of the Mixed-Layer Aurivillius-Phase Bi7Ti4NbO21

The structural behaviour of the mixed-layer Aurivillius-phase Bi7Ti4NbO21 as a function of temperature is investigated by Neutron Powder Diffraction (NPD). The first dielectric anomaly is associated to the regularisation of the “Bi4Ti3O12 part” of the structure while the “Bi3TiNbO9 part” is almost unaffected. The data recorded above the second dielectric anomaly indicate the probable existence of an intermediate orthorhombic paraelectric phase.

Martensitic phase transition in pure zirconia: a crystal chemistry viewpoint

Abstract A crystal chemistry approach was carried out in order to decipher the mechanisms involved in phase transitions of zirconia. A detailed analysis of structures, based on structural sheets of half fluorite cell thick and described in terms of SDF and HDF (for slightly and highly distorted fluorite) sheets is proposed for the first time. This approach allowed clarifying the relationships between tetragonal, orthorhombic and monoclinic forms. This concept permits to propose simple structural relationships between the structures of the different polymorphs. These relationships and their correlated transformation pathways were confirmed using symmetry group relations. The results were compared to those obtained by the phenomenological approach of the tetragonal → monoclinic (T → M) martensitic phase transition. The different lattice correspondences are revisited and discussed. For the first time, some fine structural models were proposed to describe the pathways of atomic displacements taking place during each possible phase transitions. The T→M phase transition is the main transformation pathway. It may solely gives rise to the C-type lattice correspondence, the (100)M||(100)T and [001]M||[001]T relationship allowing the minimum structural changes during the transformation. The B-type variant with (100)M||(100)T and [010]M||[001]T may be developed from an indirect pathway involving either a ferroelastic phase transition by domain switching taking place in the T phase before the T → M phase transition operates or a two stage process involving a combined T → O and O → M successive and independent transitions. The orthorhombic phase is an alternative form to the monoclinic one and is in no way an intermediate phase.

Crystal structure of the aurivillius phases in the system Bi4Ti3O12-PbTiO3

The crystal structure of four Aurivillius phases within the pseudo-binary system Bi4Ti3O12—PbTiO3 is examined using a 3 + 1D approach where these phases are treated as commensurately modulated layered structures. The structure of PbBi4Ti4O15, Pb2Bi4Ti5O18, Pb3Bi4Ti6O21 and PbBi8Ti7O27 are refined by the Rietveld method using XRPD data from single phased powders. The structure of Pb3Bi4Ti6O21 and PbBi8Ti7O27 is reported for the first time. When the lead content increases, the structure gets less distorted and approaches the prototype paraelectric phase in an apparent close relation with the onset of a “relaxor-like” ferroelectric behaviour. For these relaxor compounds, SAED investigation evidences the existence of a disorder in the form of micro-twinning.

The high-temperature and intermediate phases of ferroelectric Aurivillius compounds in superspace. A unified approach

The recently proposed superspace description of the Aurivillius compounds in their ferroelectric phase is here extended to their high-temperature phases. The probable existence of an intermediate phase between the tetragonal high-symmetry and the ferroelectric phases is stressed. The relevant superspace groups for both the tetragonal and the intermediate phases are identified. Consequently, the 3D space groups for the intermediate phases as a function of composition are predicted.