P. Garnier

Structural study of PMN ceramics by x-ray diffraction between 297 and 1023 K

Abstract The variation of the cubic parameter and thermal expansion coefficient of lead magnesium niobate (PMN) ceramics was studied by very precise X-ray diffraction at fourteen temperatures between 297 and 1023 K. The refinement of the Pb and Nb atom positions at room temperature gives the lowest value of the discrepancy factor R for Pb atoms shifts of 0.33 A along [110] direction and Nb atoms shifts of 0.18A along [110] and [111] directions. At temperatures higher than 600 K, the Nb atoms are no more located at any local disordered position and above 900 K, the Pb atoms are also on the special position of the ideal cubic perovskite structure.

Evolution des tenseurs de dilatation thermique en fonction de la temperature. I. Loi generale d'evolution de la symetrie du tenseur

Abstract In studying the variation of the symmetry of the thermal expansion tensor as a function of temperature, we first define a measure of this symmetry, called aspherism index . Then we analyze experimental results of thermal expansion measurements obtained from various authors, for a set of 8 crystallized solids which show order-disorder phase transitions and for a set of 38 solids which do not. A law relating to variation of the aspherism index with respect to temperature is given.

Mise en evidence et localisation des protons dans les bioxydes de plomb PbO2α et β chimiques et électrochimiques

Abstract Chemical and electrochemical samples of lead dioxides PbO2α (orthorhombic) and PbO2β (tetragonal) are studied by quasi-elastic neutron scattering and by X-ray diffraction between 77 and 550 K. The presence of OH−-like proton species is established: the electrochemical species are characterized by two types of local proton motions ( ΔE 1 2 = 300 μ eV , ΔE 1 2 = 40 μ eV ). The thermal expansion of the chemical and electrochemical sample shows: (1) the loss of OH−-like charged species after heating to 550 K, (2) an anisotropy of the OH ··· O bonds in the [PbO6] octahedra chains which are oriented differently in the α and β structures. An interpretation of the role of protons is proposed.

Etude des phases ferroelastiques de Pb3V2O8 : Determination de la maille cristalline de la phase α ferroelectrique basse temperature

Abstract The cell parameters of the ferroelastic and ferroelectric phase of Pb 3 V 2 O 8 (T a = 7.460 (1) A , b = 6.191 (1) A , c = 9.348 (1) A and β = 116.63 (1) A . This monoclinic cell is characterized by Z = 2 and the space group is A2. Thermal expansion tensors have been measured versus temperature for the three phases α (T 360 K). The intermediate phase β is characterized by an important thermal expansion anisotropy and its molecular volume is higher than that of the two other phases α and γ.

Neutron and X-ray Rietveld analysis of the three phases of lead orthovanadate Pb3V2O8: Importance of the electronic lone pairs in the martensitic transitions

Abstract Neutron and X-ray powder diffraction investigations have been performed for the three phases of Pb3V2O8. The structural results from the Rietveld refinements are analyzed and compared with the available data of both phases of Pb3P2O8. The rhombohedral high temperature structures consist of layers of [PO4] or [VO4] tetrahedra. Inside this skeleton of tetrahedra the lead atoms have bonds of different lengths with the oxygen atoms in order to adapt themselves to the packings. The layers of the structures are connected by the electronic lone pairs of the lead ions inside the interlayers. The different phase transitions of both compounds mainly differ by the way the steric occupation of the electronic lone pairs is realized inside the interlayers. The microscopic interpretation of the large shears observed during the martensitic-like transitions is explained on the basis of the tilting of the lone pairs at the critical temperatures.

Structure cristalline de la phase haute temperature de PbSO4.2PbO A 973K

Abstract Full profile X-ray powder diffraction structure refinements have been performed on the two polymorphs of the PbSO4.2PbO phase. Both structures can be described as infinite double-chains of edge-sharing OPb4 tetrahedra linked by the sulfate groups and running parallel to the (010) direction for the low temperature phase, and parallel to the (001) direction for the high temperature one. The mechanism of the phase transition may be described as a rotation of the sulfate group about one of its pseudo-ternary axis. The space group adopted for the refinement of the structure of the high temperature phase is Cmcm.

Analyse de rietveld de la structure de Pb1−xTixO1+x par diffraction des neutrons

Abstract The Rietveld neutron powder profile analysis of Pb 1−x Ti x O 1+x is reported. The structure is similar to the ∝-PbO one (P 4/nmm; Z=2). A contraction of the cell parameters is observed when substituting titanium for lead. The extra oxygen atom is located in the interlayer of the host structure on the site of the electron lone pair of the Pb 2+ ion; a new interlayer Pb-O bond appears and so the cohesion of the structure increases. The Ti coordination polyhedron is similar for Pb 1−x Ti x O 1+x and PbTiO 3 , whose structures are compared.

Modèle de décomposition thermique des bioxydes de plomb: rôle des protons, relation avec la structure de l'oxyde pseudocubique PbO1.57

Abstract The thermal decomposition of lead dioxides α-PbO2 and β-PbO2 is explained according to a complex model; postulating the existence of Pb24O48 and Pb24O40(OH)8 groups in the initial dioxide, it is possible to explain the various steps of the thermal decomposition from Pb24O44(OH)4 to β-PbO in relation with the various structures of these oxides. Structural data of the pseudocubic oxide PbO1.57 are used to justify this model; PbO1.57 appears as a juxtaposition of Pb24O36 and Pb24O40 zones with similar structures. The influence of the grain size and the speed of heating are also explained.

Oxydes de plomb: V. Etude de la texture des phases quadratique et orthorhombique de Pb3O4: Influence des defauts sur la transition de phase

Abstract Several samples of Pb 3 O 4 have been prepared by oxidizing PbO in air at various temperatures in the range 705–815°K. A correlation is established between the nonstochiometry of the samples and their X-ray diffraction line profiles at 295°K which are characteristic of an orthorhombic distortion of the tetragonal lattice. In the high-temperature phase ( T > 170°K), orthorhombic microdomains exist in the tetragonal matrix. The mean distortion increases with the nonstochiometry of the compound. Below 170°K Pb 3 O 4 exhibits an orthorhombic phase with orthorhombic domains according to two orientation states, and para crystalline distortion. A model of texture is proposed and compared with the high-temperature one. The pretransitional effect which is observed between 250 and 170°K is correlated with the presence of orthorhombic microdomains in the high-temperature phase (tetragonal).

Proton motions in battery lead dioxides

Abstract Proton motions in α- and β-PbO2 varieties have been studied using neutron quasielastic scattering in the temperature range 77–300 K. Four varieties of PbO2 have been characterized: α and β electrochemical PbO2 and two samples obtained from a battery after two types of cycling. Chemically prepared samples were used to compare quasielastic scattering signals. In electrochemical PbO2, two types of motions of protons are clearly evidenced: (i) a rotation-like local motion (FWHM = 280 μeV, R = 1.1A) which disappears at low temperature or after heating and (ii) a translational motion (Dt ∼ 3 × 10−6 cm2/sec or, using a jump model, τJ = 5 × 10−12 sec, dJ ∼ 1.5 A). In all samples, a systematic contribution of host lattice is observed in the elastic signals: it might be linked with disordered clusters due to the method of preparation and the presence of protons; two types of elastic contribution can be defined, one connected with some correlations of static distortions.