E. Husson

A structural model for the relaxor PbMg1/3Nb2/3O3 at 5 K

The perovskite structure of the relaxor PbMg1/3Nb2/3O3 (PMN) is studied using X-ray and neutron powder diffraction data. The static diffuse scattering (SDS) observed in the diffraction patterns at low temperatures is interpreted using a two-phase Rietveld analysis. The structural model is based on a long-range structure with an average cubic symmetry, and a short-range order due to atomic shifts involved by the formation of polar regions. The two-phase model provides a good improvement in reliability factors. The correlation length and the coexistence of different phases at low temperatures are discussed.

Short-range order in PbMg13Nb23O3 ceramics by Raman spectroscopy

Abstract The IR and Raman spectra at 20°C of PbMg 1 3 Nb 2 3 O 3 are reported and an assignment of the observed frequencies taking into account the different types of chemical bonds is proposed. The evolution of the spectra with temperature is analyzed. The Raman spectra are sensitive to a small modification of the short-range order i n the B site of the perovskite obtained by substitution of Nb V by W VI or Pb II by La III .

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.

Superstructure in PbMg13Nb23O3 ceramics revealed by high resolution electron microscopy

Abstract Lead magnesium niobate ceramics have been studied by high resolution electron microscopy (HREM). The electron diffraction patterns and high resolution images show the existence of a superstructure along the [111] direction of the perovskite cubic unit cell. The origin of this superstructure is discussed.

Comparison of the force field in various pyrochlore families. I. The A2B2O7 oxides

Abstract A complete vibrational study of various pyrochlore compounds A 2 B 2 O 7 shows the role of the different chemical bonds in the structure and how the physico-chemical characteristics of the A and B cations influence these bonds and the rigidity of the two lattices of the structure. Relations between vibrational spectra and structural features are established.

Modification of the B-site order of PbMg13Nb23O3 ceramics by thermal annealing or by La-doping

Abstract The interesting properties of the perovskite PbMg 1 3 Nb 2 3 O 3 for the manufacture of multilayer capacitors are due to its ferroelectric diffuse phase transition. The influence of annealing or La-doping of PMN ceramics on the diffuse character of the phase transition is studied. A model is considered, according to which the composition fluctuations induced by the growth of the 1:1 ordered domains promotes the diffuse behavior of the phase transition. The peculiar ferroelectric behavior of the annealed ceramics is discussed

Structural study of a poled PbMg13Nb23O3 ceramic at low temperature

Abstract The evolution of the structure of a poled PbMg 1 3 Nb 2 3 O 3 ceramic is studied by precise X-ray diffraction, between 300 and 5 K. The application of a high electric field at low temperature induces a macrodomain ferroelectric rhombohedral structure ( a = 4.040 A , α = 89.91°). By increasing the temperature, this structure disappears at T t # 200 K and it is transformed into the polar nanodomain structure observed when an unpoled ceramic is cooled. This transformation is irreversible.

Etudes structurales d'alumines de transition. I, Alumines gamma et delta

Abstract γ and δ aluminas obtained by dehydration of boehmite are characterized by SEM and TEM. X-ray and electron diffraction patterns show that γ-Al2O3 exhibits a slight tetragonal distortion of the cubic spinel structure; for δ-Al2O3 different structural models were considered: a tetragonal structure (space group P 4 m2 with a =5.599 A and c =23.657 A ) with a distribution of the cationic vacancies on octahedral sites of the spinel lattice is proposed. HRTEM studies reveal that both aluminas present a microdomain structure which could be induced by cationic vacancies and by very numerous micropores. The long-range order increases with the interpore distance.

Normal coordinate analysis for CaNb2O6 of columbite structure

The Raman and infrared spectra of CaNb2O6 are assigned by normal coordinate analysis according to Shimanouchi’s method. The Raman spectroscopy results enabled us to refine the force field, which was then checked by the infrared spectroscopy results. The obtained force constants show that there are three different types of Nb–O bonds in the columbite structure and that the Ca–O bonds are of electrostatic nature. The potential energy distribution calculation gives us a precise assignment for all the observed frequencies.

Spectres de vibration et calcul du champ de force des antimoniates et des tantalates de structure trirutile

Abstract The i.r. and Raman spectra of antimonates M II Sb 2 O 6 and tantalates M II Ta 2 O 6 belonging to the trirutile structure are studied. The vibrational assignment is obtained by means of the comparison of the spectra, the use of the correlation method and normal coordinate calculations. A force field characteristic of this type of compounds is proposed.