Philippe Sciau

Phase Transitions in Disordered Lead Iron Niobate - X-Ray and Synchrotron Radiation Diffraction Experiments

Diffraction above room temperature, on powders and crystals of Pb(Fe0.5Nb0.5)O-3, using X-rays and synchrotron radiation, led to a novel and consistent picture of the symmetry of the phases in this disordered perovskite. The compound is a monoclinic ferroelectric at 293 K, undergoes a structural transition to another ferroelectric, tetragonal state at about 355 K and finally transforms into a cubic form at approximately 376 K

Ferroelastic symmetry changes in the perovskite PbFeY0.5Ta0.5O3

The ferroelastic symmetry changes of the lead-based complex perovskite PbFeY0.5Ta0.5O3 are investigated by means of single crystal x-ray diffraction and observations in polarized light microscopy at different temperatures. The position of the 400 and 222 cubic reflections is followed between 80 K and 300 K. Results indicate the presence of two structural transitions at about 270 K and 220 K. Such temperatures define the stability range of three different phases. The high-temperature phase (T > 270 K) is the optically anomalous paraelectric prototype, the optical symmetry of which is uniaxial (tetragonal) in spite of the diffractometric (pseudo)cubic symmetry. The intermediate derived phase (270 K > T > 220 K) is tetragonal. This tetragonal phase coexists with a monoclinic one for 220 K > T > 200 K. Below 200 K only the monoclinic phase is stable. The (pseudo)cubic-to-tetragonal transition seems to be of the second order. The tetragonal-to-monoclinic one is of the first order.

Structures des phases paraélectrique et ferroélectrique de Pb2KNb5O15

The paraelectric and the ferroelectric/ferroelastic structures of Pb(2)KNb(5)O(15) have been refined by the Rietveld method from neutron and X-ray powder diffraction data. The paraelectric phase is tetragonal (space group P4/mbm) with lattice parameters a(t) = 12.646 (1) and c(t) = 3.9551 (2) Å at 790 K. The Pb- and K-atom distribution has been determined. It was established that the Pb atoms situated on the 15-coordinate sites are not on the special position (2mm) but are distributed over two disordered positions as in the tetragonal phase of Pb(2)KTa(5)O(15). The ferroelectric/ferroelastic phase is orthorhombic. The space group of the average structure is Cm2m and the lattice parameters are a(o) = 17.779 (2), b(o) = 18.015 (2), c(o) = 3.9209 (4) Å at 294 K and a(o) = 17.756 (1), b(o) = 18.019 (1), c(o) = 3.9141 (2) Å at 81 K. It is isostructural with the average ferroelectric/ferroelastic structure of PbNb(2)O(6). The orthorhombic distortion is of the same order but the Curie temperature is lowered by the substitution 2K(+)-Pb(2+). The structures of PbNb(2)O(6), PbTa(2)O(6), Pb(2)KNb(5)O(15) and Pb(2)KTa(5)O(15) are compared. The ferroelectric polarization of the niobates is due to both the Pb off-site position and the Nb displacements, which induce a distortion of the oxygen lattice. In PbTa(2)O(6), the oxygen-lattice distortion is very weak. The polarization seems to be due only to the Pb atom. In the tantalates the substitution 2K(+)-Pb(2+) should weaken the correlations between the Pb atoms and consequently suppress the ferroelectric transition. Only a local polarization should take place; this might be the origin of the dielectric relaxation observed in this compound.

Rietveld refinement of the incommensurate structure of the elpasolite (ordered perovskite)

Lead magnesium tellurate, , undergoes two structural phase transitions at about 194 K and 142 K. The structures of the cubic and of the lower-temperature phase have been refined by the Rietveld method at three different temperatures (350, 220 and 6 K). The relative simplicity of the structure has allowed the incommensurately modulated structure for powder samples to be characterized. The modulated displacements of O atoms give rise to a large spread of bond lengths. Pb atoms are disordered and their average positions are away from the threefold axis. The structural resolution is compared to that from the analysis of the symmetry-adapted coordinates of the optic modes becoming totally symmetric in the lower-temperature phase.

High temperature solution growth of perovskite Pb2CoWO6 single crystals

Single crystals of the perovskite Pb2CoWO6 (PCW) of sufficient size and homogeneity for optical and X-ray characterizations have been grown in sealed Pt crucibles by a slow cooling method of high temperature solution growth using PbO as flux. The best growth conditions were found to be: (1) 1130 to 830°C as growth temperatures, (2) (1 − x)PCW + xPbO with x (in wt%) varying from 0.35 to 0.43 as starting compositions and (3) 0.3 to 0.4°C/h as cooling rates. The common problem of separating the grown crystals from the residual flux has been solved by a flux separation technique using a porous ceramic piece which sucked the liquid at the end of the growth process inside the sealed Pt crucible. The growth morphology of PCW was found to be related to the cooling rate of the growth solution. Quasi-cubic {100} crystals with small {111} faces were grown at a small cooling rate, whereas cubo-octahedral {111}+{100}; crystals at a higher cooling rate. The PbO concentration in the starting composition was shown to have no significant effect on the growth morphology of PCW. Growth mechanisms are discussed according to the results of some surface micromorphological observations. Growth spirals and growth layers were observed on the (100) surface, giving evidence of a layer growth mechanism. A higher cooling rate induced growth instability and brought on formation of hopper crystals which are difficult to be cut and polished into thin plates necessary for subsequent characterizations. Some results of a polarized light microscopy study of domains in transmitted light and of X-ray diffraction characterizations are summarized, showing the high quality of the grown crystals.

Electron diffraction study of the Pb2CoWo6 phases

Abstract A study by electron microscopy has been started in order to clarify the crystallographic features of Pb2CoWO6. Diffraction patterns have been obtained on monodomains of the incommensurate phase (II). The average monoclinic symmetry has been confirmed and the modulated wave vector determined. For the phase III a new orthorhombic cell is proposed.

Magnetoelectric measurements on BaMnF4

Abstract A quadratic magnetoelectric effet is detected in the incommensurate antiferro-magnetic phase of BaMnF4. Two of the eight third order coefficients have been determined at 4K: βxxx = 1.1 10−19 s.A−1, βxzz = -1.6 10−19 s.A−1 (relative values, not correlated with the sign of the spontaneous polarization).

Convergent beam electron diffraction study of Pb2CoWO6 at 100K

Abstract A study by convergent beam electron diffraction has been performed in order to clarify the symmetry of the phase III of Pb2CoWO6. Microdiffractions with higher-order Laue zone on monodomain areas have confirmed the cell proposed in our preceding study. In addition, an ‘axial’ glide plane (c) has been found normal to the b axis. In the conditions of observation, the convergent beam electron diffraction patterns lead to an apparent point group mmm. No breakdown of a mirror symmetry could be found. The discrepancy between this result and the existence of a spontaneous polarisation is discussed.