Structural Phase Transition in Tetrafluoroaluminates
- PDF / 2,598,705 Bytes
- 8 Pages / 414 x 635.4 pts Page_size
- 97 Downloads / 206 Views
STRUCTURAL PHASE TRANSITION IN TETRAFLUOROALUMINATES
J.M. LAUNAY¶, J.Y. LAVAL , A. GIBAUDI, A. BULOU and J. NOUET + Laboratoire d'Etude et de Synth~se des Microstructures EPA 912 CNRS-ESPCI, 10, rue Vauquelin - 75231 PARIS cedex 05 France * Laboratoire de Physique de I'Etat Condens& ERA 682, Universitg du Maine LE MANS Cedex, France
ABSTRACT It is shown that the detailed investigation of an irreversible SPT implies the combination of X-ray, neutron and electron diffraction data. This diffraction analysis is applied to K Al F 4 and Rb Al F4. There is a good agreement between the three diffraction techniques in the case of Rb Al F where the SPT is reversible. On the other hand, for K Al F 4 room temperature quadratic phase, electron diffraction exhibits exta spots which are not evidenced by X-ray and neutron diffraction. Finally the assumption of an orthorombic lattice from neutron scattering for the low temperature phase is consistant with electron diffraction results.
INTRODUCTION The structure of the tetrafluoroaluminates A Al F4 (A = Rb, Tl, K) is derived from the ideal tetragonal TI Al F structure [1) where Al F 6 octahedron is centered in a square base paraltelepipedof cation A (Fig 1) and the space group is P 4/m m m. The Al F 6 octahedra are corner linked in a two- dimensional array normal to the tetragonal axis, while the successive octohedra sheets along this axis are disconnected (Fig 2). As a consequence, these materials exhibit marked layer properties and can be cleaved very easily. The importance of these compounds comes from the occurence of several structural phase transitions (SPT) in connection with their two-dimensional character. The present paper is devoted to the structural investigations of the low temperature phase of K Al F 4 by neutron powder diffraction, electron and X-ray diffraction on crystal. The corresponding transition in Rb Al F4 is considered as a comparison. EXPERIMENTAL PROCEDURE The crystals of K Al F and Rb Al F4 are prepared in a sealed platinum crucible by hydrothermal growth* or by the horizontal Bridgman method for larger samples. In order to get a high purity sample, the powder used in the neutron diffraction experiments is obtained by pulverizing crystals. However, this powder gives enlarged diffraction lines and it must be annealed before being studied by the neutron powder profile refinement method. Neutron powder diffraction patterns were recorded on D I A at I.L.L. Grenoble with a 1,909 A and 2,98 A wavelength. For electron diffraction experiments the samples have been thinned by cleavage parallel to the basal plane. Electron diffraction diagrams have been recorded at 100 KeV in T.E.M. on a Siemens Elmiskop IA and on a Jeol 100 CX fitted with cooling and regulating device and also at higher voltage ie 1000 KeV on the CNRS-ONERA microscope. The symmetry and intensity ratio of spots have been tested and measured with a double beam micro densitometer. * J.L. FOURQUET, Laboratoire des Fluorures et oxyfluorures ioniques EPA 609 Universit6 du Maine - LE MANS FRANCE Mat.
Data Loading...
