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.
Sciau
P
Centre d'Elaboration de Matériaux et d'Etudes Structurales, UPR CNRS 8011, BP 4347, 31055 Toulouse CEDEX 04, France, and Laboratoire de Chimie Physique du Solide, Ecole Centrale Paris, URA CNRS 453, 92295 Chatenay Malabry CEDEX, France. sciau@cemes.fr
Calvarin
G
Ravez
J