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.