Acta Crystallographica Section B

Structural Science

Volume 55, Part 4 (August 1999)

Acta Cryst. (1999). B55, 459-466    [doi:10.1107/S0108768198017984]

Structures des phases paraélectrique et ferroélectrique de Pb₂KNb₅O₁₅

Ph. Sciau, G. Calvarin et J. Ravez

Abstract: The paraelectric and the ferroelectric/ferroelastic structures of Pb₂KNb₅O₁₅ 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 Pb₂KTa₅O₁₅. 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 PbNb₂O₆. The orthorhombic distortion is of the same order but the Curie temperature is lowered by the substitution 2K⁺-Pb²⁺. The structures of PbNb₂O₆, PbTa₂O₆, Pb₂KNb₅O₁₅ and Pb₂KTa₅O₁₅ 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 PbTa₂O₆, the oxygen-lattice distortion is very weak. The polarization seems to be due only to the Pb atom. In the tantalates the substitution 2K⁺-Pb²⁺ 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.

Online 1 August 1999

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