Structures des phases paraélectrique et ferroélectrique de Pb2KNb5O15

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 at = 12.646 (1) and ct = 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 ao = 17.779 (2), bo = 18.015 (2), co = 3.9209 (4) Å at 294 K and ao = 17.756 (1), bo = 18.019 (1), co = 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.