Instituto de Ciencia de Materiales de Madrid

ABSTRACT The transition between the relaxor and ferroelectric states for 0.8Pb(Mg1/3Nb2/3)O3–0.2PbTiO3 and 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 polycrystalline aggregates has been studied by measurements of the dielectric and elastic properties as a function of temperature. The ferroelectric to relaxor transition seems to present a regime of ferroelectric fluctuations that is sharply interrupted by the appearance of the polar nanodomains. It also presents thermal hysteresis, not only in the transition temperature, Tc, but in the characteristic time scale of the ferroelectric fluctuations that is longer on cooling than on heating. The latter effect is much more significant for 0.8Pb(Mg1/3Nb2/3)O3–0.2PbTiO3 (Tc = 346 K on heating), than for 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 (Tc ∼ 408 K on heating). This indicates that there is a sharp slowing down of the kinetics of the phase transition below 346 K.

We describe the preparation, the crystal structure refined from neutron powder diffraction (NPD) data, and study of the permittivity of two related double perovskites, Pb2Sc(Ti0.5Te0.5)O6 and Pb2Sc(Sc0.33Te0.66)O6. These compounds were synthesized by standard ceramic procedures; Rietveld refinements from room temperature NPD data show that the crystal structures are well defined in a cubic unit cell (space group Fm3m) with double parameter, a = 2a0 ≈ 8 Å. They contain a completely ordered array of ScO6 and (B,Te)O6 (B = Sc, Ti) octahedra sharing corners; the PbO12 polyhedra present an off-center displacement of the lead atoms along the [1 1 1] directions, due to the electrostatic repulsion between the Pb(2+) 6 s electron lone-pair and the Pb-O bonds of the cuboctahedron. Both compounds present a low temperature, highly dispersive maximum in permittivity, the position of which follows the Vogel-Fulcher relation with freezing temperatures of 156 and 99 K for Pb2Sc(Ti0.5Te0.5)O6 and Pb2Sc(Sc0.33Te0.66)O6, respectively, exhibiting a typical phenomenology of relaxors.

The new double perovskite Pb2ScSbO6 was synthesized by standard ceramic procedures; the Rietveld refinement of room temperature neutron powder diffraction data shows that the crystal structure is well defined in the space group Fm3[combining macron]m. It contains a completely ordered array of alternating ScO6 and SbO6 octahedra sharing corners; the PbO12 polyhedra present an off-center displacement of the lead atoms along the [111] direction, due to the electrostatic repulsion between the Pb2+ 6s lone pair and the Pb-O bonds of the cuboctahedron. Dielectric permittivity measurements show a peak near 343 K, with a Curie-Weiss response above this temperature, which suggests an antiferroelectric behavior. Finally we present a DFT study of the electronic structure of Pb2ScSbO6, showing a great difference between the electronic density within SbO6 and ScO6 octahedra.

ABSTRACT Phases appearing in lanthanum-modified lead titanate thin films prepared by a diol-based sol-gel method and crystallized by rapid heating were studied. The results clearly indicate that a phase transformation from a pyrochlore structure to the perovskite phase occurs in Pb-deficient films during the thermal treatment, which involves a heating rate higher than 500 °C min−1. The rate of this transformation is a function of the lead content of the films, decreasing as lead volatilizes. Temperatures higher than 650 °C or soak times longer than 2 h make possible the complete pyrochlore-to-perovskite transformation without any lead excess in the films.