Lead Stereochemistry in Incommensurate Ferroelectric Perovskites and in Incommensurate Lead Monoxide
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Lead stereochemistry in incommensurate ferroelectric perovskites and in incommensurate lead monoxide Gianguido Baldinozzi, Jean-Marc Raulot and Ram Seshadri1 SPMS, CNRS Ecole Centrale Paris, 92295 Châtenay-Malabry, France SSSCU, Indian Institute of Science, Bangalore, India
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ABSTRACT We have performed Rietveld refinements on neutron and synchrotron diffraction patterns and density functional calculations on various ferroelectric lead perovskites and on α lead monoxide (litharge). These structural data have allowed to shed some light on lead stereochemistry in these compounds. In particular, we discuss the changing in the lead behaviour between the paraelectric cubic phases and the low temperature anti or ferroelectric phases in Pb2CoWO6 and Pb2MgTeO6 (both incommensurate), in Pb2MgWO6 (antiferroelectric) and in PbMg1/3Nb2/3O3 (relaxor). The possible phase transition mechanisms are reviewed and the bonds are compared to those in the aperiodic structure of α-lead monoxide. INTRODUCTION The interesting dielectric properties of ferroelectric materials spring from the coupling of the electric field to polar distortions of the crystal lattice. In the soft mode framework, the progressive softening of a lattice vibrational mode near the phase transition is responsible for the large rise in the value of the dielectric constant. The resulting very large relative permitivities (in the thousands range) have found important technological applications as high density capacitors, actuators, pyro-electric detectors and piezo-electric transducers [1]. From a practical point of view, however, the development of these materials faces the difficulty of trying to split experimentally the influences of various effects: composition, structure, domain configuration [2-9]. The object of this work is to go beyond the average structure approximation usually obtained in conventional Rietveld refinements and to compare different approaches to better model the deviations from the ideal structure (superspace approach, anharmonic treatments …). The knowledge of these deviations from the ideal structure at different temperatures are necessary for the understanding of the relationships between structural modifications and physical properties. Moreover, since the charge density is not easily accessible from diffraction experiments because of the heavy atoms in the structure, density functional calculations of the electronic structure were performed in lead complex perovskites and in lead monoxide to provide a better understanding of the behaviour of Pb-O bonding in these compounds.
EXPERIMENTAL We have spanned a very large portion of the Ewald sphere up to a Q momentum transfer of about 15 Å-1 using synchrotron radiation at BM 16 beamline at ESRF or, to a lesser extent, using D12.7.1 Downloaded from https://www.cambridge.org/core. La Trobe University, on 20 Jun 2020 at 14:12:24, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-718-D12.7
neutron diffraction at D1a at ILL.
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