Finite-Temperature Properties of Ba(Zr,Ti)O3_3 Relaxors From First Principles

A first-principles-based technique is developed to investigate properties of Ba(Zr,Ti)O$_3$ relaxor ferroelectrics as a function of temperature. The use of this scheme provides answers to important, unresolved and/or controversial questions, such as: what do the different critical temperatures usually found in relaxors correspond to? Do polar nanoregions really exist in relaxors? If yes, do they only form inside chemically-ordered regions? Is it necessary that antiferroelectricity develops in order for the relaxor behavior to occur? Are random fields and random strains really the mechanisms responsible for relaxor behavior? If not, what are these mechanisms? These {\it ab-initio-based} calculations also leads to a deep microscopic insight into relaxors.Comment: 3 figures + Supplemen

Random Electric Field Instabilities of Relaxor Ferroelectrics

Relaxor ferroelectrics are complex oxide materials which are rather unique to study the effects of compositional disorder on phase transitions. Here, we study the effects of quenched cubic random electric fields on the lattice instabilities that lead to a ferroelectric transition and show that, within a microscopic model and a statistical mechanical solution, even weak compositional disorder can prohibit the development of long-range order and that a random field state with anisotropic and power-law correlations of polarization emerges from the combined effect of their characteristic dipole forces and their inherent charge disorder. We compare and reproduce several key experimental observations in the well-studied relaxor PbMg1/3Nb2/3O3–PbTiO3.Universidad de Costa Rica/[816-B7-601]/UCR/Costa RicaBasic Energy Sciences, Department of Energy/[contract no. DE-AC02-06CH11357]//Estados UnidosUCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigación en Ciencia e Ingeniería de Materiales (CICIMA

Finite-temperature properties of the relaxor PbMg 1/3 Nb 2/3 O 3 from atomistic simulations

International audienceAn atomistic numerical scheme is developed and used to study the prototype of relaxor ferroelectrics, that is PbMg 1/3 Nb 2/3 O 3 (PMN), at finite temperatures. This scheme not only reproduces known complex macroscopic properties of PMN, but also provides a deep microscopic insight into this puzzling system. In particular, relaxor properties of PMN are found to originate from the competition between (1) random electric fields arising from the alloying of Mg and Nb ions belonging to different columns of the Periodic Table within the same sublattice; (2) the simultaneous condensation of several off-center k points as a result of a specific short-range, antiferroelectriclike interaction between lead-centered dipoles; and (3) ferroelectriclike interactions. Such origins contrast with those recently proposed for the homovalent Ba(Zr,Ti)O 3 solid solution, despite the fact that these two materials have similar macroscopic properties—which therefore leads to a comprehensive understanding of relaxor ferroelectrics

Morphotropic phase boundary of heterovalent perovskite solid solutions: Experimental and theoretical investigation of PbSc1/2Nb1/2O3-PbTiO3

International audienceX-ray and neutron diffraction techniques are combined with first-principles-based simulations to derive and understand the structural properties of Pb(Sc,Nb,Ti)O-3 (PSN-PT) near its morphotropic phase boundary (MPB). An analysis of our measurements yields, at room and low temperatures, an overall tetragonal T-monoclinic M-C-monoclinic M-B-rhombohedral R path (when adopting the notations of Vanderbilt and Cohen, Phys. Rev. B 63, 94108 (2001) for the monoclinic phases) as the Ti composition decreases across the MPB. A composition- and temperature-dependent significant mixing between some of these phases is also measured and reported here. The overall T-M-C-M-B-R path, which has also been proposed for Pb(Mg,Nb,Ti)O-3 [A. K. Singh and D. Pandey, Phys. Rev. B 67, 64102 (2003)] is rather complex since it involves a change in the polarization path: this polarization first rotates in a (100) plane for the T-M-C part of the path and then in a (1-10) plane for the M-B-R part of the path. Moreover, a comparison between these measurements and first-principles-based calculations raises the possibility that this complex path, and the associated M-C and M-B phases, can only occur if the samples exhibit a deviation from a perfectly homogeneous and disordered situation, e.g. possess nanoscale chemically-ordered regions. If not, homogeneously disordered PSN-PT is predicted to exhibit at low temperature the same polarization path as Pb(Zr,Ti)O-3, that is T-monoclinic M-A-R which involves a "single" polarization rotation in a (1-10) plane. Nanoscale inhomogeneity may thus play a key role on the macroscopic properties of PSN-PT, in particular, and of other heterovalent complex solid solutions, in general, near their MP

A new temperature scale T* in lead-based relaxor systems

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Cationic-competition-induced monoclinic phase in high piezoelectric (PbSc1/2Nb1/2O3)1-x-(PbTiO3)x compounds

International audienceA global picture for the structural evolution in the relaxor-ferroelectric solid solution (PbSc1/2Nb1/2O3)1-x-(PbTiO3)x is proposed. Thanks to x-ray profile analysis and Rietveld neutron powder refinement, a monoclinic phase has been evidenced in the morphotropic region (i.e., x≈0.43). This lower-symmetry phase “bridges” the rhombohedral Ti-poor phase (x0.55), in a similar way as in PbMg1/3Nb2/3O3-PbTiO3 or Pb(Zn1/3Nb2/3)O3-PbTiO3. For weak titanium concentration, we observe a macroscopic rhombohedral state with local monoclinic symmetry resulting from the combination between Pb and Sc/Nb/Ti shifts along [001] and [111] directions, respectively. Cationic competition with Ti doping increases the coherence length of this short-range monoclinic phase, which becomes long range in the morphotropic region. This intermediate monoclinic phase is in complete agreement with our first-principles calculations which predict Pm or Cm space groups. It has been shown that these ones are very close to each other in the free-energy space, and a minor change of atomic distribution and/or a slight modification in composition or in stoichiometry is enough to alter the space group of the monoclinic ground state. Finally, in the Ti-rich region, the monoclinic ground state is destroyed in favor of a tetragonal phase. © 2003 The American Physical Society

Polar and chemical order in relation with morphotropic phase boundaries and relaxor behaviour in bulk and nanostructured PSN-PT

We have recently evidenced the existence of a complex path of polarisation rotation via two monoclinic phases in the giant-piezoelectric materials PSN PT that is similar to the PMN-PT system. In this article we have presented the state of knowledge on the PSN-PT system and have given new results showing the close connection between the local inhomogeneous chemical order of Sc/Nb/Ti cations and the stability of different ferroelectric phases. The notion of morphotropic phase boundaries is discussed in relation with the gradual appearance of long-range polar order with increasing Ti content in PSN-PT. Comparison with the films and nanopowders/ceramics has also been made