Effects of coupling between octahedral tilting and polar modes on the phase diagram of PbZr1-xTixO3 (PZT)

The results are presented of anelastic and dielectric spectroscopy measurements on large grain ceramic PZT with compositions near the two morphotropic phase boundaries (MPBs) that the ferroelectric (FE) rhombohedral phase has with the Zr-rich antiferroelectric and Ti-rich FE tetragonal phases. These results are discussed together with similar data from previous series of samples, and reveal new features of the phase diagram of PZT, mainly connected with octahedral tilting and its coupling with the polar modes. Additional evidence is provided of what we interpret as the onset of the tilt instability, when is initially frustrated by lattice disorder, and the long range order is achieved at lower temperature. Its temperature T_IT(x) prosecutes the long range tilt instability line T_T(x) up to T_C, when T_T. It is proposed that the difficulty of seeing the expected 1/2 modulations in diffraction experiments is due to the large correlation volume associated with that type of tilt fluctuations combined with strong lattice disorder. It is shown that the lines of the tilt instabilities tend to be attracted and merge with those of polar instabilities. Not only T_IT bends toward T_C and then merges with it, but in our series of samples the temperature T_MPB of the dielectric and anelastic maxima at the rhombohedral/tetragonal MPB does not cross T_T, but deviates remaining parallel or possibly merging with T_T. These features, together with a similar one in NBT-BT, are discussed in terms of cooperative coupling between tilt and FE instabilities, which may trigger a common phase transition. An analogy is found with recent simulations of the tilt and FE transitions in multiferroic BiFeO3. An abrupt change is found in the shape of the anelastic anomaly at T_T when x passes from 0.465 to 0.48, possibly indicative of a rhombohedral/monoclinic boundary.Comment: 15 pages, 11 figure

Piezoelectric softening in ferroelectrics: ferroelectric versus antiferroelectric PbZr1−x_{1-x}Tix_{x}O3_{3}

The traditional derivation of the elastic anomalies associated with ferroelectric (FE) phase transitions in the framework of the Landau theory is combined with the piezoelectric constitutive relations instead of being explicitly carried out with a definite expression of the FE part of the free energy. In this manner it is shown that the softening within the FE phase is of electrostrictive and hence piezoelectric origin. Such a piezoelectric softening may be canceled by the better known piezoelectric stiffening, when the piezoelectric charges formed during the vibration are accompanied by the depolarization field, as for example in Brillouin scattering experiments. As experimental validation, we present new measurements on Zr-rich PZT, where the FE phase transforms into antiferroelectric on cooling or doping with La, and a comparison of existing measurements made on FE PZT with low frequency and Brillouin scattering experiments

Effects of aging and annealing on the polar and antiferrodistortive components of the antiferroelectric transition in PZT

The antipolar and antiferrodistortive (AFD) components of the antiferroelectric (AFE) transition in PbZr1-xTixO3 (x=0.054) can occur separately and with different kinetics, depending on the sample history, and are accompanied by elastic softening and stiffening, respectively. Together with the softening that accompanies octahedral tilting in the fraction of phase that is not yet transformed into AFE, they give rise to a variety of shapes of the curves of the elastic compliance versus temperature. All such anomalies found in samples with x=0.046 and 0.054, in addition to those already studied at x=0.050, can be fitted consistently with a phenomenological model based on the simple hypothesis that each of the polar and AFD transitions produces a step in the elastic modulus, whose position in temperature and width reflect the progress of each transition. The slowing of the kinetics of the transformations is correlated with the formation of defect structures during aging in the ferroelectric or AFE state, which are also responsible for a progressive softening of the lattice with time and thermal cycling, until annealing at high temperature recovers the initial conditions

Phase transitions and phase diagram of the ferroelectric perovskite NBT-BT by anelastic and dielectric measurements

The complex elastic compliance and dielectric susceptibility of (Na_{0.5}Bi_{0.5})_{1-x}Ba_{x}TiO_{3} (NBT-BT) have been measured in the composition range between pure NBT and the morphotropic phase boundary included, 0 <= x <= 0.08. The compliance of NBT presents sharp peaks at the rhombohedral/tetragonal and tetragonal/cubic transitions, allowing the determination of the tetragonal region of the phase diagram, up to now impossible due to the strong lattice disorder and small distortions and polarizations involved. In spite of ample evidence of disorder and structural heterogeneity, the R-T transition remains sharp up to x = 0.06, whereas the T-C transition merges into the diffuse and relaxor-like transition associated with broad maxima of the dielectric and elastic susceptibilities. An attempt is made at relating the different features in the anelastic and dielectric curves to different modes of octahedral rotations and polar cation shifts. The possibility is also considered that the cation displacements locally have monoclinic symmetry, as for PZT near the morphotropic phase boundary.Comment: 11 pages, 9 figures, submitted to Phys. Rev.

SVOM pointing strategy: how to optimize the redshift measurements?

The Sino-French SVOM mission (Space-based multi-band astronomical Variable Objects Monitor) has been designed to detect all known types of gamma-ray bursts (GRBs) and to provide fast and reliable GRB positions. In this study we present the SVOM pointing strategy which should ensure the largest number of localized bursts allowing a redshift measurement. The redshift measurement can only be performed by large telescopes located on Earth. The best scientific return will be achieved if we are able to combine constraints from both space segment (platform and payload) and ground telescopes (visibility).Comment: Proceedings of Gamma-Ray Bursts 2007 conference, Santa Fe, USA, 5-9 November 2007. Published in AIP conf. proc. 1000, 585-588 (2008

Interacting Binaries with Eccentric Orbits. III. Orbital Evolution due to Direct Impact and Self-Accretion

The rapid circularization and synchronization of the stellar components in an eccentric binary system at the onset of Roche lobe overflow (RLO) is a fundamental assumption common to all binary stellar evolution and population synthesis codes, even though the validity of this assumption is questionable both theoretically and observationally. Here we calculate the evolution of the orbital elements of an eccentric binary through the direct three-body integration of a massive particle ejected through the inner Lagrangian point of the donor star at periastron. The trajectory of this particle leads to three possible outcomes: direct accretion (DA) onto the companion star within a single orbit, self-accretion (SA) back onto the donor star within a single orbit, or a quasi-periodic orbit around the companion star. We calculate the secular evolution of the binary orbit in the first two cases and conclude that DA can increase or decrease the orbital semi-major axis and eccentricity, while SA always decreases the orbital both orbital elements. In cases where mass overflow contributes to circularizing the orbit, circularization can set in on timescales as short as a few per cent of the mass transfer timescale. In cases where mass overflow increases the eccentricity, the orbital evolution is governed by competition between mass overflow and tidal torques. In the absence of tidal torques, mass overflow resulting in DI can lead to substantially subsynchronously rotating donor stars. Contrary to common assumptions, DI furthermore does not always provide a strong sink of orbital angular momentum in close mass-transferring binaries; in fact we instead find that a significant part can be returned to the orbit during the particle orbit. The formulation presented here can be combined with stellar and binary evolution codes to generate a better picture of the evolution of eccentric, RLO binary star systems.Comment: 15 pages, 10 figures, Accepted for publication in Ap

Aurophilic interactions in polynuclear Au(I) complexes with phosphine ligands containing imidazole rings

Aurophilic interactions in polynuclear Au(I) complexes with phosphine ligands containing imidazole rings Fiorella Bachechiaa*, Alfredo Burinib, Rossana Galassib aIstituto di Strutturistica Chimica, C.N.R., Area della Ricerca di Roma, C.P. 10, 00016 Monterotondo St. (Roma), Italy., bDipartimento di Scienze Chimiche, Università di Camerino, 62032 Camerino, Italy. E-mail: [email protected] Keywords: gold complexes, aurophilicity, crystal structures The tertiary phosphines of the type (Bzim)3-nPhnP, where Bzim is 1-benzyl-2-imidazolylphosphine and n varies from 0 to 2, were synthesised as part of a program aimed to the study of the coordination chemistry of phosphines containing imidazole rings. The softer character of imidazole, with respect to other heterocycles, turned out to be useful in the stabilization of complexes with soft metals as those of Group 11 in low oxidation states. These ligands can behave as monodentate or as P, N bidentate forming dinuclear cationic complexes. With Au(I) they were found to be able to favour the formation of intraand intermolecular aurophilic interactions. TheAu(I) complexes assume considerable interest when aurophilic bonds occur between gold atoms, since their chemical and physical properties change to particular characteristics with potential applications in electronic, optical or sensor devices. In particular short Au•••Au interactions are considered related to optoelectronic properties as observed in a number of polynuclear gold compounds. Here the structures of two Au(I) complexes are reported. The binuclear complex [μ-(Bzim)Ph2PAu2(C6F5)2] adopts a folded conformation with parallel, eclipsed C6F5 rings and a corresponding short AucccAu contact of 3.033(2)Å. The tetranuclear cluster complex {[μ-N,N’-(Bzim)3PAuCl]2 Au2}2+c AuCl2 - c AuCl4 - consists in a 12-membered macrocycle with two gold atoms coordinated to two (Bzim)3P molecules, in bridging mode, through the nitrogen atoms of the imidazole rings and two gold atoms coordinated to the phosphorus atoms of the (Bzim)3P ligands. Two chloride atoms complete the Au bicoordination. Three strong aurophilic interactions, Au(1)cccAu(4) [2.989(2)Å], Au(2)cccAu(3) [3.014(2)Å] andAu(1)cccAu(2) [3.257(3)Å], occur.Aweak gold-gold contact, Au(3)cccAu(4) [3.766(2)Å], is also present. Understanding molecular interactions 23r