A combined theoretical and experimental study of the low temperature properties of BaZrO3

Low temperature properties of BaZrO3 are revealed by combining experimental techniques (X-ray diffraction, neutron scattering and dielectric measurements) with theoretical first-principles-based methods (total energy and linear response calculations within density functional theory, and effective Hamiltonian approaches incorporating/neglecting zero-point phonon vibrations). Unlike most of the perovskite systems, BaZrO3 does not undergo any (long-range-order) structural phase transition and thus remains cubic and paraelectric down to 2 K, even when neglecting zero-point phonon vibrations. On the other hand, these latter pure quantum effects lead to a negligible thermal dependency of the cubic lattice parameter below ~ 40 K. They also affect the dielectricity of BaZrO3 by inducing an overall saturation of the real part of the dielectric response, for temperatures below ~ 40 K. Two fine structures in the real part, as well as in the imaginary part, of dielectric response are further observed around 50-65 K and 15 K, respectively. Microscopic origins (e.g., unavoidable defects and oxygen octahedra rotation occurring at a local scale) of such anomalies are suggested. Finally, possible reasons for the facts that some of these dielectric anomalies have not been previously reported in the better studied KTaO3 and SrTiO3 incipient ferroelectrics are also discussed.Comment: 8 pages, 5 figures, submitted to Physical Review

A tetragonal-to-monoclinic phase transition in a ferroelectric perovskite: the structure of PbZr(0.52)Ti(0.48)O3

The perovskite-like ferroelectric system PbZr(1-x)Ti(x)O3 (PZT) has a nearly vertical morphotropic phase boundary (MPB) around x=0.45-0.50. Recent synchrotron x-ray powder diffraction measurements by Noheda et al. [Appl. Phys. Lett. 74, 2059 (1999)] have revealed a new monoclinic phase between the previously-established tetragonal and rhombohedral regions. In the present work we describe a Rietveld analysis of the detailed structure of the tetragonal and monoclinic PZT phases on a sample with x= 0.48 for which the lattice parameters are respectively: at= 4.044 A, ct= 4.138 A, at 325 K, and am= 5.721 A, bm= 5.708 A, cm= 4.138 A, beta= 90.496 deg., at 20K. In the tetragonal phase the shifts of the atoms along the polar [001] direction are similar to those in PbTiO3 but the refinement indicates that there are, in addition, local disordered shifts of the Pb atoms of ~0.2 A perpendicular to the polar axis.. The monoclinic structure can be viewed as a condensation along one of the directions of the local displacements present in the tetragonal phase. It equally well corresponds to a freezing-out of the local displacements along one of the directions recently reported by Corker et al.[J. Phys. Condens. Matter 10, 6251 (1998)] for rhombohedral PZT. The monoclinic structure therefore provides a microscopic picture of the MPB region in which one of the "locally" monoclinic phases in the "average" rhombohedral or tetragonal structures freezes out, and thus represents a bridge between these two phases.Comment: REVTeX, 7 figures. Modifications after referee's suggestion: new figure (figure 5), comments in 2nd para. (Sect.III) and in 2nd & 3rd para. (Sect. IV-a), in the abstract: "...of ~0.2 A perpendicular to the polar axis.

Nanomechanical and nanowear properties of Cr3C2 thin films deposited by rf sputtering

In this investigation, nanoindentation, nanoscratch and nanowear tests were done in order to study the nanomechanical and nanotribologicalproperties of the chromium carbide nanostructured thin films. Chromium carbide films were synthesized by rf sputtering from a Cr3C2 target onmagnesium oxide substrate. The samples were characterized by X-ray diffraction and grazing X-ray reflectivity. All the films have a thickness of300 nm. A pyramidal Berkovich indenter diamond tip was used for nanomechanical testing. The residual wear depth of the samples was examinedusing an atomic force microscope (AFM). In this study, an experimental method adapted to the nanowear properties was explored.We have studiedthe influence of the substrate temperature on the structural, mechanical and tribological properties of chromium carbide films.We have shown thatCr3C2 films deposited at a substrate temperature up to 600 ◦C exhibit a better wear resistance and a hardness value higher than what is measuredin the case of the films deposited at room temperature

Syntheses of relaxor/ferroelectric superlattice thin films PB(Sc1/2NB1/2)O3/PbTi03 and their dielectric properties

Superlattice (SL) thin films with different periods and composition ratios of Pb(Sc1∕2Nb1∕2)O3 and PbTiO3 (PT), inside and outside the morphotropic phase boundary, are synthesized by a pulsed laser deposition method. High order SL reflections are observed by x-ray diffraction, from which the SL period and correlation length are determined. The coherency of the SL structure is found to extend over the whole film in the thinner films. A maximum dielectric constant of 1800 with a dielectric loss of 0.05 is obtained in the frequency range of 1–104Hz for a SL with average composition of 25% of PT

Is K(1-x)LixTa03 a real relaxor ?

Li-doped KTaO3, K(1-x)Li x TaO3 (KTL) has been reported to be a relaxor with an isovalent substitution of Li+ for K+. For confirming the idea, we performed second-harmonic generation microscopic observations, X-ray diffraction experiments and dielectric measurements of KTL with the critical concentration of Li (x = 3%). These reveal that an intermediate phase exists between 2 characteristic temperatures, Td and Tp. The former corresponds to an elastic change accompanying small deformation and the latter to a dipolar change of the crystal. It is highly probable that the polar-nano regions appear below Td and transformed into ferroelectric domains at Tp. The present experiments support that KTL is a real relaxor

Experimental and fist-principles study of the structure and dielectric response of the high-temperature quantum paraelectric La1/2 Na1/2 Ti03

The low-temperature structural evolution and dielectric and dynamic properties of perovskite La1∕2Na1∕2TiO3 (LNTO) have been investigated by the means of x-ray and neutron diffractions, dielectric measurement, and first-principles density-functional calculations. The structural evolution has been clarified and reveals that LNTO behaves as a high-temperature quantum paraelectric, with a structure isomorphous to that of LaTiO3 (space group Pnma). Using ab initio methods, two chemical orderings, respectively, along the [001] and [111] directions of the perovskite structure, have been studied, first in their corresponding high-symmetry structures and then in low-symmetry tilted structures, analogous to I4∕mcm and Pnma. For each case, the optic dielectric tensor, the Born effective charges, and the phonon modes at various points of the Brillouin zone are computed, revealing several instabilities with different natures. In the low-symmetry phases studied, weak ferroelectric instabilities suggest that ordered LNTO structures are quantum paraelectrics. The chemical disorder on the A site is evidenced to decrease the dielectric response and the structural distortions. The calculations are compared to the experimental situation and are discussed in relation to other quantum paraelectrics

Synthesis of nanometric cubic BaTiO3 by using and original chemical route: Freeze-drying method

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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