‘Soft’ phonon modes,structured diffuse scattering and the crystal chemistry of Fe-bearing sphalerites

Electron diffraction has been used to carefully investigate the reciprocal lattices of a range of iron-bearing sphalerites looking for evidence of Fe clustering and/or Fe/Zn ordering in the form of either additional satellite reflections or a structured diffuse intensity distribution accompanying the strong Bragg reflections of the underlying sphalerite-type average structure. While a highly structured diffuse intensity distribution in the form of transverse polarized f110g sheets of diffuse intensity has been detected and found to be characteristic of all compositions,it does not appear to arise from Fe clustering and/or Fe/Zn ordering. Rather inherently low frequency,and therefore strongly thermally excited,phonon modes propagating along reciprocal space directions perpendicular to each of the six /110S real space directions of the average structure are suggested to be responsible for these f110g sheets of diffuse intensity. Monte Carlo simulation (for a range of Zn–S,Zn–Zn and S–S interaction strengths) and subsequent Fourier transformation is used to confirm the existence of these low-frequency phonon modes of distortion as well as to show that they are an intrinsic,predictable property of the corner-connected tetrahedral structure of sphalerite. The low-frequency phonon modes involve coupled (Zn,Fe) and S motion in one-dimensional strings along /110S real space directions

Compositional uniformity, domain patterning and the mechanism underlying nano-chessboard arrays

We propose that systems exhibiting compositional patterning at the nanoscale, so far assumed to be due to some kind of ordered phase segregation, can be understood instead in terms of coherent, single phase ordering of minority motifs, caused by some constrained drive for uniformity. The essential features of this type of arrangements can be reproduced using a superspace construction typical of uniformity-driven orderings, which only requires the knowledge of the modulation vectors observed in the diffraction patterns. The idea is discussed in terms of a simple two dimensional lattice-gas model that simulates a binary system in which the dilution of the minority component is favored. This simple model already exhibits a hierarchy of arrangements similar to the experimentally observed nano-chessboard and nano-diamond patterns, which are described as occupational modulated structures with two independent modulation wave vectors and simple step-like occupation modulation functions.Comment: Preprint. 11 pages, 11 figure

Dipolar-glass-like relaxor ferroelectric behaviour in the 0.5BaTiO3-0.5Bi(Mg1/2Ti1/2)O3 electroceramic

In this study, the dielectric and ferroelectric switching behaviour of 0.5BaTiO3-0.5Bi(Mg1/2Ti1/2)O3 (BT-BMT) ceramics are investigated. The BT-BMT ceramic exhibits a typical dipolar-glass-like, dielectric polarisation relaxation. This is attributed to the 15 distinct possible local A4B2 configurations around the O ions and the effect this unavoidable local compositional variability has on the dipole relaxation behaviour of inherent {1-D h111} dipole chains, arising from correlated off-centre displacements of Bi3+ and Ti4+ ions along local {111} directions. On the other hand, switchable polarisation under strong applied electric fields is observed on different length scales accompanied by the appearance of strong polarisation relaxation, as observed via time-delayed piezoresponse hysteresis loop measurements. These experimental results demonstrate that this BT-BMT ceramic is relaxor ferroelectric in nature, although it exhibits dipolar-glass-like dielectric relaxation behaviour.The authors J.W., Y.L., and R.L.W. acknowledge the support of the Australian Research Council (ARC) in the form of Discovery projects. Y.L. also appreciates support from the ARC Future Fellowships program

Composition-induced structural phase transitions in the (Ba1xLax)2In2O5+x (0pxp0.6) system

Composition-induced structural phase changes across the high temperature, fast oxide ion conducting (Ba1xLax)2In2O5+x, 0pxp0.6, system have been carefully analysed using hard mode infrared (IR) powder absorption spectroscopy, X-ray powder diffraction and electron diffraction. An orthorhombic brownmillerite to three-dimensionally disordered cubic perovskite phase transition in this system is signalled by a drastic change in slope of both wavenumber and average line widths of IR spectra as a function of composition. Some evidence is found for the existence of an intermediate tetragonal phase (previously reported to exist from electron diffraction data) around x 0:2: The new spectroscopic data have been used to compare microscopic and macroscopic strain parameters arising from variation in composition. The strain and spectroscopic data are consistent with firstorder character for the tetragonal-orthorhombic transition, while the cubic-tetragonal transition could be continuous. Differences between the variation with composition of spectral parameters and of macroscopic strain parameters are consistent with a substantial order/disorder component for the transitions. There is also evidence for precursor effects within the cubic structure before symmetry is broken

Ferroelectric and non-linear dielectric characteristics of Bi₀.₅Na₀.₅TiO₃ thin films deposited via a metallorganic decomposition process

Polycrystalline Bi₀.₅Na₀.₅TiO₃ (NBT) thin films have been successfully fabricated via a metal organic decomposition process on Pt/Ti/SiO₂/Si substrates. The structural evolution of the as-prepared thin filmsannealed over the moderate temperature range 500–700 °C is studied. NBT thin filmsannealed at 700 °C are of single phase NBT perovskite type. They exhibit a well-defined P-E hysteresis loop at room temperature. The measured dielectric constant is 465–410 over the frequency range of 1 kHz to 1 MHz. The corresponding dielectric loss is ∼10⁻². The measured capacitance-voltage curve shows strong non-linear dielectric behavior leading to a high tunability of the dielectric constant, up to 14% at 1 MHz.J.X. acknowledges financial support from the Department of Education, Science and Training DEST in the form of an Endeavor Australian Research Fellowship Award. J.X., Y.L., and R.L.W. acknowledge financial support from the Australian Research Council ARC in the form of ARC Discovery Grants

Observation of short-lived local polar states induced by applied tip biases in BaTiO3-based relaxor ferroelectric ceramics

Using piezoresponse force microscopy imaging and spectroscopy methods, surface polar states induced by applied tip biases in the ergodic/paraelectric phase of a BaTiO3-based relaxor ferroelectric ceramic have been observed. The induced polar state appears to be short-lived (typical lifetime only a few seconds) and dependent on the voltages applied and their duration. The observed piezoresponse hysteresis and relaxation behavior is primarily interpreted with respect to the picture of the polar nano-region dynamics, as proposed for lead-based relaxors. Spatially resolved piezoresponse relaxation mapping reveals the presence of sub-micron correlated features, presumably due to A-site chemical inhomogeneity as supported by site-correlated elemental mapping microanalysis.Q.L., Y.L., and R.L.W., acknowledge the support of the Australian Research Council (ARC) in the form of ARC Discovery Grants. Y.L. also acknowledges support from the ARC Future Fellowships Program

A combined diffraction (XRD, electron and neutron) and dielectric study of Na3MoO3F3

For the dielectric measurements, Na3MoO3F3 ceramics were cut and polished down to a disc with a thickness of 0.5 mm and a diameter of 4.79 mm. Gold electrodes were then sputtered onto both surfaces of the sample. Its polarisation hysteresis loop was characterised by a ferro- and piezo-electric measuring system (TA Analyser 2000, aixACT). The temperature dependence of the dielectric constant and the dissipation factor of Na3MoO3F3 was investigated at 10KHz by a high precision LCR meter (3522, Hioki) between room temperature and 180oC using a scan rate of 5oC/min

Fully-inverted piezoresponse hysteresis loops mediated by charge injection in 0.29Pb(In1/2Nb1/2)O3–0.44Pb(Mg1/3Nb2/3)O3–0.27PbTiO3 single crystals

The domain structure and local switching behavior of ternary relaxor (001) 0.29Pb(In1/2Nb1/2)O3–0.44Pb(Mg1/3Nb2/3)O3–0.27PbTiO3single crystals are studied using piezoresponse force microscopy. The as-grown crystals exhibit a labyrinthine domain pattern similar to other relaxor-based ferroelectrics. Abnormally switched domains are observed for both positive and negative tip-voltages, with sign-dependent thresholds and growth rates on the poled crystals. Further piezoresponse hysteresis loop measurements show that fully inverted loops can be observed under high switching voltages, mediated by injected charge fields. The dynamic behavior of the observed abnormal switching is qualitatively analyzed and the underlying mechanisms discussed.Q.L., Y.L., and R.L.W. acknowledge financial support from the Australian Research Council ARC in the form of an ARC Discovery Grant No. DP0877069

A correlated electron diffraction, in situ neutron diffraction and dielectric properties investigation of poled (1-x)Bi0.5Na0.5TiO3-xBaTiO3 ceramics

A correlated electron diffraction, temperature-dependent in situ neutron diffraction, and temperature-dependent dielectric properties investigation of poled (1-x)Bi0.5Na0.5TiO3-xBaTiO3 (BNTBT100x) (x ¼ 0.04, 0.07, and 0.12) samples has been carried out. The results show that the depolarization temperature, Td, of the rhombohedral BNTBT 4 sample is associated with the disappearance of G 6 1=2 [111]*p satellite reflections and aaa- octahedral tilting while that of the BNTBT 12 sample is associated with a metrically tetragonal to metrically cubic or pseudo-cubic symmetry. In the case of the poled BNTBT 7 sample in the MPB region, the dielectric properties show a quite distinct two stage transition from a room temperature clearly metrically tetragonal phase again to a metrically cubic or pseudo-cubic symmetry above 150 C. There is no apparent change in its average structure in vicinity of Td in BNTBT 7. Electron diffraction shows the presence of considerable octahedral tilt twin disorder in all three samples.J.W., Y.L., R.L.W., Q.L., and Y.P.G. appreciate the support of the Australian Research Council (ARC) in the form of a Discovery Grant. Y.L. also appreciates support from the ARC Future Fellowships program