Stochastic Polarization Instability in PbTiO_{3}.

Although discussions of structural phase transitions in prototypical ferroelectric systems with the perovskite structure, such as BaTiO_{3} and PbTiO_{3}, started almost seventy years ago, an atomic-level description of the polar characteristics as a function of temperature, pressure, and composition remains topical. Here we provide a novel quantitative description of the temperature-driven local structural correlations in PbTiO_{3} via the development of characteristic relative cationic shifts. The results give new insights into the phase transition beyond those reliant on the long-range order. The ferroelectric-to-paraelectric transition of PbTiO_{3} is realized by the extent of a stochastic polarization instability driven by a progressive misalignment instead of a complete disappearance of the local dipoles, which further suggests that such polarization instability is chemically induced at the morphotropic phase boundary of PbTiO_{3}-based solid solutions with giant piezoelectric effect. As such, our results not only identify the evolving atomistic disorder in a perovskite-based ferroelectric system, but also suggest that polarization instability can serve as a generic fingerprint for phase transitions as well as for better understanding structure-property relationships in PbTiO_{3}-based ferroelectric solid solutions.

Structural investigations of amorphous metal-organic frameworks formed via different routes.

The structures of chemically identical amorphous zeolitic amorphous frameworks (ZIFs), which were prepared from crystalline ZIF-4 via three different routes, are compared by refining atomistic models against their neutron and X-ray total scattering data. The diffraction data are very similar at all but the lowest values of momentum transfer and this is reflected in the ability of models with the same continuous random network topology to fit the data from each of the three amorphous ZIFs. Despite this there are differences in the detail; the relative positions of the lowest-Q peak in the Zn-Zn partial structure factors are consistent with differences in the densities of the different amorphous samples, and peaks in the ZIF-4 glass total scattering structure factors are in general broader, suggesting shorter-ranged correlations.

Local-scale structures across the morphotropic phase boundary in PbZr1-x Ti x O3.

Lead zirconate titanate (PZT) is one of the most widely studied piezoelectric materials, mainly because of its 'mysterious' relationship between the so-called morphotropic phase boundary (MPB) and its strong piezoelectric coupling factor. Using results from a pair distribution function analysis, this paper examines how the complex local structure in PZT affects the long-range average structure across the MPB. A monoclinic M C type structure is discovered in PZT. A first-order transformation between the monoclinic M A and M C components in both the average and local structures explains the sudden change in piezoelectric effect around these compositions. The role of polarization rotation in the enhancement of the piezoelectric properties is discussed with respect to the composition of PZT. The structure-property relationship that is revealed by this study explains the unique properties of PZT, and may be applicable in the design of new MPB-type functional materials.

Emergence of Long-Range Order in BaTiO_{3} from Local Symmetry-Breaking Distortions.

By using a symmetry motivated basis to evaluate local distortions against pair distribution function data, we show without prior bias, that the off-center Ti displacements in the archetypal ferroelectric BaTiO_{3} are zone centered and rhombohedral-like across its known ferroelectric and paraelectric phases. We construct a simple Monte Carlo model that captures our main experimental findings and demonstrate how the rich crystallographic phase diagram of BaTiO_{3} emerges from correlations of local symmetry-breaking distortions alone. Our results strongly support the order-disorder picture for these phase transitions, but can also be reconciled with the soft-mode theory of BaTiO_{3} that is supported by some spectroscopic techniques.

The missing boundary in the phase diagram of PbZr(1-x)TixO3.

PbZr(1-x)Ti(x)O3 (PZT) is one of the most important and widely used piezoelectric materials. The study of its local and average structures is of fundamental importance in understanding the origin of its high-performance piezoelectricity. Pair distribution function analysis and Rietveld refinement have been carried out to study both the short- and long-range order in the Zr-rich rhombohedral region of the PZT phase diagram. The nature of the monoclinic phase across the Zr-rich and morphotropic phase boundary area of PZT is clarified. Evidence is found that long-range average rhombohedral and both long- and short-range monoclinic regions coexist at all compositions. In addition, a boundary between a monoclinic (M(A)) structure and another monoclinic (M(B)) structure has been found. The general advantage of a particular monoclinic distortion (M(A)) for high piezoactivity is discussed from a spatial structural model of susceptibility to stress and electric field, which is applicable across the wide field of perovskite materials science.

Flexibility of zeolitic imidazolate framework structures studied by neutron total scattering and the reverse Monte Carlo method.

The zeolitic imidazolate framework ZIF-4 undergoes an amorphization transition at about 600 K, and then transforms at about 700 K to ZIF-zni, the densest of the crystalline ZIFs. This series of long-range structural rearrangements must give a corresponding series of changes in the local structure, but these have not previously been directly investigated. Through analysis of neutron total diffraction data by reverse Monte Carlo modelling, we assess the changes in flexibility across this series, identifying the key modes of flexibility within ZIF-4 and the amorphous phase. We show that the ZnN4 tetrahedra remain relatively rigid, albeit less so than SiO4 tetrahedra in silicates. However, the extra degrees of freedom afforded by the imidazolate ligand, compared to silicate networks, vary substantially between phases, with a twisting motion out of the plane of the ligand being particularly important in the amorphous phase. Our results further demonstrate the feasibility of reverse Monte Carlo simulations for studying intermolecular interactions in solids, even in cases, such as the ZIFs, where the pair distribution function is dominated by intramolecular peaks.

From quantum disorder to magnetic order in an s=1/2 kagome lattice: a structural and magnetic study of herbertsmithite at high pressure.

The structural and magnetic properties of deuterated herbertsmithite have been studied by means of neutron powder diffraction and magnetic susceptibility measurements in a wide range of temperatures and pressures. The experimental data demonstrate that a phase transition from the quantum-disordered spin-liquid phase to the long-range ordered antiferromagnetic phase with the Néel temperature T(N)=6 K is induced at P=2.5 GPa. The observed decrease of T(N) upon compression correlates with the anomalies in pressure behavior of Cu-O bond length and Cu-O-Cu bond angles. The reasons for the observed spin-freezing transition are discussed within the framework of the available theoretical models and the recent observation of the field-induced spin freezing.

Vibrational origin of the thermal stability in the high-performance piezoelectric material GaAsO4.

Theoretical calculations and experiments show the absence of libration modes of the tetrahedra in GaAsO(4), the most α-quartz-type distorted material. In consequence, the degree of dynamic disorder at high temperature is very low, making GaAsO(4) of high interest for high-temperature applications. This paper shows the importance of the theoretical calculations of vibration in oxide materials. In this way, it could be possible to extend this result to other materials and predict the thermal stability of the materials and their potential applications at high temperature.

Ionic diffusion within the α(*) and ß phases of Ag(3)SI.

The ionic diffusion mechanism of mobile ions within an underlying body-centred cubic (bcc) sublattice of immobile counterions is discussed. In particular, the case of equal numbers of two ionic species forming long-range ordered and disordered bcc arrays is considered, since these form the basis of the cubic perovskite and α-AgI-type crystal structures, respectively. Their structural behaviour, and its influence on the dynamic ionic disorder which characterizes superionic conduction, is illustrated for the case of Ag(+) diffusion within the ß and α(*) phases of Ag(3)SI. The calculated behaviour obtained by molecular dynamics (MD) computer simulations is validated with reference to published neutron diffraction and ionic conductivity measurements of Ag(3)SI, and used to examine the preferred diffusion pathways. The relevance of these findings for the anion conduction mechanisms within perovskite structured compounds is briefly discussed.

Strength training can improve steadiness in persons with essential tremor.

We evaluated the effect of a strength-training program on the ability of persons with essential tremor to exert steady forces with the index finger. Thirteen subjects with a diagnosis of essential tremor were assigned to three different groups: one group trained with heavy loads, one with light loads, and one did not perform any training. Subjects attempted to generate steady contractions during both postural and constant-force tasks. Steadiness was quantified by the root mean square amplitude of acceleration during postural tasks and the standard deviation and coefficient of variation of force during the constant-force tasks. Subjects who performed the training program with heavy loads experienced an increase in steadiness around the target force during the constant-force tasks. Subjects in the other two groups did not exhibit any changes. These findings suggest that strength training can decrease the magnitude of tremor. However, we did not observe any associated improvements in functional abilities.

An expert virtual instrument approach to the automated, data dependent MS/MS and LC/MS/MS analysis of proteins.

Mass spectrometry has become an indispensable analytical tool for studies related to the structure and function of peptides and proteins. The variety of analytical methods, the range of instrument capabilities, and the complexity of the data obtained make it difficult for most laboratories to acquire the necessary expertise to make optimal use of their instrumentation.We describe an expert system approach to automating specific types of analyses in a way that makes it easier to transfer the capability to do specific experiments to other laboratories. Central to the approach is the creation of a computer program (ie, a virtual instrument) that controls the operation of physical components, analyzes incoming data, automatically adjusts instrument parameters to achieve the goal of the analysis, and reports the results. By interacting with the mass spectrometer through the computer operating system, it is possible to add useful functions to the system without altering any of the manufacturer-controlled data system software. The usefulness of this approach is illustrated by the automation of experiments to confirm the sequences of synthetic peptides and perform LC/MS/MS peak parking experiments and real-time database searches.

Strength training improves the steadiness of slow lengthening contractions performed by old adults.

When old adults participate in a strength-training program with heavy loads, they experience an increase in muscle strength and an improvement in the steadiness of submaximal isometric contractions. The purpose of this study was to determine the effect of light- and heavy-load strength training on the ability of old adults to perform steady submaximal isometric and anisometric contractions. Thirty-two old adults (60-91 yr) participated in a 4-wk training program of a hand muscle. Both the light- and heavy-load groups increased one-repetition maximum and maximal voluntary contraction (MVC) strength and experienced similar improvements in the steadiness of the isometric and shortening and lengthening contractions. The increase in MVC strength was greater for the heavy-load group and could not be explained by changes in muscle activation. Before training, the lengthening contractions were less steady than the shortening contractions with the lightest loads (10% MVC). After training, there was no difference in steadiness between the shortening and lengthening contractions, except with the lightest load. These improvements were associated with a reduced level of muscle activation, especially during the lengthening contractions.

Training-related enhancement in the control of motor output in elderly humans.

The increase in motor unit force that occurs with aging has been hypothesized to cause a decline in the ability to maintain a constant submaximal force. To test this hypothesis, young and elderly subjects performed a 12-wk strength-training program that was intended to increase motor unit force. The training program caused similar increases (%initial) in the training load (137.4 +/- 17.2%), twitch force (23.1 +/- 7.4%), and maximum voluntary contraction force (39.2 +/- 6.8%) of the first dorsal interosseus muscle for the young and elderly subjects. The increase in strength was associated with a modest increase in muscle volume (7% of initial value) and a nonmonotonic increase in the surface-recorded electromyogram that was significant at week 8 but not at week 12. The elderly subjects reduced the variability in force at the lower target forces (2.5, 5.0, and 20.0% maximum voluntary contraction force). This improvement, however, was unrelated to changes in the distribution of motor unit forces, which was not consistent with the hypothesis that the greater coefficient of variation for the force fluctuations is due to increased motor unit forces.