Anisotropic thermal expansion of La(n)(Ti,Fe)(n)O(3n + 2) (n = 5 and 6).

Crystal structures are reported for two perovskite-related compounds with nominal compositions La5(Ti(0.8)Fe(0.2))5O17 and La6(Ti(0.67)Fe(0.33))6O20 at seven different temperatures between 90 and 350 K. For both compounds no evidence of a structural phase transition in the investigated range of temperatures was found. The thermal expansions are found to be anisotropic, with the largest thermal expansion along a direction parallel to the slabs of these layered compounds. The origin of this anisotropy is proposed to be a temperature dependence of tilts of the octahedral (Ti,Fe)O6 groups. It is likely that the same mechanism will determine similar anisotropic thermal behaviour of other compounds A(n)B(n)O(3n + 2). The crystal structures have revealed partial chemical order of Ti/Fe over the B sites, with iron concentrated towards the centers of the slabs. Local charge compensation is proposed as the driving force for the chemical order, where the highest-valent cation moves to sites near the oxygen-rich borders of the slabs. A linear dependence on the site occupation fraction by Fe of the computed valences leads to extrapolated valence values close to the formal valence of Ti(4+) for sites fully occupied by Ti, and of Fe(3+) for sites fully occupied by Fe. These results demonstrate the power of the bond-valence method, and they show that refined oxygen positions are the weighted average of oxygen positions in TiO6 and FeO6 octahedral groups.

Two-dimensional magnetic clusters in La(n)(Ti(1-x)Fe(x))(n)O(3n+2) (n = 5 with x = 0.2 and n = 6 with x = 0.33).

Crystal structure analyses of the layered compounds La(n)(Ti(1-x)Fe(x))(n)O(3n+2), with nominal compositions x = 0.2 for n = 5 and x = 0.33 for n = 6, show that the iron is concentrated at the centers of the slabs. The spatial arrangements of the iron ions can be approximated by two-dimensional square lattices with strong magnetic interactions between neighboring sites, albeit with fractional occupancies of the sites of an average of 0.67 magnetic ions in n = 6 and 0.42 magnetic ions in n = 5 compounds. A previously described (Lichtenberg et al 2008 Prog. Solid State Chem. 36 253) crossover of magnetic behavior of n = 6 at room temperature is explained by the formation of two-dimensional, ferromagnetically organized magnetic clusters with an average size of 52 Fe(3+) ions. The absence of long-range magnetic order follows from the fractional occupancy of the sites by the magnetic ions. The lower concentration of magnetic ions in n = 5 explains why crossover behavior is not found and clusters do not form in that compound.

Modulated anharmonic ADPs are intrinsic to aperiodic crystals: a case study on incommensurate Rb2ZnCl4.

A combination of structure refinements, analysis of the superspace MEM density and interpretation of difference-Fourier maps has been used to characterize the incommensurate modulation of rubidium tetrachlorozincate, Rb(2)ZnCl(4), at a temperature of T = 196 K, close to the lock-in transition at T(lock-in) = 192 K. The modulation is found to consist of a combination of displacement modulation functions, modulated atomic displacement parameters (ADPs) and modulated third-order anharmonic ADPs. Up to fifth-order Fourier coefficients could be refined against diffraction data containing up to fifth-order satellite reflections. The center-of-charge of the atomic basins of the MEM density and the displacive modulation functions of the structure model provide equivalent descriptions of the displacive modulation. Modulations of the ADPs and anharmonic ADPs are visible in the MEM density, but extracting quantitative information about these modulations appears to be difficult. In the structure refinements the modulation parameters of the ADPs form a dependent set, and ad hoc restrictions had to be introduced in the refinements. It is suggested that modulated harmonic ADPs and modulated third-order anharmonic ADPs form an intrinsic part, however small, of incommensurately modulated structures in general. Refinements of alternate models with and without parameters for modulated ADPs lead to significant differences between the parameters of the displacement modulation in these two types of models, thus showing the modulation of ADPs to be important for a correct description of the displacive modulation. The resulting functions do not provide evidence for an interpretation of the modulation by a soliton model.

Dynamic torque during a precision grip task comparable to picking a raspberry.

Numerous studies have shown torque control to be an important factor in grip-force control. This study introduces a novel task which allows quantification of the dynamics of torque development while increasing grip forces during a task comparable to picking a raspberry. The performance of this task was analysed in two healthy subjects and two cerebellar patients. Individual grip forces and finger positions on a grip rod were analysed using a recently developed technique [Kutz DF, Woelfel A, Timmann D, Kolb FP. Detection of changes in grip forces on a sliding object. J Neurosci Methods 2007;166:250-8]. Levers and torques were derived from grip forces and geometric properties of the grip rod. The analysis of this task performance provides evidence that healthy subjects are able to minimise torque despite increasing grip force, whereas the cerebellar patients tested increased torque disproportionately with increasing grip forces, whereby these high torques were due primarily to the patients' inability to optimise individual finger positions on the rod. Patients tried to compensate their ataxia-based insecurity by employing higher grip forces, resulting in disproportionately higher torques and increased instability, whereupon they again increased grip force, thus establishing a vicious circle. The analysis of this task suggests that effective rehabilitation strategies must be aimed at interrupting this circle.

Spatio-Temporal Human Grip Force Analysis via Sensor Arrays.

This study describes a technique for measuring human grip forces exerted on a cylindrical object via a sensor array. Standardised resistor-based pressure sensor arrays for industrial and medical applications have been available for some time. We used a special 20 mm diameter grip rod that subjects could either move actively with their fingers in the horizontal direction or exert reactive forces against opposing forces generated in the rod by a linear motor. The sensor array film was attached to the rod by adhesive tape and covered approximately 45 cm(2) of the rod surface. The sensor density was 4/cm(2) with each sensor having a force resolution of 0.1 N. A scan across all sensors resulted in a corresponding frame containing force values at a frame repetition rate of 150/s. The force value of a given sensor was interpreted as a pixel value resulting in a false-colour image. Based on remote sensed image analysis an algorithm was developed to distinguish significant force-representing pixels from those affected by noise. This allowed tracking of the position of identified fingers in subsequent frames such that spatio-temporal grip force profiles for individual fingers could be derived. Moreover, the algorithm allowed simultaneous measurement of forces exerted without any constraints on the number of fingers or on the position of the fingers. The system is thus well suited for basic and clinical research in human physiology as well as for studies in psychophysics.

Detection of changes in grip forces on a sliding object.

Holding a slipping object in hand requires adjustment of grip forces. The aim of the study was to develop a method for measuring the temporal and spatial distribution of grip forces during the holding of a slipping object in the hand. A special grip rod with a measuring film containing 200 resistor-based pressure sensors equally distributed over 50 cm(2) was developed, providing a system that has a spatial resolution of 5 mm, a temporal resolution of 1/150 Hz and a force resolution 0.05 N. A force-change-detection algorithm was constructed to detect and separate pressure and position of individual fingers. The algorithm is a modification of a classical Gaussian random field theory algorithm for detecting significant data [Rogerson PA. Change detection thresholds for remotely sensed images. J Geog Syst 2002;4:85-97]. The modification takes the signal strength into account to reduce false positive detection in low grip force situations. The grip force measuring system and the force-change-detection algorithm allow measurement of the forces exerted by any number of fingers simultaneously without any constraints on finger position and are suitable for basic and clinical research in human and animal physiology as well as for psychophysics studies.