Fast polarization mechanisms in the uniaxial tungsten-bronze relaxor strontium barium niobate SBN-81

The high-frequency dielectric response of the uniaxial strontium barium niobate crystals with 81% of Sr has been studied from 1 kHz to 30 THz along the polar c axis by means of several techniques (far infrared, time domain terahertz, high-frequency and low-frequency dielectric spectroscopies) in a wide temperature interval 20–600 K. Relaxor properties were observed in the complex dielectric response and four main excitations were ascertained below the phonon frequencies. These fast polarization mechanisms take place at THz, GHz and MHz ranges and show different temperature evolution. The central mode excitation in the THz range, related to anharmonic dynamics of cations, slightly softens from high temperatures and then hardens below T ~ 400 K. Below the phase transition (at T ~ 330 K) an additional microwave excitation appears near 10 GHz related to micro domain wall oscillations. The strongest relaxation appears in the GHz range and slows down on cooling according to the Arrhenius law. Finally, another relaxation, present in the MHz range at high temperatures, also slows down on cooling at least to the kHz range. These two relaxations are due to polar fluctuations and nanodomains dynamics. Altogether, the four excitations explain the dielectric permittivity maximum in the kHz range.

Electric field and aging effects of uniaxial ferroelectrics Sr x Ba1-x Nb2O6 probed by Brillouin scattering

Static and dynamic heterogeneity of disordered system is one of the current topics in materials science. In disordered ferroelectric materials with random fields, dynamic polar nanoregions (PNRs) appear at Burns temperature and freeze into nanodomain state below Curie temperature (T _C). This state is very sensitive to external electric field and aging by which it gradually switches into macrodomain state. However, the role of PNRs in such states below T _C is still a puzzling issue of materials science. Electric field and aging effects of uniaxial ferroelectric Sr _x Ba_1-x Nb₂O₆ (x = 0.40, SBN40) single crystals were studied using Brillouin scattering to clarify the critical nature of PNRs in domain states below T _C. On field heating, a broad anomaly in longitudinal acoustic (LA) velocity at low temperature region was due to an incomplete alignment of nanodomains caused by the interaction between PNRs. A sharp anomaly near T _C was attributed to the complete switching of nanodomain to macrodomain state owing to the lack of interaction among PNRs. After isothermal aging below T _C, the noticeable increase of LA velocity was observed. It was unaffected by cyclic temperature measurements up to T _C, and recovered to initial state outside of a narrow temperature range above and below aging temperature.

Temperature Effect on the Stability of the Polarized State Created by Local Electric Fields in Strontium Barium Niobate Single Crystals

The stability of ferroelectric domain patterns at the nanoscale has been a topic of much interest for many years. We investigated the relaxation of the polarized state created by application of a local electric field using a conductive tip of a scanning probe microscope for the model uniaxial relaxor system Sr_xBa_1−xNb₂O₆ (SBN) in its pure and Ce-doped form. The temporal relaxation of the induced PFM contrast was measured at various temperatures. The average value of the induced contrast decreases during heating for all investigated crystals. Below the freezing temperature the induced state remains stable after an initial relaxation. Above the freezing temperature the induced state is unstable and gradually decays with time. The stability of the induced state is strongly affected by the measuring conditions, so continuous scanning results in a faster decay of the poled domain. The obtained effects are attributed to a decrease of the induced polarization and backswitching of the polarized area under the action of the depolarization field.

Dynamics of nanoscale polarization fluctuations in a uniaxial relaxor

We have studied neutron diffuse scattering in a Sr(0.61)Ba(0.39)Nb(2)O(6) single crystal by neutron backscattering at sub-μeV energy resolution. We can identify two response components with transverse polarization: an elastic (resolution limited) central peak, which monotonically increases with decreasing temperature, and a quasielastic central peak, having a maximum intensity around the ferroelectric phase transition close to 350 K. In contrast to previous neutron experiments on this and other relaxor materials, we were able to observe a temperature dependence of the characteristic frequency of these fluctuations, obeying the same Vogel-Fulcher law as the dynamic part of the dielectric permittivity of this material. In this way our findings provide a first direct link between the Vogel-Fulcher-type frequency dependence of dielectric permittivity and dynamic nanoscale lattice modulations with a transverse correlation length of about 5-10 unit cells.

Multiple soft-mode vibrations of lead zirconate

Polarized Raman, IR, and time-domain THz spectroscopy of orthorhombic lead zirconate single crystals have yielded a comprehensive picture of temperature-dependent quasiharmonic frequencies of its low-frequency phonon modes. It is argued that these modes primarily involve vibrations of Pb ions and librations of oxygen octahedra. Their relation to phonon modes of the parent cubic phase is proposed. Counts of the observed IR and Raman active modes belonging to distinct irreducible representations agree quite well with group-theory predictions. Analysis of the results yields insight into the phase transition mechanism, involving a soft ferroelectric branch coupled by a trilinear term to another two oxygen octahedra tilt modes.

Freezing of the Nb⁵+ ion dynamics in AgNbO₃ studied by linear and nonlinear dielectric response

Linear and nonlinear dielectric measurements of AgNbO₃ ceramics and single crystals were carried out for the M phases (77-673 K). The linear dielectric response is dominated by the contribution of the submillimetre relaxational mode related to the Nb⁵+ ion dynamics (M₂-M₃). On the other hand, nonlinear dielectric χ₃' susceptibility revealed anomalies at three characteristic temperatures: 90, 325 and 448 K. Two later ones are connected with changes of the Nb⁵+ ion dynamics. At T(f) = 448 K a partial freezing of the Nb⁵+ ion displacement to the anti-polar, antiferroelectric array takes place. At 325 K further freezing of Nb and Ag displacements to the polar weak relaxor ferroelectric or dipolar glass transition occurs. This polar state coexists with the ground antiferroelectric one.

Computer-controlled susceptometer for investigating the linear and nonlinear dielectric response

A fully automatized alternating current (ac) susceptometer is constructed for simultaneous measurements of the phase resolved complex linear and complex nonlinear ac susceptibilities of lossy and dispersive dielectric materials. This relatively simple setup allows measurements over a wide range of experimental variables, such as ac amplitudes up to 40 V, frequencies from 10(-2) to 10(3) Hz, and temperatures from 100 to 600 K utilizing only current/voltage and analog/digital converters and a personal computer. In contrast with the commonly used analysis of the charge accumulated on a standard capacitor in series with the sample our method is based on the analysis of the current flowing directly through the sample. Absence of any capacitive voltage dividers in the measurement circuit eliminates uncontrolled phase shifts. This is why the instrument provides high quality, nonlinear susceptibility data and in particular appears as a very convenient tool for discrimination between continuous and discontinuous phase transitions when determining the sign of the real part of the third order dielectric susceptibility.

Creep and relaxation dynamics of domain walls in periodically poled KTiOPO4

Creep and relaxation of domain walls under ac electric fields are observed in an ideal model system, periodically poled KTiOPO4, to occur in different regimes, which are separated by dynamic phase transitions at frequencies f(m)(T)=f(m0)exp((-DeltaE/k(B)T), with f(m0)=3 x 10(9) Hz and DeltaE=0.6 eV. Power law dispersion of the creep susceptibility, chi proportional to 1+(iomegatau)(-beta), with beta approximately equal to 0.4, and large nonlinearity encountered at f < f(m), is contrasted with Cole-Cole-type relaxational dispersion, chi proportional to (1+[iomegatau](1-alpha))(-1), with alpha approximately 0.3, at f > f(m).

Birefringence of lead titanate (PbTiO3)

Since lead titanate was first grown in single-crystal form in the 1930s, several independent reports of its linear birefringence have been published. Whilst there is overall qualitative agreement on its behaviour with temperature, the absolute values show some degree of variation. At 298 K they range from −0.009 to −0.013, a relative spread of 40%, whilst at 673 K the discrepancy is even greater. This paper reports unexpected results obtained from independent measurements of a selection of crystals that have been grown recently, in which the birefringence is some four times greater than hitherto believed. This has important consequences for the use of birefringence measurement as a means of studying phase transitions in crystals. Intrinsic values of birefringence for lead titanate are measured for the first time.

[Intertrochanteric fractures of the hip treated with dynamic hip screws (DHS)]

The history and concept of sliding fixation is presented. Treatment results of 72 cases with 76 intertrochanteric fractures treated with DHS were analysed. Fractures were classified according to the Kyle scale into 4 different types. 61 males and 11 women age ranging from 46 to 72 years (average age: 65 years) were treated surgically. In 48 cases good results were achieved, in 22 cases results were satisfactory and in 6 cases results were poor. The use of DHS screws is an effective way of treating intertrochanteric fractures among patients with osteoporosis, as well as young patients requiring anatomic reduction of bone fragments. This technique allows early mobilization and weight bearing.

The application of allogenic ligament grafts in traumatic and orthopedic surgery

Deep frozen and lyophilized allografts constitute a good alternative to the widely-used autografts in ligament reconstruction. To date the chief disadvantage of allografts has been the risk of transmitting viral infections, especially HIV and various kinds of viral hepatitis. The use allografts shortens operating time, reduces the extent of surgical damage, and preserves the patient's own tissue, which makes it possible to perform multiplex reconstruction and repair damaged autografts. The article described the techniques used to collect, sterilize, and store allografts, as well as the intraoperative procedure.

Dehalogenation of xenobiotics as a consequence of binding to humic materials

Chlorinated phenols and anilines were transformed by oxidoreductive catalysts with release of chloride ions in both the absence and the presence of humic substances (syringaldehyde, catechol, and humic acid). Dehalogenation of these xenobiotics resulted from oxidative coupling reactions occurring at the chlorinated sites of the substrates. The effect of humic substances on dehalogenation depended on the mechanism of oxidative coupling. In a free-radical reaction mediated by peroxidase, laccase, or birnessite (delta-MnO2), syringaldehyde enhanced the dehalogenation of most of the chlorinated phenols, but it did not enhance the dehalogenation of the chloroanilines. With catechol, which does not form free radicals, dehalogenation was reduced or remained the same for both the chlorophenols and the chloroanilines. However, in tyrosinase-mediated reactions controlled by nucleophilic addition, catechol enhanced the dehalogenation of most of the chlorophenols, whereas syringaldehyde had little effect. Humic acid in most cases enhanced the dehalogenation of the chlorophenols, but it had little effect on the dehalogenation of the chloroanilines. On a molar basis, changes in dehalogenation caused by humic substances were proportional to the respective changes in substrate transformation. Only syringaldehyde was capable of releasing disproportionately high amounts of chloride ions from chlorophenols, apparently as a result of multiple crosscouplings to one molecule of the substrate.

[Densitometry for assessment of periacetabular bony changes after Parhofer-Mönch total hip replacement]

Densitometry for assessment of mineral changes after Parhofer-Mönch total hip replacement 10 mm around the acetabular component has been done in 53 patients (31 females and 22 males, mean age 52.1 years) with unilateral hip arthritis. Lunar DPX apparatus has been used, "manual analysis" of "Orthopedic" software and modification of DeLee and Charnley zones for DEXA technique was employed. After 6 months follow-up BMC and BMD decreased in all zones analyzed, especially in zone 2. These values increased in next 6 and 12 months but did not compensate for the primary loss.

[Pathomechanics of spinal and rib cage deformity in idiopathic scoliosis]

Idiopathic scoliosis is a triplanar deformity associated with a rib hump. The quantitative assessment of the principal elements of spinal and rib cage deformity based on the chosen parameters measured on a-p radiographs and CT scans was done in a group of 89 children with idiopathic scoliosis. The average Cobb angle was 63 degrees. In regression and correlation analysis the statistically significant correlation among spinal lateral curve, axial rotation and kyphosis/lordosis index was found. Significant correlation between the elements of spinal deformity and the size of the rib hump were also found. The rib cage may play an important role in the pathogenesis of idiopathic scoliosis.

Investigation into the Crystal Structure of the Perovskite Lead Hafnate, PbHfO3

The room-temperature crystal structure of the perovskite lead hafnate PbHfO3 is investigated using both low-temperature single crystal X-ray diffraction (Mo Kα radiation, λ = 0.71069 Å) and polycrystalline neutron diffraction (D1A instrument, ILL, λ = 1.90788 Å). Single crystal X-ray data at 100 K: space group Pbam, a = 5.856 (1), b = 11.729 (3), c = 8.212 (2) Å, V = 564.04 Å3 with Z = 8, μ = 97.2 mm−1, F(000) = 1424, final R = 0.038, wR = 0.045 over 439 reflections with F >1.4σ(F). Polycrystalline neutron data at 383 K: a = 5.8582 (3), b = 11.7224 (5), c = 8.2246 (3) Å, V = 564.80 Å3 with χ2 = 1.62. Although lead hafnate has been thought to be isostructural with lead zirconate, no complete structure determination has been reported, as crystal structure analysis in both these materials is not straightforward. One of the main difficulties encountered is the determination of the oxygen positions, as necessary information lies in extremely weak l = 2n + 1 X-ray reflections. To maximize the intensity of these reflections the X-ray data are collected at 100 K with unusually long scans, a procedure which had previously been found successful with lead zirconate. In order to establish that no phase transitions exist between room temperature and 100 K, and hence that the collected X-ray data are relevant to the room-temperature structure, birefringence measurements for both PbZrO3 and PbHfO3 are also reported.

A Re-investigation of the Crystal Structure of the Perovskite PbZrO3 by X-ray and Neutron Diffraction

The crystal structure of the perovskite lead zirconate PbZrO3 has been redetermined using single-crystal X-ray diffraction (Mo Kα radiation, λ = 0.71069 Å). Single-crystal data at 100 K: space group. Pbam, a = 5.884 (1), b = 11.787 (3), c = 8.231 (2) Å, V = 570.85 Å3 with Z = 8, μ = 612.6 cm−1, D x = 8.06 Mg m−3, F(000) = 1168, final R = 0.033, wR = 0.061 over 555 reflections with I > 2σ(I). An investigation is made into previous contradicting reports of a possible disorder in the O atoms and their origin by examining the crystal pseudo-symmetry. Information distinguishing an ordered and disordered oxygen substructure is shown to reside in weak l odd reflections. Because of their extremely low intensities these reflections have not contributed sufficiently in previous X-ray structure investigations and hence, to date, conclusive evidence differentiating between ordered and disordered models has not been possible. By collecting single-crystal X-ray data at low temperature and by using exceptionally long scans on selected hkl, l odd, reflections, a new accurate structure determination is presented and discussed, showing the true ordered oxygen positions. Because of the large difference in scattering factors between lead and oxygen when using X-rays, a neutron diffraction Rietveld refinement using polycrystalline samples (D1A instrument, ILL, λ = 1.90788 Å) is also reported as further evidence to support the true ordered oxygen structure revealed by the low-temperature X-ray analysis.

Instrument for measuring the switching transition in ferroelectric thin films

A description is given of the construction and operating principles of an instrument for investigation of the process of polarization reversing in ferroelectric thin films in an electric field arising from rectangular impulses. The system constructed enables investigation of changes of polarization as a function of intensity of electric field in a wide range of time intervals by means of an integrating system, and investigation of the time-course of polarization reversing as a function of intensity of electric field by means of a differentiating system.