Showing papers by "Robert Blinc published in 2005"

NMR study of disorder in Ba Ti O 3 and Sr Ti O 3

Abstract: The observation of first order satellites in the NMR spectra of $^{47}\mathrm{Ti}$ and $^{49}\mathrm{Ti}$ in the cubic phases of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ and $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ as well as $^{18}\mathrm{O}$ enriched $\mathrm{Sr}\mathrm{Ti}^{18}\mathrm{O}_{3}$ demonstrates the presence of nonzero quadrupole coupling at the Ti sites in all these systems. This is incompatible with the central position of Ti in the oxygen octahedron and requires the presence of off-center Ti sites and dynamic disorder even in the paraelectric cubic phase. The Ti potential surface does not seem to change significantly at the cubic-tetragonal transitions. The pure 8-site ``order-disorder'' model however does not agree with the experiment. A good agreement with the experimental data is obtained if in addition to the dynamic Ti disorder the ``displacive'' soft mode induced deformation of the unit cell shape in the dynamic tetragonal nanodomains, existing in the cubic phase, is taken into account. The simultaneous presence of the Ti disorder and the soft mode leads to a special type of phase transition with combined ``displacive'' and ``order-disorder'' character.

Order-disorder component in the phase transition mechanism of 18O enriched strontium titanate.

Abstract: Ti and Sr nuclear magnetic resonance spectra of 18O enriched SrTiO3 (STO-18) provide direct evidence for Ti disorder already in the cubic phase and show that the ferroelectric transition at T(C)=24 K occurs in two steps. Below 70 K rhombohedral polar clusters are formed in the tetragonal matrix. These clusters subsequently grow in concentration, freeze out, and percolate, leading to an inhomogeneous ferroelectric state below T(C). This shows that the elusive ferroelectric transition in STO-18 is indeed connected with local symmetry lowering and implies the existence of an order-disorder component in addition to the displacive soft mode one. Rhombohedral clusters, Ti disorder, and a two-component state are found in the so-called quantum paraelectric state of STO-16 as well. The concentration of the rhombohedral clusters is, however, not high enough to allow for percolation.

Calorimetric and dielectric studies of ferroelectric sodium nitrite confined in a nanoscale porous glass matrix.

Abstract: Heat-capacity measurements of the sodium nitrite confined in a nanoscale porous glass matrix show that the intermediate incommensurate phase, present in the bulk, has disappeared and that the first-order ferroelectric transition becomes suppressed and gradual. The ferroelectric transition temperature is shifted considerably to lower temperatures. Two noncritical dielectric modes were observed; however, the observed giant growth of the dielectric constant on heating through the transition temperature TC is shown to be mainly due to the electrode polarization effect.

Spherical model of relaxor polymers

Abstract: Recent dielectric experiments have revealed a striking analogy between inorganic relaxor ferroelectrics and disordered organic ferroelectric polymers. We propose a model description of these relaxorlike polymers by considering a statistical ensemble of randomly oriented polar nanoregions, embedded in an amorphous matrix. In analogy to the spherical random bond-random field (SRBRF) model of inorganic relaxor ferroelectrics, we assume that the polar nanoregions are coupled through infinitely ranged random interactions of a spin-glass type and are subject to random local fields. The second moment of the probability distribution of dipole moments is required to have a fixed value, which is equivalent to a global spherical condition. A general solution of the SRBRF model is presented, and the linear and nonlinear static dielectric susceptibilities are calculated and compared with the experimental data. The phenomenon of giant electrostriction in relaxor polymers is also discussed.

Low-temperature phase of SrTiO3

Abstract: The linear and nonlinear dielectric properties of SrTiO3 (STO16) and SrTiO318 (STO18) are described by an order-disorder component represented by the spherical random-bond-random-field model, up until now applied only to relaxor ferroelectrics. It is shown that, due to the contribution of domain walls, the dielectric response of STO18 below Tc increases with increasing the measuring field. As similar behavior is observed in STO16 also and, furthermore, the field-cooled-zero-field-heated polarization is different from zero below 55 K, the high values of the linear dielectric constant in the low-temperature phase of STO16 are ascribed to the dynamic response of the polar clusters.

Temperature dependence of17O and14N NQR frequencies in commercial TNT

Abstract: The temperature dependence of the17O nuclear quadrupole resonance (NQR) frequencies has been measured in commercial trinitrotoluene by highly sensitive nuclear quadrupole double resonance techniques. The14N NQR frequencies have been measured in the same sample for comparison. In contrast to the14N quadrupole coupling constant, the17O quadrupole constant increases with increasing temperature.

Proton NMR study of molecular motion in bulk and in highly drawn fiber polyamide-6

Abstract: The proton motion in bulk and highly drawn fiber polyamide-6 has been studied by field cycling relaxometry and proton line shape measurements. The dips in theT1 dispersion allowed for the determination of the14N quadrupole coupling tensor. The fact that only one set of14N nuclear quadrupole resonance lines, has been found shows that all N-H groups in nylon-6 are H-bonded. A striking difference in the main line width transition and the low-frequency molecular dynamics has been observed between a slowly cooled “bulk” polyamide-6 sample and a rapidly cooled and highly drawn “fiber” sample by wide line proton nuclear magnetic resonance line shape and spin-lattice relaxation time measurements. This result is consistent with the picture that shearing melt processing procedures, such as spinning, plant stable and long-lived crystallization nuclei into the amorphous phase which impose additional motional constraints on the surroundings and inhibit the self-diffusion process.

Electron paramagnetic resonance of stressed fibre nylon 6: annealing effects

Abstract: The temperature dependence of the electron paramagnetic resonance (EPR) spectra of 'high tension' (HT) and 'low tension' (LT) nylon 6 fibres has been measured in the X-band. The spectra show a characteristic hyperfine structure owing to coupling to the nitrogen nucleus and occur due to nitroxide additives. The linewidth transitions signalling the onset of self-diffusion occur at much higher temperatures than in the proton NMR spectra. The higher resolution of the EPR data allows for discrimination between HT and LT fibres and the observation of the effects of the super-structure induced by different thermo-mechanical treatments on the molecular self-diffusion.

Angular Dependence of 93Nb NMR in KTa1−x NbxO3

Abstract: The 93 Nb NMR spectra of a mixed KTa 1−x Nb x O 3 (KTN) single crystal with x = 15% have been measured The observation of an unresolved quadrupole-induced first-order satellite background below the sharp central 1/2 ⟷ −1/2 transition already in the cubic phase demonstrates that the Nb ions occupy off-center positions rather than the high symmetry central perovskite site The angular dependence of the second moments of the satellite background further shows that these distortions are of rhombohedral symmetry, ie the Nb ions are displaced along the [111] body diagonals

Dynamic Disorder in Perovskites: Local Symmetry Breaking at the Sr Site in SrTiO3

Abstract: Perovskites such as SrTiO3 and BaTiO3 have been considered to be classical displacive systems where the phase transitions are connected with a soft mode instability. Only recently a disorder component has been detected in addition to the soft mode. It has been shown [1–3] by quadrupole perturbed Ti (I = 5/2) and Ti (I = 7/2) NMR that in the high temperature cubic phases of BaTiO3 and SrTiO3 the Ti ions are dynamically disordered between 8 off-center sites so that the ideal Oh symmetry is locally broken. The electric field gradient (EFG) tensor is non-zero at the Ti sites whereas it should be zero by symmetry in the ideal perovskite lattice with the Ti ion in the center of the oxygen octahedron. The Ti off-center sites are located at the cubic body diagonals in agreement with the extended X-ray absorption fine structure (EXAFS) data [4, 5], the early X-ray work of Comes et al. [6], and the theoretical model of Chaves et al. [7] (Fig. 1). The off-centering of the Ti ions is accompanied by a local tetragonal distribution of the unit cell, so that fluctuating tetragonal nanodomains (+a, − a, + b, − b, + c, − c) are formed in the cubic phase (Fig. 2). The macroscopic cubic symmetry is the result of space and time averaging. Below the cubic-tetragonal transition long range correlations appear and the nanodomains transform into macroscopic 90◦ domains. ∗corresponding author