Field cooled and zero field cooled 207Pb NMR and the local structure of relaxor PbMg1/3Nb2/3O3

An isotropic 207Pb NMR spectrum corresponding to the glassy matrix with spherical shell type Pb shifts from the cubic sites, as well as an anisotropic spectral component corresponding to polar nanoclusters with a Pb shift parallel to the [111] direction, have been observed in a PbMg(1/3)Nb(2/3)O3 (PMN) single crystal. This represents a microscopic confirmation of the model of relaxors first proposed by Burns and Dacol. A sudden increase in the intensity of the anisotropic cluster line is seen for electric fields larger than the critical field around 210 K. This demonstrates the occurrence of an orientational percolation type transition to the field-induced ferroelectric phase with about 50% of the Pb nuclei still remaining in the spherical glass matrix. A similar though smaller increase of the intensity of this line is also seen in the zero field cooled data, demonstrating that relaxor PMN is an incipient ferroelectric.     

Criticality controlled by cross-linking density in liquid single-crystal elastomers

A high-resolution calorimetry and deuteron-nuclear magnetic resonance study of a paranematic-nematic phase transition was performed on liquid single-crystal elastomers. We show that density variations of both rodlike and pointlike cross-links strongly affect the mean value and the dispersion of local mechanical fields. The system exhibits an inherent weakly disordered orientational state composed of regions with the temperature profile of the nematic order parameter ranging from first order to supercritical. On increasing the cross-linking density the predominantly first order thermodynamic response transforms into a predominantly supercritical one.     

Chemical and chromatographic stability of methacrylate-based monolithic columns

Chemical and chromatographic stability of methacrylate-based monolithic columns bearing 3-N,N-diethylamino-2-hydroxypropyl (DEAE) and quarternary amine (QA) groups was studied. The leakage products from both monolithic columns were determined and the leakage of amines has been quantified in alkali solutions. Monolithic columns bearing QA functional groups being exposed to 1M sodium hydroxide solution for up to 3 months caused reduction of ion-exchange groups for approximately 12%, while for DEAE monolithic columns was only around 3% in 1 year. In 0.1M NaOH and 20% ethanol degradation was significantly lower. The main leaking compound from DEAE monolith was found to be 3-(diethylamino)-1,2-propanediol and 2,3-dihydroxypropyltrimethylammonium salt for QA monolith. During repeated 50 cleaning-in-place (CIP) cycles, no changes in chromatographic properties were detected.     

Synthesis and self-assembly of thio derivatives of calix[4]arene on noble metal surfaces

Self-assembled monolayers (SAMs) provide a simple route to functionalize electrode surfaces with organic molecules. Herein we use cavity-containing derivatives of calix[4]arenes in SAMs. Bound to noble metal surface, the assembled molecules are candidates to serve as molecular sieves for H 2 molecules and H (+) ions, which could have relevance for fuel cell applications. Tetra- O-alkylated calix[4]arenes with thiolacetate and thiolamide wide-rim anchoring groups in cone and partial-cone conformations were designed, synthesized and self-assembled onto Au, Pt, and Pd surfaces. The resulting SAMs were systematically examined. Single crystal X-ray diffraction of 5,11,17,23-tetrakis(thioacetyl)-25,26,27,28-tetra- i-propoxycalix[4]arene confirmed the cone conformation and revealed the cavity dimensions of the SAMs that were formed by immersing noble metal substrates (Au, Pt and Pd deposited on Si-wafers) in solutions of calix[4]arenes. Surface characterization techniques including ellipsometry, cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) were used, indicating that the metal surface is terminated with a monomolecular layer. Experimental thicknesses obtained from the ellipsometry are consistent with the calculated values. CV results showed 50 to 80% physical passivation against the Fe(CN) 6 (3-/4-) couple, implying an overall relatively low concentration of defects and pinholes in the films. The binding energies of the S2p core level in the XPS were consistent with the literature values and revealed that up to 3.2 out of four anchoring groups were bonded to the noble metal surface.     

PdGa intermetallic hydrogenation catalyst: an NMR and physical property study

The PdGa intermetallic compound is a highly selective and stable heterogeneous hydrogenation catalyst for the semi-hydrogenation of acetylene. We have studied single crystals of PdGa grown by the Czochralski technique. The (69)Ga electric-field-gradient (EFG) tensor was determined by means of NMR spectroscopy, giving experimental confirmation of both the recently refined structural model of PdGa and the theoretically predicted Pd-Ga covalent bonding scheme. The hydrogenation experiment has detected no hydrogen uptake in the PdGa, thus preventing in situ hydride formation that leads to a reduction of the catalytic selectivity. We have also determined bulk physical properties (the magnetic susceptibility, the electrical resistivity, the thermoelectric power, the Hall coefficient, the thermal conductivity and the specific heat) of single-crystalline PdGa. The results show that PdGa is a diamagnet with metallic electrical resistivity and moderately high thermal conductivity. The thermoelectric power is negative with complicated temperature dependence, whereas the Hall coefficient is positive and temperature-dependent, indicating complexity of the Fermi surface. Partial fulfillment of the NMR Korringa relation reveals that the charge carriers are weakly correlated. Specific heat measurements show that the density of electronic states (DOS) at the Fermi energy of PdGa is reduced to 15% of the DOS of the elemental Pd metal.     

Structural and magnetic changes in hydrogenated TiFe1−xNix alloys

A series of crystalline B2 and amorphous TiFe_1−xNi_x samples with different compositions and hydrogen loading ratios was characterized by XRD, VSM, TG, mass-spectrometry, and ¹H NMR in order to investigate possible correlations between the sample magnetization and hydrogen content. In crystalline samples, a reversible second-order phase transition from α-phase (B2 with soluble hydrogen) through a mixture of α- and ß-phase (B19 TiFe_1−xNi_xH_y) into a pure ß-phase was found. An increase of magnetization upon hydrogen loading was observed, depending on the concentration of iron in the samples. No unambiguous correlation between the hydrogen content and the saturation magnetization was found. The saturation magnetization of as-prepared amorphous samples was found to be significantly higher than in as-prepared crystalline ones. For both types of samples, magnetization remained high after dehydrogenation as well. Thermal desorption mass-spectra was found to strongly depend on composition, whereas structure, charging pressure, and H-content did not have tremendous effect. By ¹H NMR, enhanced relaxation due to strong magnetic moments was observed; however, by TEM the existence of Fe-clusters in hydrogenated samples could not be verified.