M. Klanjšek

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.

Origin of the maximum in the temperature-dependent electrical resistivity of quasicrystals

We discuss the origin of the maximum in the electrical resistivity ρ(T) of quasicrystals (QCs) and show that it can be consistently explained as a magnetic effect due to the presence of localized magnetic scattering centres within the quasiperiodic lattice. On the experimental side we present a comparative electrical resistivity–magnetic susceptibility study of icosahedral AlPdMn and CdCa QCs and show correlation between the temperature of the ρ(T) maximum and the electronic paramagnetic magnetization of localized magnetic centres. We propose a theoretical explanation in terms of the Korringa–Gerritsen (KG) model, originally developed for noble metals with diluted transition metal impurities. Unlike the other existing models of electrical conduction in QCs, the KG model describes the ρ(T) maximum as a magnetic effect and predicts its shift to higher temperatures for an increased concentration of magnetic moments.

Experimental study of the electronic density of states in aluminium-based intermetallics

We report on an experimental investigation of the electronic density of states (DOS) in aluminium-based intermetallics by three experimental techniques: electrical resistivity, NMR spin–lattice relaxation and soft x-ray emission spectroscopy (SXES). The investigated samples were alloys of Al with transition elements Cu, Fe, Cr, Pd and Mn and sometimes also a small amount of B. The samples were structurally very different and included (1) regular periodic intermetallic compounds, (2) large-unit-cell intermetallics that are translationally periodic on the scale of several nanometres and exhibit local polytetrahedral order (like quasicrystals) on the scale of interatomic distances and (3) quasicrystals. Correlation analysis between the DOS parameters determined independently by the three techniques employed showed that the order of samples with decreasing metallic character was the same in all experiments. Quantitative evaluation of the DOS at EF has shown that in regular alloys the DOS is reduced to about 50% of the free-electron-like value in fcc Al metal, whereas the reduction becomes increasingly larger on going to large-unit-cell periodic solids and quasicrystals. Our results also demonstrate that low resistivities are accompanied by positive temperature coefficient (PTC) variation, whereas samples with large resistivity exhibit negative temperature coefficient (NTC). Samples with a resistivity of about 200 µΩ cm appear to be at a crossover from PTC to NTC resistivity, resulting in a temperature-compensated resistivity with essentially zero temperature coefficient. Magnetic properties of the samples are also presented.

Basics of NMR line shape in quasicrystals

The measurement and analysis of broad nuclear magnetic resonance (NMR) spectra of quasicrystals require experimental methods and theoretical interpretations different from NMR investigations of regular periodic crystals. Frequency- and field-sweep methods for recording quasicrystalline NMR spectra are described and compared with the measurement of27Al NMR spectra of icosahedral AlPdMn and decagonal AlNiCo quasicrystals. The nuclear spin interactions that determine the NMR line shape are the same for both types of the above Al-based quasicrystals, where the electric quadrupolar interaction with the broad distribution of its electric field gradient parameters predominantly determines the shape of the broad satellite “background” intensity. The essential observations are an almost isotropic27Al NMR spectrum of the icosahedral quasicrystals and a strong angular dependence of the spectrum of decagonal quasicrystals.

Nuclear magnetic resonance reveals ‘forbidden’ symmetries in quasicrystals and related metallic alloys with giant unit cells

We report on measurements of angular dependences of the selectively excited NMR intensity in a decagonal AlNiCo quasicrystal, icosahedral AlPdMn quasicrystal and orthorhombic ξ′-AlPdMn single crystal with approximately 258 atoms in the unit cell. In all three cases, we found 10-fold rotation patterns, in agreement with structural properties of the investigated samples. However, in the orthorhombic ξ′-AlPdMn complex metallic alloy, a non-perfect 10-fold symmetry was observed, which is a consequence of periodicity that strictly forbids non-crystallographic rotational symmetries. These measurements represent a starting point for testing structural models of quasicrystals and related metallic alloys with giant unit cells by means of NMR.

Modelling electric field gradients of icosahedral quasicrystals

A simple auxiliary model is presented that enables the extraction of general features of the electric field gradient (EFG) tensor distribution in icosahedral quasicrystals directly from the orientation dependence of the quadrupole-perturbed NMR spectrum. The only assumption of the model is a macroscopic icosahedral symmetry of the sample under study. It is shown that the model serves as a good filter since on the basis of the measured orientation-dependent NMR spectrum it can directly differentiate between qualitatively different icosahedral distributions of the EFG tensor orientations.

Amorphization of Al32Ge68 in a slow heating run studied by NMR

Amorphization of Al32Ge68 at normal pressure, starting from the thermobarically quenched high-pressure crystalline metallic γ-phase, was studied in a slow heating run by performing NMR field-sweep experiments. In the temperature interval from 77 to 300 K the 27Al central line exhibits a continuous broadening and a positive frequency shift. Close to 300 K rather abrupt changes were observed, where the shift changed its sign, and the line width jumped to its highest value. The discontinuous course of the phase changes indicates that the amorphization process proceeds via a sequence of intermediate metastable states, which can be ‘overheated’ in a slow heating run and consequently transform in an explosion-like manner to the final amorphous state at a well-defined temperature. The frequency shift could be decomposed into the negative second-order quadrupolar shift and the positive Knight shift. The change of the shift sign from positive to negative at 300 K reflects the vanishing tendency of the Knight shift upon heating and is compatible with the conduction-electron localization upon structure amorphization.

Paramagnetic NMR relaxation in icosahedral quasicrystals

Abstract The nuclear spin-lattice relaxation in icosahedral quasicrystals containing magnetic transition-metal atoms is governed by two mechanisms of competing strength: the relaxation via conduction electrons and the relaxation via paramagnetic centers in combination with spin diffusion. The 27Al NMR relaxation experiments on icosahedral AlPdMn and AlCuFe have shown that the paramagnetic relaxation dominates at temperatures below 100 K.

1-D and 2-D NMR of broad-line solids: an application to quasicrystals

Nuclear magnetic resonance (NMR) techniques for measuring one-dimensional absorption spectra and two-dimensional exchange spectra of solids with extremely inhomogeneously broadened lines are discussed. Among various “broad-line” solids, quasicrystals represent alloys of metallic elements, the structures of which include “forbidden” symmetry elements. NMR absorption lines of quasicrystals exhibit a strong electric-quadrupole-induced inhomogeneous broadening that originates from the lack of translational periodicity of the otherwise perfectly long-range-ordered quasiperiodic lattice. Recording an NMR spectrum of a quasicrystalline sample requires a magnetic field-sweep technique. The two-dimensional exchange experiment on quasicrystals can be performed on selectively excited portions of the NMR spectrum only. Due to the off-resonance effects in a selective excitation, the use of a simple three-pulse stimulated-echo exchange sequence is preferred. The27Al spectra of the Al-Pd-Mn and Al-Pd-Re families show interesting features like temperature-dependent frequency shifts and exchange effects due to atomic motion.