O. J. Zogal’s research while affiliated with Włodzimierz Trzebiatowski Institute of Low Temperature and Structure Research and other places

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We report experimental results of 139La pulse NMR studies in LaOs4As12. Measurements have been performed on a powder sample obtained from high quality single crystals. For the first time the pattern of quadrupole echoes for 139La nuclei (I=7/2) was obtained. All the allowed quadrupolar echoes expected for spin I=7/2 were observed at times t=(4/3)tau, (3/2)tau, (5/3)tau, 2tau, (5/2)tau, 3tau, 4tau. The presence of quadrupolar echoes is the fingerprint of the deviation from perfect cubic symmetry of the structure and can be used as a simple and fast test of the sample quality.

We have grown a single crystal of ReB ₂ and refined its crystal structure. Our structural studies confirmed the hexagonal structure (space group P6₃/mmc ) with lattice parameters a=2.8982(1) Å and c=7.4723(3) Å . We also report the observation of first order satellites in the ¹¹ B nuclear magnetic resonance, which indicated the presence of a nonzero quadrupole coupling frequency, ν_Q=276±3 kHz , and an asymmetry parameter η=0 at the boron atom sites. These values are in excellent agreement with electric-field-gradient (EFG) tensor calculations based on first principles. These calculations showed that the principal axis of the most negative EFG-tensor component, V_ZZ , is parallel to the c -axis of the crystal. This behavior is in agreement with the observed excess of B p_z charge ( c direction) over the p_x and p_y charges and is related to stronger metal-boron bonds compared to previously studied YB ₁₂ and LaB ₆ , where the B–B bonds are stronger. Finally, the bonding properties of ReB ₂ are discussed in terms of densities of states, valence-electron densities, and partial charges.

Both one dimensional H-2 MAS NMR and two dimensional (EXSY) NMR were used to study lanthanum sub-stoichiometric trideuteride (LaD2.85) at room temperature. Remarkably narrow (8 and 10 Hz) intense lines were observed at 3.06 and 2.97 ppm in addition to two additional (31 and 2 35 Hz wide) at 2.62 and 1.92 ppm. The shifts were measured with respect to the H NMR line in PMMA (poly(methylmethacrylate)). The positions and intensities of the lines are discussed in terms of the order-disorder situation in the octahedral D-sublattice of the lanthanum matrix.

Proton magnetic resonance absorption spectra of yttrium dihydride (YH(2+x)), with x = 0.10, were recorded in the temperature ranges 4.2-310 K at 36.01 MHz and 150-400 K at 299.8 MHz. The evidence of proton self-diffusion follows from the changes of linewidth with temperature. The second moment of the resonance lines was determined from the experimental spectra and was compared with values calculated from the crystallographic data. The averaging effect of diffusion on the second moment was taken into account through Monte Carlo simulations of the diffusion process. The simulation was performed in a block of unit cells 5 x 5 x 5 with periodic boundary conditions. They compensated the effect of finite dimensions of the block. The calculated temperature dependence of the proton second moment values was fitted to the experimental ones. The fitting parameters were: the attempt frequency v0 and the activation energy Ea for hydrogen diffusion, assuming Arrhenius behavior of the jump frequencies vc = v0 exp(-Ea/k(B)T). In these preliminary studies, the Monte Carlo simulations were performed for tetrahedral-octahedral exchanges while direct tetrahedral-tetrahedral jumps were neglected for simplicity. Three models of hydrogen diffusion, differing in the maximum jump lengths allowed for a given model, were considered. These lengths were taken as the distances from the hydrogen attempting to jump to the first (1NN), second (2NN), and third (3NN) nearest neighbor position able to accept the jumping atom. Assuming the same attempt frequency v0 = 6.0 x 10(12)s(-1) for all three models, the activation energies giving the best fit to experimental data were 0.5, 0.54, and 0.55 eV for 1NN, 2NN, and 3NN models, respectively.

We have studied the structure of hydrogen loaded YyTb1−y alloys, with y=0.2 and 0.9, by X-ray diffraction (XRD) techniques. The room-temperature XRD profiles of Y0.2Tb0.8Hx exhibit, in addition to the peaks of the h.c.p. metal lattice, those of the f.c.c. dihydride for all x>0. In the case of Y0.9Tb0.1Hx, the f.c.c. peaks show up only for x=0.20, indicating H-solubility for smaller hydrogen concentrations in this alloy. As for the two YyGd1−y (y=0.3 and 0.4) alloys, we observe in both cases precipitation of the dihydride for the smallest H concentrations. These results complement earlier resistivity measurements on the same systems, by presenting the evolution of the respective lattice parameters and their c/a ratios upon hydrogen addition and, in particular, specifying the influence of the metal lattice environment upon the state of the precipitating hydride.

Like in lithium oxide, it is easy to nucleate very pure metallic lithium precipitates by electron-irradiation of lithium fluoride crystals. Irradiations performed near room temperature give metallic colloids which are characterized by electron spin resonance (ESR). The metallic character of the precipitates is demonstrated by the temperature behaviour of the ESR line intensity, which follows the temperature-independent Pauli law. A typical value for the metal concentration is about a few percent. Varying the irradiation parameters, i.e. the total fluence and/or the instantaneous flux, affects considerably the ESR properties of the colloids: first, the ESR line width changes by almost two orders of magnitude, indicating that the sizes of the precipitates strongly depend on these irradiation conditions. Second, when measuring the variation of the ESR intensity, proportional to the metal content, two very different behaviours are observed during recovery (dynamic or isochronal). When the flux is high, an intense and narrow metallic line is obtained, which disappears rapidly with annealing (300 °C). For lower flux, the colloids have a broader ESR signal, i.e. a much smaller size, and are not destroyed by annealing until the LiF crystal melts (870 °C). 7Li NMR measurements extending earlier NMR experiments on neutron-irradiated LiF indicate that the Knight-shifted signal is split into two components with different widths. The position of the narrower component seems to correspond to big lithium particles having the features of bulk metallic lithium. The second component is likely to regroup particles of different sizes and geometry with different hyperfine interaction strength.

When introduced into the h.c.p. rare earths, hydrogen either precipitates, at low enough temperatures, into the β-phase dihydride or forms the peculiar α*-phase composed of H–R–H pairs occupying second-neighbour tetrahedral sites along the c-axis. In order to understand the origin of these structures, we have investigated the interaction of H with the magnetic configurations present in some Y–R systems by electrical resistivity measurements. Recent experiments on YyTb1−yHx have shown a correlation between the (y-dependent) existence of the α*-phase and eventual modulated magnetism, via the Fermi surface topology. The present results obtained on the (Y–Gd)Hx system seem to confirm the idea regarding the analogy between magnetic and structural modulations (spin–density vs. charge–density waves).

Li-7 MAS NMR spectra of 2.5 MeV electron-irradiated LiF crystals have been measured in a field of 9.4 T. Besides the resonance line of the ionic compound, a second well-separated spectrum is observed in the region of the Knight shift value for metallic lithium. At room temperature, the latter can be decomposed into two components with different Knight shift and linewidth values. When the temperature is increased, line narrowing takes place at first, indicating shortening of correlation times for self-diffusion, independently in both components. Above 370 K, both lines broaden and approach each other before collapsing into a single line. The high ppm component disappears after crossing the melting temperature of metallic lithium (454 K). The two lines are attributed to different types of metallic Li: one to bulk-like metal, the other to Li present initially under pressure and relaxing to the former under thermal treatment.

Magnetic susceptibility measurements were performed to analyze the influence of hydrogenation on superconducting properties of magnesium diboride. Powdered MgB2 was used to perform hydrogen absorption at a constant temperature and hydrogen gas pressure of 0.5 to 20 bars. Modified McMillan equations were used to analyze phonon mediated superconductor with two opposing directions of superconductivity. Observations indicated that hydrogenation decreased the diamagnetic signal of hydrogenated, commercial MgB2 powder whereas the critical temperature remained unchanged.