B. Nowak

Nuclear magnetic resonance study of the dihydride phase of the Ti-V-H system

Abstract We report the results of nuclear magnetic resonance (NMR) measurements of 1. (1) the line shape, the second and fourth moments and the spin-lattice relaxation of the 1 H resonance, and 2. (2) the Knight shift and spin-lattice relaxation of the 51 V resonance in the dihydride phase Ti 1− x V x H 2 (0.04 ⩽ x ⩽ 0.65). The X-ray and magnetic susceptibility data are also reported. A two-line system was observed in both the 1 H and 51 V NMR spectra for x = 0.65 indicating that the range of existence of the single dihydride phase in vanadium-rich dihydrides is much narrower than reported previously. Comparison of the experimental and theoretical values of the linewidth and the second and fourth moments of 1 H NMR shows that the dipolar interaction is the main source of the line broadening. The temperature dependence of the 1 H value T 1 T and the magnetic susceptibility in dilute vanadium hydrides is interpreted in terms of the transition from an f.c.c. phase to a face-centred tetragonal phase induced by the Jahn-Teller effect. Comparison of the values of vanadium Knight shifts in Ti 1− x V x H 2 with existing data for the titanium Knight shift in TiH 2 strongly suggests that the local densities of states at titanium and vanadium sites in the dilute vanadium hydrides Ti 1− x V x H 2 are different. Opposite dependences of the 1 H and 51 V spinlattice relaxation rates on the vanadium concentration support this suggestion.

X-ray diffraction and 1h and 51V NMR study of the structure of a TiVH alloy in relation to preparation conditions

Abstract It is shown that the alloy Ti 0.35 V 0.65 (b.c.c. solid solution) is separated into a few phases by hydrogenation. The structure, concentration and composition of each phase are determined by X-ray diffraction and 1 H and 51 V NMR measurements. It is concluded that the precipitation of titanium dihydride phase is vanishingly small if the hydrogen pressure used in the synthesis of the hydrides is of the order of atmospheric pressure, and it is suggested that high hydrogen pressure favours the precipitation of titanium dihydride.

91Zr NMR in non-stoichiometric zirconium hydrides, ZrHx (1.55 ⩽ x ⩽ 2)

Abstract 91Zr nuclear magnetic resonance (NMR) has been observed in f.c.c. (δ phase) and f.c.t. (ϵ phase) ZrHx (1.55 ⩽ × ⩽ 2) in the temperature range 150–294 K. A systematic increase in the linewidth with decreasing hydrogen content was observed and possible sources of the line broadening are discussed. Both the line shapes and 91Zr Knight shifts were found to be temperature independent. In the ϵ phase the shift, which results from competition between positive d-orbital and negative d-core polarization contributions, was found to be concentration dependent, exhibiting a local minimum near x=1.8. This behaviour is consistent with the hydrogen spin-lattice relaxation rate measured previously by several authors and confirms the existence of a local maximum in the electronic density of states at the Fermi level, N(EF), at x=1.8 in the ZrHx system. In the δ phase the shift is about 85% larger than in the ϵ phase and its magnitude results mainly from d-orbital interactions.

47Ti and 49Ti nuclear magnetic resonance in TiH2

Abstract The NMR of 47Ti and 49Ti has been observed in TiH2 in the temperature range 155–310 K. The Knight shift and spin-lattice relaxation rate (TlT)− were found to be temperature dependent. The shift varied from 0.245 ± 0.002% at 310 K to 0.319 ± 0.002% at 155 K, whereas T1T varied from 22 ± 1 s K at 310 K to 48.8 ± 3 s K at 155 K. From the temperature dependences of Knight shift and magnetic susceptibility a core polarization hyperfine field of − (126 ± 8) kOe was deduced. Applying the tight binding approximation the data for the cubic phase (310 K) have been partitioned into spin (s,p,d) and orbital (p,d) contributions. In the tetragonal phase (below 310 K) a temperature dependence of the d band density of states at the Fermi level was deduced. An influence of the lower symmetry on the spin-lattice relaxation behaviour is discussed.

1H NMR and magnetization measurements of a nanostructured composite material of the Mg2NiH system synthesized by reactive mechanical grinding

Abstract 1 H NMR spectra and magnetization have been measured for a nanostructured Mg 2 NiH system. 1 H MAS NMR spectra indicate the presence of four phases; α-Mg 2 NiH, β-Mg 2 NiH 4 , α-MgH, and β-MgH 2 . Most of the hydrogens are contained in the disordered β-Mg 2 NiH 4 phase. Magnetization consists of superparamagnetic and paramagnetic ones. The former has its origin in Ni particles formed during the hydrogenation. The latter is greater than the values of crystalline α-Mg 2 NiH, and β-Mg 2 NiH 4 . 1 H NMR line width increases with the decrease in temperature linearly, most of which is of dipolar origin. The hydrogen is more mobile in the disordered β-Mg 2 NiH 4 phase than in the crystalline counterpart.

Magnetic properties of cubic and hexagonal chromium hydrides: a comparison of the magnetic susceptibility with the 53Cr NMR Knight shift

Abstract We have measured the 53 Cr Knight shift in cubic and hexagonal chromium hydrides with atomic hydrogen-to-chromium ratio H/Cr≈1 in the temperature range 3–300 K. The shifts (0.30% in cubic phase, 0.53% in hexagonal phase) are temperature independent and imply that the intrinsic magnetic susceptibilities of the respective hydrides are also temperature independent. The values of intrinsic magnetic susceptibilities have been determined from the analysis of magnetisation isotherms recorded in the temperature range 1.7–300 K, and in magnetic field strengths up to 55 kOe. The isotherms indicate the presence of paramagnetic and ferromagnetic impurities in the samples, the magnetisation of which was subtracted from the measured magnetisation data. The cubic and hexagonal chromium hydrides are Pauli paramagnets with intrinsic susceptibilities of 5.9·10 −8 and 5.0·10 −8 m 3 /kg, respectively.

29Si NMR and magnetic susceptibility of U3Si2

Abstract The magnetic susceptibility, Дm, of U3Si2 has been measured in the temperature range 4–1000 K. It exhibits a broad maximum ( T max ⋍300 K ) which may relate to the spin fluctuation process. Below Tmax the microscopic magnetic properties of U3Si2 have been investigated by the 29Si Knight shift (K) and spin-lattice relaxation rate (1/T1). The Knight shift versus susceptibility relation gives a hyperfine coupling constant Hhf=48 kOe/μB. From the extrapolated limit of the linear K vs Дm curve a large positive temperature-independent contribution to Дm is obtained. This is attributable to the Van Vleck-type orbital paramagnetism, to be expected in metals with unfilled degenerate bands. The temperature dependencies of Дm, K and (1/T1) give evidence for the fluctuations of the uranium 5f moments. The characteristic fluctuation rate is ~3kBTmax/ ħ when an effective number of uranium ions involved in the transferred hyperfine interaction is limited to first neighbours.

51V nuclear magnetic resonance and magnetic susceptibility study of the electronic structure in Ti-V dihydrides

Abstract The 51 V nuclear spin-lattice relaxation time T 1 , 51 V Knight shift K V and magnetic susceptibility χ in nearly stoichiometric Ti 1− x V x H 2 (0.04 ⩽ x ⩽ 0.65) have been measured. It was found that ( T 1 T ) v −1 in dilute Ti-V hydrides is twice as large as ( T 1 T ) v −1 in VH 2 , decreasing gradually with increasing vanadium concentration. The separation of ( T 1 T ) v −1 and K v into d spin and d orbital contributions for each vanadium concentration is reported, and the resulting densities N d v ( E F ) of states as well as the d spin and d orbital susceptibilities at vanadium sites are discussed. The results are compared with existing data for binary TiH 2 and VH 2 .

27Al and 65Cu NMR study in UCu5Al.

We have studied the microscopic properties of the tetragonal UCu5Al Kondo compound by 27Al and 63,65Cu NMR in the paramagnetic state. NMR and susceptibility measurements performed on the powdered sample, but oriented along the applied field, showed chi(parallel) > chi(perpendicular). Plots of K(parallel)(T) against chi(parallel)(T) at temperatures T > or = 100 K yield the transferred hyperfine fields of +5.9 kOe/micron(B) for 27Al nuclei, and +5.3 and -7.0 kOe/micron(B) for 65Cu nuclei in crystallographically inequivalent Cu(2) and Cu(1) sites, respectively. The Knight shift vs. susceptibility plots for T < 100 K exhibit a deviation from the linear behaviour (absolute values of shifts become smaller than expected). We attribute this finding to the crystalline electric field effect in similar way as it was reported for several Ce-based compounds. The random distribution of the Al and Cu(2) atoms in the crystal lattice we consider as a reason of an unusual broadening of the NMR spectra, particularly at low temperatures.

Comment on the reference compound for chemical shift and Knight shift determination of 209Bi nuclei

Several groups exploring the (209)Bi NMR in solids, including usual insulators, metallic and magnetic materials and recently diamagnetic topological materials, use different standards (usually old and invalid) for chemical shift (Knight shift) determination, ignoring IUPAC recommendations. As a consequence the published shift values exhibit considerable differences (up to 17,500 ppm).