D. R. Torgeson

NMR search for evidence of hydrogen pairing in h.c.p. yttrium

Abstract We report the results of a proton nuclear magnetic resonance (NMR) study of hydrogen in solid solution in h.c.p. yttrium (α-YH 0.2 ) over the temperature range 77–715 K. The concept of hydrogen “pairing” in closely adjacent tetrahedral (T) sites in the metal lattice is not supported by lineshape measurements, and the dipolar contribution to the spin-lattice relaxation time ( T 1 ) is consistent with random occupancy of T-sites. The T 1 measurements yield an activation energy for hydrogen diffusion of 0.51 eV atom − 1 , in excellent agreement with recent inelastic neutron-scattering results.

Conduction electron density of states and proton spin-lattice relaxation in the dihydrides of scandium, yttrium, lanthanum and lutetium☆

Abstract We report determinations of the proton Korringa product T1eT in the dihydride phases of scandium, yttrium, lanthanum and lutetium based on samples prepared from the highest purity metals available in the Ames Laboratory and in addition in some cases utilizing the technique of partial deuteration to suppress further the paramagnetic impurity contribution to the spin-lattice relaxation rate. We conclude that the quantity (T 1e T) − 1 2 , which is essentially proportional to the electronic density of states N(EF), has the value 0.055 ± 0.002 s − 1 2 K − 1 2 at the dihydride composition in all systems. The dependence of ( T 1e T) − 1 2 on hydrogen concentration within the dihydride phase appears to be weak; however, in LaHx (T1eT)− 1 2 decreases substantially for x ⪢ 2 indicating a decrease in N(Ef) in this hydrogen concentration range. The implications of these results for relative s and d band contributions and hyperfine fields are discussed in the light of recent low temperature heat capacity measurements.

Dynamical evidence of hydrogen sublattice melting in metal-hydrogen systems

Abstract Measurements of the temperature dependence of the proton and deuteron spin-lattice relaxation time T1 in cubic fluorite (CaF2) structure dihydrides and dideuterides (MH2 and MD2) show a second high temperature turndown in T1 in addition to the usual minimum associated with the independent hopping motion of H(D) among tetrahedral (T) interstitial sites at lower temperatures. The close analogy to the motion of anions in fluorite structure superionics suggests that the high temperature minimum indicates the onset of strongly correlated hydrogen motion, possibly accompanied by the occurrence of long-lived hydrogen clusters.

Paramagnetic impurity effects in nuclear magnetic resonance determinations of hydrogen diffusion and electronic structure in metal hydrides: Cerium in YH2☆

Abstract We report the results of a preliminary survey of the effects on the proton spin-lattice relaxation time T1 resulting from the presence of controlled low levels of the Kramers ions cerium, neodymium, gadolinium, dysprosium and erbium in YH2. The Ce3+ results in particular are presented in some detail because the cerium ion behaves very differently from the others, reflecting the fact that it is an extremely “fast relaxing” ion. From the measured proton T1 we conclude that the Ce3+ ion spin-lattice relaxation time τi is 1.65 × 10−12s at 77 K.

Proton nuclear magnetic resonance study of crystal field levels in praseodymium hydrides

We report proton (1H) T1, measurements in the temperature range 8 K ⩽ T ⩽ 300 K for PrH2.0 and PrH2.5, as well as for praseodymium-doped YH≈2. The measured temperature dependence of T1 in PrH2.0 and PrH2.5 is entirely accounted for by the proton spin interaction with fluctuating Pr3+ electronic moments, and its behavior in PrH2.0 differs markedly from that in PrH2.5. The non-zero magnetic moment of the crystalline electric field (CEF) ground state Γ5 triplet in PrH2.0 causes the rate R1 = T1−1 to increase with decreasing temperature, yielding very short T1 values ranging from 100 μs at 300 K to only 30 μs at 8 K. In PrH2.5 the orthorhombic CEF results in nine singlet levels, with the result that the 1H T1, is roughly 1000 times longer than in PrH2.0, and its temperature dependence behaves in the opposite fashion, decreasing with increasing temperature from 250 ms at 8 K to 1 ms at 280 K to 1 ms at 280 K. In praseodymium-doped YH≈2 the magnitude and temperature dependence of R1p show the ground state CEF level to be a non-magnetic singlet.

Magnetic resonance determination of the electronic density of states and the metal-non-metal transition in the LaH2LaH3 system

Abstract Proton spin-lattice relaxation rate R1 and 139La Knight shift measurements have been utilized to study the electronic density of states N(EF) at the Fermi level and the metal-non-metal (MNM) transition in the lanthanum hydrides, LaHx(2 ⩽ x ⩽ 3), in samples prepared from highest purity Ames Laboratory lanthanum as well as from lanthanum containing controlled low levels (less than 300 ppm) of gadolinium. In the pure samples, free-electron-like metallic behavior is found for all compositions up to x ≈ 2.8 at temperatures T ⩽ 200 K. In the gadolinium-doped samples the Gd3+ ion spin fluctuations, characterized by the ion spin-lattice relaxation time τi,contribute an additional proton spin relaxation rate. Measurements of this rate for x 250 K, τi ∝ T−5, showing that Gd3+ spin relaxation is then governed by the two-phonon process typical of insulating solids.