P. J. Mendes

Study of NbH phases using perturbed angular correlation techniques

Abstract Niobium hydride phases were investigated by the perturbed angular correlation technique. By this method the electric field gradient (EFG) at radioactive probe atoms inside the NbH lattice is measured. We find two distinct EFGs in the region of the ϵ phase of NbH whereas, in the temperature range of the ζ and β phases, only a single EFG with a spread around the mean value is observed. Only a very weak disturbance of the cubic symmetry is found in the α phase. The observed EFGs are discussed in the frame of the point charge model.

A magnetization and neutron powder diffraction study of compounds (R = Y, Dy, Ho, Er)

Neutron powder diffraction experiments and magnetic measurements were performed on compounds of the series (R = Y, Dy, Ho and Er). The influence of the R element on both the structural and the magnetic properties of the different compounds is discussed, as well as the possible correlation between the iron environments and the local moments. Comparison is made with a previous Mossbauer study on the same compounds.

PAC study of O-H and O-D in Ta

A PAC experiment was performed on a181HfTa sample which contained ∼ 0. 1 at. oxygen and ∼1 at. hydrogen. At low temperature a new quadrupole interaction withνQ=470 MHz, η=0.95 was found. This interaction is attributed to an O-H complex. At 55 K this interaction disappears and the well known oxygen frequency shows up, indicating a breakup of the O-H pair at 55 K. With deuterium instead of hydrogen the same frequency but a different (105 K) breakup temperature was found.

Phase transitions in the tantalum-hydrogen system observed by PAC

The perturbation felt by181Hf probes in a181HfTa lattice loaded with 30 at% hydrogen was observed by PAC as a function of temperature. Three different interactions were identified: 1) ΝQ1=433 (6) MHz, η=0.45 2) ΝQ2=142 (9) MHz, η=0.9, and 3) ΝQ≃0, σ=4–14 Μ−1 which are attributed to the Β−, ε− and α-phase in TaH system, respectively.

Magnetic phase transitions in RFe9.5Mo2.5 intermetallics studied by 57Fe Mössbauer spectroscopy, magnetisation and μ+SR

Abstract A series of RFe 12− x M x -type alloys were stabilised with M = Mo and x = 2.5 for R = Y, Nd, Dy and Er. Magnetisation, ac susceptibility measurements and μ + SR spectroscopy results are in good agreement with a systematic 57 Fe Mossbauer spectroscopy analysis. The strength of all local iron magnetic moments (and the related hyperfine fields), as well as the long range magnetic ordering, have been found to be markedly affected by the substitution of iron by molybdenum in Fe(8i) sites. It was found that the temperature dependence of the order/disorder phenomenology is well described by local magnetic correlation.

Electric field gradients in different niobium hydride phases

Abstract Various phases of the NbH system were investigated by the perturbed angular correlation technique. The 181HfNb samples were charged with various hydrogen concentrations (atomic ratios of 0.20, 0.75 and 0.90). For each sample, measurements were performed at several temperatures. The observed interactions are discussed in terms of the phases involved in the NbH phase diagram. Asymmetry parameters are compared with point-charge model calculations.

Level crossing resonance in ice: Identification of two diamagnetic muon states

Level crossing resonance studies of ice, doped with HF and NH3 to enhance and suppress the concentration of orientational defects, support the hypothesis that muons can trap at such defects to form complexes of the type H2O-Mu+-OH2. The doping alters the partition of the total diamagnetic fraction between this state, which is stable against charge migrationvia proton rearrangement at low temperature, and the neutral HMuO state, the isotopically substituted water molecule.

γ-γ Perturbed angular correlation studies of dysprosium hydrides☆

Abstract Perturbed angular correlation measurements have been performed on two dysprosium hydrides (DyHx with x ≈ 2 and x ≈ 3) using the 181Ta nucleus as a substitutional probe. When x ≈ 2, the observed electric field gradients result from hydrogen vacancies in a cubic lattice. The contribution of the conduction electron density to the electric field gradient is discussed. In the trihydride the electric field gradient has an axial symmetry.

Study of RFe12 xMox (R = Y, Ho) compounds by neutron powder diffraction, ac susceptibility and magnetization

Neutron powder diffraction, magnetization and ac susceptibility measurements were performed on compounds of the series RFe12 x Mox (RD Y and Ho,xD 1, 2, 3). The influence of the Mo content on both structural and magnetic properties is discussed. Comparison with published data for different Mo concentrations is made. It is found that the effect of Mo substitution for Fe on structural and magnetic properties can be described by two regimes separated by a critical Mo content aroundxD 2.

Muon level crossing resonance in aluminium

Cross relaxation between implanted positive muons and27Al nuclei in Al metal, lightly doped with Cu to impede the muon diffusion, shows weak resonances at 2.3 and 4.0 mT. Assignment of these to polarization transfervia the 1/2 ↔ 3/2 and 3/2 ↔ 5/2 transitions of the (J=5/2) spins leads to a quadrupole coupling constante2qQ/h=1.8(1) MHz,i.e. an electric field gradient on Al nuclei immediately adjacent to the muon siteq=0.048(3) a.u., with a small departure from axial symmetry.