Vacancy motion in rare-earth-deficient R_(1-x)Ni_2 Laves phases observed by perturbed angular correlation spectroscopy

Rare-earth-deficient R_(1-x)Ni_2 Laves phases, which reportedly crystallize in a C15 superstructure with ordered R vacancies, have been investigated by perturbed angular correlation (PAC) measurements of electric quadrupole interactions at the site of the probe nucleus ^111Cd. Although ^111Cd resides on the cubic R site, a strong axially symmetric quadrupole interaction (QI) with frequencies ν(q)approximate to265-275 MHz has been found in the paramagnetic phases of R_(1-x)Ni_2 with R=Pr,Nd,Sm,Gd. This interaction is not observed for the heavy R constituents R=Tb,Dy,Ho,Er. The fraction of probe nuclei subject to the QI in R_(1-x)Ni_2, R=Pr,Nd,Sm,Gd, decreases from 100% at low temperatures to zero at T>300 K and 500 K for R=Sm,Gd and R=Pr,Nd, respectively. At T=100 K the QI is static within the PAC time window, but at T=200 K fluctuations with correlation times τ_(C) 500 K nuclear spin relaxation related to vacancy hopping is observed in nearly all R_(1-x)N_i2. Auxiliary ^111Cd PAC measurements have been carried in Sc_0.95Ni_2, ScNi_2, ScNi_0.97, Gd_2Ni_(17), GdNi_5, GdNi_3, and GdNi

A Density Functional Study of Atomic Hydrogen and Oxygen Chemisorption on the Relaxed (0001) Surface of Double Hexagonal Close Packed Americium

Ab initio total energy calculations within the framework of density functional theory have been performed for atomic hydrogen and oxygen chemisorption on the (0001) surface of double hexagonal packed americium using a full-potential all-electron linearized augmented plane wave plus local orbitals method. Chemisorption energies were optimized with respect to the distance of the adatom from the relaxed surface for three adsorption sites, namely top, bridge, and hollow hcp sites, the adlayer structure corresponding to coverage of a 0.25 monolayer in all cases. Chemisorption energies were computed at the scalar-relativistic level (no spin-orbit coupling NSOC) and at the fully relativistic level (with spin-orbit coupling SOC). The two-fold bridge adsorption site was found to be the most stable site for O at both the NSOC and SOC theoretical levels with chemisorption energies of 8.204 eV and 8.368 eV respectively, while the three-fold hollow hcp adsorption site was found to be the most stable site for H with chemisorption energies of 3.136 eV at the NSOC level and 3.217 eV at the SOC level. The respective distances of the H and O adatoms from the surface were found to be 1.196 Ang. and 1.164 Ang. Overall our calculations indicate that chemisorption energies in cases with SOC are slightly more stable than the cases with NSOC in the 0.049-0.238 eV range. The work functions and net magnetic moments respectively increased and decreased in all cases compared with the corresponding quantities of bare dhcp Am (0001) surface. The partial charges inside the muffin-tins, difference charge density distributions, and the local density of states have been used to analyze the Am-adatom bond interactions in detail. The implications of chemisorption on Am 5f electron localization-delocalization are also discussed.Comment: 9 Tables, 5 figure

Magnetoelastic effects in Jahn-Teller distorted CrF2_2 and CuF2_2 studied by neutron powder diffraction

We have studied the temperature dependence of crystal and magnetic structures of the Jahn-Teller distorted transition metal difluorides CrF$_2$ and CuF$_2$ by neutron powder diffraction in the temperature range 2-280 K. The lattice parameters and the unit cell volume show magnetoelastic effects below the N\'eel temperature. The lattice strain due to the magnetostriction effect couples with the square of the order parameter of the antiferromagnetic phase transition. We also investigated the temperature dependence of the Jahn-Teller distortion which does not show any significant effect at the antiferromagnetic phase transition but increases linearly with increasing temperature for CrF$_2$ and remains almost independent of temperature in CuF$_2$. The magnitude of magnetovolume effect seems to increase with the low temperature saturated magnetic moment of the transition metal ions but the correlation is not at all perfect

Pressure-Induced Order-Disorder Transitions in RNi2 Compounds.

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe

Structural stability of YM2\mathrm{YM_2} compounds (M = Al, Ni, Cu) studied by abab initioinitio total-energy calculations and high-pressure x-ray diffraction

A combination of ab initio total-energy calculations and high-pressure x-ray diffraction experiments has been used to study the phase stability of Y-based 1:2 compounds. The motivation for these investigations was to clarify the origin of the change of the structure among the compounds (T = 3d transition element) between the neighbouring elements Ni (: defect superstructure of cubic Laves phase C15) and Cu (: orthorhombic -type structure). As a test of the ab initio calculations the cubic Laves phase compound has been included in the investigations.The ab initio calculations confirm the reversal of the relative stabilities of the and C15 phases in and and give also the correct results for . Furthermore a pressure-induced structural transition is predicted for .The high-pressure x-ray experiments show that there is very good agreement between the calculated and the measured pressure dependence of the lattice parameters of up to about 10 GPa. Above 10 GPa the structure starts to become irreversibly amorphous. This instability may be a hint at the structural phase transition predicted by the calculations

Compressibility of YMn2, Y6Mn23, YMn12 and ScMn2 Studied by Synchrotron Powder Diffraction.

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe

Homogeneity Range and Order-Disorder Transitions in R1-xNi2 Laves Phase Compounds

The range of homogeneity has been studied in R1-xNi2 Laves phases (R=Ce, Gd, Tb) by x ray diffraction, microprobe analysis and density measurements. In these compounds the number of R vacancies varies as a function of the nature of the rare earth and the nominal composition. For R=Tb the number of vacancies varies by 0 £ x £ 0.5, from a pure C15 structure to a 2a superstructure accompanied by a cell volume decrease. The order-disorder transition of the rare earth vacancies has been studied under applied pressure for La7Ni16 and versus temperature for all the -xNi2 compounds. The evolution of the pressure and temperature transition, which reflects the binding energy of the vacancies, depends not only on the radius of the R element but also on the mass of the R atom.JRC.E.6-Actinides researc

High-Pressure X-Ray Diffraction UxLa1-xS Solid Solution.

Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe