Rare-earth-deficient R1-xNi2 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 v q≈265-275 MHz has been found in the paramagnetic phases of R 1-xNi2 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 R1-xNi2, 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 τC<10-6 s, have been detected. These observations can be explained consistently by two assumptions: (i) the mother isotope 111In of the PAC probe 111Cd constitutes an attractive potential for vacancies and (ii) the R vacancies in R1-xNi 2 are highly mobile at temperatures T<300 K, which is incompatible with a static vacancy superstructure. The measurements indicate a decrease of the vacancy-probe binding energy from the light to the heavy R constituents of R1-xNi2. For R=Pr, Nd, Sm, Gd the binding energy is in the range 0.15-0.40 eV. The activation energy EA for vacancy jumps near the probe derived from the temperature dependence of the nuclear spin relaxation at 200 K≤T≤300 K is small. The values observed in different samples cover a range of 0.1 eV≤EA≤0.23 eV. The trial frequency w 0 of these jumps appears to be correlated to the activation energy: In w0(MHz)≈58EA(eV). At high temperatures T>500 K nuclear spin relaxation related to vacancy hopping is observed in nearly all R1-xNi2. Auxiliary 111Cd PAC measurements have been carried in Sc0.95Ni2, ScNi2, ScNi 0.97, Gd2Ni17, GdNi5, GdNi 3, and GdNi.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - Jul 2004|