Vacancy motion in rare-earth-deficient R_1-xNi₂ Laves phases observed by perturbed angular correlation spectroscopy

Rare-earth-deficient R_1-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 R_1-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 τ_C<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 R_1-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 R_1-xNi₂. For R=Pr, Nd, Sm, Gd the binding energy is in the range 0.15-0.40 eV. The activation energy E_A 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≤E_A≤0.23 eV. The trial frequency w ₀ of these jumps appears to be correlated to the activation energy: In w₀(MHz)≈58E_A(eV). At high temperatures T>500 K nuclear spin relaxation related to vacancy hopping is observed in nearly all R_1-xNi₂. Auxiliary ¹¹¹Cd PAC measurements have been carried in Sc_0.95Ni₂, ScNi₂, ScNi _0.97, Gd₂Ni₁₇, GdNi₅, GdNi ₃, and GdNi.