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

Abstract

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)<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-x)Ni_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-x)Ni_2. 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 ≤ 0.23 eV. The trial frequency w_(0) of these jumps appears to be correlated to the activation energy: ln w_(0)(MHz)approximate 58E_(A)(eV). At high temperatures T > 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.