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



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Forker, M. and Presa Muñoz del Toro, Patricia de la and Müller, S: and Lindbaum, A. and Gratz, E. (2004) Vacancy motion in rare-earth-deficient R_(1-x)Ni_2 Laves phases observed by perturbed angular correlation spectroscopy. Physical review B, 70 (1). ISSN 1098-0121

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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.

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©2004 The American Physical Society.
Two of the authors would like to express their gratitude for financial support, P.P. from “Deutscher Akademischer Austauschdienst” (DAAD) and A.L. from the Austrian Academy of Sciences (APART 10739) and the Austrian Science Fund FWF (P14932).

Uncontrolled Keywords:Electric hyperfine interactions; Quadrupole interactions; ^111Cd; Temperature; RCo_2
Subjects:Sciences > Physics > Materials
Sciences > Physics > Solid state physics
ID Code:45593
Deposited On:04 Dec 2017 12:16
Last Modified:04 Dec 2017 12:18

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