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M. Forker, P. de La Presa, S. Müller, A. Lindbaum, E. Gratz:
"Vacancy motion in rare-earth-deficient R_1-xNi₂ Laves phases observed by perturbed angular correlation spectroscopy";
Physical Review B, 70 (2004), 014302-1 - 014302-12.

Vacancy motion in rare-earth-deficient R1-xNi2 Laves phases observed by perturbed angular correlation spectroscopy

M. Forker, P. de La Presa, and S. Müller
Helmholtz-Institut für Strahlen- und Kernphysik, Nussallee 14-16, D-53115 Bonn, Germany

A. Lindbaum and E. Gratz
Institut für Festkörperphysik, Vienna University of Technology, Wiedner Hauptstr. 8-10, A-1040 Vienna, Austria

(Received 2 December 2003; revised 21 April 2004; published 19 July 2004)

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 nuq[approximate]265-275 MHz has been found in the paramagnetic phases of R1-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 tauC<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-xNi2 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 w0 of these jumps appears to be correlated to the activation energy: ln w0(MHz)[approximate]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, ScNi0.97, Gd2Ni17, GdNi5, GdNi3, and GdNi. ©2004 The American Physical Society

URL: http://link.aps.org/abstract/PRB/v70/e014302
doi:10.1103/PhysRevB.70.014302
PACS: 76.60.Es, 61.72.Ji, 76.80.+y

http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC04970240

http://link.aps.org/abstract/PRB/v70/e014302