Motional Correlation Time of Dilute 111 Cd Impurities in Se-Rich Liquid Se-Te Alloys
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MOTIONAL CORRELATION TIME OF DILUTE ALLOYS
369
t1
1Cd IMPURITIES IN Se-RICH LIQUID Se-Te
D. K. GASKILL, J. A. GARDNER, K. S. KRANE, and K. KRUSCH Department of Physics, Oregon State University, Corvallis, Oregon
97331
R. L. RASERA Department of Physics, Oregon State University and University of Maryland Baltimore County, Catonsville, Maryland 21228 ABSTRACT The motional correlation time, Tc, in liquid Se and Se-rich Se-Te alloys has been investigated between 500 and 9000 C using time differential perturbed angular correlations of y-rays from dilute 111Cd impurities. In all alloys we find T Tc M exp (E0/kT) at low T where E0 = 0.36 eV. c deviates from this relation at high T. At low T, Tccis tentatively identified as the lifetime of a Cd to host molecule bond, and at high T as the average lifetime of bonds in the host molecule. INTRODUCTION The Perturbed Angular Correlation (PAC) experimental technique is not really a resonance method, but because it is a measurement of interactions between nuclei and their environment, it is informative to include it in this symposium.
Physical information from PAC experiments is quite similar to that
obtained from M6ssbauer, NMR, and ESR hyperfine measurements.
PAC utilizes a radioactive isotope which decays via a 2-step gamma-ray cascade. The experimental work reported in this paper utilizes the isotope 11 1ln, which decays to 111Cd* by electron capture. The lllCd* subsequently decays by a 171 keV gamma ray (yl) to an intermediate 111Cd state with lifetime TN = 121 nsec, which then decays to the 111Cd ground state by emission of a 245 keV gamma (Y2). In the absence of magnetic fields or electric field gradients at the Cd nucleus, Y2 would be emitted with an anisotropic probability proportional to W(e) = [I + A2 P2 (cosa) + A4 P4 (cosa)]
(1)
where e is the angle of emission of Y2 with respect to the direction of Y1 , and the nuclear angular correlation constants are given by A2 = -0.180, A4 = 0.002, for this cascade. In general, however, a Cd nucleus in condensed matter will be subject to magnetic and/or electric fields due to nearby electrons and ions. Thus the nucleus will be subjected to torque during the time between emission of y, and Y2, so in general the angular distribution of Eq. 1 must be modified to include the effects of reorientation of the nuclear ensemble by interaction with these fields. For a polytropic sample (i.e., one with no preferred overall orientation such as a liquid or powder) then W(6,t) = [1 + A2 G2 (t)P 2 (cose) + A4 G4 (t)P 4 (cosa)]
(2)
370 The functions G2 (t) and G4 (t) are normalized to unity at t = 0 and are governed by the hyperfine interactions. The purpose of a PAC experiment is to measure G2 and G4 and relate them to the electronic properties of the material being investigated. Because in our case A4 is quite small, the last term in Eq. 2 contributes negligibly to W(e,t), and we shall drop it. It is easy to show [1] that G (t) can be determined experimentally by measuring W(e,t) at o = 900 and 180°. Then 0 = 2 W(180°,t) - W(90 ,
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