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Vacancy Jumps in PdIn: Reconciling Nuclear Relaxation and Diffusion Measurements

Vacancy jump frequencies in PdIn were determined in the range 888 to 1288 K from analysis of nuclear relaxation of monovacancy signals detected by perturbed angular correlation of gamma rays. The relaxation is attributed to jumps of Pd-vacancies out of, within, or into the first neighbor shell of In/Cd probe atoms. These frequencies, measured for samples with 48.5 at.% Pd, are compared with frequencies derived from Pd-diffusivities measured for 49 at.% Pd by Hahn, Frohberg and Wever [phys. stat. sol. (a)79, 559 (1983)]. Jump frequencies are found to be in excellent agreement at high temperature. An Arrhenius plot of PAC jump frequencies exhibits curvature that could be fitted well assuming a superposition of two independent jump mechanisms: one with activation energy fixed at the value 2.15 eV found by Hahn et al. and the other fitted to 0.49(14) eV. The low activation energy is attributed to a cyclic diffusion mechanism in which the first diffusion barrier has a height of 0.49 eV and in which the correlation factor is small, so that lots of jumps take place without mass transport. A possible candidate for such a jump is an In atom jumping into a Pd vacancy next to the probe. Introduction Microscopic methods can help identify diffusion mechanisms in complex materials such as intermetallic compounds, where a number of alternative mechanisms may be possible. In this paper, the method of perturbed angular correlation of gamma rays (PAC) is used to determine vacancy jump frequencies in Pd-poor PdIn through relaxation of nuclear quadrupole interactions caused by stochastic jumps of vacancies near In/Cd probe atoms. The jumps cause decoherence of quadrupole interactions through reorientation of the axes of the electric-field-gradient (efg) tensor and/or through a change in magnitude of the efg. PdIn has the CsCl-structure and a wide phase field. Deviations from stoichiometry are accommodated on the Pd-poor side by structural vacancies on the Pd-sublattice. Additional vacancies form as a result of thermal activation of defects but it can be shown that these are few under conditions of our measurements. There is some controversy as to the identity of the hightemperature equilibrium defect. Assuming it is a Schottky vacancy pair (VPd+ VIn) or tripledefect (2VPd+ PdIn), formation energies were determined to be 0.9(2) and 1.6(2) eV, respectively, from Pd-vacancy concentrations measured by PAC and sample compositions [1]. Applying the methodology in ref. [1] to a sample with 48.5 at.% Pd, the vacancy concentration at 1288 K would increase to 6.2% (Schottky pair) or 5.83% (triple-defect) compared with a concentration 5.8 at.% for structural vacancies alone. Thus, for practical purposes the Pd-vacancy concentration can be taken to be constant at about 6 at.% on the Pd sublattice. Experiments were carried out to determine vacancy jump frequencies in PdIn as a function of temperature. A partial account of this work appeared previously [2]. Experiment and Data Analysis A 100-mg sample of PdIn doped with the PAC probe In at the level of 10 parts-per-billion was made by arc-melting high-purity metals and carrier-free activity together in a small argon arcfurnace, followed by a annealing at 1273 K for 1 hour under hydrogen while wrapped in Mo foil to reduce evaporation of In. Since weight losses were very small, the nominal composition 48.5 at.% Pd was taken to be the actual one. PAC measurements were carried out using a fourdetector spectrometer of standard design [3]. In Fig. 1 are shown PAC spectra measured at the indicated temperatures. All exhibit a prominent signal with fundamental frequency 103 Mrad/s. 0 100 200 300 400 G2(t)