L Journal of Alloys and Compounds 253–254 (1997) 360–363 Reorientation of the Ce–D pairs in a Pd Ce D alloy 95 5 n a a a a a, b * B. Coluzzi , A. Biscarini , R. Campanella , G. Floridi , F.M. Mazzolai , Y. Sakamoto , c F.A. Lewis a Department of Physics, Univ. of Perugia, Perugia, Italy b Department of Materials Science and Eng., Nagasaki Univ., Nagasaki, Japan c Department of Chemistry, Queen’ s Univ. of Belfast, Belfast, UK Abstract Internal friction measurements have been carried out in a Pd Ce alloy containing different amounts of D (0.003,n 5D/(Pd1Ce), 95 5 D 0.335) at vibration frequencies in the kHz range. Three IF peaks P , P and P have been observed at around 160, 115 and 103 K, 1 2 3 respectively. The highest temperature peak P is due to D-dislocation interactions (Snoek-Koester type of relaxation), the peak P to 1 2 stress-induced reorientation of Ce–D pairs and the peak P is due to stress-induced changes in the short-range order of D within the 3 21 12 21 sublattice of the octahedral interstitial sites (Zener type of relaxation). The relaxation parameters of peak P are: t 57310 s and 2 0 W50.22 eV, while an estimate of the anisotropy factor for the Ce–D pairs gives a value of 0.038. Keywords: Mechanical relaxation; Elastic properties; Lattice defects; Pd–Ce alloys 1. Introduction argon atmosphere. The button was rolled and machined to 3 obtain bars of 0.5436.95344.6 mm dimensions, which 26 It is well known that hydrogen and deuterium may were then annealed in vacuo (P54310 torr) at about interact with substitutional atoms in dilute alloys giving 1300 K for 3 h. The doping of the samples with deuterium rise to diatomic complexes, both in bcc and fcc lattices was carried out at temperatures ranging between about 400 [1–8]. Ni-, Fe- and Co-based alloy systems have been and 500 K in a D atmosphere at gas pressures of between 2 most extensively investigated, either by magnetic disac- 150 and 700 torr and with doping times of between 1 and 2 comodation or internal friction (IF) techniques. Recently, h. Deuterium contents n ( n5D/(Pd1Ce)) were determined also, Pd containing Y as a substitutional impurity has been gravimetrically with an accuracy better of than 0.001. found to display an internal friction relaxation which has The internal friction measurements were carried out by been attributed to reorientations of Y–H pairs [9,10]. monitoring the number of oscillations included between Cerium atoms are expected to behave similarly to Y two fixed amplitude levels during the free decay of the atoms when in solution in the Pd lattice; thus, they should vibrations of the sample, which had been initially excited both give rise to the formation of pairs with H(D). in one of its free–free resonant modes. All the data were 23 However, the form of pressure-composition isotherms at taken at an average cooling rate of approximately 5310 21 room temperature suggests that for the case of Ce as a Ks . solute the impurity–H(D) binding enthalpy is very small; thus, the question arises whether this impurity dissolved in Pd may actually constitute a trapping centre for H(D) 3. Results [11,12]. The aim of the present work was to detect IF peaks due to Ce–D pairs in Pd. The temperature dependence of the elastic energy 21 dissipation coefficient Q and of the Young’s modulus E for a specimen charged with gradually increasing 2. Experimental deuterium contents n ( n 5D/(Pd1Ce)) is shown in Fig. D D 1a Fig. 1b Fig. 2, respectively. At deuterium levels n # D The alloy used in the present work was prepared by 0.007 a single peak (P ) occurs (see Fig. 1a) at a 2 arc-melting 99.95% pure Pd with 99.9% pure Ce under an temperature T which remains constant with changing D 2M content ( T 5115 K). For 0.007,n ,0.05 the IF shows 2M D * Corresponding author. two closely spaced bumps and the temperature of the 0925-8388 / 97 / $17.00  1997 Published by Elsevier Science S.A. All rights reserved PII S0925-8388(96)03043-5