XPS studies of superconducting Mo–Ru–Rh–Pd alloy

Journal of Alloys and Compounds 298 (2000) 291–294 L www.elsevier.com / locate / jallcom XPS studies of superconducting Mo–Ru–Rh–Pd alloy Jong-Gyu Lee a , *,1 , Yong Joon Park a , Hyung Yeol Pyo a , Jong Goo Kim a , Kwang Yong Jee a , Won Ho Kim a , Yongseog Jeon b a Korea Atomic Energy Research Institute, Nuclear Chemistry Laboratory, P.O. Box 105, Yusong, Taejon, 305 -600, South Korea b Jeonju University, Department of Physics, Jeonju, 560 -759, South Korea Received 31 July 1999; accepted 19 October 1999 Abstract Superconducting quaternary alloys, Mo 25 Ru 50 Rh 12.5 Pd 12.5 , Mo 30 Ru 45 Rh 12.5 Pd 12.5 , Mo 40 Ru 40 Rh 10 Pd 10 and Mo 50 Ru 20 Rh 15 Pd 15 have been prepared using an arc melting furnace. X-ray photoelectron spectroscopic (XPS) measurements indicate that the binding energies of Pd 3d 3 / 2 and 3d 5 / 2 peaks increased by 0.5–0.9 eV. XPS measurements for several binary alloys, Pd 12x Mox , Pd 12x Rux , Pd 12x Rhx and Pd 12x Ag x (x50.25, 0.5, 0.75 and 1.0) were also carried out to study the origin of the binding energy shifts of Pd 3d 3 / 2 and 3d 5 / 2 peaks. The binding energy shifts are found to be irrespective of the electronegativity of the constituent elements in the quaternary alloys, but rather closely related to the final state effects.  2000 Elsevier Science S.A. All rights reserved. Keywords: Alloy; Superconducting; Insoluble residue; XPS; Core level 1. Introduction The chemical state of most of the fission products in irradiated or simulated nuclear oxides is now well understood, since powder X-ray diffraction and electron probe microscopic analyses have been performed [1]. However, the chemical state of insoluble residue like multinary alloys was not understood well in nuclear fuels, even though the alloys among the many fission products have received much attention during the dissolution and recovery studies in both spent and simulated nuclear fuels of types such as the Pressurized Water and Fast Breeder Reactors [2–4]. XRD measurements revealed very broad diffraction peaks, suggesting the possibility of heterogeneous mixtures of multinary alloys, Mo–(Tc)–Ru–Rh–Pd, when included in the UO 2 matrix [5]. In fact, the ´-phase region of the hexagonal close packed structure is very large, as already reported in the phase diagram. Recently, superconductivity has been found at 3–5 K from the DC *Corresponding author. Tel.: 182-42-868-8153; fax: 182-42-8688148. 1 Present address: POSTECH, Department of Chemistry, Pohang, Korea 790-784. magnetic susceptibility measurements in the quaternary alloys, Mo–Ru–Rh–Pd [6]. X-ray photoelectron spectra of SIMFUEL, simulated high-burnup UO 2 fuel, were analyzed by Sunder et al. in 1994 [7]. They determined the approximate ratio of U 61 to U 41 as well as the oxidation state of the dopant elements in SIMFUEL, where Mo, Ru, Rh and Pd are present in the metallic state. However, the detailed core level shifts of Pd and Mo in the metallic alloy were not reported, probably because of the weak intensities of the peaks. On the other hand, the final state or relaxation contribution to the core level binding energy shift was calculated for several noble metal / transition metal alloy systems, using the pseudopotential linear response method developed previously for pure metals [8,9]. The effect of final state and initial state shifts on the binding energies of core level electrons were compared from the experimental data. In this report, the X-ray photoelectron spectra of the quaternary alloys, Mo–Ru–Rh–Pd, as well as several binary alloys, Pd 12x A x (A5Mo, Ru, Rh, Ag; x50, 0.25, 0.5, 0.75, 1.0), are presented. The origin of the binding energy shifts of Pd 3d 3 / 2 and 3d 5 / 2 peaks observed in the quaternary alloys are discussed in terms of the final state effect and compared with the binding energy shifts of Pd 3d 3 / 2 and 3d 5 / 2 peaks in Pd 12x Ag x . 0925-8388 / 00 / $ – see front matter  2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00664-7 292 J.-G. Lee et al. / Journal of Alloys and Compounds 298 (2000) 291 – 294 2. Experimental The quaternary alloys, Mo–Ru–Rh–Pd (25:50:12.5:12.5, 30:45:12.5:12.5, 40:40:10:10, 50:20:15:15), and the binary alloys, Pd 12x A x (A5Mo, Ru, Rh, Ag; x50, 0.25, 0.5, 0.75, 1.0), were prepared using an Ar arc melting furnace (Ace Vacuum, Korea). The stoichiometric powdered mixture of Mo (Aldrich, 99.95%), Ru (Aldrich, 99.9%), Rh (Aldrich, 99.99%), Pd (Aldrich, 99.91%) and Ag (Kojima Chemical Co., 99.99%) was pelletized and completely melted three times by turning it upside down in the arc furnace. The powder X-ray diffraction (Siemens D5000) patterns were recorded in the 2u range of 30–908 using an internal standard Pt. The platinum wire on the alloy was pressed flat together in a hydraulic press. The unit cell parameters were obtained by the least square refinements of the diffraction data. The X-ray photoelectron spectra of the quaternary alloys have been obtained using ESCALab 220i (VG Scientific) equipped with a full 1808 hemispherical electrostatic analyzer to examine the chemical state of the constituent elements. As a photon source, Mg Ka and Al Ka radiation were used. The half-width at half-maximum of the 4f 7 / 2 line in the XPS spectrum of gold obtained by our XPS spectrometer was smaller than 1.0 eV. The energy scale of the spectrometer was calibrated using Au 4f 7 / 2 (84.0 eV) or Ag 3d 5 / 2 (368.3 eV), which was also pressed flat on the alloy. The surface of the alloy was cleaned by Ar sputtering in the preparation chamber for |15 min. 3. Results and discussion The quaternary alloys, Mo–Ru–Rh–Pd (25:50:12.5:12.5, 30:45:12.5:12.5, 40:40:10:10, 50:20:15:15), were prepared successfully using an arc melting furnace [6]. The powder XRD studies of the quaternary alloys resulted in the same hexagonal symmetry as the ´-phase reported previously in the literature [1,5,10]. The unit cell parameters, a and c of the quaternary alloys Mo–Ru–Rh–Pd were obtained by least squares refinements based on the space group P63 /mmc. Both unit cell parameters, a and b, increased from 2.735 and 4.363 to ˚ respectively, as the Mo content 2.765 and 4.431 A, increased in the quaternary alloys (Fig. 1). The powder X-ray diffraction measurements of the binary alloys, Pd 12x A x (A5Mo, Ru, Rh, Ag; x50, 0.25, 0.5, 0.75, 1.0) also showed the formation of solid solution, as can be seen in the phase diagram of those binary alloys [11]. The chemical state of the elements in the alloys was studied in the energy range 25 to 400 eV using XPS. The binding energies data measured for the 3d 5 / 2 and 3d 3 / 2 peaks of Ru, Rh and Pd indicate that these elements exist in metallic state. However, those of Mo 3d 5 / 2 and 3d 3 / 2 Fig. 1. Variation of unit cell parameters of quaternary alloys, Mo–Ru– Rh–Pd. (1) 25:50:12.5:12.5; (2) 30:45:12.5:12.5; (3) 40:40:10:10; (4) 50:20:15:15. peaks appeared as a triplet, rather than a doublet, suggesting the overlap of the two doublets of elemental and fully oxidized Mo (Fig. 2). The MoO 3 3d 5 / 2 peak at 231.75 eV and the Mo 3d 3 / 2 peak at 231.14 eV overlapped to give a broad peak at |231.6 eV. In fact, a slight oxidation on the surface of the alloy occurred during preparation in the arc melting furnace as well as upon exposure in the air. The MoO 3 3d 5 / 2 and 3d 3 / 2 peaks at 231.75 and 235.92 eV disappeared after Ar sputtering of the surface of the alloys in the preparation chamber for |15 min. A close examination of the Pd 3d 5 / 2 and 3d 3 / 2 peaks was carried out after calibration of the energy scale of the spectrometer using the Au 4f 7 / 2 peak (84.0 eV). The binding energies of the Pd 3d 5 / 2 and 3d 3 / 2 are found to be shifted to higher energy by |0.5–0.9 eV for the quaternary alloys, for example, |0.5 eV for Mo 25 Ru 50 Rh 12.5 Pd 12.5 and |0.9 eV for Mo 50 Ru 20 Rh 15 Pd 15 . The binding energy shift of Pd 3d 5 / 2 and 3d 3 / 2 peaks was confirmed in different measurements using a Ag standard, since the Au 4d 5 / 2 peak (335.1 eV) is overlapped with the Pd 3d 5 / 2 peak (335.2 eV). The binding energy shifts of Pd 3d 5 / 2 and Mo 3d 5 / 2 peaks was compared with those of Pd and Mo in the Fig. 2. XPS spectra of the quaternary alloy, Mo 50 Ru 20 Rh 15 Pd 15 . J.-G. Lee et al. / Journal of Alloys and Compounds 298 (2000) 291 – 294 pressed pellet of the powdered mixture of the elemental Mo, Ru and Pd without melting down in the arc melting furnace. In the powdered mixture of elemental Mo, Ru and Pd, the Pd 3d 5 / 2 and 3d 3 / 2 peak positions were exactly matched to the reported values at |335.2 and |340.5 eV, respectively [12] (Fig. 3). In order to study the origin of the binding energy shift of Pd 3d 5 / 2 and Mo 3d 5 / 2 peaks, the X-ray photoelectron spectra of several binary, Pd 12x A x (A5Mo, Ru, Rh; x50, 0.25, 0.5, 0.75, 1.0) were recorded in the range 25 to 400 eV. The effect of Mo, Ru and Rh on the binding energy of Pd was tested, respectively. For the Pd 12x Ru x and Pd 12x Rh x , there was no shift in the binding energy of Pd 3d 5 / 2 and 3d 3 / 2 peaks, as Ru and Rh content increased from 0.25 to 0.75 in the binary system (Fig. 4a and b). There was also no shift in the binding energy of 3d 5 / 2 and 3d 3 / 2 peaks of Ru and Rh as Pd content varies in the Pd 12x Ru x and Pd 12x Rh x . For the Pd 12x Mo x , however, the binding energy of Pd 3d 5 / 2 and 3d 3 / 2 increased as Mo content increased (Fig. 5). For x50.75, the binding energy 293 Fig. 5. XPS spectra of Pd 3d 5 / 2 and 3d 3 / 2 in Pd 12x Mo x (x50.00, 0.25, 0.50, 0.75). of Pd 3d 5 / 2 was increased from 334.92 to 335.39 eV (0.47 eV shift). X-ray photoelectron spectroscopy core-level binding energy shifts are often used to study the electronic Fig. 3. XPS spectra of the quaternary alloy, Mo 25 Ru 50 Rh 12.5 Pd 12.5 , after arc melting and the powdered mixture of Mo, Ru and Pd (25:50:25). Fig. 4. XPS spectra of the Pd 3d 5 / 2 and 3d 3 / 2 in the binary alloys, (a) Pd 12x Ru x and (b) Pd 12x Rh x (x50.00, 0.25, 0.50, 0.75). 294 J.-G. Lee et al. / Journal of Alloys and Compounds 298 (2000) 291 – 294 redistribution or charge transfer upon alloying. The binding energy shift of core electrons depends on changes in the bulk charge around an atomic site. The general rule is that the core level binding energy of the central atom increases as the electronegativity of the attached atoms or groups increases [13]. Electronegativity is a measure of the ability of one element to compete with another for valence electron charge; it is thus relevant to see whether Pd does or does not gain charge upon alloying with the more electronegative metallic elements [14]. Since Pd is more electronegative than Mo according to Pauling’s electronegativity table, one would expect that the Pd core level shift toward the lower binding energy with increasing Mo concentration in the Pd–Mo system [14]. From Fig. 5, it is clear that the general rule based on the electronegativity table fails to explain the observed Pd 3d 5 / 2 and 3d 3 / 2 core level shift. It is, however, known from several experimental examples that, in alloys, a direct correlation between core-level shifts and charge transfer is not straightforward due to final state effects and volume change [15,16]. The final state contribution is generally found to be important and sometimes dominant [8,17,18]. Here the binding energy shifts of Pd 3d 5 / 2 and 3d 3 / 2 levels is correlated to the charge redistribution when chemical bonding between Pd and Mo takes place. This observation is consistent with the work of Chae et al. who investigated the charge redistribution in ion-beam-mixed and bulk Pd–Ag alloys [19,20]. We find that the shifts of core level binding energy of Pd 3d 5 / 2 and 3d 3 / 2 levels in Pd–Mo systems are roughly equal to those of Pd–Ag alloys. The core level binding energy shift is precisely the behavior one would expect from Mo–Pd bonding through hybridization. For the other elements, Ru and Rh, there is no discernable shift compared to the reported values of metallic Ru and Rh within an experimental error. The valence band peaks in the quaternary alloys, Mo– Ru–Rh–Pd are very broad as we can expect from the Fig. 6. Typical valence level spectra of the quaternary alloy, Mo 30 Ru 45 Rh 12.5 Pd 12.5 . overlap of 4d and 5s orbitals, cutting 0 eV (Fig. 6). There was no discernable change in the valence level spectrum due to hybridization of Mo and Pd. Acknowledgements The authors greatly acknowledge the financial support (Nuclear Development Fund) from the Korea Ministry of Science and Technology. We also thank Dr. Keun Hwa Chae for helpful discussion in the interpretation of XPS data. References [1] J.I. Bramman, R.M. Sharpe, D. Thom, G. Yates, J. Nucl. Mater. 25 (1968) 201–215. [2] T. Matsui, M. Ohkawa, R. Sasaki, K. 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