Physical properties of icosahedral and glassy PdUSi alloys

Materials Science and Engineering, 99 (1988) 357 360

357

Physical Properties of Icosahedral and Glassy Pd-U-Si Alloys* P. GRt0TTER, H. BRETSCHER, G. INDLEKOFER, H. JENNY, R. LAPKA, P. OELHAFEN, R. WIESENDANGER, T. ZINGG and H.-J. G~INTHERODT

lnstitut .flit Physik, Universitiit Basel, Klingelbergstrasse 82, CH-4056 Basel (Switzerland) J.-B. SUCK

Kernforschungszentrum Karlsruhe, Institutfiir Nukleare Festk6rperphysik, P.O.B. 3640, D-7500 Karlsruhe (F.R.G.)

Abstract

Various physical properties have been measured on glassy, crystalline and on well-characterized quasicrystalline samples of the P d - U - S i system. In contrast to the strong differences observed between the static structure factors of quasicrystalline and glassy P d - U - S i samples, their dynamical properties, as revealed by neutron inelastic scattering, are nearly identical and quite different from those of the ,fully crystallized samples. The electronic structure has been investigated by photoelectron spectroscopy and the top surface layers have been probed by ion scattering spectroscopy. The general features of the valence electron structure in the quasicrystalline phase are very similar to those in the corresponding glassy alloy and similar intensities at the Fermi energy are observed. The electrical resistivity, the Hall coefficient and the magnetic susceptibility have been measured to high temperatures. The electronic transport and the magnetic properties in the icosahedral and glass), phases above room temperature are very similar. I. Introduction

Recently, a transformation of a glassy (g) P d - U - S i alloy into an icosahedral (i) phase, i Pd6oU2oSi2o, upon annealing has been observed [1]. This alloy has the advantage, compared with the original quasicrystal i Als6Mn~4 or i AlsoMn2o [2], that it can be prepared with nearly one hundred per cent of the i phase and thus provides an ideal system for the investigation of structural, transport, magnetic and electronic properties of an i solid. Comparison with the g state can also be made. The samples studied in this paper have been prepared by splat cooling or melt spinning techniques.

*Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montr6al, August 3-7, 1987. 0025-5416/88/$3.50

The g state obtained in this fashion formed the starting point for the preparation of the i state by thermal annealing.

2. Results

A sensitive and convenient way of monitoring transitions between the g state and energetically favoured states is by means of differential scanning calorimetry (DSC). Therefore we have studied the transition of g Pd6oU2oSi2o and g Pdss.sU2o.6Si2o.6 into the i state using a Perkin-Elmer DSC II. Concurrently, the observed phases were characterized or identified by X-ray diffraction (XRD). In addition, the problem of finding the optimum conditions for obtaining an i sample with minimal c (less than one percent) and g (less than ten per cent) fractions by annealing from the g state, was solved [3]. DSC experiments performed on Pdss.sU2o.6Si20.6 both in the dynamic and isothermal mode give comparable activation energies of 7 eV for the g --, i transition and 6 eV for the i ~ c transition, independently of the evaluation method employed. The DSC traces obtained in the dynamic mode with a heating rate of 40 K min t after 600 s are shown in Fig. 1 at various indicated annealing temperatures. The two transitions observed were identified as g--.i and i--,c by XRD. For isochronal annealing of 600 s at temperatures above 790 K no g ~ i transition is observed, as the sample is already fully transformed to the i phase. The optimal annealing condition is to heat the sample for 600 s at a temperature of 787 K. Higher temperatures lead to c fractions, detectable by XRD. By this method we obtained large quantities of nearly one hundred per cent i Pd U-Si, as needed for neutron inelastic scattering (NIS) [4, 5, 6] or static structure factor determination by X R D and neutron scattering [7]. In contrast to the static structure factor S(Q), where the long-range bond orientational order leads © Elsevier Sequoia/Printed in The Netherlands

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to pronounced diffraction peaks with finite halfwidth, the dynamic structure factor exhibits little or no influence from the long-range bond orientational order on the atomic dynamics of i Pd-U-Si in the range of frequencies (0.5 ~
observed. (ii) A small narrow peak at Er is visible in UV photoelectron VB spectra measured with high resolution [3]. (iii) Comparison with a dilute g alloy shows a significant contribution of uranium (5f+ 6d) electron states to the VB density of states between Ef and a binding energy of about 1.5 eV. (iv) The uranium 4f core spectrum reveals that the dominant contribution to the core line is similar to that observed in alloys in which uranium has a tetravalent 5 f 2 configuration. A second (minor) contribution can be associated with the 5f 3 configuration as found in pure or-uranium and this might reflect the rather complex surface composition profile. However, a final conclusion regarding this point cannot be reached since the uranium surface segregation observed by ion scattering spectroscopy does not correlate directly with the proportion of the 5f 3 configuration estimated from the X-ray photoelectron 4f core-level spectra. The magnetic susceptibility of g and i Pd6oU2oSi2o was measured in order to extend the low temperature measurements of Poon et al. [1] to higher temperatures and even into the liquid state. The liquid state data are helpful in understanding the formation of local moments [3]. The susceptibility shows a Curie-Weiss-like behaviour in the g, i, c and liquid states [3]. The susceptibility of the i phase is about 5% smaller than in the g state over the whole temperature range. In the c state, the susceptibility has almost the same value and

359 shape as in the g state. The magnetic susceptibility and its temperature coefficient show the following behaviour in the liquid state. The alloy PdTsSi25 is diamagnetic, and pure liquid uranium is paramagnetic but shows no magnetic moment [9]. Both PdTsSi2s and uranium show a slightly positive temperature coefficient of the susceptibility. By adding uranium to Pd75Si25, the susceptibility and the slope of the susceptibility become positive, indicating the formation of a magnetic moment. The magnetic moment exists in the range of 6 to about 40 at.% uranium [9]. The electrical resistivity and the Hall coefficient are very sensitive probes of structural differences in the c and g states. The electrical resistivity and the Hall coefficient of g, i and c Pd6oU20Si2o have been measured at temperatures above room temperature by an a.c. technique, extending earlier low temperature data by Wong and Poon [10]. The upper half of Fig. 2 shows the results of these measurements. At room temperature our results are in agreement with the low temperature values reported by Poon e t a l . [1]. In the g and i states the magnitude of the electrical resistivity is very large and the corresponding temperature coefficients are negative. In the c state the value of the electrical resistivity is smaller, comparable with the measured values for UPd3 [9], and the temperature coefficient is positive.

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For comparison, the electrical resistivity of PdslSil9 in the g and c states is presented and the resistivity of pure liquid uranium is indicated (with an arrow) in the lower part of Fig. 2. Since pure uranium does not exist in the g state, we take the resistivity in the liquid state as representative of the g state. A remarkable increase of the electrical resistivity upon alloying with uranium is observed. The temperature coefficient of the resistivity changes from positive for Pds~Sil9 to zero and negative for Pd6oU2oSi2o. It is important to note that pure liquid uranium does not exhibit a magnetic moment, whereas the uranium in g and i Pd6oU~oSi2o does form a magnetic moment. The existence of this magnetic moment is expected to account for the large resistivity values, as the localized electrons, forming the magnetic moment of uranium, are removed from the valence band. The electrical resistivity of i Pd6oU2oSi2o (225 pfl cm) is considerably larger than the values of around 150 #fl cm found for i A1-Mn alloys. In the discussion of the resistivities of i A1-Mn alloys it is often not realized that pure maganese in its non-crystalline state (the liquid state) already shows a very high resistivity value of 200/~fZ cm. However, it is worthwhile to note that the the electrical resistivity of liquid and glassy binary alloys of iron, cobalt, nickel and uranium show a maximum of only 1 4 0 / ~ cm. We suggest that any theory of the electrical resistivity in the i phase should follow the framework of those

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360 used to describe electrical resistivity in the glassy state and liquid state (see ref. 3 and references cited therein). An influence of the developing local magnetic moment on the electronic transport properties is also seen clearly in the measured Hall coefficients for these alloys (Fig. 3) [3]. The Hall coefficient for i and g Pd6oU20Si2o is positive and follows a Curie-Weisslike behaviour. Small deviations from this behaviour are observed in the c state, where the room temperature value lies at R H = 8 3 x 1 0 l~m 3 ( A s ) - ~ . An analysis of g (Pd(~_x)Ux)s3Si~7 for various x leads to the conclusion that the Hall coefficient depends on the magnetic contribution [3, I l].

3. Discussion We would like to point out that our experimental results are only representative of a poly-quasicrystalline and poly-crystalline state of the sample, which means that observed properties represent an average over many grains. We have shown that in contrast to the strong differences observed between the static structure factors of i and g P d - U - S i samples, a variety of physical properties (atomic dynamics, electronic structure, electronic transport, magnetic properties) are rather similar in the two states. All these results might suggest that there should be similarities in the atomic structure, particularly in the short-range order, and in the electronic structure between the g and the i state. Open questions quite naturally lead to the suggestion that metallic glasses and quasicrystals should be studied in parallel. Two most interesting questions are raised. Firstly, where are the atoms? Secondly, is there an underlying general principle that explains negative temperature coefficients of the electrical resistivity and positive Hall coefficients in the the strong scattering regime ofg and i structures? Clearly, we need reliable experimental data on other i systems before a satisfactory theoretical treatment can be developed.

Most important might be data for systems which do not contain d electrons near the Fermi energy. Several experiments (magnetic susceptibility, Hall coefficient and electron spectroscopy) have established that a valence change or the development of a local moment of uranium occurs in the concentration range of 6--40 at.% uranium in the P d - U - S i alloys, which includes the i phase at 20 at.% uranium. It is not yet clear if there is any relation between the existence of the quasicrystal and the valence change or the local magnetic moment.

Acknowledgments We are very grateful to Harry Thomas for many fruitful discussions. We would like to thank H. Breitenstein and P. Reimann for the preparation of the g P d - U - S i ribbons. We are all indebted to the Swiss National Science Foundation and the Kommission zur F6rderung der wissenschaftlichen Forschung for financial support.

References 1 S. J. Poon, A. J. Drehmann and K. R. Lawless, Phys. Rev. Lett., 55(1985) 2324. 2 D. Shechtman, I. Blech, D. Gratias and J. W. Cahn, Phys. Rev. Lett., 53 (1984) 1951. 3 H. Bretscher, P. Griitter, G. lndlekofer, H. Jenny, R. Lapka, P. Oelhafen, R. Wiesendanger, T. Zingg and H.-J. Giintherodt, Z. Phys. B, 68(1987) 313. 4 J.-B. Suck, H. Bretscher, H. Rudin, P. Griitter and H.-J. Giintherodt, Phys. Rev. Lett., 59 (1987) 102. 5 J.-B. Suck, H. Bretscher, H. Rudin, P. Griitter and H.-J. Giintherodt, Z. Phys. Chem., 156-157 (1987). 6 J.-B. Suck, H. Bretscber and H.-J. Giintherodt, Z. Phys. B, 68 (1987) 285. 7 R. Fuchs, S. B. Jost, H. Rudin, H.-J. Giintherodt and P. Fischer, Z. Phys. B, 68 (1987) 309. 8 G. Indelkofer, P. Oelhafen and H.-J. Giintherodt, in S. Steeb and H. Warlimont, (eds.) Rapidly Quenched Metals, North-Holland, Amsterdam, 1985, p. 1011. 9 H. Jenny, et al. to be published. 10 K. M. Wong and S. J. Poon, Phys. Rev. B, 34(1986) 7371. 11 A. Tschumi, T. Zingg and H.-J. Giintherodt, Z. Phys. Chem., 156-157 (1987).