Interstitial site of Cu in Ca

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Solid State Con~nunicatlons, Vol.46,No.6, pp.455-456, Printed in Great Britain.

0038-1098/83/180455-02503.00/0 Pergamon P r e s s L t d .

1983.

INTERSTITIAL SITE OF CU IN CA M. A. Marcus Bell Laboratories, 600 Mountain Ave., Murray Hill, N.J. 07974 and C. L. Tsai and B. C. Giessen Northeastern University, Boston, Mass 02115 (Recieved 8 December 1982 by A. Chynoweth) Fhiorescence EXAFS was ttsed to study the siting of a dilute solution of Cu in Ca. The Cu is interstitial in Ca, and no more than .4A. away from the center of the octahedral site. No lattice dilation was seon about the Cu atom, even though Cu is "too big" to fit into the interstitial site.

1. The Problem 15 There has been much interest in metallic glasses, in particular in the criteria for glass formationI. It is of interest to know how the atoms of a glass forming alloy interact, and what is "special" about this interaction. To this end, we are using the EXAFS technique to investigate atomic siting and vibrations in dilute alloy systems representative of metallic glass-formers in order to learn about their interatomie potentials. In this, the first such study, we investigate the siting of Cu in the crystalline Ca lattice.

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% According to the Hume-Rothery rules for predicting solid solubility 2, Cu is "too big" to fit the crystalline Ca interstitial, and "too small" to be substititionally soluble. The equilibrium phase diagram shows very little solubility, as expected. However, what happens if Cu is "forced" into solution by rapid cooling? It is of interest to discover which site the Cu atoms "choose" and how much distortion they cause in the surrounding Ca lattice. This paper reports the results of an examination of the Cu sites in a rapidly-quenched, crystalline solid solution of .5at. % Cu in Ca.

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EXAFS data for Ca-.5%Cu at room temperature. This data was taken in fluorescence, and has been backgroundsubtracted, normalized, and k3-multiplied.

2. Method The sample was prepared by arc melting followed by quenching in a piston-type apparatus3, all under vacuum. This process produced a foil about 40t~ thick and 2 cm. in diameter. The sample was kept in a sealed ampule until the start of the experimental run at the synchrotron.

The Fourier transform of the composite data showed a single peak, with a height fourteen times that of the tallest noise peak. There was no sign of a beat note or a second shell. Since we had no Cu-Ca model compound, the TeoLee theoretical phase shifts4 were used to derive a value for the Cu-Ca distance in the manner discussed by Kincaid 5. The TeoLee phase-shifts are known to be good to .01-.02,~. A cheek was performed by replacing the assumed Cu central-atom phase shift by the difference between the experimental Cu-Cu shift from Cu foil and the theoretical Cu backscatter phase shift. This replacemerit made less than .02.~ difference in the distance. The distance thus derived is 2.76A. The effects of noise could be checked by deriving a distance from the data for each temperature. Such distanees differed from the "averaged" value by no more than .025,~. Therefore, we quote the Cu-Ca distance as 2,76_+.03,~. The Eo values for each temperature were the same to within 3eV. Therefore, the E0 uncertainty was not a problem.

The EXAFS data were taken on beamline II-3 at Stanford Synchrotron Radiation Laboratory, under dedicated running conditions (3 GeV, 60mA). Nine plastic scintillators collected the fluorescence radiation from the sample. A Ni filter was used to reduce background radiation. The sample was mounted in a Helitran cold stage so that its temperature could be varied from 6-300" K. 3. Analysis The background-removed, k3-multiplied data for the sample at room temperature are shown in Fig. 1. The data for other temperatures are similar in quality to the set shown. There is no clear evidence for a large distance shift due to thermal expansion, so a good value of distance could be gotten using a composite data set produced by averaging the data for all temperatures.

The Ca lattice is foe, with a lattice parameter of 5.56.~.. If the Cu were in the octahedral site, the Cu-Ca distance

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INTERSTITIAL SITE OF CU IN CA

would be 2.78A, whereas the distance for a substitutional site is 1.1A longer. The Cu is thus very close to the octahedral site. It turns out that the most sensitive indicator of displacements from the oetahedral site is a change in the amplitude of the EXAFS wiggles. If the Cu atom is off-center, then it "sees" different distances to neigboring Ca atoms, and there is a beat-note effect in the amplitude function. Unfortunately, the "no-beat" amplitude is not known, except from theoretical calculations which do not get the absolute value correct. However, the amplitude shows a shape similar to that produced by other third-row scatterers. Let us take as a criterion that the amplitude is different by no more than a factor of five at k-8/~-1 from the undistorted value. Such distortion would be rather noticeable, even without a model against which to compare it, as the shape of the amplitude vs. k curve would be differ visibly from that seen for a typical third-row scatterer. Calculations show that this assumption puts a limit of .15A on displacements along the (110) direction. For (111), the displacement could be either less than .14A, or .43+.08A. There are two possible values in this case because the first beat node could be at very low or very high k values. The distance measurement limits the (100) displacement to .4A..

Vol. 46, No.

by more than .4,~ in the (100) direction or .15,~ in the (110) direction. The displacement in the (111) direction is either less than .14.~ or .43+.08A. The Cu-Ca distances in CaCus are 2.94.~ and 3.27A. If we assume that the shorter distance is the "ideal" Ca-Cu separation in the alloy, then the distance in the octahedral site is .16A smaller than the "ideal'. A simple-minded calculation using the "springs" model of Marcus6 yields an effective Ca-Cu spring constant less than 1/7 that for Ca-Ca. This calculation assumes that the Ca-Cu ideal distance is as assumed above. Clearly, the Cu atom is "accommodating" itself to the demands of the Ca lattice. This result is in line with the results of Turnbull7 concerning partial molar volumes of metalloids in metal-metalloid glasses.

Acknowledgements

- We acknowledge S. Davis for sample preparation, J. Banzahf, A. Waleh, and L. Powers for help with beam-line operation, and the SSRL floor staff. SSRL is supported by the NSF through the Div. of Materials Research and the NIH (IP41 RR01209-02).

REFERENCES All of these displacement values are small enough so that the Cu cannot be in any cluster, such as a di-substitutional. Also, the trivial possibility that the Cu has all precipitated out as a Cu-rich phase is ruled out because the observed phase shifts do not match the Cu-Cu phase shifts in Cu metal. Finally, the nearestneighbor octahedron dilates less than .03,g, about the Cu atom. A sample with 2 at. % Cu was made, and its spectrum was indistinguishable from that of the .5 at. % sample. It is therefore very unlikely that Cu-Cu pairs contribute much to the observed signal, since the fraction of Cu atoms thus paired would be expected to change when the Cu concentration is quadrupled. 4. Conclusion We find that the Cu atoms in Ca metal take up interstitial sites, with no detectable lattice relaxation. The Cu atoms cannot be displaced from the center of the octabedral site

D. E. Polk, B. C. Giessen, "Overview of Principles and Applications" in Metallic Glasse_~s Chapter I (ASM Materials Science Seminar, Metals Park, Ohio, 1976) 2

C . E . Birchenall, physical (McGraw-Hill, 1959)

M_..._etallurgy, pp.

102-108,

M. Ohring, A. Haldipur, Rev. Sci. Inst. 42, 530(1971) B. K. Teo, P. A. Lee, J. Am. Chem. Soe, 101, 2815(1979) P. A. Lee, P. H. Citrin, P. Eisenberger, B. M. Kincaid, Rev. Mod. Phys. 53, 769(1981) M. Marcus, Sol. State Commun. 38_~251(1981) D. Turnbull, Scripta Met. 11, 1131(1977)