: 17 :. . . .ME LE K L .ll . . T 1 . ' .. UNCLASSIFIED ORNL '' ! Y 77 " i . " . 1 . MY . 400 . NEMOTY WWW T ' U " . L . P ".. . .', ; . .. . .. Orna- Poxoo CONF-651-9 Bli 51964 A CORRELATION OF THE MOSS BAUER ISOMER SHIFT AND THE RESIDUAL ELECTRICAL RESISTIVITY FOR 197A3 ALLOYS* Louis D. Roberts, Richard L. Becker, F. E. Obenshein, Oak Ridge National Laboratory Oak Ridge, Tennessee and w musy. o to, . A privately owned m s danomato, o prorda bo via the Cornulou, or wo element with such y me repose of contractor of the Cou peyes or contacter of the COURRON or de oud in the move, "person athegou may heloration, inint, med de Couston, ; or may talarmathon, yanto, method of procent ploto, or wtalow of the laboration A. Miw my warreaty of reprenesution, apo to, uy tailortattoo frent my liablitos no roupect to the wol, or for de May pria atting a ston, or The report we womernd un uw of Government wort, Matter to Valitud rector. LEGAL NOTICE - play , or recono delow is the reprto plogue of much contractor ll of of all of the Conta" wchody Weapon cloud in the room can a contractor todo or leplied, with respect to the Kry- du tror, or time : or contract to the extent that trom mana, no intring the J. 0. Thomson University of Tennessee, Knoxville, Tennessee I. INTRODUCTION In the theoretical discussion of metals 1t 18 often convenient to describe the metallic sample by using a single potential well for the conduction electrons. The Bloch wave functions which extend throughout the entire sample give an approximate solution to the corresponding quantum mechanical problem. When an impurity is dissolved in the metal, the electron wave functions again will, in general, extend throughout the sample. One may then expect a correlation between different physical phenomena associated with the impurity which may each depend dominantly on the different regions of the wave function. For a suitable host and impurity one should observe a correlation between the charge density at the impurity nucleus pro) and the transport cross section per impurity atom or which the impurity atoms present to the host conduction electrons. For impurity atoms whose nuclei have resonant gamma ray transitions suitable for Mössbauer effect studies", a measurement of the isomer shift will give information about plo). Por suitable dilute alloys, 'the measurement of the residual electrical resistivity per atomic per cent AR/C, will give or Thus one may expect a correlation between the 1somer shift ve "Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation. . :. .. . : .. 'i -. ! E ' -2 : - of the resonance gamma ray energy E, of the nucleus of an impurity in a dilute alloy with AR c due to that impurity in the alloy. The 1sotope 19 Au has a suitable gamma ray for recoilless radiation (Mössbauer) Investigations' and, since gold 18 a noble metal, 1t 18 particularly suitable for a study of the correlation of v, with AR/c. Thus, if we assume that only the 68 shell of gold 18 appreciably modified when gold 10 dissolved in a host metal, measurements of V, and of AR/C may be used in conjunction to give information about the s-band wave function in the gold alloy system. In the following we present measurements of V, and of A R/C for gold alloyed in a variety of host metals and show a correlation between these measured quantities using a simple theoretical model which 18 based on the use of an impurity potential. Information 18 obtained about the impurity potential and about the s-band filling. The 18omer shift v, aribes as a result of the Coulomb (electric monopole) interaction between the nucleus and the atomic s-electrons which penetrate it. In first order perturbation theory the Coulombic energy shift of a nuclear state * produced by a single Dirac s-electron 18 D14.(0)|2 R, 18 assured to be 11.dependent of k. Here v (r) or v (r) 18 taken to be the wave function at the bottom of the s-band and within the Wigner-Seltz cell of the pure host metal A or of the pure solute metal B. The radius R wiu be taken as the radius of the Wigner-Seitz celi of the pure metal B. It will be assumed that Un and V, have the same amplitude at the surface of their respective Wigner-Seitz spheres, 1.e., U(R) = 0,(R). The charge density Pn (0) at the origin, 1.e., at the Impurity nucleus B in the alloy, arising from the s-partial wave of a given k state 18 then PENCO) - lug(0)12.18660312 where k 18 the electron momentum vector. For pure gold Paklo) - \U,(0)|2 and thus the isomer shift arising from a given k state vrk may be written PIK - 8/09(0)12 (143812 - 1) (7) To obtain the 18omer shift, Vipe must be multiplied by the density of states n(k) and Integrated over filled k states. Assuming n(k) to be proportional to k', and where kr 18 the s-band electron momentum at the Fermi surface of the host metal we have V د مرسده (6) Yg + 62 | 0360)12 saix (1460)/2 - 1) x2 (8) +811440)12 ( - 1). (80) 1. . . . . . . -7 r ariwis. riscont b are t i .**--'-i i where 8, 18 a constant. It 18 now necessary to obtain an estimate of 102(0)/. In the asymptotic region k(r) = sin (kr + Sc(k))/kr. Here So(k) 18 the phase shift of the s-partial wave arising from the effect of the impurity atom on a conduction 8-band electron state of energy Eck It will be assumed that at the Fermi surface only so(k) and Sq(Kp) (where S18 the phase shift of the p-partial wave) are appreziably different from zero. Then an estimate of So(k) and S,(k) may be obtained using the measured A R/c and the Friedel bum rue. For infinitely dilute alloys Huang' has given the result .. .. . . .. .. AR/c = (g/0.703 mm'3) Şle + 1) sinº [fe(key) - letz(ka) (10) which describes A R/C in terms of the phase shifts -S,(kg) of the l-th partial wave produced at the Fermi surface by the impurity. Here r. 18 the Wagner-Seitz cell radius 1n atomic units and YA 18 the number of b-band electrons per atom for pure metal A. The Friedel sum rule, 2 Ş (2 + 1) Se(key) (11) where 2 16 the difference of charge between impurity and host atoms, gives a second condition on the phase shifts. With the assumption mentioned, that only do and S, are appreciable, and given the value for 2, the simultaneous solution of Eqs. (10) and (11) will give an estimate of (k). The difference of lonic charge z between impurity and host, Eq. (11), 16 assumed to have the form 2-178-Ya!-10A (12) Tale - -8. Here you like which was defined earlier, 16 the number of s-band electrons per atom in the pure metal B, and ( 2 2 ) gives the ionic charge on the impurity to be screened by the s-band of the host metal A. The second term on the right in Eq. (12) gives the contribution to 2 which arises from the dilation of the host lattice A by the impurity B and 18 of the form used by Blatt. Here vw 16 Poisson's ratio and Sa/a 18 the change of lattice parameter in per cent of metal A per atomic per cent of metal B dissolved, Table I. Then to obtain an estimate of 104(0)2 we assume that folkp) 18 produced by a square well of radius R and of a suitably adjusted depth k?/2. Using this well we may then obtain the result 7-1 10.60)2 - 1 - (13) [ (x2 + xəyfə 61m2 (VIII a form given previously by Daniel' in obtaining an estimate of the Knight shift in dilute alloys. The use here of Eq. (13) amounts to assuming only that, in its effect on 8-waves over a limited range of energy, the pseudo- potential may be replaced by a roughly equivalent square well. By sub- stituting Ey. (13) into Eq. (82) and performing the indicated integration an estimate of v, may be obtained from sR/c. In obtaining Sc(k) and f (kg) from R/c and the sun rule, there are in general two sets of solutions because Eq. (10) involves the squares of the sine functions. We give below the results of the calculations for both sets of solutions. For the cases we have treated one value of f(ke) 18 postive and the other negative. If the potential is weak, a positive -9- Solkype) corresponds to an attractive potential for s-waves, and a negative S (K) arises from a repulsive potential. Consequently the two solutions give strikingly different results for the isomer shift. IV. COMPARISON OF THEORY WITH EXPERIMENT As shown in Eq. (80) the 1somer shift V, 18 proportional to (p - 1). This (Ž - 1) 18 estimated from A R/C by the calculation outlined in Section III where, in the calculation, there are two free parameters, the 5-band filling in the host metal Ya, and the choice between the two possible solutions of Eq8. (10), (11) and (12) from which the impurity potential 18 obtained. Because the experimental isomer shifts are all of the same sign and % 18 less than or equal to me, for the alloys discussed here, one must have the same kind of potential, attractive or repulsive, associated with the s-partial waves for all of the alloys. In F18. 1 the calculated (P - 1) 18 presented for Au alloyed in Cu, A8, Pa, Pt and Ni as a function of s-band filling for the case where the impurity potential is attractive and in Fig. 2 for the case where this potential 16 repulsive. The proportionality constant Q = 87/.o)l2 of Eqs. (8), which relates V, and († - 1), 18 obtained by normalizing the theoretical value of Ž - 1 at na = 1 for Au In Cu to the experimental value va = 4.4 1 0.2 mm/sec. This must be done separately for the attractive well solution, yielding a constant G = 8.0 mm/sec, and for the repulsive barrier, giving a constant GR = - 31 mm/sec. The quantities (B-1) are also given as functions of Ya in Fig. 1, and the quantities Q (- 1) are given similarly in M8. 2. For Cu and Ag, na - - - ... te pozwontman Vonomi - S MANN -10- 1s taken to be unity to the experimental 18omer shifts are plotted as points at Y. -1 with the experimental errors indicated. For the three transition metal alloys the value of 18 lese certain, so that the experimental 18omer shifts are plotted as crosshatched bands with heights equal to the experimental errors. -.-.- An examination of Fig. 1 shows a good measure of agreement between theory and experiment for the attractive potential case. In detail, ex- .-.-.-.-.-.-.-.-. periment and theory agree for Au in Pd with a Po 8-band filling in the -. -.-.-.-. range 0.56 to 0.60. This is in quite reasonable agreement with the usual interpretation of the results of magnetic measurements on Pa. For the case of Pt as host, the s-band filling at which experiment and calculation agree, Fig. 1, 18 in the range 0.35 to 0.38. There have been several previous papers on the subject of the 8-band filling in Pt. Some years ago, Wohlfarthº gave an interpretation of measurements of the magnetic susceptibility and of the specific heat of pure pt in which it was suggested that the number of holes in the d-band was in the vicinity of 0.2 - 0.3. The number of electrons in the s-band would then be expected to lie in a similar range. A recent treatment of the Knight shift in Pt by Clogston et al. also indicates an 8 band filling in this range. Thus there is reasonable agreement between these results for the s-band filling in Pt and the result obtained here. A much higher value of 0.58 for this g-band filling has been suggested by Budworth, Hoare, and Preston'from a rigid band model Interpretation of their specific heat measurements on platinum-gold alloys. Perhaps estimates of the band filling of pure Pt may be derived more easily 1. AN " : -110 frca Investigations of the pure or nearly pure metal as in the work of Wohlfarth, Clogston et al. or from the results reported here, than from a rigid band model interpretation of the properties of concentrated alloys. for the case of Au in Ag, Mg. 1, our experimental isomer shirt lies about 30% below the theoretical curva. For the case of Au la Ni at any -0.6 the experimental result lies about 60% higher than the theoretical curve. The agreement between theory and experiment for the case of Au io Ag could be made to appear better by changing the normalization constant to a somewhat lower value near , * 7.0 m/sec. The experimental results for both Au la Ag and Au in Cu would then lie almost within their experimental errors of the theoretical curves and the results for the 8-band rilling ia Ad and Pt would be very little changed. On the other hand the disagreement between theory and experiment for Au in Ni would be somewhat worse. In our calculation of the isomer shift from the electrical re- 818 tance, ferromagnetism of the host was not taken into account in any way. We find a reasonable agreement between theory and experiment for the para- magnetic or nonagnetic cases but not where ferromagnetien occurs. As was observed earlier, AR/C for Au in NI 16 strongly temperature dependent." While this temperature dependence must be due mainly to magnetic scattering 1t 18 not absolutely clear that 1 R/C at T - Oºk gives a useful measurement of the transport cross section associated with the Au impurity in the context of our model. Turaing to fig. 2, we may now lavestigate the comparison between experiment and the theoretical model for the case of a repulsive potential. PA 1. -12- With theory and experiment again normalized for Au in Cu we find . 31 /060. Here, however, for Au In 18, Pd, and Pt the theoretica nalwes for vel are all greater than for Au la Cu while the experimental values are all less. There is no agreement for these cases between theory and experiment for any value of the s-band filling y. In the case Au in Ni, theory and experiment still do not agree at n - 0.6 although the disagreement in this case so less severe than for the attractive potential Pig. l. Considering all of these cases, the use of a repulsive well in the above calculation is not consistent with experimental results. The results presented in Fig. 1 thus indicate that in Au alloys or compounds where v, 1o greater than zero, the electron charge density near the gold nucleus 1. greater than the density in pure gold, 12:,(0) · P., (0)) > 0. This result is in agreement with the conclusion of Mott? that gold when alloyed in silver bould present an attractive potential to valence band electrons. Prom a study of the correlation between the 1somer shift of gold in a variety of alloys with the electro- negativity difference between gold and the host metal, Shirley et al.14 have also concluded that ve > ņ corresponds to (Paulo) - Pau (0)) > 0. We note from Eq. (1) in agreement with Shirley et al., that with Paulo) - Pau (0)) > 0 one has ( 29) - (120) >0. This result 18 in agreement as to sign with the description of 197Au given by zeldes 25 using the shell model. In the use of the theoretical model given above to describe the 1somer shift, the range of the pseudo-potential was taken to be the radius . NO . . Ypl . d :* M w ' WW . IN 2 UiT L picy I UA TIX 12 G' WA6 .". Tom W -13- of a gold aton. Strictly, this assumption to not required in the above theoretical model. We have thus repeated the above calculation for an R of 0.25 rg, 0.50 rg, 0.75 rg and 1.25 rg. The best correlation between 1somer shift and residual resistance was found for the case presented in detail, namely R = rp . . ....UTE WWW .... .. . .. Y .PRWINI , .. . VIJI TLALU ": , 974 1710 . "71 IT W ITAMIN ", . 1. .. .." " . .' --- wwM .' PA ... - - TY TNI. w ... . AND . .. . -14- REFERENCES 1. R. L. Mössbauer. 2. Phy., 151, 124 (1958). 2. 0. C. Kistner and A. W. Sunyar, Phys. Rev. Lett., 4, 412 (1960). 3. L. D. Roberts and J. 0. lhomson, Phys. Rev., 129, 664 (1963). L. D. Roberts, A. Pomerance, J. 0. Thomson, and C. p. Dan, Bull. Am. Phyns. Soc., 1, 565 (1962). 4. L. D. Roberto, R. L. Becker and J. 0. Thonson, Bull. Am. Phys. Soc. 8, 42 (1963); R. L. Becker, L. D. Roberts and J. 0. Thomson, Bull. Am. Phys. Soc., 8, 558 (1953). L. D. Roberts, R. L. Becker, F. E. Obenshain, and J. 0. Thomson (to be published). 5. L. D. Roberts, F. E. Obehshain, R. L. Becker, and J. 0. Thomson, Bull. Am. Phys. Soc., 2, 398 (1964). J. Priedel, Phil. Mag. 43, 153 (1952); Adv. in Phys., 3, 446 (1954). P. de Faget de Casteljau and J. Friedel, J. Phys. Radium, 27, 27 (1956). J. Priedel, Il Nuovo Cimento Ser. X, 1, 287 (1958). 7. K. Auang, Proc. Phys. Soc., 60, 161 (1948). 8. F. J. Blatt, Phys. Rev., 108, 285 (1957); Phys. Rev., 108, 1204 (1957). E. Daniel, J. Phys. Chem. Solid8, 10, 174 (1959). 10. E. P. Wohlfarth, Proc. of the Leeds Philosophical and Lit. Society, Sci. Sect., 1948, p. 89-101. 11. A. M. Clogston, V. Jaccarino and Y. Yafet, Phys. Rev. 134A, 650 (1964). 12. D. W. Budworth, P. E. Hoare, and J. Preston, Proc. Roy. Soc., A257, 250 (1960). ó IT MAMA . -15- 13. N. 7. Mott, Proc. Camb. Phil. 800., 22, 281 (1936). 14. D. A. Shirley, Rev. Mod. Phys., 26, 339 (1964); P. 8. Barrett, R. W. Orant, M. Kaplan, and D. A. Shirley, J. Chem. Phys., 29, 1035 (1963). 15. N. Zldes, Muc. Phys., 3, 1 (1956/57). TABLE I Parameters uscd in the correlation of the isomer shirt with the residual resistance for Au. The phase shirts S and S, are given for the case of a potential attractive for s-waves and for thc valencen, also given below, whith results in the best correlation, Fig. 7. The quantity k/2 18 the corresponding well depth in ev. Host mexperimental residual resistivity Experimental Isomer shirt Poisson's Host valence used Phase shirts ratio Well depth k2/2 (ev) Aplechts (www/sec) 0.52 + 0.02 0.364 0.02 0.38 0.02 0.700.02 1.550.02 4.4 0.2 2.1 + 0.2 5.4 + 0.2 2.4 t 0.2 1.4 1 0.2 0.157 0.364 -0.00735 0.37 0.209 0.31 0.05280,0.398 0.040 0.39€ 1.337 0.984 1.397 1.120 1.091 1.0 1.0 0.6 0.58 0.37 0.0562 -0.1951 0.2155 -0.06343 0.2872 -0.01113 0.423 +0.0425 0.624 +0.102 0.305 1.07 1.407 1.935 2.597 a) L. Vegard and A. Kloster, Zeit. Krist. 89, 560 (1934); C. S. Smith, Min. and Met. 9, 158 (1928). b) Q. Sachs and J. Weerts, Zeit. Phys. 60, 481 (1930). c) 3. C. Elwood and K. Q. Bagley, J. Inst. Metals, London, 80, 617 (1951/2). a) V. G. Kuznecov, Izv. Sek. Platiny Akad. Nauk. SSSR, 20, 5 (1946), see Structure Reports, 10, 54 (1945/6). e) 1. . Darling, R. A. Mintern and J. C. Chastoa, J. Inst. Metals, london, 81, 125 (1952/3). 8) from the A.I.P. Handbook, McGraw Hill, 1957. 8) Assumed the value for Pt. b) y = 3(1-0)/(1+0) 1 FIGURE CAPTIONS Fig. 1. Comparison between calculated and experimental 1soner shifts for an attractive potential. The calculated values have been normalized to give agreement for Au in Cu, with Q = 8.0 mm/sec, Eq. (86). M8. 2. Comparison between calculated and experimental isomer shifts for a repulsive potential. The calculated values have been normalized to give agreement for Au in Cu, with a = - 31.0 m/sec, Bq. (80). UNCLASSIFIED ORNL-DWG 64-2846 - - kriminimai ir 11" (P-1) FOR ATTRACTIVE POTENTIAL W V1, ISOMER SHIFT (mm/sec) N s S * A Pay Ever.. in. - 0.3 0.4 9.0 9.1 0.5 0.6 0.7 0.8 0.9 mu, S-BAND FILLING OF HOST Fig. 1. - -- -- *** - - - IN UNCLASSIFIED ORNL-DWG 64-2847 Ag NI MILIONIZIMIT (P-1) FOR REPULSIVE POTENTIAL Tvil, ISOMER SHIFT (mm/sec) MIPA 0.4 0.5 0.6 0.7 0.8 0.9 na , S-BAND FILLING OF HOST 9.0 9.1 Fig. 2. DATE FILMED 11/ 125 / 164 . 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