Hyperfine Interactions 27 (1986) 437-440                                                            437

438           P.M.L.O. Scholte, et al., Influence of the conduction electrons on the EFG

P.M.L.O. Scholte, et al., Influence o f the conduction electrons on the EFG                                               ...
440     P.M.L.O. Scholte, et aL, Influence of the conduction electrons on the EFG

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1986 the influence of conduction electrons on the efg of amorphous intermetallic alloys


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1986 the influence of conduction electrons on the efg of amorphous intermetallic alloys

  1. 1. Hyperfine Interactions 27 (1986) 437-440 437 THE INFLUENCE OF THE CONDUCTION ELECTRONS ON THE EFG IN AMORPHOUS INTERMETALLIC ALLOYS P.M.L.O. SCHOLTE(1), M. TEGZE(2), F. VAN DER WOUDE(1), K.H.J. BUSCHOW (3) and I. VINCZE (1) Solid State Physics Laboratory, Materials Science Center, University of Groningen, 1 Melkweg, 9718 EP Groningen, The Netherlands (2) Central Research Institute for Physics, P.O.Box 49, H-1525 Budapest, Hungary (3) Philips Research Laboratories, 5600 MD Eindhoven, The Netherlands The quadrupole splitting in alloys is determined by the structural and electronic properties of the alloy. Only in cases where the density of states at the Fermi level is constant or known as a function of the concentration, relevant structural information can be obtained from the quadrnpole splitting. i. INTRODUCTION The electric field gradient (EFG) is often used to study the local configurations in amorphous metals. The EFG usually is decomposed into aelattice iat . I contribution V and a contrlbution of the conduction electrons V . In a zg . . . z large number of Intermetalllc systems the electronlc part is proportlona~ to the lattice contribution /1,2/. Therefore the EFG in amorphous metals often is calculated using a simple point charge model with a Coulomb potential # between the constituent atoms /3/. 1 qn = ~ ! r~ (I) vt~ (l-7oo)Vlzz + Velzz at (2) where 7~ is the Sternheimer antishielding factor, qn and r are the charge and radial distance of atom n. Furthermore n V el = -K V lat (3) zz zz with K=-2 for IIIB and IV and K=+3 for the other elements /2/. However not all intermetallic systems follow the trend of the universal correlation. Verma et al. observed in STFeZr a large value of K=9 /4/. In this paper we will show the importance of---a screening term for the electronic contribution of the EFG. The possible role of screening can also be concluded from the observation of Moruzzi et al. /5/ that the Fermi level in amorphous TM-TM alloys is located at, or near, a maximum in the density of states. The screening term s(rn) will modify the potential ~ as expressed in 1 qn s( (4) n n Consequently in alloys where screening is important an independent source for s(r ) is necessary if one wants to obtain structural information from qua~rupole splittings as obtained from MES. 9 J.C. Baltzer A.G., Scientific Publishing Company
  2. 2. 438 P.M.L.O. Scholte, et al., Influence of the conduction electrons on the EFG 0.6 - --- --- - ..... Fc c. Dodec. B cc-12 Bcc-8 J - - Tetra. 0,41 0.4 0.3g E E // / / ~ 0.37 // I I E ,'// / s 0.2 ,!,: I / / /// /// / 03~ ,!/// ~,'x / ,,2/ 0,33 2'0 4 6'0 do loo ~ i , i , k/Ann i i , 10-1 , i i I i i 100 at% Zr Fig.2. Quadrupole splitting as a func- tion of screeninglength ~ in a number Fig.l. Quadrupole split- of "quasi-crystalline" models. Ann is ting in a statiscally the nearest neighbour distance; Bcc-12 disordered alloy without resp. -8 is a Bcc structure with 12, chargetransfer, resp. 8 neighbours. 2. EXPERIMENTAL 2.1. Sample preparation Amorphous ribbons ( F e . ~ Zr , x=]0,60-80) of about 1 mm wide and 9 I U--X X 20 pm thick were prepared ~y means of meltspinning in an atmosphere of purified Argon. Amorphous films ( F e 1 ~ Zr , X=IO-80; (Fel00_vCUv)lOO~xZrx , x,y=lO,20,40,80) of about 5006UUA x w~re produced by coevaporatiOn ot the elements onto an aluminium substrate in UHV-equipment /6/. The noncrystallinity of these samples was confirmed by X-ray diffraction. 2.2. Mossbauer experiments The room temperature Mossbauer spectra were recorded using a conventional constant acceleration type spectrometer with a moving source ( STCo in Rh) and a stationary absorber. To eliminate substrate effects and to reduce the measuring time we used conversion electron Mossbauer spectroscopy to obtain the spectra of the thin films. Some films were measured also in transmission geometry, but the spectra were identical to the CEMS spectra. The spectra were analysed using the deconvolution method of Vincze /7/. 3. RESULTS AND DISCUSSION 3.1. The model We calculated the EFG using a Thomas-Fermi screening, i.e. in (4) s(r) = exp(-r/~) (5)
  3. 3. P.M.L.O. Scholte, et al., Influence o f the conduction electrons on the EFG 439 ~I O 8 8,, A 0.7 ~20 Y (Fe 100-yCUy ) 100-xZTx A40 o60 oz~ 80 / O3 A < o / / E 0.4 E 05 / / E AA AA o iI ___./ i 0.3 I ~ 20 8b loo o 20 0 60 80 100 Fe Zr x (at% Zr) x (Qt % Z r ) Fig.3. Average quadrupole splitting in Fig.4. Average quadrupole splitting in amorphous Fe-Zr alloys prepared by amorphous (Fe,Cu)-Zr alloys. The dashed meltquenching ( 0 ) and by coevaporation line indicates the splittings in (A). amorphous Fe-Zr alloys. where ~ is the screening length. The EFG-tensor can be expressed as X - r2 1 3Xnl nj n~ij Vii - 4~s0 ~ qn S(rn) rs (6) n n with the screening term S(r) S(r) = (I + rlX + ~<r/%) 2) exp(-r/X) (7) are the coordinates of atom n and 6 . is the Kronecker delta. Xni The summation is calculated o v ~ the nearest neighbours surrounding a central Fe atom. In crystalline materials with long range order (LRO) this may lead to erroneous results, but because of the absence of LRO in amorphous alloys the symmetry of the next coordination shells becomes more spherical. As a consequence the contribution of these shells will be small. Also the introduction of the screening term causes the lattice sum in (6) to converge more rapidly. The amorphous "quasi-crystalline" and DRPHS clusters are constructed as described in /8/. The charge transfer and the changes in the interatomic distances as a function of the concentration were calculated using the Miedema-Niessen theory /9/. In fig. 1 the concentration dependence of the quadrupole splitting in a statistically disordered alloy is shown, assuming an infinite screening length and no charge transfer. One obtains a parabolic-like behaviour. In fig. 2 we plot the quadrupole splitting as a function of the screening length. The quadrupole splitting shows a clear sensitivity to small changes in the screening length, when X is not too large. In the free electron model the screening length is connected to the density of states at the Fermi level n(Ef)
  4. 4. 440 P.M.L.O. Scholte, et aL, Influence of the conduction electrons on the EFG 1 e 2 n(Ef) (8) ~2- ~0 Vm Where V is the molar volume. Consequently a decrease of n(Ef) will cause an 9 m increase of the quadrupole splitting. Additional information about the concentration dependence of the density of states may be obtained from the specific heat, the magnetic susceptibility and from UPS experiments. 3.2. The experimental quadrupole splitting The concentration dependence of the quadrupole splitting in amorphous Fe-Zr alloys was presented in /8/ and is shown in fig. 3. In fig. 4 we show the splittings in a number Qf amorphous (Fe,Cu)-Zr alloys; the Fe-Zr splittings are indicated by the dashed line. In both systems we observe a rather steep increase of the splitting as a function of the concentration. The splitting in the Fe-Zr alloys is more or less constant up to 60 at.% Zr. The concentration dependence does not resemble fig. I. Within our model this steep increase is caused by a lowering of the Fermi level d.o.s, with increasing Zr content. This is corroborated by the Work of Matsuura et al./lO/, who observed a steep lowering of the linear term in the specific heat at 60 at.% Zr. This causes a lowering of the Fermi level d.o.s.; though this interpretation is not straightforward because of the magnetic and electron-fonon interactions, which also contribute to the specific heat. But after correcting for these effects Matsuura et al. calculate a d.o.s, which decreases from 70 at.% Zr /i0/. 3.3. Conclusion We conclude that the quadrupole splitting in these alloys is complicated by the contribution of the conduction electrons, which is not simply an amplification effect. Since the sensitivity of the splitting to the screening length is not very much dependent on the structural model used it is obvious that structural information is not easily obtained from the Mossbauer quadrupole splitting. Additional information concerning the d.o.s, is necessary. ACKNOWLEDGEMENTS This investigation forms part of the research program of the "Stichting voor Fundamenteel Onderzoek der Materie" (Foundation for Fundamental Research on Matter - FOM) and was made possible by financial support from the "Nederlandse Organisatie voor Zuiver- Wetenschappelijk Onderzoek" (Netherlands Organization for the Advancement of Pure Research - ZWO) REFERENCES / i/ R.S. Raghavan, E.N. Kaufmann, and P. Raghavan, Phys. Rev. Lett. 34(1975) 1280. / 2/ H. Ernst, E. Hagn and E. Zech, G. Eska, Phys. Rev. B19(1979)4460. / 3/ G. Czjek, J. Fink, F. Gotz, H.Schmidt, J.M.D. Coey, J.-P. Rebouillat and A. Lienard, Phys. Rev. B23(1981)2513. / 4/ H.C. Verma, J. Chappert and G.N. Rao, Hyp. Int. Ii(1981)45~ / 5/ V.L. Moruzzi, P. Oelhafen, A.R. Williams, R. Lapka, H.J. Guntherodth and J. Kubler, Phys. Rev. B27(1983),2049. / 6/ A.M. van der Kraan and K.H.J. Buschow, Phys. Rev. B27(1983)2693. / 7/ I. Vincze, Nucl. Instr. Meth. 199(1982)247. / 8/ P.M.L.O. Scholte, M. Tegze, F. van der Woude, K.H.J. Buschow and I. Vincze, Proc. RQM V, Wurzburg, 1984, ed. S.Steeb and H. Warlimont (Elsevier Science Publishers B.V., 1985) p.541. / 9/ A.R. Miedema and A.K. Niessen, Physica I14B(1982)367 /I0/ M. Matsuura, U. Mizutani and K. Fukamichi, Proc. RQM V, Wurzburg, 1984, ed. S.Steeb and H. Warlimont (Elsevier Science Publishers B.V., 1985) p. 1019.