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Short range order in amorphous Ni50Ta50 alloys by means of X-ray and neutron diffraction
Journal of Non-Crystalhne Sohds 156-158 (1993) 165-168 North-Holland
~
J O U R N A L OF
U
Short range order in amorphous Nis0Ta50 alloys by means of X-ray and neutron diffraction H. U h l i g a, L. R o h r b H.-J. G i i n t h e r o d t b, p. F i s c h e r ¢, P. L a m p a r t e r a a n d S. S t e e b a a Max-Planek-InsUtut fur Metallforsehung, Insutut fur Werkstoffwtssenschaft, W-7000 Stuttgart, Germany b Insntut fiir Phystk, Umversttdt Basel, CH-4056 Basel, Swttzerland c Labor fitr Neutronenstreuung, ETHZ, CH-5232 Vilhgen PSI, Swttzerland
In the present work, amorphous Nls0Tas0 samples with a high crystalhzauon temperature of 985 K were mveshgated To evaluate the three partial structure factors, an X-ray diffraction experiment with Ni50Tas0 and two neutron dfffrachon experiments with NIs0Tas0 and with C%0Ni40Ta50, respectwely, were performed The nearest nelghbour distance Js d = 2 82 ,~ for Ni-Nl, d = 2.91 ,~ for Ta-Ta, and d = 2 44 A for NI-Ta. The coordination numbers are NN,N,= 4 9, NTdT~= 8 2 and NN,Ta= 6 0. The chemical short-range order and the chemical bounding in a-Nls0Ta50 are reported
1. Introduction
2. Theoretical background
Amorphous Ni~Tal_ x alloys (0.3 < x < 0.68) were prepared using the splat cooling method with levitation melting in vacuum [1]. These alloys show rather high crystallization temperatures between 1030 and 950 K [1]. The Vickers hardness has the rather high value of about 900 k p / m m 2. The resistivity and Hall effect were also measured [2]. In the present paper we report on the determination of partial structure factors by a combination of X-ray and neutron diffraction of the amorphous Nis0Ta50 alloys (crystallization temperature 985 K; melting point of the corresponding crystalline alloy 1930 K). Three diffraction experiments, which have to differ in the scattering length of at least one of the components, are necessary. We show that in the present case Ni can be isomorphously substituted by Co. Neutron diffraction was applied to a-Nis0Tas0 and to aCOl0Nia0Taso and an X-ray diffraction experiment was performed with a-NisoTaso.
From the coherently scattered intensity, Icoh(Q), one obtains the total structure factor, sFZ(Q), according to Faber and Ziman [3]:
Correspondence to Professor Dr S. Steeb, Max-Planck-InstltUt liar Metallforschung, Inshtut fur Werkstoffw~ssenschaft, Seestrasse 92, W-7000 Stuttgart 10, Germany. Tel +49-711 2095 384 Telefax. +49-711 2265 722.
sFZ(Q) = [/¢oh(e)-c~c2(b,-b2)2]/(b)2,
(1) where Q = 4"rr(sin0)/A is the modulus of the momentum transfer, 20 is the diffraction angle, A is the wavelength of the scattered radiation, c, is the concentration of component t in atomic fractions, b, is the scattering length of component i for coherent scattering, and ( b ) = clb 1 + c z b 2. The term q c 2 ( b I - b2 )2 in eq. (1) is called the monotonic Laue scattering. This diffuse scattering is caused by the statistical distribution of the two types of atom in the sample, sFZ(Q) is the weighted sum of the partial structure factors s v z ( Q ) , which describe the contribution of ij pairs to the structure factor:
cZb 2 SFZ(Q) = - ~1S n1( Q )
+
c2b 2 + (b)2S22(Q).
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B V. All rights reserved
2clczblb2 ~ $12(Q)
(2)
166
H Uhhg et al. / SRO in a-NtsoTa50
By Fourier transformation, one obtains the total reduced pair distribution function, G FZ(R), which is composed of the partial functions, G,j(R), using the same weighting factors as in eq. (2). For the interpretation of structural results, it is useful to adopt the description according to Bhatia and Thornton [4]. For the total Bhatia-Thornton structure factor we have sBT(Q) =
Icoh(Q) (b 2)
( b)2 (b z) SNN(Q)
Qcz(b 1 - b2) 2 +
+2
( b 2)
Scc(Q)
(b)(b 1-b2) (b2) SNc(Q ),
(3) where SNN(Q) is the contribution of density-density correlations, where Scc(Q) is the contribution of concentration-concentration correlations, SNc(Q) is the contribution of concentration-density correlations, and ( b 2) = Cl b2 + c2 b2. The Fourier transform of sBT(Q) yields the BT total reduced pair distribution function, GBT(R), which is composed of the partial BT pair distribution functions, GNN(R), Gcc(R), and GNc(R)-
three experimental total structure factors with different weighting factors, i.e., three independent equations which present a linearly independent 3 × 3 system of linear equations. In the present work we applied the combination of two neutron with one X-ray diffraction experiment. For the X-ray experiment, we used the complex X-ray scattering-amplitudes b(Q) = bo(Q) + b' + ib". The values of bo(Q) were taken from ref. [5], and values of b' and b" from ref. [6]. Therefore the weighting factors according to eq. (2) show a Q dependence. Whereas the weighting factors WN,N1and WN1Tadecrease slightly, the weighting factor WTaTa increases with increasing Q. For calculating the weighting factors for the neutron diffraction experiments, the scattering lengths of ref. [7] were used. These values are independent
of Q. We performed an X-ray diffraction experiment and a neutron diffraction experiment with NisoTaso and a neutron diffraction experiment with COl0Ni40Tas0, in which a fraction of Ni atoms is substituted isomorphously by the chemically similar element Co. The amount of 10% Co atoms causes no change of structure. This fact was checked by performing a neutron diffraction experiment with an amorphous CosoTas0 alloy.
3.3. Diffractton experiments 3. Experimental
3.1. Specimen preparation From Ta (purity 99.9%), Ni (purity 99.98%), and Co (purity 99.99%), alloy samples were made by melting the constituents in an argon atmosphere. The samples were turned over and remelted several times in order to ensure homogeneity. The alloys were broken into 400 mg sections and rapidly quenched from the melt within a two-piston splat-cooling device. The resulting splats were 40-60 ixm thick and 15-25 mm wide.
3.2. Calculation of partial structure factors For the evaluation of the partial Faber-Ziman structure factors according to eq. (2), one needs
X-ray diffraction with a-NisoTas0 was done in the reflection mode using Ag Kot radiation (h = 0.559 ,~) with a graphite monochromator in the diffracted beam. The experimentally obtained intensity curve was corrected for polarization [8], Compton scattering [5,8], and then normalized to S(Q) [9]. Neutron diffraction with the a-NisoTa5o specimen (10 g) was performed with the powder diffractometer DMC [10] using A = 1.0862 A at the research reactor Saphir of the Paul Scherrer Institute (Villigen, Switzerland). Neutron diffraction with the a-Col0Ni40Tas0 specimen (4.23 g) was done with the two-axes diffractometer 7C2 using A = 0.699 ,~ at the hot source of the research reactor Orph6e (Saclay, France). The experimentally obtained intensity curves were corrected for absorption [11], multiple scattering [12],
H. Uhhg et aL / SRO m a-MsoTa5o
167
inelastic scattering [13], and then normalized to
S(Q) [9]. To eliminate the influence of rounding errors in the calculation of the partial structure factors, it is recommended that one apply the method of iterative refinement [14].
_=
5
r.f3
1
0 4. R e s u l t s
2
Figure 1 shows the partial structure factors as obtained using the method of iterative refinement. In all three curves, the main peak is followed by a double peak (second maximum) as well as a third and fourth peak. At about 1.6 A* - 1, we observe in the Ni-Ni curve a prepeak indicating compound formation. In fig. 2, we present the partial Bhatia-Thornton structure factors for a-NisoTas0. They were calculated from the experimental saT(Q) via eq. (3) using the method of iterative refinement. The SNN(Q) curve represents the topological arrangement of the atoms. The Scc curve is determined by the distribution of the atoms of different types within the specimen. Its pronounced oscillations are a hint to strong chemical ordering effects.
4 5 8 Q {k ~ }
11]
Fig. 2. Partial Bhatia-Thornton structure factors of amorphous NisoTas0
tial structure factors in fig. 1. All three Faber-Ziman partial pair correlation functions show oscillations up to 18 ,~. This indicates a strong shortrange order. Table 1 shows the atomic distances and partial coordination numbers. The experimental Ni-Ta distance of 2.44 ~, is smaller than the sum of the atomic radii of Ni and Ta, which means strong Ni-Ta bonding. The experimental Ta-Ta distance of 2.91 A corres2onds well to the metallic diameter DTa = 2.98 A. The experiment shows, further, that the Ni-Ni distance ois larger than the metallic diameter DN, = 2.82 A, i.e., the Ni atoms do not occur in close contact. The short Ni-Ta distance, ac-
5. D i s c u s s i o n
5.1. Faber-Ziman pair correlation functions
5~,~
~
^
N,-N
Figure 3 shows the Faber-Ziman partial pair correlation functions which follow from the par-
2-
s
1 C2~
re-3
Ta-Ta
1 NI -Ta
2
4
5
Q
[ ~-1 ]
8
i0
Fig 1. Partial Faber-Ziman structure factors of amorphous NlsoTa 50.
0
0
5
10
15
20
R {A} Fig. 3. Partial Faber-Zlman
pair correlation fl]nctlons of
amorphous Ni50Taso
H Uhhg et al / SRO m a-NtsoTaso
168
Table 1 Amorphous N150Tas0 atomic d~stances, R,j, partial coordination numbers, Z,:, and atomic diameters, Oatom [15] Pair t - j
R,j (A)
Zzj
Datom (.~)
NI-Nl Ta-Ta N1-Ta Ta-Nl
2.82 + 0.05 2 91 :t: 0.05 2.44 :lz0 05 2.44 + 0 05
4.9 + 0.5 8 2 _+0.5 6 0 + 0.5 6.0 + 0 5
2.48 2 98 2.73 2 73
tal total structure factors, the rather ill-conditioned systems of linear equations in the present paper or in ref. [18], respectively, confront us with very contradictory results.
6. Conclusions
companied by a distinct short-range order can be explained by a d - d electron interaction of the two components. Such features were predicted by Wang's [16] computer simulations for inter-transition-metal glasses.
The shortened Ni-Ta distance of 2.44 ,~ is the result of a strong chemical interaction. A direct Ni-Ni contact does not occur. We found strong discrepancies with earlier experiments on Ni55Ta4~. Neutron diffraction experiments using the method of isotopic substitution are needed for final clarification.
5.2. Bhatia-Thornton partial pair correlation functtons
Thanks are due to R. Bellissent, LLB Scalay, for beamtime at the 7C2 instrument.
In ref. [17] we presented the Bhatia-Thornton partial functions GNN(R), Gcc(R), and GNc(R). The GNN(R) curve represents the topological order. The high amplitude of the oscillations of the Gcc(R) curve and the change of sign at R = 2.58 A point to a distinct chemical ordering effect. The GNc(R) function is determined by the difference between the atomic volumes and shows the usual behaviour.
5.3. Comparison with amorphous N155Ta 45 In ref. [18] three partial functions were evaluated from one neutron and one X-ray diffraction experiment with meltspun a-Ni55Ta45 as well as one neutron diffraction experiment with meltspun a-Ni62Ta38. According to ref. [18], for a-Ni55Ta45, the nearest-neighbour distances are Ni-Ni = 2.52 ,~ (2.82 ~, in this paper), Ta-Ta = 3.06 A (2.91 A) and Ni-Ta = 2.78 A (2.44 A). The discrepancy between ref. [18] and the present paper cannot be explained by the fact that in the present paper splat-cooled samples were used, whereas in ref. [18] melt-spun samples were investigated. Also the different alloy compositions, namely Ni55Ta45 in ref. [18] compared with NisoTas0, cannot explain the discrepancies. At this moment we can only state that, despite the very good experimen-
References [1] T Richmond, L. Rohr, P Retmann, G L e e m a n and H-J. Gtintherodt, m' RQ7, Proc 7th Int. Conf. on Rapidly Quenched Metals (Elsevier, Amsterdam, 1991) p 63 [2] L. Rohr, P Relmann, T. Richmond and H.-J Gtintherodt, in RQ7, Proc 7th Int. Conf. on Rapidly Quenched Metals (Elsevier, Amsterdam, 1991) p 715 [3] T.E Faber and J M Ziman, Phdos Mag 11 (1965) 153. [4] A. Bhatia and D E. Thornton, Phys. Rev. B2 (1970) 3004 [5] J H Hubbell et al., J Phys Chem. Ref. Data 4 (1975) 471 [6] Y. Waseda, Novel Apphcation of Anomalous (Resonance) X-ray Scattering for Structural Characterization of Disordered Materials (Springer, Berhn, 1984) [7] L. Koester, H Rauch and E. Seymann, At. Data Nucl. Data Tables 49 (1991) 65 [8] C N J. Wagner, J. Non-Cryst. Sohds 31 (1978) 1. [9] J Krogh-Moe, Acta Crystallogr 9 (1956) 951 [10] J Schefer, P. Fischer, H Heer, A. Isacson, M. Koch and R Thut, Nucl. Instrum. and Methods A288 (1990) 477. [11] H.H Paalman and L.J. Pings, J Appl Phys 33 (1962) 2635 [12] V F Sears, Adv Phys. 24 (1975) 1 [13] G. Placzek, Phys. Rev. 31 (1952) 377. [14] .~. Bjbrck and G Dahlquist, Numerlsche Methoden (Oldenbourg, Munich, 1972). [15] Catalogue Number 9-18806 (Sargent-Welsh Scientific Comp, 1968) p 2 [16] R. Wang, Nature 278 (1979) 700 [17] H Uhlig, diploma thesis, Umversity of Stuttgart (1992). [18] H E. Fenglai, P P. Gardner, N. Cowlam and H.A. Davies, J. Phys. F17 (1987) 545.
~
J O U R N A L OF
U
Short range order in amorphous Nis0Ta50 alloys by means of X-ray and neutron diffraction H. U h l i g a, L. R o h r b H.-J. G i i n t h e r o d t b, p. F i s c h e r ¢, P. L a m p a r t e r a a n d S. S t e e b a a Max-Planek-InsUtut fur Metallforsehung, Insutut fur Werkstoffwtssenschaft, W-7000 Stuttgart, Germany b Insntut fiir Phystk, Umversttdt Basel, CH-4056 Basel, Swttzerland c Labor fitr Neutronenstreuung, ETHZ, CH-5232 Vilhgen PSI, Swttzerland
In the present work, amorphous Nls0Tas0 samples with a high crystalhzauon temperature of 985 K were mveshgated To evaluate the three partial structure factors, an X-ray diffraction experiment with Ni50Tas0 and two neutron dfffrachon experiments with NIs0Tas0 and with C%0Ni40Ta50, respectwely, were performed The nearest nelghbour distance Js d = 2 82 ,~ for Ni-Nl, d = 2.91 ,~ for Ta-Ta, and d = 2 44 A for NI-Ta. The coordination numbers are NN,N,= 4 9, NTdT~= 8 2 and NN,Ta= 6 0. The chemical short-range order and the chemical bounding in a-Nls0Ta50 are reported
1. Introduction
2. Theoretical background
Amorphous Ni~Tal_ x alloys (0.3 < x < 0.68) were prepared using the splat cooling method with levitation melting in vacuum [1]. These alloys show rather high crystallization temperatures between 1030 and 950 K [1]. The Vickers hardness has the rather high value of about 900 k p / m m 2. The resistivity and Hall effect were also measured [2]. In the present paper we report on the determination of partial structure factors by a combination of X-ray and neutron diffraction of the amorphous Nis0Ta50 alloys (crystallization temperature 985 K; melting point of the corresponding crystalline alloy 1930 K). Three diffraction experiments, which have to differ in the scattering length of at least one of the components, are necessary. We show that in the present case Ni can be isomorphously substituted by Co. Neutron diffraction was applied to a-Nis0Tas0 and to aCOl0Nia0Taso and an X-ray diffraction experiment was performed with a-NisoTaso.
From the coherently scattered intensity, Icoh(Q), one obtains the total structure factor, sFZ(Q), according to Faber and Ziman [3]:
Correspondence to Professor Dr S. Steeb, Max-Planck-InstltUt liar Metallforschung, Inshtut fur Werkstoffw~ssenschaft, Seestrasse 92, W-7000 Stuttgart 10, Germany. Tel +49-711 2095 384 Telefax. +49-711 2265 722.
sFZ(Q) = [/¢oh(e)-c~c2(b,-b2)2]/(b)2,
(1) where Q = 4"rr(sin0)/A is the modulus of the momentum transfer, 20 is the diffraction angle, A is the wavelength of the scattered radiation, c, is the concentration of component t in atomic fractions, b, is the scattering length of component i for coherent scattering, and ( b ) = clb 1 + c z b 2. The term q c 2 ( b I - b2 )2 in eq. (1) is called the monotonic Laue scattering. This diffuse scattering is caused by the statistical distribution of the two types of atom in the sample, sFZ(Q) is the weighted sum of the partial structure factors s v z ( Q ) , which describe the contribution of ij pairs to the structure factor:
cZb 2 SFZ(Q) = - ~1S n1( Q )
+
c2b 2 + (b)2S22(Q).
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B V. All rights reserved
2clczblb2 ~ $12(Q)
(2)
166
H Uhhg et al. / SRO in a-NtsoTa50
By Fourier transformation, one obtains the total reduced pair distribution function, G FZ(R), which is composed of the partial functions, G,j(R), using the same weighting factors as in eq. (2). For the interpretation of structural results, it is useful to adopt the description according to Bhatia and Thornton [4]. For the total Bhatia-Thornton structure factor we have sBT(Q) =
Icoh(Q) (b 2)
( b)2 (b z) SNN(Q)
Qcz(b 1 - b2) 2 +
+2
( b 2)
Scc(Q)
(b)(b 1-b2) (b2) SNc(Q ),
(3) where SNN(Q) is the contribution of density-density correlations, where Scc(Q) is the contribution of concentration-concentration correlations, SNc(Q) is the contribution of concentration-density correlations, and ( b 2) = Cl b2 + c2 b2. The Fourier transform of sBT(Q) yields the BT total reduced pair distribution function, GBT(R), which is composed of the partial BT pair distribution functions, GNN(R), Gcc(R), and GNc(R)-
three experimental total structure factors with different weighting factors, i.e., three independent equations which present a linearly independent 3 × 3 system of linear equations. In the present work we applied the combination of two neutron with one X-ray diffraction experiment. For the X-ray experiment, we used the complex X-ray scattering-amplitudes b(Q) = bo(Q) + b' + ib". The values of bo(Q) were taken from ref. [5], and values of b' and b" from ref. [6]. Therefore the weighting factors according to eq. (2) show a Q dependence. Whereas the weighting factors WN,N1and WN1Tadecrease slightly, the weighting factor WTaTa increases with increasing Q. For calculating the weighting factors for the neutron diffraction experiments, the scattering lengths of ref. [7] were used. These values are independent
of Q. We performed an X-ray diffraction experiment and a neutron diffraction experiment with NisoTaso and a neutron diffraction experiment with COl0Ni40Tas0, in which a fraction of Ni atoms is substituted isomorphously by the chemically similar element Co. The amount of 10% Co atoms causes no change of structure. This fact was checked by performing a neutron diffraction experiment with an amorphous CosoTas0 alloy.
3.3. Diffractton experiments 3. Experimental
3.1. Specimen preparation From Ta (purity 99.9%), Ni (purity 99.98%), and Co (purity 99.99%), alloy samples were made by melting the constituents in an argon atmosphere. The samples were turned over and remelted several times in order to ensure homogeneity. The alloys were broken into 400 mg sections and rapidly quenched from the melt within a two-piston splat-cooling device. The resulting splats were 40-60 ixm thick and 15-25 mm wide.
3.2. Calculation of partial structure factors For the evaluation of the partial Faber-Ziman structure factors according to eq. (2), one needs
X-ray diffraction with a-NisoTas0 was done in the reflection mode using Ag Kot radiation (h = 0.559 ,~) with a graphite monochromator in the diffracted beam. The experimentally obtained intensity curve was corrected for polarization [8], Compton scattering [5,8], and then normalized to S(Q) [9]. Neutron diffraction with the a-NisoTa5o specimen (10 g) was performed with the powder diffractometer DMC [10] using A = 1.0862 A at the research reactor Saphir of the Paul Scherrer Institute (Villigen, Switzerland). Neutron diffraction with the a-Col0Ni40Tas0 specimen (4.23 g) was done with the two-axes diffractometer 7C2 using A = 0.699 ,~ at the hot source of the research reactor Orph6e (Saclay, France). The experimentally obtained intensity curves were corrected for absorption [11], multiple scattering [12],
H. Uhhg et aL / SRO m a-MsoTa5o
167
inelastic scattering [13], and then normalized to
S(Q) [9]. To eliminate the influence of rounding errors in the calculation of the partial structure factors, it is recommended that one apply the method of iterative refinement [14].
_=
5
r.f3
1
0 4. R e s u l t s
2
Figure 1 shows the partial structure factors as obtained using the method of iterative refinement. In all three curves, the main peak is followed by a double peak (second maximum) as well as a third and fourth peak. At about 1.6 A* - 1, we observe in the Ni-Ni curve a prepeak indicating compound formation. In fig. 2, we present the partial Bhatia-Thornton structure factors for a-NisoTas0. They were calculated from the experimental saT(Q) via eq. (3) using the method of iterative refinement. The SNN(Q) curve represents the topological arrangement of the atoms. The Scc curve is determined by the distribution of the atoms of different types within the specimen. Its pronounced oscillations are a hint to strong chemical ordering effects.
4 5 8 Q {k ~ }
11]
Fig. 2. Partial Bhatia-Thornton structure factors of amorphous NisoTas0
tial structure factors in fig. 1. All three Faber-Ziman partial pair correlation functions show oscillations up to 18 ,~. This indicates a strong shortrange order. Table 1 shows the atomic distances and partial coordination numbers. The experimental Ni-Ta distance of 2.44 ~, is smaller than the sum of the atomic radii of Ni and Ta, which means strong Ni-Ta bonding. The experimental Ta-Ta distance of 2.91 A corres2onds well to the metallic diameter DTa = 2.98 A. The experiment shows, further, that the Ni-Ni distance ois larger than the metallic diameter DN, = 2.82 A, i.e., the Ni atoms do not occur in close contact. The short Ni-Ta distance, ac-
5. D i s c u s s i o n
5.1. Faber-Ziman pair correlation functions
5~,~
~
^
N,-N
Figure 3 shows the Faber-Ziman partial pair correlation functions which follow from the par-
2-
s
1 C2~
re-3
Ta-Ta
1 NI -Ta
2
4
5
Q
[ ~-1 ]
8
i0
Fig 1. Partial Faber-Ziman structure factors of amorphous NlsoTa 50.
0
0
5
10
15
20
R {A} Fig. 3. Partial Faber-Zlman
pair correlation fl]nctlons of
amorphous Ni50Taso
H Uhhg et al / SRO m a-NtsoTaso
168
Table 1 Amorphous N150Tas0 atomic d~stances, R,j, partial coordination numbers, Z,:, and atomic diameters, Oatom [15] Pair t - j
R,j (A)
Zzj
Datom (.~)
NI-Nl Ta-Ta N1-Ta Ta-Nl
2.82 + 0.05 2 91 :t: 0.05 2.44 :lz0 05 2.44 + 0 05
4.9 + 0.5 8 2 _+0.5 6 0 + 0.5 6.0 + 0 5
2.48 2 98 2.73 2 73
tal total structure factors, the rather ill-conditioned systems of linear equations in the present paper or in ref. [18], respectively, confront us with very contradictory results.
6. Conclusions
companied by a distinct short-range order can be explained by a d - d electron interaction of the two components. Such features were predicted by Wang's [16] computer simulations for inter-transition-metal glasses.
The shortened Ni-Ta distance of 2.44 ,~ is the result of a strong chemical interaction. A direct Ni-Ni contact does not occur. We found strong discrepancies with earlier experiments on Ni55Ta4~. Neutron diffraction experiments using the method of isotopic substitution are needed for final clarification.
5.2. Bhatia-Thornton partial pair correlation functtons
Thanks are due to R. Bellissent, LLB Scalay, for beamtime at the 7C2 instrument.
In ref. [17] we presented the Bhatia-Thornton partial functions GNN(R), Gcc(R), and GNc(R). The GNN(R) curve represents the topological order. The high amplitude of the oscillations of the Gcc(R) curve and the change of sign at R = 2.58 A point to a distinct chemical ordering effect. The GNc(R) function is determined by the difference between the atomic volumes and shows the usual behaviour.
5.3. Comparison with amorphous N155Ta 45 In ref. [18] three partial functions were evaluated from one neutron and one X-ray diffraction experiment with meltspun a-Ni55Ta45 as well as one neutron diffraction experiment with meltspun a-Ni62Ta38. According to ref. [18], for a-Ni55Ta45, the nearest-neighbour distances are Ni-Ni = 2.52 ,~ (2.82 ~, in this paper), Ta-Ta = 3.06 A (2.91 A) and Ni-Ta = 2.78 A (2.44 A). The discrepancy between ref. [18] and the present paper cannot be explained by the fact that in the present paper splat-cooled samples were used, whereas in ref. [18] melt-spun samples were investigated. Also the different alloy compositions, namely Ni55Ta45 in ref. [18] compared with NisoTas0, cannot explain the discrepancies. At this moment we can only state that, despite the very good experimen-
References [1] T Richmond, L. Rohr, P Retmann, G L e e m a n and H-J. Gtintherodt, m' RQ7, Proc 7th Int. Conf. on Rapidly Quenched Metals (Elsevier, Amsterdam, 1991) p 63 [2] L. Rohr, P Relmann, T. Richmond and H.-J Gtintherodt, in RQ7, Proc 7th Int. Conf. on Rapidly Quenched Metals (Elsevier, Amsterdam, 1991) p 715 [3] T.E Faber and J M Ziman, Phdos Mag 11 (1965) 153. [4] A. Bhatia and D E. Thornton, Phys. Rev. B2 (1970) 3004 [5] J H Hubbell et al., J Phys Chem. Ref. Data 4 (1975) 471 [6] Y. Waseda, Novel Apphcation of Anomalous (Resonance) X-ray Scattering for Structural Characterization of Disordered Materials (Springer, Berhn, 1984) [7] L. Koester, H Rauch and E. Seymann, At. Data Nucl. Data Tables 49 (1991) 65 [8] C N J. Wagner, J. Non-Cryst. Sohds 31 (1978) 1. [9] J Krogh-Moe, Acta Crystallogr 9 (1956) 951 [10] J Schefer, P. Fischer, H Heer, A. Isacson, M. Koch and R Thut, Nucl. Instrum. and Methods A288 (1990) 477. [11] H.H Paalman and L.J. Pings, J Appl Phys 33 (1962) 2635 [12] V F Sears, Adv Phys. 24 (1975) 1 [13] G. Placzek, Phys. Rev. 31 (1952) 377. [14] .~. Bjbrck and G Dahlquist, Numerlsche Methoden (Oldenbourg, Munich, 1972). [15] Catalogue Number 9-18806 (Sargent-Welsh Scientific Comp, 1968) p 2 [16] R. Wang, Nature 278 (1979) 700 [17] H Uhlig, diploma thesis, Umversity of Stuttgart (1992). [18] H E. Fenglai, P P. Gardner, N. Cowlam and H.A. Davies, J. Phys. F17 (1987) 545.