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Comparison of amorphous and liquid alloys by photoelectron spectroscopy
Materials Science and Engineering, 99 (1988) 257-260
257
Comparison of Amorphous and Liquid Alloys by Photoelectron Spectroscopy* G. INDLEKOFER, A. PFLUGI, P. OELHAFEN and H.-J. GI~NTHERODT Institut fiir Physik, Universitiit Basel, CH-4056 Basel (Switzerland) P. H~USSLER, H.-G. BOYEN and F. BAUMANN Physikalisches Institut der Universitiit Karlsruhe, D- 7500 Karlsruhe 1 (F.R.G.)
Abstract
Photoemission data of liquid A u - S n alloys are presented and compared with corresponding measurements on amorphous vapour-quenched films. A fairly close general similarity between the valence bands of the two phases is observed. This result supports earlier studies o f the atomic structure o f the amorphous films which have been interpreted in terms of a liquid-like structure with a strong tendency to compound formation within the nearest-neighbour arrangements. Distinct deviations of spectra measured on the liquid samples compared with the amorphous phase have been observed; the Au 5d band is shifted to lower binding energies and no density-of-states minimum is observed at Ev. The Au d band exhibits a continuous temperature-dependent shift in the liquid state which has been observed for the first time and which can be extrapolated to 193 ' C in the amorphous phase. The origin o f this temperature effect is discussed in terms o f changes in the electron density and structural alterations in the short-range order at the gold site.
study, we compare the UV photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) data obtained from liquid alloys with those of the corresponding amorphous phases prepared by in situ vapour quenching. The valence bands of tin alloys such as Cu-Sn, Ag-Sn and Au-Sn have previously been studied by photoelectron spectroscopy in the amorphous state [3]. Evidence for a structure-induced minimum in the density of states at the Fermi energy E~ has been found at noble-metal-rich concentrations which exhibit the highest stability against crystallization. The low evaporation rates of these systems offer the possibility of performing photoemission studies also in the liquid state and therefore a direct comparison of the electronic structures of the two phases is feasible. The same holds for A ~ S i near the eutectic composition for which the glassy state of metals formed by rapid quenching from the liquid was firstly reported by Duwez and coworkers [4]. 2. Results and discussion
I. Introduction
The atomic structure of amorphous alloys has been a topic for a variety of experimental investigations. The amorphous state is often considered to have a structure similar to that of a frozen liquid. Hence it is of fundamental interest to compare other properties such as the electronic structure of liquid and amorphous alloys which itself is sensitive to the atomic structure. Whilst the electronic structure of amorphous metals has been the subject of many photoemission studies, the corresponding liquid phases have been investigated very little, mainly because of experimental difficulties in preparing clean liquid surfaces. A new effective mechanical cleaning procedure [1, 2] was therefore applied for this work. In our
*Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montr6al, August 3-7, 1987. 0025-5416/88/$3.50
The UPS valence band spectra of liquid AusoSns0 recorded with different photon energies are shown in Fig. 1. The spectra are dominated by the two smooth peaks near 4.5 and 6.4 eV binding energy originating from Au 5d band states. With increasing photon energy the ratio of the 5d3/2 to 5d5/2 emission intensity increases in a similar way to the case of liquid AuslSil9 [2]. At the same time the relative intensity at the Fermi level decreases. These effects reflect the sensitivity of the partial photoionization cross-sections on the photon energy. When the photon energy is raised from 16.8 to 40.8 eV, the ratio of the Sn 5s to Sn 5p excitation is dramatically increased as observed for example for liquid tin [5] and as indicated by cross-section calculations for atomic core states [6]. According to these calculations, the total Au 5d emission intensity varies comparatively little in the applied range of photon energies. Therefore the obtained spectra can be interpreted as a first approximation in terms of © Elsevier Sequoia/Printed in The Netherlands
258 I
t"-I
..~ ~ I ' 3Au75Sn25 >z iii I--z
Z
LU I--
AusoSn5o
z
Au25Sn75- Au30SnTo...... AuloSngo
10 8 6 4 2 EF=0 BINDING ENERGY (eV) Fig. 1. Valence band spectra of liquid AusoSnso measured at 440 "C with UPS Ne I (hv = 16.8 eV), He I (hv = 21.2 eV) and He II (hv = 40.8 eV) excitation normalized to the same maximum intensity. The He I and He 1I spectra are corrected for UV satellites. weighted sums of the partial (Au 5d, Au 6s, Sn 5s and Sn 5p) density of occupied states dominated by the two Au 5d peaks and the Sn 5p band at lower binding energies. A comparison o f valence band spectra o f liquid and vapour-quenched amorphous A u - S n is depicted in Fig. 2. A close similarity can be observed between the spectra o f the liquid and the amorphous phase regarding the general shape of the valence bands including 5d band features and the plateaux near EF. The intensities in these two regions which are directly related to the alloy compositions via the A u 5d and Sn 5p electron states reveal identical concentrations in the corresponding liquid and a m o r p h o u s alloys. In addition, the XPS core level analysis shows that the concentrations are close to the nominal concentrations apart from a minor gold enrichment in both phases (Fig. 4), in contrast with a minor tin enrichment found by XPS results published earlier [7]. A decrease in the gold content from 75% to 10% causes d band shifts to higher binding energies which amount for example to 0.8 eV for the 5d5/2 peak in the liquid state. A closer look at the spectra reveals the following distinct differences between the liquid phases and the corresponding amorphous solids.
10
8 6 4 2 EF=0 BINDING ENERGY (eV) Fig. 2. Comparison of UPS He I (hv = 21.2 eV) valence band spectra of liquid (--) and vapour-quenched amorphous (.... ) A~Sn alloys (sample temperatures: vapour-quenched Au-Sn, -193 °C; liquid Au75Sn25, 480 °C; liquid AusoSnso, 440 °C; liquid Au25Sn75, 350 °C; liquid AuloSngo, 300 °C). The area of the Au 5d band emission was chosen to be proportional to the gold content. All spectra were recorded under identical spectrometer conditions and are corrected for the weak He 1 satellite lines. (i) The A u 5d bands exhibit generally lower binding energies. (ii) There is no indication of a minimum in the valence band spectra near EF. (iii) A higher background of inelastically scattered electrons is observed at high binding energies. Point (ii) is of particular interest since the depression of the intensity from a binding energy of 1 eV towards EF observed in the amorphous solids has been attributed to a structure-induced minimum in the density of states near EF [3] as proposed by Nagel and Tauc [8] for the stabilization of the amorphous phase. Additional information can be obtained from the temperature dependence of the spectra. Au25Sn75, which has a low melting point, was measured in the liquid state from 325 to 550 °C, as shown in Fig. 3(a). On heating, there is a decrease in the binding energy of the A u 5d band peaks. A linear temperature dependence of - 0 . 6 meV K-~ (denoted as the d band temperature coefficient ( D T C ) in the following) is found
259
I
I
t
I
I
I
I
I
ib
I
5.4.
a
7.4 Au30SnTo, amomnous
72
s
,,=,
~5.0
Au3oSnTo
; 4 " amorphous ..' ",-,,.~..,.,," T = - 193°C.j
S
i 's' ......
7.0
i ~
68
~
t
4.6
- 6.6 1
4.4
k
I
-200
I
I
I
0
¥
64
I
'1 '¥
200
400
600
TEMPERATURE (°C) v
>-
i-
,iqud
/
/I
co z LU z I
I
I
~ . -0.6 ~
L
E
C
-0.4~
{
v
{
-0.2 I 8
I I I 6 4 2 BINDING E N E R G Y (eV)
L
a
0.0 Sn
EF=0
, , L I 20 40 60 80 Au - content (at %)
Au
Fig. 3. Temperature dependence of the Au 5d state binding energies in Au-Sn: (a) UPS He I (hv = 21.2 eV) valence band spectra of tin-rich liquid and glassy alloys; (b) 5d peak positions as a function of the sample temperature; (c) d band temperature coefficients as a function of the gold content for disordered Au-Sn alloys and pure polycrystalline gold. which turns out to fit even the 5d band positions of the a m o r p h o u s alloy, as demonstrated in Fig. 3(b). It emerges clearly from Fig. 2 that the temperature effect is reduced at higher gold concentrations. Similar d band shifts ( - 0.18 + 0.02 eV) have already been observed for amorphous and liquid Cu85Sn~5 (the values denote surface concentrations [1]) for a temperature difference of 750 K and similarly in A u - S i alloys [9]. In order to obtain more information about the DTCs of the A u - S n alloys, we measured the valence band and core level binding energies of polycrystalline gold at - 193 and 600 C . Distinct temperature effects were observed in both the valence band and the core states which results in temperature coefficients of - 0 . 2 5 + 0.03 meV K - ~ and - 0 . 1 5 + 0.04 meV K respectively. The DTCs are shown in Fig. 3(c). The A u 4f7.,2 and Sn 3d5/2 core level binding energy shifts are summarized in Table 1. The figures clearly show that the binding energy shifts measured at the liquid samples are reduced compared with those of the a m o r p h o u s phase. These differences are closely re-
lated to the DTCs. In fact, very similar shifts are found for the Au 4f7/2 core states and the Au 5d band (with respect to the a m o r p h o u s phase). The great similarity of the corresponding valence band spectra of the liquid and a m o r p h o u s phases clearly indicates the close resemblance of the atomic
10 ,y, J
I
I • 0
0
5
I 0
0
O A
133 i
o~ oO Q_ X
I
u oo
0 i
-5 Sn
i
i
i
20 40 60 80 Au - content (at %)
Au
Fig. 4. The deviation of the surface gold concentration determined by XPS from the bulk concentration in liquid (A) and glassy (O) Au Sn alloys.
260 TABLE l Core level binding energy shifts with respect to polycrystalline pure gold and tin at room temperature (experimental errors, __0.05 eV)
Nominal composition
T (~C)
AEB(Au 41"7/2)
AEB(Sn3d5/2)
(eV)
(eV)
Au75Sn25 Au75Sn25 AusoSnso AusoSnso Au25Sn75 Au25Sn75 Au30SnTo AuloSngo AuioSngo
+480 - 193 +440 - 193 + 550 +350 -193 +590 +300
+0.31 +0.57 +0.44 +0.85 + 0.57 +0.63 +1.11 +0.53 +0.67
+0.20 +0.35 +0.05 +0.23 0.00 -0.02 +0.14 -0.05 -0.07
structures. In fact, electron diffraction patterns and resistivity measurements on vapour-quenched A u - S n and C u - S n films have been interpreted in terms of a "liquid-like structure" [10]. In addition, a close similarity in the nearest-neighbour organization in the amorphous films and the liquid alloys has been found. The obvious differences (similarly observed in C u Sn and A u - S i alloys) between the spectra of the two phases are the decreased d band binding energies and the loss o f the minimum in the valence bands of the liquid phase. The latter effect represents a challenging problem and could stimulate discussion of the early Nagel and Tauc [8] model. The temperature effects in the valence bands and core states observed for the first time in the present form (Fig. 3(b)) seem to be caused by more than one effect. A D T C of - 0 . 2 5 meV K ~ was measured for pure gold; this could be interpreted in terms of a change in the electron density n which is responsible for a change in the Fermi energy. In a free-electron model with 1 dEv Er d T
2 l dn
3 n dT
a temperature coefficient of - 0 . 4 6 m e V K t is obtained (with E ~ = 8 e V and ( 1 / p ) ( d p / d T ) = - 8 . 7 x 10 -5 °C-I). This clearly indicates that the observed thermal effects can at least partly be explained simply by density changes of the electron gas.
The DTC value of pure gold provides a reliable estimate of the magnitude o f this effect that is expected for the alloys. However, the large increase in the DTC values near 20 at.% A u (Fig. 4(c)) is hard to explain on the same basis. A continuous change in the shortrange order (for which a strong tendency to compound formation has been found [10]) has to be considered. In fact, the d band lowering on alloying is raised to b o n d formation between unlike atoms. An increase in temperature might reduce the short-range order at the gold sites, leading to a decrease in the d band binding energy. However, more information is required in order to explain conclusively the present phenomena.
Acknowledgment
Financial support of the Swiss National Science F o u n d a t i o n is gratefully acknowledged.
References
I G. Indlekofer, P. Oelhafen, H.-J. Giintherodt, C. F. Hague and J.-M, Mariot, in D. M. Zehner and D. W. Goodman (eds.), Materials Research Society Syrup. Proc., Vol. 83, Materials Research Society, Pittsburgh, PA. 2 G. lndlekofer, P. Oelhafen, H.-J. G/intherodt, C. F. Hague and J.-M. Mariot, Proc. 6th Conf. on Liquid and Amorphous Metals, Garmisch-Partenkirchen, August 24-29, 1986, in Z. Phys. Chem., 156-157(1987) in press. 3 P. Hfiussler, F. Baumann, J. Krieg, G. lndlekofer, P. Oelhafen and H.-J. Giintherodt, Phys. Rev. Lett., 51 (1983) 714. P. Hfiussler, F. Baumann, U. Gubler, P. Oelhafen and H.-J. Gfintherodt, in S. Steeb and H. Warlimont (eds.), Rapidly Quenched Metals, North-Holland, Amsterdam, 1985, p. 1007. 4 W. Klement, Jr., R. H. Willens and P. Duwez, Nature (London), 187(1960) 869. 5 G. Indlekofer, P. Oelhafen, R. Lapka and H.-J. Giintherodt, Proc. 6th Conf. on Liquid and Amorphous Metals, GarmischPartenkirchen, August 24-29, 1986, in Z. Phys. Chem., 156157 (1987). 6 J. J. Yeh and I. Lindau, At. Data Nucl. Data Tables, 32 (1985) 1. 7 T. Ichikawa, Phys. Status Solidi A, 32 (1975) 369. 8 S. R. Nagel and J. Tauc, Phys. Rev. Lett., 35(1975) 380. 9 G. Indlekofer et al., to be published. 10 H. Leitz, Z. Phys. B, 40 (1980) 65, and references cited therein.
257
Comparison of Amorphous and Liquid Alloys by Photoelectron Spectroscopy* G. INDLEKOFER, A. PFLUGI, P. OELHAFEN and H.-J. GI~NTHERODT Institut fiir Physik, Universitiit Basel, CH-4056 Basel (Switzerland) P. H~USSLER, H.-G. BOYEN and F. BAUMANN Physikalisches Institut der Universitiit Karlsruhe, D- 7500 Karlsruhe 1 (F.R.G.)
Abstract
Photoemission data of liquid A u - S n alloys are presented and compared with corresponding measurements on amorphous vapour-quenched films. A fairly close general similarity between the valence bands of the two phases is observed. This result supports earlier studies o f the atomic structure o f the amorphous films which have been interpreted in terms of a liquid-like structure with a strong tendency to compound formation within the nearest-neighbour arrangements. Distinct deviations of spectra measured on the liquid samples compared with the amorphous phase have been observed; the Au 5d band is shifted to lower binding energies and no density-of-states minimum is observed at Ev. The Au d band exhibits a continuous temperature-dependent shift in the liquid state which has been observed for the first time and which can be extrapolated to 193 ' C in the amorphous phase. The origin o f this temperature effect is discussed in terms o f changes in the electron density and structural alterations in the short-range order at the gold site.
study, we compare the UV photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) data obtained from liquid alloys with those of the corresponding amorphous phases prepared by in situ vapour quenching. The valence bands of tin alloys such as Cu-Sn, Ag-Sn and Au-Sn have previously been studied by photoelectron spectroscopy in the amorphous state [3]. Evidence for a structure-induced minimum in the density of states at the Fermi energy E~ has been found at noble-metal-rich concentrations which exhibit the highest stability against crystallization. The low evaporation rates of these systems offer the possibility of performing photoemission studies also in the liquid state and therefore a direct comparison of the electronic structures of the two phases is feasible. The same holds for A ~ S i near the eutectic composition for which the glassy state of metals formed by rapid quenching from the liquid was firstly reported by Duwez and coworkers [4]. 2. Results and discussion
I. Introduction
The atomic structure of amorphous alloys has been a topic for a variety of experimental investigations. The amorphous state is often considered to have a structure similar to that of a frozen liquid. Hence it is of fundamental interest to compare other properties such as the electronic structure of liquid and amorphous alloys which itself is sensitive to the atomic structure. Whilst the electronic structure of amorphous metals has been the subject of many photoemission studies, the corresponding liquid phases have been investigated very little, mainly because of experimental difficulties in preparing clean liquid surfaces. A new effective mechanical cleaning procedure [1, 2] was therefore applied for this work. In our
*Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montr6al, August 3-7, 1987. 0025-5416/88/$3.50
The UPS valence band spectra of liquid AusoSns0 recorded with different photon energies are shown in Fig. 1. The spectra are dominated by the two smooth peaks near 4.5 and 6.4 eV binding energy originating from Au 5d band states. With increasing photon energy the ratio of the 5d3/2 to 5d5/2 emission intensity increases in a similar way to the case of liquid AuslSil9 [2]. At the same time the relative intensity at the Fermi level decreases. These effects reflect the sensitivity of the partial photoionization cross-sections on the photon energy. When the photon energy is raised from 16.8 to 40.8 eV, the ratio of the Sn 5s to Sn 5p excitation is dramatically increased as observed for example for liquid tin [5] and as indicated by cross-section calculations for atomic core states [6]. According to these calculations, the total Au 5d emission intensity varies comparatively little in the applied range of photon energies. Therefore the obtained spectra can be interpreted as a first approximation in terms of © Elsevier Sequoia/Printed in The Netherlands
258 I
t"-I
..~ ~ I ' 3Au75Sn25 >z iii I--z
Z
LU I--
AusoSn5o
z
Au25Sn75- Au30SnTo...... AuloSngo
10 8 6 4 2 EF=0 BINDING ENERGY (eV) Fig. 1. Valence band spectra of liquid AusoSnso measured at 440 "C with UPS Ne I (hv = 16.8 eV), He I (hv = 21.2 eV) and He II (hv = 40.8 eV) excitation normalized to the same maximum intensity. The He I and He 1I spectra are corrected for UV satellites. weighted sums of the partial (Au 5d, Au 6s, Sn 5s and Sn 5p) density of occupied states dominated by the two Au 5d peaks and the Sn 5p band at lower binding energies. A comparison o f valence band spectra o f liquid and vapour-quenched amorphous A u - S n is depicted in Fig. 2. A close similarity can be observed between the spectra o f the liquid and the amorphous phase regarding the general shape of the valence bands including 5d band features and the plateaux near EF. The intensities in these two regions which are directly related to the alloy compositions via the A u 5d and Sn 5p electron states reveal identical concentrations in the corresponding liquid and a m o r p h o u s alloys. In addition, the XPS core level analysis shows that the concentrations are close to the nominal concentrations apart from a minor gold enrichment in both phases (Fig. 4), in contrast with a minor tin enrichment found by XPS results published earlier [7]. A decrease in the gold content from 75% to 10% causes d band shifts to higher binding energies which amount for example to 0.8 eV for the 5d5/2 peak in the liquid state. A closer look at the spectra reveals the following distinct differences between the liquid phases and the corresponding amorphous solids.
10
8 6 4 2 EF=0 BINDING ENERGY (eV) Fig. 2. Comparison of UPS He I (hv = 21.2 eV) valence band spectra of liquid (--) and vapour-quenched amorphous (.... ) A~Sn alloys (sample temperatures: vapour-quenched Au-Sn, -193 °C; liquid Au75Sn25, 480 °C; liquid AusoSnso, 440 °C; liquid Au25Sn75, 350 °C; liquid AuloSngo, 300 °C). The area of the Au 5d band emission was chosen to be proportional to the gold content. All spectra were recorded under identical spectrometer conditions and are corrected for the weak He 1 satellite lines. (i) The A u 5d bands exhibit generally lower binding energies. (ii) There is no indication of a minimum in the valence band spectra near EF. (iii) A higher background of inelastically scattered electrons is observed at high binding energies. Point (ii) is of particular interest since the depression of the intensity from a binding energy of 1 eV towards EF observed in the amorphous solids has been attributed to a structure-induced minimum in the density of states near EF [3] as proposed by Nagel and Tauc [8] for the stabilization of the amorphous phase. Additional information can be obtained from the temperature dependence of the spectra. Au25Sn75, which has a low melting point, was measured in the liquid state from 325 to 550 °C, as shown in Fig. 3(a). On heating, there is a decrease in the binding energy of the A u 5d band peaks. A linear temperature dependence of - 0 . 6 meV K-~ (denoted as the d band temperature coefficient ( D T C ) in the following) is found
259
I
I
t
I
I
I
I
I
ib
I
5.4.
a
7.4 Au30SnTo, amomnous
72
s
,,=,
~5.0
Au3oSnTo
; 4 " amorphous ..' ",-,,.~..,.,," T = - 193°C.j
S
i 's' ......
7.0
i ~
68
~
t
4.6
- 6.6 1
4.4
k
I
-200
I
I
I
0
¥
64
I
'1 '¥
200
400
600
TEMPERATURE (°C) v
>-
i-
,iqud
/
/I
co z LU z I
I
I
~ . -0.6 ~
L
E
C
-0.4~
{
v
{
-0.2 I 8
I I I 6 4 2 BINDING E N E R G Y (eV)
L
a
0.0 Sn
EF=0
, , L I 20 40 60 80 Au - content (at %)
Au
Fig. 3. Temperature dependence of the Au 5d state binding energies in Au-Sn: (a) UPS He I (hv = 21.2 eV) valence band spectra of tin-rich liquid and glassy alloys; (b) 5d peak positions as a function of the sample temperature; (c) d band temperature coefficients as a function of the gold content for disordered Au-Sn alloys and pure polycrystalline gold. which turns out to fit even the 5d band positions of the a m o r p h o u s alloy, as demonstrated in Fig. 3(b). It emerges clearly from Fig. 2 that the temperature effect is reduced at higher gold concentrations. Similar d band shifts ( - 0.18 + 0.02 eV) have already been observed for amorphous and liquid Cu85Sn~5 (the values denote surface concentrations [1]) for a temperature difference of 750 K and similarly in A u - S i alloys [9]. In order to obtain more information about the DTCs of the A u - S n alloys, we measured the valence band and core level binding energies of polycrystalline gold at - 193 and 600 C . Distinct temperature effects were observed in both the valence band and the core states which results in temperature coefficients of - 0 . 2 5 + 0.03 meV K - ~ and - 0 . 1 5 + 0.04 meV K respectively. The DTCs are shown in Fig. 3(c). The A u 4f7.,2 and Sn 3d5/2 core level binding energy shifts are summarized in Table 1. The figures clearly show that the binding energy shifts measured at the liquid samples are reduced compared with those of the a m o r p h o u s phase. These differences are closely re-
lated to the DTCs. In fact, very similar shifts are found for the Au 4f7/2 core states and the Au 5d band (with respect to the a m o r p h o u s phase). The great similarity of the corresponding valence band spectra of the liquid and a m o r p h o u s phases clearly indicates the close resemblance of the atomic
10 ,y, J
I
I • 0
0
5
I 0
0
O A
133 i
o~ oO Q_ X
I
u oo
0 i
-5 Sn
i
i
i
20 40 60 80 Au - content (at %)
Au
Fig. 4. The deviation of the surface gold concentration determined by XPS from the bulk concentration in liquid (A) and glassy (O) Au Sn alloys.
260 TABLE l Core level binding energy shifts with respect to polycrystalline pure gold and tin at room temperature (experimental errors, __0.05 eV)
Nominal composition
T (~C)
AEB(Au 41"7/2)
AEB(Sn3d5/2)
(eV)
(eV)
Au75Sn25 Au75Sn25 AusoSnso AusoSnso Au25Sn75 Au25Sn75 Au30SnTo AuloSngo AuioSngo
+480 - 193 +440 - 193 + 550 +350 -193 +590 +300
+0.31 +0.57 +0.44 +0.85 + 0.57 +0.63 +1.11 +0.53 +0.67
+0.20 +0.35 +0.05 +0.23 0.00 -0.02 +0.14 -0.05 -0.07
structures. In fact, electron diffraction patterns and resistivity measurements on vapour-quenched A u - S n and C u - S n films have been interpreted in terms of a "liquid-like structure" [10]. In addition, a close similarity in the nearest-neighbour organization in the amorphous films and the liquid alloys has been found. The obvious differences (similarly observed in C u Sn and A u - S i alloys) between the spectra of the two phases are the decreased d band binding energies and the loss o f the minimum in the valence bands of the liquid phase. The latter effect represents a challenging problem and could stimulate discussion of the early Nagel and Tauc [8] model. The temperature effects in the valence bands and core states observed for the first time in the present form (Fig. 3(b)) seem to be caused by more than one effect. A D T C of - 0 . 2 5 meV K ~ was measured for pure gold; this could be interpreted in terms of a change in the electron density n which is responsible for a change in the Fermi energy. In a free-electron model with 1 dEv Er d T
2 l dn
3 n dT
a temperature coefficient of - 0 . 4 6 m e V K t is obtained (with E ~ = 8 e V and ( 1 / p ) ( d p / d T ) = - 8 . 7 x 10 -5 °C-I). This clearly indicates that the observed thermal effects can at least partly be explained simply by density changes of the electron gas.
The DTC value of pure gold provides a reliable estimate of the magnitude o f this effect that is expected for the alloys. However, the large increase in the DTC values near 20 at.% A u (Fig. 4(c)) is hard to explain on the same basis. A continuous change in the shortrange order (for which a strong tendency to compound formation has been found [10]) has to be considered. In fact, the d band lowering on alloying is raised to b o n d formation between unlike atoms. An increase in temperature might reduce the short-range order at the gold sites, leading to a decrease in the d band binding energy. However, more information is required in order to explain conclusively the present phenomena.
Acknowledgment
Financial support of the Swiss National Science F o u n d a t i o n is gratefully acknowledged.
References
I G. Indlekofer, P. Oelhafen, H.-J. Giintherodt, C. F. Hague and J.-M, Mariot, in D. M. Zehner and D. W. Goodman (eds.), Materials Research Society Syrup. Proc., Vol. 83, Materials Research Society, Pittsburgh, PA. 2 G. lndlekofer, P. Oelhafen, H.-J. G/intherodt, C. F. Hague and J.-M. Mariot, Proc. 6th Conf. on Liquid and Amorphous Metals, Garmisch-Partenkirchen, August 24-29, 1986, in Z. Phys. Chem., 156-157(1987) in press. 3 P. Hfiussler, F. Baumann, J. Krieg, G. lndlekofer, P. Oelhafen and H.-J. Giintherodt, Phys. Rev. Lett., 51 (1983) 714. P. Hfiussler, F. Baumann, U. Gubler, P. Oelhafen and H.-J. Gfintherodt, in S. Steeb and H. Warlimont (eds.), Rapidly Quenched Metals, North-Holland, Amsterdam, 1985, p. 1007. 4 W. Klement, Jr., R. H. Willens and P. Duwez, Nature (London), 187(1960) 869. 5 G. Indlekofer, P. Oelhafen, R. Lapka and H.-J. Giintherodt, Proc. 6th Conf. on Liquid and Amorphous Metals, GarmischPartenkirchen, August 24-29, 1986, in Z. Phys. Chem., 156157 (1987). 6 J. J. Yeh and I. Lindau, At. Data Nucl. Data Tables, 32 (1985) 1. 7 T. Ichikawa, Phys. Status Solidi A, 32 (1975) 369. 8 S. R. Nagel and J. Tauc, Phys. Rev. Lett., 35(1975) 380. 9 G. Indlekofer et al., to be published. 10 H. Leitz, Z. Phys. B, 40 (1980) 65, and references cited therein.