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EXAFS characterization of partial pair correlation functions in Au1−xNix solid solutions
58
PhysicaB 158 (1989)58-59 North-Holland. Atnsterdarn
EXAFS CHARACTERIZATION OF PARTIAL PAIR CORRELATION FUNCTIONS IN Au,_xNi x SOLID SOLUTIONS.
G. Renaud, N. Motta*, M. Belakhovsky DRF/CEN-G, BP 85X - 38041 Grenoble Cedex, FRANCE. *Dipartimento di Fisica, II Universita di Roma, 00173 ROMA, ITALY
Many enced the
properties
by the 14 % size
been
of the Au,_,Ni,
local
distortions
difference
performed
over
between
the
solid
solutions
of the fee
whole
Au and
lattice, Ni atoms.
composition
range
are strongly which
An EXAFS to
influ-
originate study
measure
in has
these
distortions.
Low (Au-h
temperature
EXAFS
spectra
were
measured
above
both edges
and Ni-K) using fluorescence and transmission detection. The pre-
cise
way the electronic functions required for EXAFS fitting were obtained has been described in details elsewhere (1). The partial phaseshifts
Ni,
(SC ) were
absorber
refined and
first calculated for the four kinds of atoms (Au &
and
scatterer)
independently by
within
fitting the
the
EXCURV86
program, and next
first four shells of the pure Ni
Au spectra; the backscattering factors were then recalculated using
a spherical wave correction (2), taking into account the difference betp- and d- wave scattering. For the Au scatterer, the effect of the
ween wave
curvature can
below 5 A“
not be
neglected until k exceeds 12 A“
; further,
it is necessary to distinguish between the two photoelectron
wave symmetries. The
phase functions
determined, together
with amplitudes empiri-
cally extracted from the pure metal spectra, were used to fit the spectra for concentrated alloys. The Au-Ni distances obtained above both edges
are equal
within 0.1
A, which confirms that these functions are
adequate. However, the amplitudes used for the unlike atoms pairs (extracted from the Au spectrum for the Ni-Au pair, and from the Ni one for the Au-Ni pair) were clearly inadequate since Au and Ni exhibit differences in many electrons amplitude factors S$, in core hole width, and also, experimental resolutions differ. These factors have been carefully analyzed on dilute alloys spectra. When fitting them above 6 A-', an underestimation of the Ni-Au coordination number by 0.67, and an overestimation extract
of the
Au-Ni one
the appropriate
by the same factor were found. This led us to Xi-Au
amplitudes
from
the Ni-edge
spectrum
of
G. Renaudet
al.
/Aul_,Ni, solid solutions
59
the dilute AupS NiZ alloy. These phases and amplitudes were finally used to fit again all the Inspection of
data.
the structural results, with use of several consrelating them to the average fee lattice parameters showed that
traints
the static disorder was too strong for the EXAFS be properly modelled by usual function . It has been necessary to introduce the third cumu-
the lant
of the
partial pair correlation functions (3).The partial mean NN
distances Rab and second, <,, correlation functions
pair
(top) are
tances
dependence
distinct by as much as 0.1 A, and exhibit a different
on composition. Clearly, the
atoms
do
size"
in the
NN
and third,
not
retain
their
"original
2.65
solid solution. The three
pair correlation functions are broad
(middle), and display a marked asymmetry (bottom).
Both width
and asymmetry in-
crease
with
x (l-x).
tions,
they are more and more marked in
At
2.65
all composi-
the order Au-Au, Au-Ni, and Ni-Ni. Finally,
one sees that the so-cal-
"elastic core effect" is quite large : it amounts to -0.053A f 0.02A in
led
the low Ni-content alloys, and + 0.074 ? 0.02A
in the low Au-content ones. It is
well
reproduced by
the theory proposed
Froyen et al (5) which yields by - 0.05lk and 0.072 A respectively. In conclusion, we have shown that careful1 EXAFS analysis can provide very detailed gical,
information, about the
mostly topolonearest neighbor par-
tial pair correlation functions in solid solution with size mismatch.
(1) G. Renaud, N. Motta, F. Lancon and M. Belakhovsky, Phys. Rev. B in Press. (2)
J.J. Rehr, R.C. Albers, C.R. Natoli and
-. -
E.A. Sterns, Phys. Rev. B&, 4350 (1986) (3) B.A. Bunker, Nucl. Inst. and Meth. -I 207 437 (1983) (5) 2,
S.Froyen and C. Herring, J. Appl. Phys. 7165 (1981)
0
20
40 z
(at.
60
80 X)
100
PhysicaB 158 (1989)58-59 North-Holland. Atnsterdarn
EXAFS CHARACTERIZATION OF PARTIAL PAIR CORRELATION FUNCTIONS IN Au,_xNi x SOLID SOLUTIONS.
G. Renaud, N. Motta*, M. Belakhovsky DRF/CEN-G, BP 85X - 38041 Grenoble Cedex, FRANCE. *Dipartimento di Fisica, II Universita di Roma, 00173 ROMA, ITALY
Many enced the
properties
by the 14 % size
been
of the Au,_,Ni,
local
distortions
difference
performed
over
between
the
solid
solutions
of the fee
whole
Au and
lattice, Ni atoms.
composition
range
are strongly which
An EXAFS to
influ-
originate study
measure
in has
these
distortions.
Low (Au-h
temperature
EXAFS
spectra
were
measured
above
both edges
and Ni-K) using fluorescence and transmission detection. The pre-
cise
way the electronic functions required for EXAFS fitting were obtained has been described in details elsewhere (1). The partial phaseshifts
Ni,
(SC ) were
absorber
refined and
first calculated for the four kinds of atoms (Au &
and
scatterer)
independently by
within
fitting the
the
EXCURV86
program, and next
first four shells of the pure Ni
Au spectra; the backscattering factors were then recalculated using
a spherical wave correction (2), taking into account the difference betp- and d- wave scattering. For the Au scatterer, the effect of the
ween wave
curvature can
below 5 A“
not be
neglected until k exceeds 12 A“
; further,
it is necessary to distinguish between the two photoelectron
wave symmetries. The
phase functions
determined, together
with amplitudes empiri-
cally extracted from the pure metal spectra, were used to fit the spectra for concentrated alloys. The Au-Ni distances obtained above both edges
are equal
within 0.1
A, which confirms that these functions are
adequate. However, the amplitudes used for the unlike atoms pairs (extracted from the Au spectrum for the Ni-Au pair, and from the Ni one for the Au-Ni pair) were clearly inadequate since Au and Ni exhibit differences in many electrons amplitude factors S$, in core hole width, and also, experimental resolutions differ. These factors have been carefully analyzed on dilute alloys spectra. When fitting them above 6 A-', an underestimation of the Ni-Au coordination number by 0.67, and an overestimation extract
of the
Au-Ni one
the appropriate
by the same factor were found. This led us to Xi-Au
amplitudes
from
the Ni-edge
spectrum
of
G. Renaudet
al.
/Aul_,Ni, solid solutions
59
the dilute AupS NiZ alloy. These phases and amplitudes were finally used to fit again all the Inspection of
data.
the structural results, with use of several consrelating them to the average fee lattice parameters showed that
traints
the static disorder was too strong for the EXAFS be properly modelled by usual function . It has been necessary to introduce the third cumu-
the lant
of the
partial pair correlation functions (3).The partial mean NN
distances Rab and second, <,, correlation functions
pair
(top) are
tances
dependence
distinct by as much as 0.1 A, and exhibit a different
on composition. Clearly, the
atoms
do
size"
in the
NN
and third,
not
retain
their
"original
2.65
solid solution. The three
pair correlation functions are broad
(middle), and display a marked asymmetry (bottom).
Both width
and asymmetry in-
crease
with
x (l-x).
tions,
they are more and more marked in
At
2.65
all composi-
the order Au-Au, Au-Ni, and Ni-Ni. Finally,
one sees that the so-cal-
"elastic core effect" is quite large : it amounts to -0.053A f 0.02A in
led
the low Ni-content alloys, and + 0.074 ? 0.02A
in the low Au-content ones. It is
well
reproduced by
the theory proposed
Froyen et al (5) which yields by - 0.05lk and 0.072 A respectively. In conclusion, we have shown that careful1 EXAFS analysis can provide very detailed gical,
information, about the
mostly topolonearest neighbor par-
tial pair correlation functions in solid solution with size mismatch.
(1) G. Renaud, N. Motta, F. Lancon and M. Belakhovsky, Phys. Rev. B in Press. (2)
J.J. Rehr, R.C. Albers, C.R. Natoli and
-. -
E.A. Sterns, Phys. Rev. B&, 4350 (1986) (3) B.A. Bunker, Nucl. Inst. and Meth. -I 207 437 (1983) (5) 2,
S.Froyen and C. Herring, J. Appl. Phys. 7165 (1981)
0
20
40 z
(at.
60
80 X)
100