PHYSICAL REVIEW B
VOLUME 30, NUMBER 7
X-ray-absorption
OCTOBER 1984
near-edge-structure study of EuPd3B„:
A mixed-valence system
B. Darshan
Department
1
of Physics,
and
Indian Institute
B. D. Padalia
of Technology, Bombay 400076, India
R. Nagarajan, S. K. Dhar, S. K. Malik, and R. Vijayaraghavan
Tata Institute
of Fundamental Research,
Homi Bhabha Road,
Bombay 400005, India
(Received 26 August 1983; revised manuscript received 20 January 1984)
L3 absorption spectra of Eu in boron-containing alloys, EuPd3B„(x =0, 0.25, 0.5, and 1.0) have been
recorded. Direct evidence for the mixed-valence (Eu + and Eu +) behavior of Eu in EuPd3Bp5 and
EuPd3B has been provided by x-ray-absorption near-edge structure (XANES). The value of the average
valence of Eu in EuPd3B and EuPd3Bp5 alloys is estimated and it is found to be temperature independent.
The XANES results in conjunction with the Mossbauer measurements lead to the conclusion that EuPd3B
is an inhomogeneous mixed-valence system.
INTRODUCTION
Recently
it has been reported'
EXPERIMENT
that alloying
boron with
formation
R Pd3B„
expansion
of the cell volume in almost all cases and the increase is
much larger for EuPd3B„alloys (and also for CePd3B„)
compared with other members of the series. The magnetic
susceptibility of EuPd3 alloys has been found to increase
with increase in boron content, changing from almost temperature independent
(for x =0) to strongly temperature
dependent (for x = 1). These features have been interpreted in terms of a change in the valence state of the Eu ion
from Eu'+ to mixed valence (Eu'+ and Eu'+) in EuPd3B„
alloys.
Further, the Mossbauer spectrum of ' 'Eu in
EuPd3B at 300 K exhibits a single line which lies in between
the values expected for Eu + and Eu + systems. At low
112 K), splitting of the Mossbauer spectemperatures (
trum
into two lines has been observed
and this
phenomenon
(not observed earlier in any metallic system)
has been attributed to reduction in the valence fluctuation
It may be noted that the valence fluctuation
frequency.
10 '3 sec) is faster than the Mossbauer probing
time (
time (
10 9 sec), so that in this respect Mossbauer is a
slow technique and provides an average effect in the case of
systems exhibiting valence fluctuation phenomenon.
X-ray-absorption
near-edge structure (XANES) studies
offer a simple and powerful method for the investigation of
the phenomenon of valence fluctuation, mixed valence, or
fluctuation.
It is a fast (probing time
interconfiguration
10 '6 sec) technique and provides signatures of the two
different ionic configurations in a mixed-valence system.
Unlike x-ray photoelectron spectroscopy (XPS), the results
obtained from XANES are free from surface effects. The
XANES method has been effectvely employed in our laboratory to understand the valence fluctuation phenomenon
in some mixed-valence
systems. ~6 We present here, the
results of the L3 absorption near-edge structure studies of
Eu in EuPd3B„(x = 0, 0.25, 0.5, and 1.0).
8 P13 (8 = rare earth) compounds results in the
of a new series of compounds of formula,
(0~x « I). The addition of boron leads to an
—
—
—
—
30
The EuPd3B„(x=0, 0.25, 0.50, and 1.0) alloys were
prepared by arc melting the constituent elements in argon
atmosphere. An x-ray diffraction method was used to check
the formation of single phase materials. The L3 absorption
spectra were recorded using a focusing x-ray spectrograph of
Cauchois type. A molybdenum target x-ray tube was used
as a laboratory source of continuous radiation.
For lowtemperature studies, a cold-finger-type liquid nitrogen cryostat was employed.
Other details regarding experimental
setup are described elsewhere. "
RESULTS AND DISCUSSIONS
The L3 absorption near-edge spectra of Eu in EuPd3B„
0.5, and 1.0) are shown in Fig. 1. The single absorption peak corresponding to the trivalent state of Eu 'in
EuPd3 is evident. This peak arises due to an electron transition from 2p3j2 core level to the first unoccupied level in the
Sd6s conduction band, above the Fermi level, Ef. A single
absorption peak (not shown in the figure) has also been observed for EuPd3Bp 25 at the same energy position as that of
Eu'+ in EuPd3. This implies that Eu in EuPd3Bp25 exists in
the trivalent state. The present result is consistent with the
susceptibility' and Mossbauer measurements
on EuPd3Bp25.
The L3 absorption curves for EuPd3Bp 5 and EuPd3B each
show two distinct peaks separated by
7 eV. One of these
peaks is at the same energy position as that of trivalent Eu
in EuPd3. The second peak appears on the lower energy
side of the Eu + peak and is assigned to Eu +. It is interesting to note that the measured value of the energy
7 ev) between the two absorption peaks asseparation (
signed to Eu + and Eu + agrees with the theoretical results
of Herbst and %ilkins and also with the experimental reports' on other Eu-based mixed-valence systems. The
present study suggests that the Eu ions have undergone
valence change from Eu + in pure EuPd3 to mixed-valence
state (Eu3+ and Eu2+) in EuPd3B„(x = 0.5 and 1).
(x=0,
—
—
4031
1984
The American Physical Society
~RyPF RE~
4032
30
1
I
I
I
pK
QQ-
D
CQ
5K
I
I
I
2+
7eV
I
698'
g960
iE
PN&RG~&
«cp
r ion profi}es for
3 0.5 and EuPd3B are
that (i) the Hne s
onvoluted
i espo11ds to that of Eu
rea'
tr'
'
ilation o f
696
"'
FK" l ' L absorptlp' n spectra oof Fu in Eu 3
(„=o, o5.
)) at 30 ' . . e lntensisities have been norm alized wit'th respect to
fhe dptte d curves rep resent the
the Peak Pppsltion 0f the curves
3+ pmppnents.
E 2+ an
and
deconvoluted Eu
I
698o
FKJ. 2.
3
95 K The
p eak p pslflpn
Although
absprpt'pn
have been noprmalized
intens
O
f th curves
oron co ncentr ation. Th
o
f the averagee va enc
1
d
in conjunction
in
is at
ments
ii1
'
to
u '+) i
pec
(—
g c va
p
'
EuPd3 B05 is also temper
t}1
ere
ty ratio o
EUPd3
tha
f
f Euu
'
inst
es
B is temperatuture inin EuPd3B
th
bl
wit
die curve) an
arent from the
tion of temp
ab o p ion
emperature
'
on t e
and
ve
recor e
the (Eu
wl
esp
~
the XANES study establishes s the fact that Eu
'O'"" '" F
2+
Eu+/Eu
7OQO
to,
tarts splitting in o
perature
mer shift of
ec
tgur
BRIEF REPORTS
30
I
EuPd&B'
~
~ ~~
0
~O
—
Op
~4
49—
4 ~
~ ~o
OO
5.5
~
~
~ ~
~
~
5+
~
+
~ +A pg
~
~
~ ~+
~
~
~
w.
~
~ o+
0 ~0
e ~Iy
~ ~~ ~
~
~
5-4
5K
II
t
~
..
~
g
~
~
~
~%
~
'r~
&g
~
.
'«&
~
~ ~~ ~
~
~
..
~ e
~
~ ~
~
~
~1
~ ~~ ~ ~
~
~
~
~
'~
~
~ ~
~
~ ~
~
~
~v
~
~4
~
'
~~
53
~
5
~~
1
~ ~ ~ ~ ~~
~
~ ~
rQ+'
50
~
C
~0
~.rg
~o ~ ~
+~ ~ ~a
~
~
~
~~
~~
0
r»4
~
~
Oy
4
~
~0
88K
0 ~
~~ ~
~O
w
~we
se
~
go
~0
o+
~
~ 0
~
~ ~eo
~0
J
t
—IO0
—50
V E LOCITY
FIG. 3.
88 K.
I
Eu Mossbauer
spectra
0
50
—
—
0
4 ~~~
~
~
O
~
Similar behavior has been noted for Eu3S4 by others. s
The observed splitting of Mossbauer spectrum of "'Eu in
112 K requires some discussion. If the obEuPd3B at
served splitting is due to reduction in the valence fluctuation frequency so that it becomes comparable with the
10 9 sec), the x-ray absorption
Mossbauer probing time (
10 ' sec) should not exhibit change
spectra (probe time
in the spectral features around 112 K. It is noted that the
Ls absorption spectra recorded at 95 and 125 K (Fig. 2)
indeed show no change in the 1.3 absorption near-edge
structure, as is expected.
Further Mossbauer work on EuPd3B at 4.2 K, exhibits
—6.7 mm sec ' and 3.5 mm sec ' corretwo lines at
sponding to Eu + and Eu'+ ions. Below 4.2 K, EuPd3B orders magnetically, and at 1.4 K the Mossbauer absorption
corresponding to Eu2+ ions shows magnetic hyperfine splitting. It is likely that the crystal structure of EuPd3B undergoes a lattice distortion at low temperatures, stabilizing europium ions in two different valence states (mixed integral
valence or inhomogeneous mixed valence).
It may be noted that the average valence of Eu in EuPd3B
as deduced from the relative areas under the two peaks in
the Mossbaue~ spectrum at 88 K is 2.4+0.1 which is in excellent agreement with the present XAS results.
It may be stated that the XANES results, in conjunction
lead to the conclusion
with the Mossbauer measurements,
that EuPd3B is an inhomogeneous mixed-valence system.
—
~
BOOK
50—
~
4033
10
0
(mm/sec)
of EuPd3B at 300, 115, and
'To whom all correspondence should be addressed.
S. K. Dhar, S. K. Malik, and R. Vijayaraghavan, Mater. Res. Bull.
16, 1557 (1981).
2S. K. Dhar, S. K. Malik, D. Rambabu, and R. Vijayaraghavan, J.
Appl. Phys. 53, 8077 (1982).
S. K. Dhar, R. Nagarajan, S. K. Malik, and R. Vijayaraghavan, in
Proceedings of' the International Cortference on the Applications of
Mossbauer Effect, edited by. V. C. Bhide (Indian National Science
Academy, New Delhi, 1982), p. 595.
4T. K. Hatwar, R. M. Nayak, B. D. Padalia, M. N. Ghatikar, L. C.
and R. . Vijayaraghavan,
Solid
Gupta, E. V. Sampathkumaran,
Thanks are due to the Council of Scientific and Industrial
Research, New Delhi, for the financial support given to one
of us (B.D.).
State Commun. 39, 617 (1980).
5R. Nagarjan, E. V. Sampathkumaran,
L. C, Gupta, R. Vijayaraghaand B. D. Padalia, Phys. Lett. 81A, 397
van, Bhaktdarshan,
(1981).
R. Nagarajan,
L. C. Gupta, R. ViE, V. Sampathkumaran,
jayaraghavan, V. Prabhawalkar, Bhaktdarshan, and B. D. Padalia,
Phys. Lett. 84A, 275 (1981).
J. F. Herbst and J. W. Wilkins, Phys. Rev. B 26, 1689 (1982).
Q. Berkooz, M, Malamud, and S. Shtrikman, Solid State Commun.
6, 185 {1968).
~H. Micklitz (private communication).