ARTICLE IN PRESS
N. Ogita et al. / Journal of Solid State Chemistry 177 (2004) 461–465
462
atmosphere. The sample characterization was done
by a powder X-ray diffraction, and the obtained
patterns have confirmed the single phase of the cubic
hexaboride structure. The Curie temperature of CaB6
was 650 K, determined by the magnetic susceptibility
measurements.
The single crystal YbB6 was grown by a floating-zone
method using an imagefurnace with four Xe lamps [7].
Other single crystals of RB6 (R=Pr, Ce) were grown by
a floating-zone method [8].
Raman scattering spectra were measured by the
following multichannel detection systems. An Ar ion
laser operated at 514.5 nm with an output power of
10 mW was used. We employed two different measure-
ments: the so-called macro- and micro-Raman systems.
The scattered light was analyzed by a triple mono-
chromator, and the analyzed light was detected by a
liquid N2 cooled CCD detector. In order to avoid a local
heating due to the incident beam, a He gas was
employed for both measurements of high and low
temperatures.
Three phonons are Raman-active for the hexaborides
with the cubic symmetry: A1g þ Eg þ T2g: These vibra-
tions are due to only Batoms ( Fig. 2(c)), because Ca
atom is located on an inversion center. The assignment
of observed phonons is given by the following polariza-
tion dependence. A1g appears in the (x; x) geometry, Eg
in (x; x) and (x þ y; x ꢁ y), and T2g in (x; y), where x and
y correspond to [100] and [010] axes, respectively.
According to the theory [2,6], the most plausible
symmetry is tetragonal. In this symmetry, the Eg mode
in the cubic phase becomes A1g þ B1g:
R.T.
(x+y,x+y)
A1g
Eg
T2g
CaB6
YbB6
x5
CeB6
PrB6
*
*
0
500
1000
1500
Energy Shift ( cm1 )
Fig. 1. Raman scattering spectra of RB6 (R=Ca, Yb, Ce, and Pr) at
room temperature in the (x þ y; x þ y) geometry. The asterisks,
arrows, and triangles are the additional peak, CEF excitations, and
anomalous low energy excitations, respectively.
observed below 500 cmꢁ1, the detailed discussions will
be reported elsewhere [9]. In this paper, we report about
the high-energy excitations above 500 cmꢁ1
.
It is well known that trivalent rare-earth hexaborides
are monovalent metals. Actually, LaB6 is one of the
most typical monovalent metal [10]. SmB6 is a semi-
conductor and special compound since the 4f 6 and 4f 5
configurations in the Sm ions (4f 6 : 4f 5C3 : 7) [11–13].
On the other hand, non-magnetic divalent rare-earth
hexaborides such as YbB6 are known as a typical
narrow gap semiconductor [14]. These properties are in
good agreement with the band calculation [15]. There-
fore, the spectral difference between the divalent and
trivalent-cation hexaborides is caused by the difference
of electronic states.
3. Results and discussion
Fig. 1 shows the Raman spectra of RB6 (R=Ca, Yb,
Ce, and Pr) measured at room temperature. The
geometry of the presented spectra is (x þ y; x þ y),
where all Raman-active phonons in the cubic symmetry
appear. The valence of cation R for the upper two
spectra of CaB6 and YbB6 is divalent, and that for the
lower two spectra of CeB6 and PrB6 is trivalent. The
figure gives us the following remarkable difference
between the divalent and trivalent cation crystals; the
line shape of the Eg mode, the additional peak at about
1400 cmꢁ1 marked by asterisks, crystal-electric-field
(CEF) excitations (arrows), and anomalous low energy
excitations (triangles). The Eg peak is doublet for only
the divalent crystals, while that is a very broad single-
peak for the trivalent ones. The other features of the
peak at 1400 cmꢁ1, CEF and anomalous low energy
excitations are observed for RB6 with the trivalent
crystals. In addition, the line shape of the T2g peak is
asymmetric for rare-earth hexaboride with the trivalent
cation. For the CEF and anomalous excitations
Fig. 2 shows the polarization dependence of the
Raman scattering spectra of CaB6 and YbB6 at room
temperature. The spectra of CaB6 were measured by the
micro-Raman scattering from the rectangular-shaped
micro-crystal, 20 Â 10 mm. By this polarization depen-
dence, three strong peaks are assigned as phonon and
their corresponding irreducible representations are also
depicted in the figures. As described above, if the
doublet peak is due to tetragonal symmetry of CaB6, the
Eg mode should change to the A1g þ B1g symmetry.
However, the polarization dependence clearly shows
that both doublet peaks at B1150 cmꢁ1 have the
symmetry of Eg: Thus, the doublet peak of the divalent
crystals is not caused by the change of the crystal
symmetry.
An isotope effect of B10 and B11 is expected as one
possible origin of the doublet structure. We have
evaluated the possibility of the selective isotope ordering
of Bby the simple normal mode analysis, using the
reported force constants by Takegahara and Kasuya