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Near-edge vuv absorption spectra at Ba L edges in BaC6
PII: SOO22-3697(%)00374-6
Pergamon
J. Phys. Chrm Solids Vol 57. Nos 6-8. pp. 921-923. 1996 Copyright c 1996 Elsewr Science Ltd Printed in Great Britain. All nghts resewed OOZZ-3697/96 515.00 + 0.00
NEAR-EDGE VUV ABSORPTION SPECTRA AT Ba L EDGES IN BaC6 S. RABIIt, N. A. W. HOLZWARTHJ, G. LOUPIAS§Ij, V. CODAZZI§ and D. GUERARDq TDepartment of Electrical Engineering, University of Pennsylvania, Philadelphia, PA 19104, U.S.A.
IDepartment of Physics, Wake Forest University, Winston-Salem, NC 27109,U.S.A. §LMCP, Universite de Paris VI, case I 15,4 pl. Jussieu, 75252, Paris Cedex 05, France )ILURE, bit. 209D, UPS, 91405 Orsay Cedex, France ‘1LCMS, Universiti: de Nancy I, BP 239,54506 Vandoeuvre les Nancy Cedex, France (Received 28 May 1995; accepted 31 May 1995)
Abstract-The near-edge spectra of BaC6 have been measured at the Ba Ledges, using the polarized photon beam of the synchrotron facility at LURE, in the energy range of 5.2-6.2 keV. The polarization angles of 45” and 90” were used in the measurements. At the same time, the energy band structure of BaC6 was recalculated using the local density approximation and choosing the Bap orbitals as one of the localized components of the mixed basis for expansion of the wavefunctions. Since the transitions originated from the Ba s core level, they are only allowed to final states containing p orbitals on the Ba site. Comparison of the measured spectra and the calculated wavefunctions allows us to identify the structure in the measured spectra with transitions to specific unoccupied bands. Keywordr: A. multilayers, C. XAFS, D. electronic structure.
2.
1. INTRODUCTION Graphite intercalation compounds are layerzd crystals formed by inserting foreign atoms or molecules between the carbon layers in graphite. BaC6 is unusual among the binary donor graphite intercalation compounds in that the intercalant has the potential of donating two electrons to the carbon layers. In order to test the theoretical models of the electronic structure of this compound, polarized photoabsorption spectroscopy can be used as a site-specific probe of the unoccupied wavefunctions. We have already reported on the Ln and Lm Ba spectra [l], where the initial states are the 2pl,* and 2p312core levels of Ba and the final states are of atomic d symmetry. In the case of the LI spectrum, the initial state is Ba 2s and the final states have to be of atomic p symmetry. Due to the layered structure of BaC6, the solid wavefunctions at each atomic site contain eitherp, andp, (referred to asp,) orpz components but not both, where the z axis is chosen perpendicular to the carbon layers. If we define the polarization angle cy, as the angle between the electric field of the photons and the c-axis of the crystal, as a is decreased from 90” to zero, the peaks due to final pxv states will lose intensity and eventually disappear. The reverse is true for pz final states. An intermediate behavior occurs when bands of both symmetries are overlapping in energy.
SAMPLE
Bach was prepared through the reaction of barium vapor at 500°C on thin samples of HOPG (e M Spm). Since barium contained some cesium impurities, the metal was purified through a first intercalation with graphite powder which removed the traces of cesium, due to either higher vapor pressure of cesium or higher affinity of cesium towards graphite. The sample was transferred from the preparation tube to a bronze cell in a glove box under argon gas, purified by molecular sieves and titanium-zirconium cheaps heated to 900”. Then the BaC6 sample was characterized by X-ray analysis after transfer to the sealed cell. 3. MEASUREMENTS The experiments were carried out at Laboratoire pour I’Utilisation du Rayonnement Electromagdtique (LURE) at the EXAFS II station, using the linearly polarized synchrotron beam. A two-crystal Si (3 11) monochromator provides an X-ray beam tunable around the energy of the L absorption edges of barium. The polarization ratio is better than 99%. This high ratio is obtained by a sharply collimated white beam coming from the central part of the photon source and a vertical scattering plane for both reflections of monochromator. For each value 921
922
S. RABII et al. PZ
I 2120 1918Il-
16IS-
Fig.
1.States ofp, symmetry at Ba site.
of the polarization angle o, the spectrum of transmitted photons through the sample is recorded in the useful energy range by 0.25-eV steps. In this Bragg reflection, the second harmonic is practically absent in the monochromatic beam, due to two uncoated glass mirrors used to reject the remaining harmonics. The rejection ratio is equal to 10Y4in this energy range [2]. The experimental resolution, determined [3] by slit openings and separation, and source size, is limited to 0.4 eV. Nevertheless, the total resolution is dominated by the approximately 3-eV core-level widths.
4. THEORY
A new SCF energy band calculation was carried out for BaC6 within the density-functional-approximation [4, 51 using ab initio pseudopotentials. Both C and Ba pseudopotentials were nonlocal with s and p components used for the former and s, p and d for the latter. A mixed basis [6] of planewaves and localized orbitals was used with s and p orbitals on both C and Ba. This is in contrast to our earlier calculation [7] where s and d LCAO’s were placed on Ba. The local
L-
18 17 16 IS 14 13
Ef
Fig. 2. States ofp,YY symmetry
at Ba site.
923
Ba absorption spectra
results only extend to 15 eV above the Fermi level I$, the structure
D cannot
Fig. 2. A possible for structures
be identified
identification
with the states in
of unoccupied
states
A, B and C is as follows. The collection
of states of pz symmetry
(Fig. I), starting
at approxi-
mately 6eV above the Fermi level and covering a range of about 2eV, can be identified with A. Simic
.o ‘i
5
9
-I IA
larly, the large band of pxv states at 7.5 eV above the Fermi level (Fig. 2) can be identified with the strong peak B. Finally,
-a=90"
a = 45”
about
”
”
5980 6000 6020 6040 6060 6080
’ 6100
at or near the Fermi
observed Fig. 3. L, near-edge absorption spectra for Ba in BaCs. N is the angle between the electric field of the photon and the c-axis of the crystal (polarization angle). used for exchange-correlation
energy follows the analytic and Wang [8]. The crystal structure
representation
of Perdew
nearest
neighbor
of BaC6 were
C-C
distance)
is 4.302
~t0.006A. The actual stacking is AaAPAa . . In order to have a smaller unit cell and make the calculations more tractable, a stacking of AcrA/?AyAa . . .was used in this calculation. The p components
of the localized
part
tions. These are shown in Figs 1 and 2 for pr and pxY, respectively.
Figure 3 shows the LI absorption
edge in the energy angles 90” and
structure represents transition to final states symmetry. While if the intensity increases angle,
final states have pxY symmetry. structures
the
respectively.
of pz with
corresponding
On this basis, the
A, B, C, and D, are primarily
and px, ‘character’,
of
the measured peaks support our identification, their relative energy separations are much larger than that of the corresponding calculated final states. A similar situation was encountered in transition metal disilirelaxation
in calculations, the spacings of theoretical peaks were much smaller than the measured results. When any mode1 was introduced for electronic relaxation, the resulting theoretical separations became much larger than
in the experimental
results.
It may be that a
proper model for electronic relaxation will lead to the elimination of this disagreement between theory and experiment.
REFERENCES 1. Gutrard D., Codazzi V., Loupias G., and Rabii S., 6fh International Symposium on Inlercalation Compoun&,
Phys. 2,449 (1984).
45”. As stated before, when the intensity of a structure decreases when polarization angle is decreased, such a
polarization
due to their relatively
small overall density of states. Although the sequence and the relative intensities
Orlbans, France, May (1991). 2. Goulon J., Cortbs R., Retournard A., Georges A., Battoni J. P., F&y R. and Moravek B., Spring Proc.
5. RESULTS AND DISCUSSION
range 5970-6100 eV, for polarization
that the pxY
of the
wavefunctions at barium site were used to create partial densities of final states involved in the transi-
decreasing
in the experiment,
assumes
level (Fig. 2), are not
tides [lo], where in the absence of electronic and parameters
determined by GuCrard et al. [9]. The c-axis separation is 5.25 f 0.02 A and the in-plane parameter (fi times the second
of pz
19.5 eV above the Fermi level, could be identi-
fied with C. This identification states,
E W)
density approximation
the next major concentration
states, located along the k, axis near r and centered at
of pL, pxY, pz
Since the theoretical
3. Goulon J., Lemonnier M., Cords R., Retournard A. and Raoux D., Nucl. Instr. Meth. 208,625 (1983). 4. Hohenberg P. and Kohn W., Phys. Rev. 136, B864 (1964). 5. Kohn W. and Sham L. J., Phys. Rev. 140, Al 133 (1965). 6. Louie S. G., Ho K. M. and Cohen M. L., Phys. Rev. B19, 1774 (1979). 7. Holzwarth N. A. W., DiVincenzo D. P., Tatar R. C. and Rabii S., Inf. J. Quantum Chem. XIII, 1223 (1983). 8. Perdew J. P. and Wang I., Phys. Rev. B45,13244 (1992). 9. Gutrard D., Chabouni M., Lagrange P., El Makrini M. and Herpol A., Carbon 18,257 (1980). 10. Lerch P., Jarlborg R., Codazzi V., Loupias G. and Flank A.M., Phys. Rev. B45, 11 481 (1992).
Pergamon
J. Phys. Chrm Solids Vol 57. Nos 6-8. pp. 921-923. 1996 Copyright c 1996 Elsewr Science Ltd Printed in Great Britain. All nghts resewed OOZZ-3697/96 515.00 + 0.00
NEAR-EDGE VUV ABSORPTION SPECTRA AT Ba L EDGES IN BaC6 S. RABIIt, N. A. W. HOLZWARTHJ, G. LOUPIAS§Ij, V. CODAZZI§ and D. GUERARDq TDepartment of Electrical Engineering, University of Pennsylvania, Philadelphia, PA 19104, U.S.A.
IDepartment of Physics, Wake Forest University, Winston-Salem, NC 27109,U.S.A. §LMCP, Universite de Paris VI, case I 15,4 pl. Jussieu, 75252, Paris Cedex 05, France )ILURE, bit. 209D, UPS, 91405 Orsay Cedex, France ‘1LCMS, Universiti: de Nancy I, BP 239,54506 Vandoeuvre les Nancy Cedex, France (Received 28 May 1995; accepted 31 May 1995)
Abstract-The near-edge spectra of BaC6 have been measured at the Ba Ledges, using the polarized photon beam of the synchrotron facility at LURE, in the energy range of 5.2-6.2 keV. The polarization angles of 45” and 90” were used in the measurements. At the same time, the energy band structure of BaC6 was recalculated using the local density approximation and choosing the Bap orbitals as one of the localized components of the mixed basis for expansion of the wavefunctions. Since the transitions originated from the Ba s core level, they are only allowed to final states containing p orbitals on the Ba site. Comparison of the measured spectra and the calculated wavefunctions allows us to identify the structure in the measured spectra with transitions to specific unoccupied bands. Keywordr: A. multilayers, C. XAFS, D. electronic structure.
2.
1. INTRODUCTION Graphite intercalation compounds are layerzd crystals formed by inserting foreign atoms or molecules between the carbon layers in graphite. BaC6 is unusual among the binary donor graphite intercalation compounds in that the intercalant has the potential of donating two electrons to the carbon layers. In order to test the theoretical models of the electronic structure of this compound, polarized photoabsorption spectroscopy can be used as a site-specific probe of the unoccupied wavefunctions. We have already reported on the Ln and Lm Ba spectra [l], where the initial states are the 2pl,* and 2p312core levels of Ba and the final states are of atomic d symmetry. In the case of the LI spectrum, the initial state is Ba 2s and the final states have to be of atomic p symmetry. Due to the layered structure of BaC6, the solid wavefunctions at each atomic site contain eitherp, andp, (referred to asp,) orpz components but not both, where the z axis is chosen perpendicular to the carbon layers. If we define the polarization angle cy, as the angle between the electric field of the photons and the c-axis of the crystal, as a is decreased from 90” to zero, the peaks due to final pxv states will lose intensity and eventually disappear. The reverse is true for pz final states. An intermediate behavior occurs when bands of both symmetries are overlapping in energy.
SAMPLE
Bach was prepared through the reaction of barium vapor at 500°C on thin samples of HOPG (e M Spm). Since barium contained some cesium impurities, the metal was purified through a first intercalation with graphite powder which removed the traces of cesium, due to either higher vapor pressure of cesium or higher affinity of cesium towards graphite. The sample was transferred from the preparation tube to a bronze cell in a glove box under argon gas, purified by molecular sieves and titanium-zirconium cheaps heated to 900”. Then the BaC6 sample was characterized by X-ray analysis after transfer to the sealed cell. 3. MEASUREMENTS The experiments were carried out at Laboratoire pour I’Utilisation du Rayonnement Electromagdtique (LURE) at the EXAFS II station, using the linearly polarized synchrotron beam. A two-crystal Si (3 11) monochromator provides an X-ray beam tunable around the energy of the L absorption edges of barium. The polarization ratio is better than 99%. This high ratio is obtained by a sharply collimated white beam coming from the central part of the photon source and a vertical scattering plane for both reflections of monochromator. For each value 921
922
S. RABII et al. PZ
I 2120 1918Il-
16IS-
Fig.
1.States ofp, symmetry at Ba site.
of the polarization angle o, the spectrum of transmitted photons through the sample is recorded in the useful energy range by 0.25-eV steps. In this Bragg reflection, the second harmonic is practically absent in the monochromatic beam, due to two uncoated glass mirrors used to reject the remaining harmonics. The rejection ratio is equal to 10Y4in this energy range [2]. The experimental resolution, determined [3] by slit openings and separation, and source size, is limited to 0.4 eV. Nevertheless, the total resolution is dominated by the approximately 3-eV core-level widths.
4. THEORY
A new SCF energy band calculation was carried out for BaC6 within the density-functional-approximation [4, 51 using ab initio pseudopotentials. Both C and Ba pseudopotentials were nonlocal with s and p components used for the former and s, p and d for the latter. A mixed basis [6] of planewaves and localized orbitals was used with s and p orbitals on both C and Ba. This is in contrast to our earlier calculation [7] where s and d LCAO’s were placed on Ba. The local
L-
18 17 16 IS 14 13
Ef
Fig. 2. States ofp,YY symmetry
at Ba site.
923
Ba absorption spectra
results only extend to 15 eV above the Fermi level I$, the structure
D cannot
Fig. 2. A possible for structures
be identified
identification
with the states in
of unoccupied
states
A, B and C is as follows. The collection
of states of pz symmetry
(Fig. I), starting
at approxi-
mately 6eV above the Fermi level and covering a range of about 2eV, can be identified with A. Simic
.o ‘i
5
9
-I IA
larly, the large band of pxv states at 7.5 eV above the Fermi level (Fig. 2) can be identified with the strong peak B. Finally,
-a=90"
a = 45”
about
”
”
5980 6000 6020 6040 6060 6080
’ 6100
at or near the Fermi
observed Fig. 3. L, near-edge absorption spectra for Ba in BaCs. N is the angle between the electric field of the photon and the c-axis of the crystal (polarization angle). used for exchange-correlation
energy follows the analytic and Wang [8]. The crystal structure
representation
of Perdew
nearest
neighbor
of BaC6 were
C-C
distance)
is 4.302
~t0.006A. The actual stacking is AaAPAa . . In order to have a smaller unit cell and make the calculations more tractable, a stacking of AcrA/?AyAa . . .was used in this calculation. The p components
of the localized
part
tions. These are shown in Figs 1 and 2 for pr and pxY, respectively.
Figure 3 shows the LI absorption
edge in the energy angles 90” and
structure represents transition to final states symmetry. While if the intensity increases angle,
final states have pxY symmetry. structures
the
respectively.
of pz with
corresponding
On this basis, the
A, B, C, and D, are primarily
and px, ‘character’,
of
the measured peaks support our identification, their relative energy separations are much larger than that of the corresponding calculated final states. A similar situation was encountered in transition metal disilirelaxation
in calculations, the spacings of theoretical peaks were much smaller than the measured results. When any mode1 was introduced for electronic relaxation, the resulting theoretical separations became much larger than
in the experimental
results.
It may be that a
proper model for electronic relaxation will lead to the elimination of this disagreement between theory and experiment.
REFERENCES 1. Gutrard D., Codazzi V., Loupias G., and Rabii S., 6fh International Symposium on Inlercalation Compoun&,
Phys. 2,449 (1984).
45”. As stated before, when the intensity of a structure decreases when polarization angle is decreased, such a
polarization
due to their relatively
small overall density of states. Although the sequence and the relative intensities
Orlbans, France, May (1991). 2. Goulon J., Cortbs R., Retournard A., Georges A., Battoni J. P., F&y R. and Moravek B., Spring Proc.
5. RESULTS AND DISCUSSION
range 5970-6100 eV, for polarization
that the pxY
of the
wavefunctions at barium site were used to create partial densities of final states involved in the transi-
decreasing
in the experiment,
assumes
level (Fig. 2), are not
tides [lo], where in the absence of electronic and parameters
determined by GuCrard et al. [9]. The c-axis separation is 5.25 f 0.02 A and the in-plane parameter (fi times the second
of pz
19.5 eV above the Fermi level, could be identi-
fied with C. This identification states,
E W)
density approximation
the next major concentration
states, located along the k, axis near r and centered at
of pL, pxY, pz
Since the theoretical
3. Goulon J., Lemonnier M., Cords R., Retournard A. and Raoux D., Nucl. Instr. Meth. 208,625 (1983). 4. Hohenberg P. and Kohn W., Phys. Rev. 136, B864 (1964). 5. Kohn W. and Sham L. J., Phys. Rev. 140, Al 133 (1965). 6. Louie S. G., Ho K. M. and Cohen M. L., Phys. Rev. B19, 1774 (1979). 7. Holzwarth N. A. W., DiVincenzo D. P., Tatar R. C. and Rabii S., Inf. J. Quantum Chem. XIII, 1223 (1983). 8. Perdew J. P. and Wang I., Phys. Rev. B45,13244 (1992). 9. Gutrard D., Chabouni M., Lagrange P., El Makrini M. and Herpol A., Carbon 18,257 (1980). 10. Lerch P., Jarlborg R., Codazzi V., Loupias G. and Flank A.M., Phys. Rev. B45, 11 481 (1992).