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Photoemission spectra of Na β- and Na β″-aluminas
.___ l!lB
Js3
SOLID STATE IONICS
Solid State Ionics 86-88 (1996) 213-216
ELSEVIER
Photoemission
spectra of Na p- and Na b”-aluminas
Takeshi Hattori”‘“, Hideki Kanoua”, Itaru Kawaharada”, Yasuhisa Tezukab, Shik Shinb
Mareo Ishigame”,
“Research Institute for Scientific Measurements, Tohoku University, Sendai, 980, Japan hInstitute for Solid State Phyics, The University of Tokyo, Tanashi, Tokyo, 188, Japan
Abstract Photoemission spectra of Na p- and Na /3”-aluminas have been performed using a beam line with a synchrotron radiation to study the structures of the valence band. Both valence bands of these two materials are constructed by the 2p orbital of oxygen. On the results of this work, ionic conduction mechanism is discussed. Keywords:
Photoemission spectra; p-alumina; Superionic conductor: Oxide; Synchrotron
1. Introduction Na p- and p”-aluminas are well known superionic conductors [l]. Both have alternating structures of the conduction planes constructed by Na,O, and the spinel-like blocks of Al,O,. The main difference between both is in their structures of the conduction planes, which originate from their different crystal structures. The unit cells of p- and p”-aluminas are hexagonal and rhombohedral, respectively. In the conduction plane of p-alumina, Na and bridging oxygen ions are strictly localized in a mirror plane at fixed z-coordinates, while the Na ions in p-alumina are slightly displaced along c-direction from the basal plane, resulting in the formation of a conduction slab. Remarkable differences are observed between reflection spectra of p- and p”-aluminas in the *Corresponding author. ‘Present address: Fujitsu Ltd. 0167-2738/96/$15.00 Copyright PII SOl67-2738(96)00125-7
01996
radiation;
Electronic
structure; Valence hand
electronic band-edge region below 10 eV, as reported in previous papers [2,3]. Namely, two exciton bands are observed in Na p”-alumina at the band gap region, while one exciton band exists in Na /3alumina. However, in those previous works, we could not clearly understand the electronic band structure of these materials. Because, generally, in the reflection spectra, we observe the transitions from the valence band to the conduction band. From the reflection spectra, therefore, we can only get the information of the joint density of states modified by transition moments. It is necessary to study photoemission spectroscopy for getting the information of the occupied band (in insulator, the valence band) and bremsstrahlung isochromat spectroscopy for getting that of the unoccupied band (in insulator, the conduction band) above the Fermi level. In this work, the photoemission spectra in Na /3and Na p”-aluminas have been measured in order to identify the structures of their occupied valence bands, and to obtain more reliable information about
Elsevier Science B.V. All rights reserved
214
T. Hattori
et ul. / Solid State Ionics
86-M
(1996)
21.3-216
the reflection spectra and the electronic properties of p-alumina and other compounds in its family of superionic materials.
2. Experimental The sample preparation is the same as reported in previous papers [2,3]. A single crystal of Na p”alumina was grown by the flux method from a eutectic melt of Na,O, MgO and Al,O, [3]. A single crystal of Na p-alumina, which was grown from melt, was kindly provided by Dr. A. Imai of Nagoya Industrial Science Research Institute. Measurements were performed at room temperature by using a beam line BL-2 with synchrotron radiation from a 0.4-GeV electron storage ring at the Institute for Solid State Physics (SOR-RING), the University of Tokyo. The monochromatic light with energy from 30 to 150 eV can be available for the excitation source of photoemission spectra in this beam line. P-alumina-type crystals are insulators for electronic conduction, although they are good conductors for ionic conduction. Therefore, charge-up on sample surface occurs during the measurements of photoemission spectra. An electron-beam flood was used in order to supply electrons to the sample and to reduce charging effect. Before the measurement, the crystal was washed in acetone by using the supersonic washer. A sample with clean surface was obtained by scraping with a diamond file in a sample preparation chamber at about 10m9 mbar, and was transferred to a measurement chamber below 1 X 10 -lo mbar.
3. Results and discussion Fig. 1 shows the photoemission spectra of Na pand Na p”-aluminas at room temperature. These spectra were observed under the excitation of 120 eV. The evidence of residual charging effects compelled us to adjust the binding-energy scale, assigning to the observed band of 2p orbital of aluminum (A12p) its known energy of 72.5 eV Intensity scales of the spectra have been normalized, also, to that of the
Pap
(b) Na@“-alumina
3 .t: s-
hv=lZOeV
c5
Binding
Energy (A’)
Fig. I. Photoemission spectra of Na p- and Na p”-aluminas the excitation of 120 eV.
under
A12p band. The spectra have also been corrected for the effects of the secondary electrons. Four clear bands were observed in the spectrum of Na p-alumina. These four bands observed in Na p-alumina were also observed in Na /3”-alumina at almost the same positions in the binding energy. In Na p”-alumina, an extra band was also observed at 49 eV. Each band observed was assigned, as shown in Fig. 1, by referring the atomic energy levels and the results of cu-Al,O, reported by French [4]. The extra band at 49 eV observed in Na p”-alumina was assigned as that due to the 2p orbital of Mg as shown in Fig. 1, which was included only in Na /3”-alumina. The results shown in Fig. 1 show that both valence bands of Na p- and Na p”-aluminas are constructed by the 2p orbital of oxygen (02~). This result consists of those the valence bands of some insulating oxides [4,5]. By using the result obtained in this work, those in the previous works of the reflection spectra will be able to be analyzed, in which two exciton bands are observed in Na p”-alumina at the band gap region, while there is one exciton band in Na p-alumina 131. Since the valence bands of both materials are constructed by 02p, it is concluded that the conduction bands must be constructed by electronic orbitals of the metal ions. Therefore, it can be proposed that the structures of the energy bands of Na p- and Na p”-aluminas near
T. Hattori et al. I Solid State tonics 86-88
E
(1996) 213-216
E - -
/I’\\ lNa p-Al203 Fig. 2. Electronic
Y
A13si,,,
\/
215
02p
Nap-alumina hv=bOeV
-------
Nap”-alumina
/I\
INa f3”-A1203
energy levels near f
Binding Energy (eV)
point in Na p- and Na
p”-aluminas.
Fig. 3. Photoemission spectra of Na p- and Na p”-aluminas under the excitation of 60 eV. The spectra
zone center (f point) are as shown in Fig. 2. Although the lowest conduction band of Na paluminas is due to the 3s orbital of aluminum (A13s), there is, in Na /3”-alumina, the conduction band due to an orbital of sodium (probably Na3s orbital) under the conduction band due to 3s orbital of aluminum, as pointed out in a previous paper [3]. The result that the orbital of sodium makes a band shows that, in Na /3”-alumina, all conduction ions are not perfectly free, but construct bonds, perhaps with bridging oxygen, giving rise to an ordered structure existing in a limited region. An ordered structure or a longrange order in Na p”-alumina was also reported by Collin et al. [6]. It can be suggested, therefore, that ionic conduction in Na p”-alumina with an ordered structure of the conduction plane must be considered, as pointed out in a previous paper [3]. Namely, when we assumed that a conducting sodium ion may hop at any moment in Na p”-alumina, it may construct at the next moment a bond with the nearest bridging oxygen ion, and another sodium ion which had constructed a bond with the oxygen ion in an ordered structure may become free for ionic conduction. The photoemission spectra of both samples were also measured under the excitation of 60 eV. The spectra due to 02p (at the region of the binding energy below 14 eV) are shown in Fig. 3 in order to discuss the shape of the 02p band. In this figure, the intensity was normalized at the peak intensity of 02p band. The photoemission band due to 02p includes two components in each sample in common with that of rw-Al,O, [4]. In comparison with the result in it is considered that the lower energy (Y-&O,, component [(a) band in Fig. 31 is the non-bonding
0
below 14 eV are shown.
band and the higher one [(b) band in Fig. 31 the bonding one. There is a difference between the intensity ratios of these two components in Na pand Na jY’-aluminas as shown in Fig. 1 and Fig. 3. This result shows that there is a different feature of the bond between alkali ion and oxygen ion in both materials, and this difference will also be related to the conduction mechanism of both materials.
4. Conclusion Photoemission spectra of Na /3- and Na p”aluminas were measured. We formulated the following conclusions from the results of this work, in addition to the results of reflection spectra in previous works [3]. Both valence bands of Na /3- and Na p”-aluminas are constructed by the 2p orbital of oxygen. The structure of the energy bands of Na p- and Na p”-aluminas near zone center is proposed as shown in Fig. 2. In Na @“-alumina, all conduction ions are not perfectly free, but construct bonds, perhaps with bridging oxygen, giving rise to an ordered structure. Finally, it is pointed out that the study of the electronic state is a suitable method for the study of the ionic conduction in P-alumina-type superionic conductors. In order to obtain more reliable information, a
216
T. Hattori
et al. I Solid State Ionics
study of the photoemission spectra in /3- and p”aluminas with other conduction ions is required.
Acknowledgments The authors would like to SOR-RING of the Institute for the University of Tokyo. They thank Dr. A. Imai of Nagoya Research Institute for providing tals used in this work.
thank the staff of Solid State Physics, would also like to Industrial Science Na p-alumina crys-
86-88
(1996)
213-216
References J.H.Kennedy. in: Topics in applied physics, Vol. 21, Solid Electrolytes, ed. S. Geller (Springer, Berlin, 1977) Chap. 5. VI T. Hattori, S. Yashima, I. Kawaharada, M. Ishigame, N. Sata and S. Shin, Solid State Ionics 70/7l (1994) 493. 131T. Hattori, H. Kanou, I. Kawaharada, M. Ishigame, N. Sata and S. Shin, Solid State lonics 79 (1995) 30. [41 R.H. French, J. Am. Serm. Sot. 73 (1990) 471. f51 P. Camagni, G. Samoggia, L. Sangaletti, F. Parmigiani and N. Zema, Phys. Rev. B 50 (1994) 4292. 161G.Collin, J.P. Biolot, Ph. Colomban and R. Comes, Phys. Rev. B 34 (1986) 5838.
Js3
SOLID STATE IONICS
Solid State Ionics 86-88 (1996) 213-216
ELSEVIER
Photoemission
spectra of Na p- and Na b”-aluminas
Takeshi Hattori”‘“, Hideki Kanoua”, Itaru Kawaharada”, Yasuhisa Tezukab, Shik Shinb
Mareo Ishigame”,
“Research Institute for Scientific Measurements, Tohoku University, Sendai, 980, Japan hInstitute for Solid State Phyics, The University of Tokyo, Tanashi, Tokyo, 188, Japan
Abstract Photoemission spectra of Na p- and Na /3”-aluminas have been performed using a beam line with a synchrotron radiation to study the structures of the valence band. Both valence bands of these two materials are constructed by the 2p orbital of oxygen. On the results of this work, ionic conduction mechanism is discussed. Keywords:
Photoemission spectra; p-alumina; Superionic conductor: Oxide; Synchrotron
1. Introduction Na p- and p”-aluminas are well known superionic conductors [l]. Both have alternating structures of the conduction planes constructed by Na,O, and the spinel-like blocks of Al,O,. The main difference between both is in their structures of the conduction planes, which originate from their different crystal structures. The unit cells of p- and p”-aluminas are hexagonal and rhombohedral, respectively. In the conduction plane of p-alumina, Na and bridging oxygen ions are strictly localized in a mirror plane at fixed z-coordinates, while the Na ions in p-alumina are slightly displaced along c-direction from the basal plane, resulting in the formation of a conduction slab. Remarkable differences are observed between reflection spectra of p- and p”-aluminas in the *Corresponding author. ‘Present address: Fujitsu Ltd. 0167-2738/96/$15.00 Copyright PII SOl67-2738(96)00125-7
01996
radiation;
Electronic
structure; Valence hand
electronic band-edge region below 10 eV, as reported in previous papers [2,3]. Namely, two exciton bands are observed in Na p”-alumina at the band gap region, while one exciton band exists in Na /3alumina. However, in those previous works, we could not clearly understand the electronic band structure of these materials. Because, generally, in the reflection spectra, we observe the transitions from the valence band to the conduction band. From the reflection spectra, therefore, we can only get the information of the joint density of states modified by transition moments. It is necessary to study photoemission spectroscopy for getting the information of the occupied band (in insulator, the valence band) and bremsstrahlung isochromat spectroscopy for getting that of the unoccupied band (in insulator, the conduction band) above the Fermi level. In this work, the photoemission spectra in Na /3and Na p”-aluminas have been measured in order to identify the structures of their occupied valence bands, and to obtain more reliable information about
Elsevier Science B.V. All rights reserved
214
T. Hattori
et ul. / Solid State Ionics
86-M
(1996)
21.3-216
the reflection spectra and the electronic properties of p-alumina and other compounds in its family of superionic materials.
2. Experimental The sample preparation is the same as reported in previous papers [2,3]. A single crystal of Na p”alumina was grown by the flux method from a eutectic melt of Na,O, MgO and Al,O, [3]. A single crystal of Na p-alumina, which was grown from melt, was kindly provided by Dr. A. Imai of Nagoya Industrial Science Research Institute. Measurements were performed at room temperature by using a beam line BL-2 with synchrotron radiation from a 0.4-GeV electron storage ring at the Institute for Solid State Physics (SOR-RING), the University of Tokyo. The monochromatic light with energy from 30 to 150 eV can be available for the excitation source of photoemission spectra in this beam line. P-alumina-type crystals are insulators for electronic conduction, although they are good conductors for ionic conduction. Therefore, charge-up on sample surface occurs during the measurements of photoemission spectra. An electron-beam flood was used in order to supply electrons to the sample and to reduce charging effect. Before the measurement, the crystal was washed in acetone by using the supersonic washer. A sample with clean surface was obtained by scraping with a diamond file in a sample preparation chamber at about 10m9 mbar, and was transferred to a measurement chamber below 1 X 10 -lo mbar.
3. Results and discussion Fig. 1 shows the photoemission spectra of Na pand Na p”-aluminas at room temperature. These spectra were observed under the excitation of 120 eV. The evidence of residual charging effects compelled us to adjust the binding-energy scale, assigning to the observed band of 2p orbital of aluminum (A12p) its known energy of 72.5 eV Intensity scales of the spectra have been normalized, also, to that of the
Pap
(b) Na@“-alumina
3 .t: s-
hv=lZOeV
c5
Binding
Energy (A’)
Fig. I. Photoemission spectra of Na p- and Na p”-aluminas the excitation of 120 eV.
under
A12p band. The spectra have also been corrected for the effects of the secondary electrons. Four clear bands were observed in the spectrum of Na p-alumina. These four bands observed in Na p-alumina were also observed in Na /3”-alumina at almost the same positions in the binding energy. In Na p”-alumina, an extra band was also observed at 49 eV. Each band observed was assigned, as shown in Fig. 1, by referring the atomic energy levels and the results of cu-Al,O, reported by French [4]. The extra band at 49 eV observed in Na p”-alumina was assigned as that due to the 2p orbital of Mg as shown in Fig. 1, which was included only in Na /3”-alumina. The results shown in Fig. 1 show that both valence bands of Na p- and Na p”-aluminas are constructed by the 2p orbital of oxygen (02~). This result consists of those the valence bands of some insulating oxides [4,5]. By using the result obtained in this work, those in the previous works of the reflection spectra will be able to be analyzed, in which two exciton bands are observed in Na p”-alumina at the band gap region, while there is one exciton band in Na p-alumina 131. Since the valence bands of both materials are constructed by 02p, it is concluded that the conduction bands must be constructed by electronic orbitals of the metal ions. Therefore, it can be proposed that the structures of the energy bands of Na p- and Na p”-aluminas near
T. Hattori et al. I Solid State tonics 86-88
E
(1996) 213-216
E - -
/I’\\ lNa p-Al203 Fig. 2. Electronic
Y
A13si,,,
\/
215
02p
Nap-alumina hv=bOeV
-------
Nap”-alumina
/I\
INa f3”-A1203
energy levels near f
Binding Energy (eV)
point in Na p- and Na
p”-aluminas.
Fig. 3. Photoemission spectra of Na p- and Na p”-aluminas under the excitation of 60 eV. The spectra
zone center (f point) are as shown in Fig. 2. Although the lowest conduction band of Na paluminas is due to the 3s orbital of aluminum (A13s), there is, in Na /3”-alumina, the conduction band due to an orbital of sodium (probably Na3s orbital) under the conduction band due to 3s orbital of aluminum, as pointed out in a previous paper [3]. The result that the orbital of sodium makes a band shows that, in Na /3”-alumina, all conduction ions are not perfectly free, but construct bonds, perhaps with bridging oxygen, giving rise to an ordered structure existing in a limited region. An ordered structure or a longrange order in Na p”-alumina was also reported by Collin et al. [6]. It can be suggested, therefore, that ionic conduction in Na p”-alumina with an ordered structure of the conduction plane must be considered, as pointed out in a previous paper [3]. Namely, when we assumed that a conducting sodium ion may hop at any moment in Na p”-alumina, it may construct at the next moment a bond with the nearest bridging oxygen ion, and another sodium ion which had constructed a bond with the oxygen ion in an ordered structure may become free for ionic conduction. The photoemission spectra of both samples were also measured under the excitation of 60 eV. The spectra due to 02p (at the region of the binding energy below 14 eV) are shown in Fig. 3 in order to discuss the shape of the 02p band. In this figure, the intensity was normalized at the peak intensity of 02p band. The photoemission band due to 02p includes two components in each sample in common with that of rw-Al,O, [4]. In comparison with the result in it is considered that the lower energy (Y-&O,, component [(a) band in Fig. 31 is the non-bonding
0
below 14 eV are shown.
band and the higher one [(b) band in Fig. 31 the bonding one. There is a difference between the intensity ratios of these two components in Na pand Na jY’-aluminas as shown in Fig. 1 and Fig. 3. This result shows that there is a different feature of the bond between alkali ion and oxygen ion in both materials, and this difference will also be related to the conduction mechanism of both materials.
4. Conclusion Photoemission spectra of Na /3- and Na p”aluminas were measured. We formulated the following conclusions from the results of this work, in addition to the results of reflection spectra in previous works [3]. Both valence bands of Na /3- and Na p”-aluminas are constructed by the 2p orbital of oxygen. The structure of the energy bands of Na p- and Na p”-aluminas near zone center is proposed as shown in Fig. 2. In Na @“-alumina, all conduction ions are not perfectly free, but construct bonds, perhaps with bridging oxygen, giving rise to an ordered structure. Finally, it is pointed out that the study of the electronic state is a suitable method for the study of the ionic conduction in P-alumina-type superionic conductors. In order to obtain more reliable information, a
216
T. Hattori
et al. I Solid State Ionics
study of the photoemission spectra in /3- and p”aluminas with other conduction ions is required.
Acknowledgments The authors would like to SOR-RING of the Institute for the University of Tokyo. They thank Dr. A. Imai of Nagoya Research Institute for providing tals used in this work.
thank the staff of Solid State Physics, would also like to Industrial Science Na p-alumina crys-
86-88
(1996)
213-216
References J.H.Kennedy. in: Topics in applied physics, Vol. 21, Solid Electrolytes, ed. S. Geller (Springer, Berlin, 1977) Chap. 5. VI T. Hattori, S. Yashima, I. Kawaharada, M. Ishigame, N. Sata and S. Shin, Solid State Ionics 70/7l (1994) 493. 131T. Hattori, H. Kanou, I. Kawaharada, M. Ishigame, N. Sata and S. Shin, Solid State lonics 79 (1995) 30. [41 R.H. French, J. Am. Serm. Sot. 73 (1990) 471. f51 P. Camagni, G. Samoggia, L. Sangaletti, F. Parmigiani and N. Zema, Phys. Rev. B 50 (1994) 4292. 161G.Collin, J.P. Biolot, Ph. Colomban and R. Comes, Phys. Rev. B 34 (1986) 5838.