Reflection spectra of alkali β″-aluminas in ultraviolet and vacuum-ultraviolet regions

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Solid State Ionics 79 (1995) 30-33

Reflection spectra of alkali p”-aluminas in ultraviolet and vacuum-ultraviolet regions Takeshi Hattori a, Hideki Kanou a, Itaru Kawaharada a, Mareo Ishigame a, Noriko Sata b, Shik Shin b aResearch Institute for Scientific Measurements, Tohoka University, Sendai 980, Japan b The Institute for Solid State Physics, The University of Tokyo, Tanashi, Tokyo 188, Japan

Abstract Reflection spectra of alkali (Na, K and Rb) (Y-alumina crystals were measured in vacuum-ultraviolet region by using a beam line with synchrotron radiation. Remarkable differences are observed between the spectra of alkali j3- and alkali p”-aluminas in the electronic band-edge region below 10 eV. From these differences, the structure of the conduction plane (slab) will be discussed.

Keywords: Reflection spectra; p”-alumina; Vacuum-ultraviolet; to-band transition;

Synchrotron

radiation;

Superionic

conductor;

Electronic

structure;

Band-

Exciton

1. Introduction In the previous paper [l], we have reported a result of the first observation of the vacuum-ultraviolet reflectance in p-alumina using a beam line with synchrotron radiation, in order to study the electronic-band character of this material, and also discussed the conduction mechanism. It was concluded that the measurements of the reflection spectra in vacuum-ultraviolet region are a suitable method for the study of the structure of the conduction plane in P-alumina. Moreover, it was proposed that this spectrum is useful for elucidating on the structure of crystalline superionic conductors. However, in this previous work, we could not clearly understand the band structure of p-alumina. In this work, the reflection spectra of alkali (Na, K and Rb) p”-alumina were measured in ultraviolet and vacuum-ultraviolet regions in order to understand the 0167-2738/95/$09.50 0 1995 Elsevier Science B.V. AI1 rights reserved SSDI 0167.2738(95)00025-9

electronic energy state or the band structure of palumina and other compounds in its family of superionic materials. Na p”-alumina is one of the two-dimensional superionic conductors [2]. It, as well as p-alumina, has an alternating structure of the conduction planes constructed by Na,O, and the spinel-like blocks of Al,O,. The main difference between j3- and p”aluminas is in their conduction planes which originate from their different crystal structures. The unit cell of P-alumina is hexagonal, [P~,/~~c(&,)] and that of p”-alumina is rhombohedral, [ R3m(D&)]. Therefore, both Na and bridging oxygen ions in the conduction plane of p-alumina are strictly localized in a mirror plane at fixed z-coordinates, while the Na-ion positions in p”-alumina are slightly displaced along c-direction from the basal plane, resulting in the formation of a conduction slab [3]. The conduction ions (Na ions) in Na-@“-alumina, as well as

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State Ionics 79 (1995) 30-33

those in Na+-alumina, can be easily exchanged by other cations. Those conduction ions in p-alumina are distributed in three different sites commonly referred to as Beevers-Ross (BR), anti-Beevers-Ross (aBR1 and mid-Oxygen (mO> sites [2]. Since in p”-alumina the BR and aBR sites are equivalent, the available sites in it are twice the number of those in p-alumina. Therefore, it is expected that the comparison of the reflection spectra of p- and l3”-aluminas will yield a better understanding of the electronic properties of this family of superionic materials.

2. Experimental Single crystal of Na+“-alumina was grown by the flux method from a eutectic melt of Na,O, MgO and Al,O, [4]. First, the powders of Na,CO, (32.7 mol%), MgO (6.6 mol%) and Al,O, (60.7 mol%) were mixed. The concentration of Na ions in this mixture is richer than that of Na ions in l3”-alumina. This mixture was sintered at 950°C for 4 h in a Pt crucible, and then melted at about 1700°C and kept at that temperature for 150 h. The samples of the single crystal for the measurements with a typical dimension about 5 X 5 X 0.5 (c-direction) mm3 were cut in an appropriate manner from a large section of the ingot. The samples of K and Rb+“-aluminas were prepared by immersing Na+“-alumina single crystals in KNO, and RbNO, melts for a long time, respectively. Reflection measurements were performed by using a beam line of BL-1 with synchrotron radiation from a 0.4 GeV electron storage ring at the Institute for Solid State Physics @OR-RING), the University of Tokyo. The reflection spectrum of the single crystal of p”-alumina was measured on an ab surface cleaved in air and at an incident angle of about 12”. The sample was washed in acetone just before the measurements.

Fig. 1. Reflection spectra of alkali p- and alkali @‘-aluminas in the vacuum-ultraviolet region at room temperature. That of aalumina is also shown for comparison.

1, the reflection spectrum of a-alumina is also shown. Fig. 2 shows the imaginary part of the dielectric constants (~1 obtained from the Kramer-Kronig

3. Results and discussion Fig. 1 shows the reflection spectra of Na-, Kand Rb+“-aluminas at room temperature together with those of Na-, K- and Rb+-aluminas, which were reported previously [l], for comparison. In Fig.

Phkn

Eoewm(eV)

Fig. 2. Imaginary part of the dielectric constants (E*) obtained from the Framers-Kronig analyses of the spectra of alkali p- and alkali @‘-aluminas in Fig. 1.

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analyses of the spectra shown in Fig. 1. On comparison between Figs. 1 and 2, each peak-position of Ed in Fig. 2 corresponds to that of reflection spectrum in Fig. 1, especially below 10 eV. Therefore, we will discuss the results hereafter using the reflection spectra themselves. The reflection spectra above 10 eV in p- and p”-aluminas are roughly the same except for a few bands which arise from the transitions from the energy levels of the inner shells of the conduction ions [5], as shown in the previous paper [l]. The difference in the intensities in a few bands is caused by the fact that the numbers of the conduction ions in a unit cell of v-alumina is twice that of p-alumina. Therefore, it is concluded that the spectra of alkali p”-aluminas above 10 eV, as well as those of alkali p-aluminas, are also mainly due to the interband transition from the lower valence bands of oxygen ions to the higher conduction bands of aluminum ions in the spinel-like blocks, in addition to the transitions from the inner shell levels. On the other hand, below 10 eV, the spectra of alkali p”-aluminas appreciably differ from those of alkali p-aluminas. For example, in Na-P”-aluminas, two clear peaks are observed at 7.7 and 9.1 eV, while only one clear peak is observed at 8.0 eV in Na-P-aluminas, as shown in Fig. 1. Similar features are found for K-P- and K-_P”-aluminas, and also for Rb-P- and Rb+“-aluminas. From the results of the identification of the spectrum of p-alumina in the previous paper [l], the peak near 8 eV arises from an exciton formation which is due to the transition from the valence band of 2p orbital of oxygen (02~) to the conduction band of 3s orbital of aluminum (Al3s). Generally, the binding energy of the exciton depends on the dielectric constant of the material. As shown in Fig. 1, the peak position observed depends on the types of conduction ions. Therefore, the shift of the peak position corresponds to the change of the dielectric constant of p-aluminas. The exciton bands observed may be broadened as a result of a disordered nature of l3-aluminas. In contrast with the spectra of alkali p-aluminas, those of alkali p”-aluminas below 10 eV show two exciton bands. These two bands are attributed to two exciton formations; that is, a higher energy band arises from an exciton formation in the spinel-like

State Ionics 79 (199s) 30-33

block and a lower energy band in the conduction plane (slab), because the peak position of the lower band depends on the types of conduction ions but that of the higher band does not. The reason why the exciton band of the conduction plane (slab) is observed in p”-aluminas, but not in p-aluminas, will arise from the different structures of both conduction planes. The positions of the conduction ions in @‘-alumina are slightly displaced along c-direction from the basal plane, resulting in the formation of a conduction slab [3], while the conduction plane in @alumina is strictly localized in a mirror plane. In other words, the conduction slab in v-alumina possesses a three-dimensional nature as a result of a zig-zag arrangement of the conduction ions along c-direction, while that in p-aluminas is two-dimensional. Therefore, it is considered that this three-dimensional nature of the conduction slab in l3”-aluminas causes the appearance of the exciton band of the conduction slab which is constructed by alkali and oxide ions. It can also be reasoned that the lower band in p”-aluminas is due to the exciton formation in the conduction slab. As shown in Table 1, the peak position of the exciton band in alkali halides [6] does not appear to depend on the types of alkali ions in comparison with those of halogen ions. The magnitude of the energy difference of the lower bands among alkali p”-aluminas is of the same order as that of the exciton bands among alkali halides when alkali ion is exchanged. This coincidence will be significant although there are differences between halides and oxides. The appearance of the exciton band of the conduction slab implies an ordered form of the bonding of alkali and oxygen ions in the conduction slab, in

Table 1 The peak energies of the lowest bands in alkali p”-ahtminas a) and those of the exciton bands in alkali halides at room temperature. AR energies are in electron volts (eV). The values for alkali halides are obtained from the figures of the optical absorption spectra of their thin films deposited on LiF in Ref. [6].

Na K Rb

F

Cl

10.4 9.8 9.3

7.8 7.6 7.4

Br 6.5 6.6 6.4

0 a’ 7.0 7.2 7.0

7.7 8.0 7.6

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State Ionics 79 (1995) 30-33

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addition to the existence of the conduction band due to an orbital of alkali ions. That is, at least in a limited region and for a limited time, all conduction ions in alkali B”-aluminas are not perfectly free, and construct the bonds with bridging oxygen creating an ordered structure. An ordered form or a long-;ange order with the coherent length of about 70 A (at room temperature) was also reported by Collin et al. [7] from their experimental studies of crystal-structure determinations and X-ray diffuse scattering in Na l3”-aluminas. Of course, a small fraction of conduction ions may be free for ionic motion. The result described above will be directly related to the conduction mechanism of this material. Namely, we assume that a conduction ion may hop at any given moment. At the next moment, it may construct a bond with the nearest bridging oxygen ion, and another cation which had constructed a bond with the oxygen ion may become free for ionic motion. On the other hand, the peak position of the higher energy band in p”-alumina is independent of the types of conduction ions. Thus, it is considered that this higher band is due to an exciton formation in the spinel-like block. The reason why the peak position of the exciton band of the spinel-like blocks of p”-alumina is lower than that in a-alumina as shown in Fig. 1 will become apparent from the different crystal structures of both. Therefore, we conclude that two bands observed below 10 eV in alkali l3”-aluminas are due to both exciton formations in the spinel-like blocks and in the conduction slabs.

which arises from an exciton formation in the spinel-like blocks, was observed. (3) In alkali B”-a1uminas, two exciton bands were observed, which arise from both exciton formations in the spinel-like blocks and in the conduction slabs. (4) Below 10 eV, it is concluded that the difference between the spectra of alkali B- and alkali B”-aluminas is determined by the structure of the conduction plane (slab). (5) Specifically in alkali p”-aluminas, all conduction ions are not perfectly free, but construct bonds with bridging oxygen, giving rise to ordered form, existing in a limited region and for a limited time. In order to obtain more reliable information about the relationship between the spectrum and the structure, measurements for Ag- and Tl-B”-aluminas and those of the photo-emission spectra in p- and p”aluminas are in progress.

4. Conclusions

References

Reflection spectra of Na-, K- and Rb+“-alumina were measured in the vacuum-ultraviolet region. Having made the comparison between those of alkali l3- and alkali B”-aluminas, we formulated the following conclusions: (1) Above 10 eV, the structures of spectra of alkali l3”-aluminas are fundamentally the same as those of alkali (3-aluminas. They, as well as those of alkali p-aluminas, are also mainly due to the interband transition from the lower valence bands of oxygen ions to the higher conduction bands of aluminum ions in the spinel-like blocks. (2) In alkali B-aluminas, only one exciton band,

[l] T. Hattori, S. Yashima, T. Kawaharada, Y. Chiba, M. Ishigame, N. Sata and S. Shin, Solid State Ionics 70/71 (1994) 493. [2] J.H. Kennedy, in: Topics in Applied Physics, Vol. 21: Solid Electrolytes, ed. S. Geller (Springer, Berlin, 1977) chap. 5. [3] J.P. Biolot, G. Collin, Ph. Colomban and R. Comes, Phys. Rev. B22 (1980) 5912. [4] J.L. Briant and G.C. Farrington, J. Solid State Chem. 33 (1980) 385. [S] C.E. Moore, in: Atomic Energy Levels, Vols. l-3 (US Department of Commerce, National Bureau of Standards, Washington, DC, 1958). [6] J.E. Eby. K.J. Teegarden and D.B. Dutton, Phys. Rev. 116 (1959) 1099. [7] G. Collin, J.P. Boilot, Ph. Colomban and R. Comes, Phys. Rev. B34 (1986) 5838.

Acknowledgements The authors would like to thank Professor T. Ishii and the staff of SOR-RING of the institute for Solid State Physics, the University of Tokyo, for their helpful advice on the measurements. They especially thank Dr. M. Fujisawa for his advice on the measurements at the BL-1 of the SOR-RING. They would like to thank also Professor H. Arashi of Faculty of Engineering, Tohoku University for advice on the crystal growth of Na-B”-alumina.