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Unoccupied electronic states in cerium hydrides
Physica I30B (1985) 524-526 North-Holland, Amsterdam
U N O C C U P I E D E L E C T R O N I C S T A T E S IN C E R I U M H Y D R I D E S
J. O S T E R W A L D E R and L. S C H L A P B A C H
Laboratory J:or Solid State Physics, ETH, CH-8093 Ziirich, Switzerland We present UV isochromat spectra of polycrystalline CeH2~ and CeH2,~. The intensity at Ev is small in Cell2 j and vanishes in CeH2.,~ as it is expected from XPS, UPS and conductivity data. In both hydrides broad features (~2 eV FWHM) appear between 4 and 5 eV above Er.. This is in qualitative agreement with bandstructure calculations.
The c e r i u m hydrogen system CeH~ has b e c o m e a t e x t b o o k example for studying the effects of interstitial hydrogen on the electronic properties of its host metal [1-6]. The metal ions form an fcc lattice in the entire homogeneous composition r a n g e x = (t + o~ and 1.8 ~< x ~ 3.0. Theorists have calculated electronic band-structures for C e l l 2 (CaF2 structure), C e l l 3 (BiF~ structure) and intermediate, nonstoichiometric compositions [7, 8] and integrated them to the densities of occupied and e m p t y states below attd a b o v e Ev, respectively. Photoemission experim e n t s confirm the picture obtained for the o c c u pied part, i.e. that the density of states ( D O S ) diminishes at E v upon hydriding and vanishes as x a p p r o a c h e s 3.0, while a hydrogen induced band appears 5 to 6 eV below Ev [2-4]. By m e a n s of UV inverse photoemission we are now able to p r o b e the e m p t y DOS. For reasons which we discuss elsewhere [9] we do not see any 4f contribution at our low photon energy (9.7 eVL Thus we o b s e r v e the (spd)-DOS alone, which is most relevant for the chemical bonding. The inverse photoemission e q u i p m e n t consists of an energy selective iodine c o u n t e r [10] and a single lens electron gun [11], both of them m o u n t e d in a VG E S C A L A B spectrometer. The overall energy resolution of this setup is characterized by d E = 1 . 0 e V for the 1 0 % - 9 0 % intensity rise at Ev of c e r i u m metal. The hydride samples were p r e p a r e d by exposing molten c e r i u m to 0.5 bar hydrogen [2]. The surfaces were c l e a n e d in the UHV system using an Al20~ file.
Figs. 1 and 2 show our isochromat spectra of CeH2.] and CeH:,~ in comparison with b a n d structure calculations of the respective stoichiometric hydrides by A. Fujimori and coworkers [7]. The latter were o b t a i n e d by using the L C A O m e t h o d , omitting the 4f states. T h e s e arc accounted for only in the crystal potentiaL. T h e r e f o r e the calculated densities of states
UV ISOCHR. taw : 9.7 eV
I C eH ? !
,***~v,9~.%,~,,,"
,T CO LIJ p<~
°° o,r
O9
:>,I-2: bJ 12:1 >I-, Z 1.13 I-Z
t EF =0
2
4
6 (E-EF)/eV
Fig. 1. UV isochromat spectrum of ('ell2 ~ together with tile LCAO DOS of (TeHz from ref. 7.
0 3 7 8 - 4 3 6 3 / 8 5 / $ 0 3 . 3 0 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
J. Osterwalder and L. Schlapbach / Unoccupied electronic states in cerium hydrides I
I
I
I
[
I
L
I
I
UV ISOCHR. hc~ : 9,7 eV
¢ ,.,O,4.
Cell2.9
U) bJ I'-1-O9
,.
*~
I..L C, >.t--
I
~"
Z I..iJ r,,
\-
H2 /
o,,)/
/
>.I.Z tlJ I'-Z
I
E F :0
2
I
I
4
I
I
I
6 (E- E F ) / e V
Fig. 2. UV isochromat spectrum of C e l l 29 together with the LCAO DOS of Cell3 from ref. 7. The curve designated with H2 was obtained by using hydrogen gas at 10-7 mb as target.
should reflect the (spd)-DOS alone and can be compared to our spectra. CeHz~ shows an intensity at Er which is 4 to 5 times lower than in the corresponding spectrum of Ce metal (cf. ref. 9). The prominent structure which is seen in the DOS between 0 and 2 eV appears as a broad shoulder, whereas the high DOS between 3 and 6 e V corresponds to the dominating broad feature of the spectrum. A strong peak in the DOS at 6.6 eV is not seen. It has considerable s-character, whereas the rest of the DOS has mostly d-character [7]. The Cell2.9 spectrum reflects fairly well the structures in the corresponding DOS. There is no intensity at EF: CeH29 is a semiconductor (cf. ref. 1). A shoulder appears at 1 . 4 e V and a broad peak at 4.5 eV. The rising background above 6 eV is attributed to hydrogen gas desorbing from the surface of the sample [9]. The agreement between the calculated DOS and our spectra is satisfying if one takes into
525
account that our instrumental resolution is limited and that additional processes contribute to an inverse photoemission spectrum: Incoming electrons can be inelastically scattered in the crystal and probe a different final state energy [12, 13, 14]. This produces a smoothly rising background. In the case of polycrystailine Ni it has been shown (ref. 15, cf. also ref. 16) that the simple DOS picture of inverse photoemission, which implies a homogeneously smearing out of the incoming k vector over the entire Brillouin zone, does not give the correct intensity ratios of the observed structures, but that direct transitions have to be taken into account. We conclude that our spectra give a qualitative picture of the empty (spd)-DOS of CeHz~ and Cell2.9. The reduction of intensity at EF is clearly visible, and structures in the spectrum correspond to regions with a high DOS. This shows that inverse photoemission is a good tool for studying the changes in the empty electronic bandstructure of metals upon hydriding.
Acknowledgements We acknowledge helpful support by T. Riesterer, M. Erbudak, P. Brack and H.C. Siegmann. This work was financed by the Nationale Energie-Forschungs Fonds.
References [1] G.G. Libowitz and A.J. Maeland, in: Handbook on the Physics and Chemistry of Rare Earths, K.A. Gschneidner jr. and L. Eyring, eds. (North-Holland, Amsterdam, 1979), p. 319. [2] L. Schlapbach, J. Osterwalder and H.C. Siegmann, J. Less-Common Met. 88 (1982) 291. [3] A. Fujimori and N. Tsuda, phys. stat. sol. (b) 114 (1982) K139. [4] D.J. Peterman, J.H. Weaver, M. Croft and D.T. Peterson, Phys. Rev. B 27 (1982) 808. [5] J. Osterwalder, H.R. Ott, L. Schlapbach, J. Schefer and P. Fischer, J. Less-Common Met. 94 (1983) 129. [6] J. Schefer, P. Fischer, W. H~ilg, J. Osterwalder, L. Schlapbach and J.D. Jorgensen, J. Phys. C 17 (1984) 1575.
526
J. Osterwalder and L. Schlapbach / Unoccupied electronic states in cerium hydrides
[7] A. Fujimori, F. Minami and N. Tsuda, Phys. Rev. B 22 (1980) 3573. [8] A. Fujimori and N. Tsuda, J. Phys. (" 14 (1981) 1427. [9] J. Osterwalder and L. Schlapbach, Solid State Commun. 52 (1984) 503. [10] G. Denninger, V. Dose and H. Scheidt, Appl. Phys. I~; (1979) 375. [l 1] K. Designer, Diplomarbeit, Physikalisches lnstitu! der Universit~it Wiirzburg (1982).
[12] D.[,. Webster, Phys. Rev. ~ Ilt,~17) 220. [13] P.O. Nilsson and ('.(i. I,arsson, Jap. J. Appl. Phys. 17 Suppl. 17-2 ~lq7,~ 144. [14] V. Dose and (L Reusing, Appl. Phys. 23 (1980~ 131 [15] V. Dose and Th. Fauster, Solid S~ate ('ommun. 5(! t 1984) 67. [16] D.P. Woodrulf, N,V. Snlith, P.D. ,l()hnst~n ~llld W./\
Royer, Phys. Re~. B 26 ! lt~,~2) 2~43.
U N O C C U P I E D E L E C T R O N I C S T A T E S IN C E R I U M H Y D R I D E S
J. O S T E R W A L D E R and L. S C H L A P B A C H
Laboratory J:or Solid State Physics, ETH, CH-8093 Ziirich, Switzerland We present UV isochromat spectra of polycrystalline CeH2~ and CeH2,~. The intensity at Ev is small in Cell2 j and vanishes in CeH2.,~ as it is expected from XPS, UPS and conductivity data. In both hydrides broad features (~2 eV FWHM) appear between 4 and 5 eV above Er.. This is in qualitative agreement with bandstructure calculations.
The c e r i u m hydrogen system CeH~ has b e c o m e a t e x t b o o k example for studying the effects of interstitial hydrogen on the electronic properties of its host metal [1-6]. The metal ions form an fcc lattice in the entire homogeneous composition r a n g e x = (t + o~ and 1.8 ~< x ~ 3.0. Theorists have calculated electronic band-structures for C e l l 2 (CaF2 structure), C e l l 3 (BiF~ structure) and intermediate, nonstoichiometric compositions [7, 8] and integrated them to the densities of occupied and e m p t y states below attd a b o v e Ev, respectively. Photoemission experim e n t s confirm the picture obtained for the o c c u pied part, i.e. that the density of states ( D O S ) diminishes at E v upon hydriding and vanishes as x a p p r o a c h e s 3.0, while a hydrogen induced band appears 5 to 6 eV below Ev [2-4]. By m e a n s of UV inverse photoemission we are now able to p r o b e the e m p t y DOS. For reasons which we discuss elsewhere [9] we do not see any 4f contribution at our low photon energy (9.7 eVL Thus we o b s e r v e the (spd)-DOS alone, which is most relevant for the chemical bonding. The inverse photoemission e q u i p m e n t consists of an energy selective iodine c o u n t e r [10] and a single lens electron gun [11], both of them m o u n t e d in a VG E S C A L A B spectrometer. The overall energy resolution of this setup is characterized by d E = 1 . 0 e V for the 1 0 % - 9 0 % intensity rise at Ev of c e r i u m metal. The hydride samples were p r e p a r e d by exposing molten c e r i u m to 0.5 bar hydrogen [2]. The surfaces were c l e a n e d in the UHV system using an Al20~ file.
Figs. 1 and 2 show our isochromat spectra of CeH2.] and CeH:,~ in comparison with b a n d structure calculations of the respective stoichiometric hydrides by A. Fujimori and coworkers [7]. The latter were o b t a i n e d by using the L C A O m e t h o d , omitting the 4f states. T h e s e arc accounted for only in the crystal potentiaL. T h e r e f o r e the calculated densities of states
UV ISOCHR. taw : 9.7 eV
I C eH ? !
,***~v,9~.%,~,,,"
,T CO LIJ p<~
°° o,r
O9
:>,I-2: bJ 12:1 >I-, Z 1.13 I-Z
t EF =0
2
4
6 (E-EF)/eV
Fig. 1. UV isochromat spectrum of ('ell2 ~ together with tile LCAO DOS of (TeHz from ref. 7.
0 3 7 8 - 4 3 6 3 / 8 5 / $ 0 3 . 3 0 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
J. Osterwalder and L. Schlapbach / Unoccupied electronic states in cerium hydrides I
I
I
I
[
I
L
I
I
UV ISOCHR. hc~ : 9,7 eV
¢ ,.,O,4.
Cell2.9
U) bJ I'-1-O9
,.
*~
I..L C, >.t--
I
~"
Z I..iJ r,,
\-
H2 /
o,,)/
/
>.I.Z tlJ I'-Z
I
E F :0
2
I
I
4
I
I
I
6 (E- E F ) / e V
Fig. 2. UV isochromat spectrum of C e l l 29 together with the LCAO DOS of Cell3 from ref. 7. The curve designated with H2 was obtained by using hydrogen gas at 10-7 mb as target.
should reflect the (spd)-DOS alone and can be compared to our spectra. CeHz~ shows an intensity at Er which is 4 to 5 times lower than in the corresponding spectrum of Ce metal (cf. ref. 9). The prominent structure which is seen in the DOS between 0 and 2 eV appears as a broad shoulder, whereas the high DOS between 3 and 6 e V corresponds to the dominating broad feature of the spectrum. A strong peak in the DOS at 6.6 eV is not seen. It has considerable s-character, whereas the rest of the DOS has mostly d-character [7]. The Cell2.9 spectrum reflects fairly well the structures in the corresponding DOS. There is no intensity at EF: CeH29 is a semiconductor (cf. ref. 1). A shoulder appears at 1 . 4 e V and a broad peak at 4.5 eV. The rising background above 6 eV is attributed to hydrogen gas desorbing from the surface of the sample [9]. The agreement between the calculated DOS and our spectra is satisfying if one takes into
525
account that our instrumental resolution is limited and that additional processes contribute to an inverse photoemission spectrum: Incoming electrons can be inelastically scattered in the crystal and probe a different final state energy [12, 13, 14]. This produces a smoothly rising background. In the case of polycrystailine Ni it has been shown (ref. 15, cf. also ref. 16) that the simple DOS picture of inverse photoemission, which implies a homogeneously smearing out of the incoming k vector over the entire Brillouin zone, does not give the correct intensity ratios of the observed structures, but that direct transitions have to be taken into account. We conclude that our spectra give a qualitative picture of the empty (spd)-DOS of CeHz~ and Cell2.9. The reduction of intensity at EF is clearly visible, and structures in the spectrum correspond to regions with a high DOS. This shows that inverse photoemission is a good tool for studying the changes in the empty electronic bandstructure of metals upon hydriding.
Acknowledgements We acknowledge helpful support by T. Riesterer, M. Erbudak, P. Brack and H.C. Siegmann. This work was financed by the Nationale Energie-Forschungs Fonds.
References [1] G.G. Libowitz and A.J. Maeland, in: Handbook on the Physics and Chemistry of Rare Earths, K.A. Gschneidner jr. and L. Eyring, eds. (North-Holland, Amsterdam, 1979), p. 319. [2] L. Schlapbach, J. Osterwalder and H.C. Siegmann, J. Less-Common Met. 88 (1982) 291. [3] A. Fujimori and N. Tsuda, phys. stat. sol. (b) 114 (1982) K139. [4] D.J. Peterman, J.H. Weaver, M. Croft and D.T. Peterson, Phys. Rev. B 27 (1982) 808. [5] J. Osterwalder, H.R. Ott, L. Schlapbach, J. Schefer and P. Fischer, J. Less-Common Met. 94 (1983) 129. [6] J. Schefer, P. Fischer, W. H~ilg, J. Osterwalder, L. Schlapbach and J.D. Jorgensen, J. Phys. C 17 (1984) 1575.
526
J. Osterwalder and L. Schlapbach / Unoccupied electronic states in cerium hydrides
[7] A. Fujimori, F. Minami and N. Tsuda, Phys. Rev. B 22 (1980) 3573. [8] A. Fujimori and N. Tsuda, J. Phys. (" 14 (1981) 1427. [9] J. Osterwalder and L. Schlapbach, Solid State Commun. 52 (1984) 503. [10] G. Denninger, V. Dose and H. Scheidt, Appl. Phys. I~; (1979) 375. [l 1] K. Designer, Diplomarbeit, Physikalisches lnstitu! der Universit~it Wiirzburg (1982).
[12] D.[,. Webster, Phys. Rev. ~ Ilt,~17) 220. [13] P.O. Nilsson and ('.(i. I,arsson, Jap. J. Appl. Phys. 17 Suppl. 17-2 ~lq7,~ 144. [14] V. Dose and (L Reusing, Appl. Phys. 23 (1980~ 131 [15] V. Dose and Th. Fauster, Solid S~ate ('ommun. 5(! t 1984) 67. [16] D.P. Woodrulf, N,V. Snlith, P.D. ,l()hnst~n ~llld W./\
Royer, Phys. Re~. B 26 ! lt~,~2) 2~43.