- Email: [email protected]
Hydrogen-induced states in VHx and VDx observed by soft X-ray emission spectroscopy
Solid State Communications, Vol. 19, pp. 507—509, 1976.
Pergarnon Press.
Printed in Great Britain
HYDROGEN-INDUCED STATES IN VH~AND VD~OBSERVED BY SOFT X-RAY EMISSION SPECTROSCOPY Y. Fukai and S. Kazama Department of Physics, Chuo University, Kasuga, Bunkyo-ku, Tokyo, Japan and K. Tanaka and M. Matsumoto Department of Metallurgy, Nagoya Institute of Technology, Showa-ku, Nagoya, Japan (Received 13 February 1976 by Y. Toyozawa) From measurements of soft X-ray emission spectra (V—L3) of VH~and VD~,a gradual change of d-band of V metal with addition of H(D), and an appearance of H(D)-induced states immediately below the d-band, are clearly demonstrated. The states, located at 7 eV below the Fermi energy for all compositions, are interpreted as being bonding states formed by H(D)—ls and V—3d orbitals. No isotope effects were detected within the accuracy of the experiment. IN SPITE of growing interest in hydrogen in metals in these years, definitive information on its electronic structure has been extremely limited. It was only recently that H-induced levels at the bottom of a d-band were discovered 2inand PdHX by means of inadequacies photoelectron consequently, of spectroscopies,” protonic or anionic models have come to be manifest. Band calculations have shown that these induced levels are bonding states formed by H—1 s and Pd—4d orbitals, and that extra electrons donated by H atoms are accomodated by d-band holes and s—p levels pulled down to the Fermi energy by addition of H.1 These
~ VD~~C$ 1)
results have further provided a basis for understanding 8 the superconductiwty of PdHX. is intended to report on The present communication VD~by means of soft X-ray emission measurements of V—L 2P312 transition). Specimens spectra (sd-band were 3prepared by direct reaction of V foil (99.9%) with gaseous H 2(D2) as described in4 our on NMRwas Thepaper composition VH~~ (a4 our observation of similar H-induced states in VH~ and studies of H-diffusion in VHX. 1) determined to the order of L~x 0.001. The instrument 5 The bent used was described in our report. crystal used was KAP. Theprevious resolution, including the width of the 2P 3/2 level, is estimated at about 0.8 eV. A specimen, a piece of foil 10mm x 20mm x 4Opm, was mounted directly on a copper block, lightly polished 500 505 510 515 with fine MgO powder, and was excited by bombarding with electrons of lOkeV in energy, in a beam of 0.1 mm ENERGY (eV) in diameter. All measurements were performed at room Fig. 1. V—L3 emission spectra of pure V, VH~and VD~. temperature. A possible temperature rise due to electron bombardment and concomitant loss of H(D) are were carefully examined, and found to be minimized by expected to be negligibly small. The peak and backcontinuously shifting the beam position. The remaining ground counts were about 17,000 and 1700 respectively, correction was smaller than 3%. Effects of surface contamination during measurements 507 Results of the measurements are shown in Fig. 1. ________________
__________
_____________
_________
-~
______________
•
•
508
HYDROGEN-INDUCED STATES IN VH~AND VD~ ~
EF of pus’~’/ -~
-~
I
v
pure -~
V0
/ ~J
0734
I
1
4
~ ~
~
-~
2
~
I
I
-12
—10
—8
—6
—4
-2
~
2
4
F
Vol. 19, No.6 states calculated by Wakoh and Yamashita.8 It will be seen that the observed spectrum of pure V compares well with the calculated density of states, especially in regard to the position of characteristic peaks. The fact that the 11(D)-induced hump is accompanied by depression of the low-energy side of the d-band is very similar to the observation in PdHX, and suggests strongly that the hump is
due to bonding states formed by H(D)—is and V—3d orbitals. Let the bonding orbital be written as = c1 i~i~+ ~c2~,t’18, with the summation over neighboring 11(D) atoms The formation of the bonding orbital is expected to reduce the transition probability for L3of cross~transitjonH—is V—2p 2 a probability emission by approximately a factor 312 lciI should be negligibly small. A tentative estimate of the factor Ic1 12 may be derived from the ratio of the integrated intensity of the —~
E E (eV) Fig. 2. Comparison of the V—L3 emission spectra of pure V and VD0,7~and the density of states of pure ‘~~‘ calculated by APW method (reference 8). —
The spectrum pure V agrees well with previous obser6 Thereofare some conspicuous changes in the vations. spectra caused by addition of H(D) atoms, emission First, there appears a hump on the low-energy tail of a main peak (d-band), accompanied by depression of the low-energy side of the d.band, and bulging-out of the high-energy side, without any change in the energy of the peak. Second, these humps are located at 7 eV below the cut-off on the high energy side (Fermi energy) irrespective of composition, whereas the magnitude of these humps is roughly in proportion to concentration of H(D). Third, isotope effects are hardly observable within an accuracy of the present observation. Fourth, the spectra are insensitive to difference in phases: In phase, H(D) atoms are distributed randomly over T-sites of a b.c.c. metal lattice,whereas in i3~and /32 phases, they are known to occupy 0 2-sites of a b.c.t. metal lattice (slightly monoclinic in j3~)~7 In Fig. 2 are reproduced the observed spectra of pure V and VD07~,together with the total density of ~.
.
-
hump to that of the main peak. Assuming that a number of electrons in the d-band is 5, we have from Fig. 2, 1/18 0.734/5 x Ic1 2 and hence Icthe i 1~ density 0.4. The appears to be reasonable. Regarding of value states near the Fermi energy, discussion is inevitably still less precise because of appreciable self-absorption correction yet to be applied. We briefly mention here that a shift of the Fermi energy L~.EF 0.3 eV obtained from Fig. 2, multiplied by N(EF) calculated by Wakoh and Yamashita,8 gives roughly 0.6 electrons per V atom, in reasonable agreement with the number of extra electrons 0.734. It may be reminded that a protonic model, a picture that H atoms exist as protons donating their electrons to the d.band of V metal, has been deduced, in the main, from the variation of N(EF) with composition manifested by magnetic susceptibility,9 electronic specific heats,1°T~and Knight shifts of NMR.’1~12However, it is proved from the present observation that N(E~) alone shows up the ostensible rigid-band behavior, and that the density of states below the peak of the d-band is heavily deformed, and by no means rigid.
REFERENCES 1.
EASTMAN D.E., CASHION J.K. & SWITENDICK A.C., Phys. Rev. Lett. 27, 35 (1971).
2. 3.
ANTONANGELI F., BALZAROTTI A., BLANCONI A., BURATTINI E., PERFETTI F. & NISTICO N., Phys. Lett. 55A, 309 (1975). MILLER R.J. & SATTERTHWAITE C.B.,Phys. Rev. Lett. 34, 144 (1975).
4.
FUKAI Y. & KAZAMA S., Acta Met. (to be published).
5. 6.
TANAKA K., MATSUMOTO M., MARUNO S. & HIRAKI A., App!. Phys. Lett. 27, 529 (1975). NEMNONOV S.A. & BRYTOV L.A., Fiz. Metal Metalloved. 26,43 (1968); FISCHER D.W., J. Appi. Phys. 40, 4151 (1969). For structural information, see, e.g. HIRABAYSHI M., YAMAGUCHI S., ASANO H. & HIRAGA K., Order— Disorder Transformations in Alloys (Edited by WARLIMONT H.), p. 266. Springer.Verlag (1974); FUKAI Y. & KAZAMA S.,Scripta Met. 9, 1073 (1975), and references cited therein.
7.
Vol. 19, No.6 8. 9.
HYDROGEN-INDUCED STATES IN VH~AND VD~
10.
WAKOH S. & YAMASHITA J., J. Phys. Soc. Japan 35, 1394 (1973). ZANOWICK R.L. & WALLACE W.E.,J. Chem. Phys. 36, 2059 (1962); ARONSON S., REILLY J.J. & WISWALL R.H., Jr., J. Less-Common Metals 21, 439 (1970). ROHY D. & COlTS R.M.,Phys. Rev. B!, 2484 (1970) and references cited therein.
11.
KAZAMA S. & FUKAI Y.,Phys. Lett. S1A, 373 (1975) and references cited therein.
12.
ZAMIR D.,Phys. Rev. 140, A271 (1965) and references cited therm; KAZAMA S. & FUKAI Y. (to be published).
509
Pergarnon Press.
Printed in Great Britain
HYDROGEN-INDUCED STATES IN VH~AND VD~OBSERVED BY SOFT X-RAY EMISSION SPECTROSCOPY Y. Fukai and S. Kazama Department of Physics, Chuo University, Kasuga, Bunkyo-ku, Tokyo, Japan and K. Tanaka and M. Matsumoto Department of Metallurgy, Nagoya Institute of Technology, Showa-ku, Nagoya, Japan (Received 13 February 1976 by Y. Toyozawa) From measurements of soft X-ray emission spectra (V—L3) of VH~and VD~,a gradual change of d-band of V metal with addition of H(D), and an appearance of H(D)-induced states immediately below the d-band, are clearly demonstrated. The states, located at 7 eV below the Fermi energy for all compositions, are interpreted as being bonding states formed by H(D)—ls and V—3d orbitals. No isotope effects were detected within the accuracy of the experiment. IN SPITE of growing interest in hydrogen in metals in these years, definitive information on its electronic structure has been extremely limited. It was only recently that H-induced levels at the bottom of a d-band were discovered 2inand PdHX by means of inadequacies photoelectron consequently, of spectroscopies,” protonic or anionic models have come to be manifest. Band calculations have shown that these induced levels are bonding states formed by H—1 s and Pd—4d orbitals, and that extra electrons donated by H atoms are accomodated by d-band holes and s—p levels pulled down to the Fermi energy by addition of H.1 These
~ VD~~C$ 1)
results have further provided a basis for understanding 8 the superconductiwty of PdHX. is intended to report on The present communication VD~by means of soft X-ray emission measurements of V—L 2P312 transition). Specimens spectra (sd-band were 3prepared by direct reaction of V foil (99.9%) with gaseous H 2(D2) as described in4 our on NMRwas Thepaper composition VH~~ (a4 our observation of similar H-induced states in VH~ and studies of H-diffusion in VHX. 1) determined to the order of L~x 0.001. The instrument 5 The bent used was described in our report. crystal used was KAP. Theprevious resolution, including the width of the 2P 3/2 level, is estimated at about 0.8 eV. A specimen, a piece of foil 10mm x 20mm x 4Opm, was mounted directly on a copper block, lightly polished 500 505 510 515 with fine MgO powder, and was excited by bombarding with electrons of lOkeV in energy, in a beam of 0.1 mm ENERGY (eV) in diameter. All measurements were performed at room Fig. 1. V—L3 emission spectra of pure V, VH~and VD~. temperature. A possible temperature rise due to electron bombardment and concomitant loss of H(D) are were carefully examined, and found to be minimized by expected to be negligibly small. The peak and backcontinuously shifting the beam position. The remaining ground counts were about 17,000 and 1700 respectively, correction was smaller than 3%. Effects of surface contamination during measurements 507 Results of the measurements are shown in Fig. 1. ________________
__________
_____________
_________
-~
______________
•
•
508
HYDROGEN-INDUCED STATES IN VH~AND VD~ ~
EF of pus’~’/ -~
-~
I
v
pure -~
V0
/ ~J
0734
I
1
4
~ ~
~
-~
2
~
I
I
-12
—10
—8
—6
—4
-2
~
2
4
F
Vol. 19, No.6 states calculated by Wakoh and Yamashita.8 It will be seen that the observed spectrum of pure V compares well with the calculated density of states, especially in regard to the position of characteristic peaks. The fact that the 11(D)-induced hump is accompanied by depression of the low-energy side of the d-band is very similar to the observation in PdHX, and suggests strongly that the hump is
due to bonding states formed by H(D)—is and V—3d orbitals. Let the bonding orbital be written as = c1 i~i~+ ~c2~,t’18, with the summation over neighboring 11(D) atoms The formation of the bonding orbital is expected to reduce the transition probability for L3of cross~transitjonH—is V—2p 2 a probability emission by approximately a factor 312 lciI should be negligibly small. A tentative estimate of the factor Ic1 12 may be derived from the ratio of the integrated intensity of the —~
E E (eV) Fig. 2. Comparison of the V—L3 emission spectra of pure V and VD0,7~and the density of states of pure ‘~~‘ calculated by APW method (reference 8). —
The spectrum pure V agrees well with previous obser6 Thereofare some conspicuous changes in the vations. spectra caused by addition of H(D) atoms, emission First, there appears a hump on the low-energy tail of a main peak (d-band), accompanied by depression of the low-energy side of the d.band, and bulging-out of the high-energy side, without any change in the energy of the peak. Second, these humps are located at 7 eV below the cut-off on the high energy side (Fermi energy) irrespective of composition, whereas the magnitude of these humps is roughly in proportion to concentration of H(D). Third, isotope effects are hardly observable within an accuracy of the present observation. Fourth, the spectra are insensitive to difference in phases: In phase, H(D) atoms are distributed randomly over T-sites of a b.c.c. metal lattice,whereas in i3~and /32 phases, they are known to occupy 0 2-sites of a b.c.t. metal lattice (slightly monoclinic in j3~)~7 In Fig. 2 are reproduced the observed spectra of pure V and VD07~,together with the total density of ~.
.
-
hump to that of the main peak. Assuming that a number of electrons in the d-band is 5, we have from Fig. 2, 1/18 0.734/5 x Ic1 2 and hence Icthe i 1~ density 0.4. The appears to be reasonable. Regarding of value states near the Fermi energy, discussion is inevitably still less precise because of appreciable self-absorption correction yet to be applied. We briefly mention here that a shift of the Fermi energy L~.EF 0.3 eV obtained from Fig. 2, multiplied by N(EF) calculated by Wakoh and Yamashita,8 gives roughly 0.6 electrons per V atom, in reasonable agreement with the number of extra electrons 0.734. It may be reminded that a protonic model, a picture that H atoms exist as protons donating their electrons to the d.band of V metal, has been deduced, in the main, from the variation of N(EF) with composition manifested by magnetic susceptibility,9 electronic specific heats,1°T~and Knight shifts of NMR.’1~12However, it is proved from the present observation that N(E~) alone shows up the ostensible rigid-band behavior, and that the density of states below the peak of the d-band is heavily deformed, and by no means rigid.
REFERENCES 1.
EASTMAN D.E., CASHION J.K. & SWITENDICK A.C., Phys. Rev. Lett. 27, 35 (1971).
2. 3.
ANTONANGELI F., BALZAROTTI A., BLANCONI A., BURATTINI E., PERFETTI F. & NISTICO N., Phys. Lett. 55A, 309 (1975). MILLER R.J. & SATTERTHWAITE C.B.,Phys. Rev. Lett. 34, 144 (1975).
4.
FUKAI Y. & KAZAMA S., Acta Met. (to be published).
5. 6.
TANAKA K., MATSUMOTO M., MARUNO S. & HIRAKI A., App!. Phys. Lett. 27, 529 (1975). NEMNONOV S.A. & BRYTOV L.A., Fiz. Metal Metalloved. 26,43 (1968); FISCHER D.W., J. Appi. Phys. 40, 4151 (1969). For structural information, see, e.g. HIRABAYSHI M., YAMAGUCHI S., ASANO H. & HIRAGA K., Order— Disorder Transformations in Alloys (Edited by WARLIMONT H.), p. 266. Springer.Verlag (1974); FUKAI Y. & KAZAMA S.,Scripta Met. 9, 1073 (1975), and references cited therein.
7.
Vol. 19, No.6 8. 9.
HYDROGEN-INDUCED STATES IN VH~AND VD~
10.
WAKOH S. & YAMASHITA J., J. Phys. Soc. Japan 35, 1394 (1973). ZANOWICK R.L. & WALLACE W.E.,J. Chem. Phys. 36, 2059 (1962); ARONSON S., REILLY J.J. & WISWALL R.H., Jr., J. Less-Common Metals 21, 439 (1970). ROHY D. & COlTS R.M.,Phys. Rev. B!, 2484 (1970) and references cited therein.
11.
KAZAMA S. & FUKAI Y.,Phys. Lett. S1A, 373 (1975) and references cited therein.
12.
ZAMIR D.,Phys. Rev. 140, A271 (1965) and references cited therm; KAZAMA S. & FUKAI Y. (to be published).
509