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Borate substitution in YSrBaCu3O7 studied by X-ray emission spectroscopy
PHYSICA Physica C 227 (1994) 309-312
ELGEVIER
Borate substitution in YSrBaCu307 studied by X-ray emission spectroscopy E.Z. Kurmaev *, S.N. Shamin Institute of Metal Physics, Russian Academy of Sciences - Ural Division, 620219 Yekaterinburg, GSP-170, Russian Federation
P.R. Slater, C. Greaves Superconductivity Research Group, School of Chemistry, University of Birmingham, Birmingham BI5 2TT, UK
Received 22 March 1994;revised manuscript received 27 April 1994
Abstract The results of measurements ofB Kct X-ray emission spectra of YSrBaCu2.sBo.5Ox(as prepared and after high oxygenpressure treatment) are presented. The data indicate that the boron atoms form B-O bonds which are similar to those observed in B203 and FeBO3. It is concluded that the boron atoms substitute at the "chain" Cu sites in the Y 123 structure and form B-O bonds typical for isolated BO]- groups.
1. Introduction It has been reported that oxyanion substitutions (CO 2- , SO 2- , PO 3- ) at the copper sites can stabilize the Ba free phase YSr2Cu307 and allow synthesis to occur under ambient conditions [ 1,2 ]. Similar results have recently been described for borate (BO3~: ) anions [ 3,4 ] and the effects of partially replacing Cu by borate anions in the system YSr2_xBaxCu3_zB=Oy have been reported [5]. The optimum substitution level to give single-phase samples was found to be z = 0.5. Although samples of this composition were found to be non-superconducting as prepared, superconductivity could be induced by annealing in high-pressure oxygen. Using X-ray emission spectroscopy it is possible to study the local structure of boron atoms in such compounds, to estimate the actual level of boron incorporation into * Correspondingauthor.
the Y 123 lattice, and to determine any modifications after high-pressure treatment. This method has recently been used for the analysis of substitution effects in related high-To superconductors and has proved very successful for the determination of the nature of atoms substituting into the crystal structure and the preferred occupation of specific sites [6-11 ]. In the present paper the results of measurements of B Ka X-ray emission spectra (XES) of two samples YSrBaCu2.sB0.5Ox (non-superconducting) and YSrBaCu2.sBo.sOx (Tc = 25 K, annealed at high oxygen pressure) are presented.
2. Experiment The samples were prepared by using standard ceramic methods. High-purity Y2CO3, SrCO3, BaCO3, CuO and H3BO3 were used to prepare YSrBaCuE.sBo.5Ox. The intimately ground powders were
0921-4534/94/$07.00 © 1994 Elsevier ScienceB.V. All fights reserved SSD10921-4534 ( 94 ) 00260-M
E.Z. Kurmaev et aL / Phystca ('227 (1994) 309-312
310
heated in oxygen for 14 h at 1000°C, pressed into pellets, given a similar heat treatment and finally cooled slowly (24 h) in oxygen. Half the sample was annealed at 425°C for 24 h in 250 bar oxygen. Superconducting properties were investigated using a Cryogenics S100 S Q U I D susceptometer. Powder X-ray diffraction (Cu Ka radiation), Fig. 1, indicated that YSrBaCu2.sBo.5Ox was single phase as previously reported [5]. Although superconductivity could not be detected in the YSrBaCu2.sBo.50, sample as prepared, after treatment under high-pressure oxygen there was evidence for a small superconducting fraction with T~.... , of 25 K. S Q U I D data are given in Fig. 2. As is seen the signal is weak. The signal strength is improved and the divergence of FC and ZFC experiments disappears after Ca doping. B Kct XES measurements were made using an ul-
o
E O O
20
30
40
50
Scattering Angle (Beg 20) Fig. 1. X-ray diffraction profile of YSrBaCu2.sBo.50~indexed on a tetragonal unit cell (a = 3.850 ( 2 ) A, c= 11.266 ( 7 ) ~ ). 50
B D D B B D• D [] .................
/ O
.......................
0
E
[]
[]
[] D
D
x
-S0
n []
[] -100 0
10
20
30
40
Temperature (K) Fig. 2. Molar susceptibility of YBaSrCuzsBo.~Ox showing a small superconducting fraction with onset of ca. 25 K. Ca substitutions significantly improve the superconducting properties [5 ].
tra-sofl spectrometer with high spatial ( A S = 0.5 p m ) and energy resolution ( for B Kc~ A E = 1.7 eV ) [ 12 ]. The X-ray radiation was analyzed by a curved diffraction grating ( R = 2 m, :V=600 lines/mm) and a focused electron beam was used for excitation of Xray emission spectra. The X-ray tube was operated in the soft regime ( l '= 8 kcV, I = 600 hA) and the sample position with respect to the electron beam changed for every scan to avoid sample decomposition during measurements.
3. Results and discussion
X-ray emission valence spectra are generated when a hole in a core orbital is created by electron or photon bombardment and then an electron from the valence band decays into the core vacancy emitting its excess energy. Since the core orbitals correspond to relatively sharp energy levels the fine structure and width of the XES reflects the density of states in the valence band [ 13 ]. This method is especially useful for study of the electronic structure of binary and multi-component alloys and compounds because the X-ray transition is localized in the first coordination sphere of the emitting atom, and therefore allows for separation of the electron densities in the valence band which originate from different elements in the sample. In this way it is possible to examine thc changes in local environment of the emitting atom caused by doping or different treatments to the material [ 14-17 ]. In the case of B K~ XES, the allowed dipole transition corresponds to l s-2p which means that onl~ 2p electrons can decay and the intensity distribution of the B Kct XES gives direct information about the 2p density of states distribution localized at the boron atoms. It has been shown [ 1 8 - 2 0 ] that the energy and fine structure of B K~t XES is different in oxide and transition-metal betides, and therefore should allow differentiation to be made between d i f ferent environments of boron atoms in superconducting cuprates. B Kc~ XES data are shown in Fig. 3 for samples of YSrBaCuz.sBo.50~ as prepared and after high-pressure oxygen annealing. For comparison, the spectra of reference samples B203 and FeBO3 are given on the same figure. It is s e e n that the spectrum for
E.Z. Kurmaev et al. / Physica C 227 (1994) 309-312
BKa
:
~
" Z
.~
m>~ ~
~
~ g
%
. . . . .
.....
~ ~
..... .....
BzOa NlO
N9 FeBOa
j
:....-.
.-- :'-:
-
-..-.
•
i • ~;"~
~
--
-
•• :
...".
. .."
-.9
" ., ,.
~
. . •
• .-...... . • o.
".
oO° "O,o
Photon
energy
(eV).
Fig. 3. B Ktt XES of YSrBaCu2.~Bo.50~ as prepared (N 9) and after treatment under high oxygen pressure (N 10) and their comparison with spectra of B203 and FeBO3. YSrBaCu2.sBo.5Ox as p r e p a r e d (N 9) and after treatm e n t under high oxygen pressure ( N 10) have a similar fine structure: the m a i n X-ray emission sub-band centered at E = 183 eV and a low-energy sub-band at E = 168 eV. The energy position o f these spectral features and their intensity ratios are very similar to those observed for B203 and FeBO3. According to Ref. [ 18 ] the origin o f these sub-bands is attributed to O 2p and O 2s states which are revealed in B K a XES due to mixing o f B 2p states. The energy difference o f these sub-bands, AE = 15 eV, is close to the difference o f O 2p and O 2s levels in the free oxygen a t o m [ 13 ] and can serve as a d d i t i o n a l evidence in favor o f the above interpretation o f B K a XES from B203 and FeBO3. Detailed c o m p a r i s o n o f the B K a XES spectra o f the YSrBaCuz.sBo.5Ox samples and their c o m p a r i s o n with those o f B203 a n d FeBO3 clearly indicates that short B - O b o n d s exist in the B containing YSrBaCu307 samples. The full width at half m a x i m u m o f the m a i n sub-band o f B K a XES of YSrBaCu2.sB0.sOx is 1.5 times less than those o f B203 a n d FeBO3, indicating that in the case o f YSrBaCu2.sBo.5Ox boron a t o m s form isolated borate groups which do not touch in crystal structure o f this c o m p o u n d . Boron a t o m s in YSrBaCu2.sBo.sOx compounds most probably have the oxidation state B 3+
311
because o f the closeness o f their B K a XES energy position to those 0fB203 and FeBO3. We therefore infer that isolated [ B O ] - ] groups exist within the lattice, in complete agreement with structural studies o f these materials [ 4 ], which indicated partial substitution o f the " c h a i n " Cu sites by such BO 3- groups. The statistics o f the m e a s u r e d B K a XES spectra o f YSrBaCu2.sBo5Ox samples was not very good because o f the small B content o f the compounds. It is consequently difficult to find any significant difference in the B K a XES d a t a after t r e a t m e n t under high oxygen pressure. We can only point out some small b r o a d e n i n g in the X-ray emission spectrum o f the sample after such treatment, but we cannot offer any rational explanation for this fact at present.
References [ 1] Y. Miyazaki, H. Yamane, N. Ohnishi, T. Kajitani, K. Hiraga, Y. Morii, S. Funahashi and T. Hirai, Physica C 198 ( 1992 ) 7. [2] P.R. Slater, C. Greaves, M. Slaski and C.M. Muirhead, Physica C 208 ( 1993 ) 193. [3] J.Q. Li, W.J. Zhu, Z.X. Zhao and D.L. Yin, Solid State Commun. 85 (1993) 739. [4] W.J. Zhu, J.J. Yue, Y.Z. Huang and Z.X. Zhao, Physica C 205 (1993) 118. [ 5 ] P.R. Slater and C. Greaves, Physica C 215 ( 1993 ) 191. [6]S.M. Butorin, V.R. Galakhov, E.Z. Kurrnaev, S.M. Cheshnitsky, S.A. Lebedev and E.F. Kukovitzki, Phys. Rev. B47 (1993) 9035. [7] Yu.M. Yarmoshenko, V.A. Trofimova, L.V. Elokhina, E.Z. Kurmaev, S. Butorin, R. Cloots and M. Ausloos, Physica C 211 (1993)29. [8] Yu.M. Yarmoshenko, V.A. Trofimova, UV. Elokhina, E.Z. Kurmaev, S. Butorin, R. Cloots, M. Ausloos, J. Albino Aguiar and N.I. Lobatchevskaya, J. Phys. Chem. Sol. 54 (1993) 1211. [9] E.Z. Kurmaev, V.A. Trofimova, Yu.M. Yarmoshenko and V.V. Sokolov, Russian J. Supercond. 5 (1992) 1492. [ 10] Yu.M. Yarmoshenko, V.A. Trofimova, E.Z. Kurmaev, P.R. Slater and C. Greaves, Physica C 224 ( 1994 ) 317. [11] V.V. Fedorenko, V.R. Galakhov, L.V. Elokhina, L.D. Finkelstein, V.E. Naish, E.Z. Kurmaev, S.M. Butorin, E.J. Nordgren, A.K. Tyagi, U.R.K. Rao and R.M. Iyer, Physica C221 (1994) 71. [12]E.Z. Kurmaev, V.V. Fedorenko, S.N. Shamin, A.V. Postnikov, G. Wiech and Y. Kim, Phys. Scripta T 41 (1992) 288. [13] E.Z. Kurmaev, V.M. Cherkashenko and L.D. Finkelstein, X-Ray Spectra of Solids (Nauka, Moscow, 1988)p. 175. [ 14 ] E.Z. Kurmaev, V.P. Belash, R. Flukiger and A. Junod, Solid State Commun. 16 (1975) 1139.
312
E . Z Kurmaev et al. /Physica C 227 (1994) 309-312
[15] A.Z. Menshikov, E.Z. Kurmaev, Russian J. Phys. Metal Metallogr. 41 (1976) 748. [16]E.Z. Kurmaev, Yu.M. Yarmoshenko, V.E. Dolgih, S.A. Nemnonov, F. Werfel and O. Brummer, Solid Stale Commun. 29 (1979) 59. [17] V.R. Galakhov, V.M. Cherkashenko, M.Ya. Hodos, S.N. Nemnonov, E.V. Kuznetsov, E.Z. Kurmaev and V.A. Gubanov, J. Eleetr. Spectr. Relat. Phenom. 35 ( 1985 ) 87.
[ 18 ] V.A. Fomichev, Soviet J. Solid State Phys. 9 ( 1967 ) 3167. [ 19] D.W. Fischer and W.L. Baun, J. Appl. Phys. 37 (1966) 768. [20] T.B. Shashkina, I.A. Brytov, Yu.N. Romashenko and V.I. Minin, in: Proc. of Int. Sympos. Electronic Structure of Transition Metals, Their Alloys and Compounds (Naukova Dumka, Kiev, 1974) p. 182.
ELGEVIER
Borate substitution in YSrBaCu307 studied by X-ray emission spectroscopy E.Z. Kurmaev *, S.N. Shamin Institute of Metal Physics, Russian Academy of Sciences - Ural Division, 620219 Yekaterinburg, GSP-170, Russian Federation
P.R. Slater, C. Greaves Superconductivity Research Group, School of Chemistry, University of Birmingham, Birmingham BI5 2TT, UK
Received 22 March 1994;revised manuscript received 27 April 1994
Abstract The results of measurements ofB Kct X-ray emission spectra of YSrBaCu2.sBo.5Ox(as prepared and after high oxygenpressure treatment) are presented. The data indicate that the boron atoms form B-O bonds which are similar to those observed in B203 and FeBO3. It is concluded that the boron atoms substitute at the "chain" Cu sites in the Y 123 structure and form B-O bonds typical for isolated BO]- groups.
1. Introduction It has been reported that oxyanion substitutions (CO 2- , SO 2- , PO 3- ) at the copper sites can stabilize the Ba free phase YSr2Cu307 and allow synthesis to occur under ambient conditions [ 1,2 ]. Similar results have recently been described for borate (BO3~: ) anions [ 3,4 ] and the effects of partially replacing Cu by borate anions in the system YSr2_xBaxCu3_zB=Oy have been reported [5]. The optimum substitution level to give single-phase samples was found to be z = 0.5. Although samples of this composition were found to be non-superconducting as prepared, superconductivity could be induced by annealing in high-pressure oxygen. Using X-ray emission spectroscopy it is possible to study the local structure of boron atoms in such compounds, to estimate the actual level of boron incorporation into * Correspondingauthor.
the Y 123 lattice, and to determine any modifications after high-pressure treatment. This method has recently been used for the analysis of substitution effects in related high-To superconductors and has proved very successful for the determination of the nature of atoms substituting into the crystal structure and the preferred occupation of specific sites [6-11 ]. In the present paper the results of measurements of B Ka X-ray emission spectra (XES) of two samples YSrBaCu2.sB0.5Ox (non-superconducting) and YSrBaCu2.sBo.sOx (Tc = 25 K, annealed at high oxygen pressure) are presented.
2. Experiment The samples were prepared by using standard ceramic methods. High-purity Y2CO3, SrCO3, BaCO3, CuO and H3BO3 were used to prepare YSrBaCuE.sBo.5Ox. The intimately ground powders were
0921-4534/94/$07.00 © 1994 Elsevier ScienceB.V. All fights reserved SSD10921-4534 ( 94 ) 00260-M
E.Z. Kurmaev et aL / Phystca ('227 (1994) 309-312
310
heated in oxygen for 14 h at 1000°C, pressed into pellets, given a similar heat treatment and finally cooled slowly (24 h) in oxygen. Half the sample was annealed at 425°C for 24 h in 250 bar oxygen. Superconducting properties were investigated using a Cryogenics S100 S Q U I D susceptometer. Powder X-ray diffraction (Cu Ka radiation), Fig. 1, indicated that YSrBaCu2.sBo.5Ox was single phase as previously reported [5]. Although superconductivity could not be detected in the YSrBaCu2.sBo.50, sample as prepared, after treatment under high-pressure oxygen there was evidence for a small superconducting fraction with T~.... , of 25 K. S Q U I D data are given in Fig. 2. As is seen the signal is weak. The signal strength is improved and the divergence of FC and ZFC experiments disappears after Ca doping. B Kct XES measurements were made using an ul-
o
E O O
20
30
40
50
Scattering Angle (Beg 20) Fig. 1. X-ray diffraction profile of YSrBaCu2.sBo.50~indexed on a tetragonal unit cell (a = 3.850 ( 2 ) A, c= 11.266 ( 7 ) ~ ). 50
B D D B B D• D [] .................
/ O
.......................
0
E
[]
[]
[] D
D
x
-S0
n []
[] -100 0
10
20
30
40
Temperature (K) Fig. 2. Molar susceptibility of YBaSrCuzsBo.~Ox showing a small superconducting fraction with onset of ca. 25 K. Ca substitutions significantly improve the superconducting properties [5 ].
tra-sofl spectrometer with high spatial ( A S = 0.5 p m ) and energy resolution ( for B Kc~ A E = 1.7 eV ) [ 12 ]. The X-ray radiation was analyzed by a curved diffraction grating ( R = 2 m, :V=600 lines/mm) and a focused electron beam was used for excitation of Xray emission spectra. The X-ray tube was operated in the soft regime ( l '= 8 kcV, I = 600 hA) and the sample position with respect to the electron beam changed for every scan to avoid sample decomposition during measurements.
3. Results and discussion
X-ray emission valence spectra are generated when a hole in a core orbital is created by electron or photon bombardment and then an electron from the valence band decays into the core vacancy emitting its excess energy. Since the core orbitals correspond to relatively sharp energy levels the fine structure and width of the XES reflects the density of states in the valence band [ 13 ]. This method is especially useful for study of the electronic structure of binary and multi-component alloys and compounds because the X-ray transition is localized in the first coordination sphere of the emitting atom, and therefore allows for separation of the electron densities in the valence band which originate from different elements in the sample. In this way it is possible to examine thc changes in local environment of the emitting atom caused by doping or different treatments to the material [ 14-17 ]. In the case of B K~ XES, the allowed dipole transition corresponds to l s-2p which means that onl~ 2p electrons can decay and the intensity distribution of the B Kct XES gives direct information about the 2p density of states distribution localized at the boron atoms. It has been shown [ 1 8 - 2 0 ] that the energy and fine structure of B K~t XES is different in oxide and transition-metal betides, and therefore should allow differentiation to be made between d i f ferent environments of boron atoms in superconducting cuprates. B Kc~ XES data are shown in Fig. 3 for samples of YSrBaCuz.sBo.50~ as prepared and after high-pressure oxygen annealing. For comparison, the spectra of reference samples B203 and FeBO3 are given on the same figure. It is s e e n that the spectrum for
E.Z. Kurmaev et al. / Physica C 227 (1994) 309-312
BKa
:
~
" Z
.~
m>~ ~
~
~ g
%
. . . . .
.....
~ ~
..... .....
BzOa NlO
N9 FeBOa
j
:....-.
.-- :'-:
-
-..-.
•
i • ~;"~
~
--
-
•• :
...".
. .."
-.9
" ., ,.
~
. . •
• .-...... . • o.
".
oO° "O,o
Photon
energy
(eV).
Fig. 3. B Ktt XES of YSrBaCu2.~Bo.50~ as prepared (N 9) and after treatment under high oxygen pressure (N 10) and their comparison with spectra of B203 and FeBO3. YSrBaCu2.sBo.5Ox as p r e p a r e d (N 9) and after treatm e n t under high oxygen pressure ( N 10) have a similar fine structure: the m a i n X-ray emission sub-band centered at E = 183 eV and a low-energy sub-band at E = 168 eV. The energy position o f these spectral features and their intensity ratios are very similar to those observed for B203 and FeBO3. According to Ref. [ 18 ] the origin o f these sub-bands is attributed to O 2p and O 2s states which are revealed in B K a XES due to mixing o f B 2p states. The energy difference o f these sub-bands, AE = 15 eV, is close to the difference o f O 2p and O 2s levels in the free oxygen a t o m [ 13 ] and can serve as a d d i t i o n a l evidence in favor o f the above interpretation o f B K a XES from B203 and FeBO3. Detailed c o m p a r i s o n o f the B K a XES spectra o f the YSrBaCuz.sBo.5Ox samples and their c o m p a r i s o n with those o f B203 a n d FeBO3 clearly indicates that short B - O b o n d s exist in the B containing YSrBaCu307 samples. The full width at half m a x i m u m o f the m a i n sub-band o f B K a XES of YSrBaCu2.sB0.sOx is 1.5 times less than those o f B203 a n d FeBO3, indicating that in the case o f YSrBaCu2.sBo.5Ox boron a t o m s form isolated borate groups which do not touch in crystal structure o f this c o m p o u n d . Boron a t o m s in YSrBaCu2.sBo.sOx compounds most probably have the oxidation state B 3+
311
because o f the closeness o f their B K a XES energy position to those 0fB203 and FeBO3. We therefore infer that isolated [ B O ] - ] groups exist within the lattice, in complete agreement with structural studies o f these materials [ 4 ], which indicated partial substitution o f the " c h a i n " Cu sites by such BO 3- groups. The statistics o f the m e a s u r e d B K a XES spectra o f YSrBaCu2.sBo5Ox samples was not very good because o f the small B content o f the compounds. It is consequently difficult to find any significant difference in the B K a XES d a t a after t r e a t m e n t under high oxygen pressure. We can only point out some small b r o a d e n i n g in the X-ray emission spectrum o f the sample after such treatment, but we cannot offer any rational explanation for this fact at present.
References [ 1] Y. Miyazaki, H. Yamane, N. Ohnishi, T. Kajitani, K. Hiraga, Y. Morii, S. Funahashi and T. Hirai, Physica C 198 ( 1992 ) 7. [2] P.R. Slater, C. Greaves, M. Slaski and C.M. Muirhead, Physica C 208 ( 1993 ) 193. [3] J.Q. Li, W.J. Zhu, Z.X. Zhao and D.L. Yin, Solid State Commun. 85 (1993) 739. [4] W.J. Zhu, J.J. Yue, Y.Z. Huang and Z.X. Zhao, Physica C 205 (1993) 118. [ 5 ] P.R. Slater and C. Greaves, Physica C 215 ( 1993 ) 191. [6]S.M. Butorin, V.R. Galakhov, E.Z. Kurrnaev, S.M. Cheshnitsky, S.A. Lebedev and E.F. Kukovitzki, Phys. Rev. B47 (1993) 9035. [7] Yu.M. Yarmoshenko, V.A. Trofimova, L.V. Elokhina, E.Z. Kurmaev, S. Butorin, R. Cloots and M. Ausloos, Physica C 211 (1993)29. [8] Yu.M. Yarmoshenko, V.A. Trofimova, UV. Elokhina, E.Z. Kurmaev, S. Butorin, R. Cloots, M. Ausloos, J. Albino Aguiar and N.I. Lobatchevskaya, J. Phys. Chem. Sol. 54 (1993) 1211. [9] E.Z. Kurmaev, V.A. Trofimova, Yu.M. Yarmoshenko and V.V. Sokolov, Russian J. Supercond. 5 (1992) 1492. [ 10] Yu.M. Yarmoshenko, V.A. Trofimova, E.Z. Kurmaev, P.R. Slater and C. Greaves, Physica C 224 ( 1994 ) 317. [11] V.V. Fedorenko, V.R. Galakhov, L.V. Elokhina, L.D. Finkelstein, V.E. Naish, E.Z. Kurmaev, S.M. Butorin, E.J. Nordgren, A.K. Tyagi, U.R.K. Rao and R.M. Iyer, Physica C221 (1994) 71. [12]E.Z. Kurmaev, V.V. Fedorenko, S.N. Shamin, A.V. Postnikov, G. Wiech and Y. Kim, Phys. Scripta T 41 (1992) 288. [13] E.Z. Kurmaev, V.M. Cherkashenko and L.D. Finkelstein, X-Ray Spectra of Solids (Nauka, Moscow, 1988)p. 175. [ 14 ] E.Z. Kurmaev, V.P. Belash, R. Flukiger and A. Junod, Solid State Commun. 16 (1975) 1139.
312
E . Z Kurmaev et al. /Physica C 227 (1994) 309-312
[15] A.Z. Menshikov, E.Z. Kurmaev, Russian J. Phys. Metal Metallogr. 41 (1976) 748. [16]E.Z. Kurmaev, Yu.M. Yarmoshenko, V.E. Dolgih, S.A. Nemnonov, F. Werfel and O. Brummer, Solid Stale Commun. 29 (1979) 59. [17] V.R. Galakhov, V.M. Cherkashenko, M.Ya. Hodos, S.N. Nemnonov, E.V. Kuznetsov, E.Z. Kurmaev and V.A. Gubanov, J. Eleetr. Spectr. Relat. Phenom. 35 ( 1985 ) 87.
[ 18 ] V.A. Fomichev, Soviet J. Solid State Phys. 9 ( 1967 ) 3167. [ 19] D.W. Fischer and W.L. Baun, J. Appl. Phys. 37 (1966) 768. [20] T.B. Shashkina, I.A. Brytov, Yu.N. Romashenko and V.I. Minin, in: Proc. of Int. Sympos. Electronic Structure of Transition Metals, Their Alloys and Compounds (Naukova Dumka, Kiev, 1974) p. 182.