- Email: [email protected]
On the charge distribution in manganese borides studied by X-ray photoelectron and emission spectroscopy
J. Phys.
Chem.
Solids,
1975, Vol. 36, pp. 3740.
Pergamon Press. Printed
in Great Britain
ON THE CHARGE DISTRIBUTION IN MANGANESE BORIDES STUDIED BY X-RAY PHOTOELECTRON AND EMISSION SPECTROSCOPY V. V. NEMOSHKALENKO,*T. B. SHASHKINA, V. G. ALJESHIN and A. I. SENKEVICH Institute of Metal Physics, Kiev, U.S.S.R. (Received
3 July 1973; in revised form
3 April 1974)
Abstract-For the borides of the Mn-B system the values of binding energies of Mn 3p, 2p3,2, 2p,,, and Bls core levels were measured on an electron spectrometer. Depending on boride composition the variation of inner level energies is very similar, showing a change of sign in the region of the monoboride phase. The same variation has been observed for some characteristics of X-ray emission spectra from these borides. An attempt is made to understand which of the chemical bonding factors are chiefly responsible for a regular concentration dependence of the X-ray and photoelectron spectra.
INTRODUCTION As is known, both X-ray emission spectra and X-ray induced photoemission spectra can help obtain information concerning the charge distribution and the relative amounts of the covalent/ metallic/ionic bonds. In the former-positions of the higher levels containing the valence electrons are involved, in the latter-those of the inner levels. The energies of all electrons are subject to significant shifts depending on changes in the chemical environment and electronic structure. The question of charge distribution between the metal and the non-metal atoms in transition metal borides (for a survey, see [l]) still remains controversial and there are recent data some of which point to electron transfer from metal to boron atoms[2] while other support the idea that boron atoms act as electron donors [3]. It has been shown by Ramqvist et al.[4,5] that the experimental techniques of X-ray emission and electron spectroscopy can be very useful in elucidating the problem of charge transfer in transition metal-nonmetal compounds such as carbides, nitrides and borides. As an extension of previous X-ray spectroscopic studies of the compounds in the Mn-B system [6,7] we report in this paper on the core-level binding energies from the X-ray photoelectron spectra of M&B, MnzB, MnB, Mn3B4 and MnB2 borides. The
measurements were carried out with a Varian IEE-15 spectrometer using Al K, radiation for excitation. The operating vacuum was 1 x lo-’ torr. Since all compounds were powders, the samples (the same as in [6,7]) were deposited on adhesive tape. The energy calibration was based upon the position of the carbon Is-peak whose binding energy was taken as 284.0eV. All manganese borides are good metallic conductors (even better than manganese metal[S]) thus the binding energies of core electrons are determined with respect to the Fermi level. For the purpose of comparison similar measurements were run for metallic manganese and for pure boron. Prior to measurements the samples were cleaned by ion bombardment. The results are shown in Table 1 which contains the values for Mn 3p, 2~312,2p,,, and Bls electron-core level energies in the borides along with the experimental values for the pure elements. The latter values served as a reference for calculating binding energy shifts (AEb). It should be pointed out that our results for the 2~3,~and 2pln binding energies in pure manganese are in satisfactory agreement with the data of Bearden and Burr[9] and Siegbahn et al.[lO], but for the 3p level the discrepancy is somewhat larger. According to Bearden and Burr the Bls-energy is 188 eV[9], which does not agree with the values found by Hendrickson et al. [l l] (187.5 eV) and by Bremser and Linnemann[l2] (186.8 eV). Our result is exactly the same as received by the latter investigators. All boride samples used in this work showed two lines in the Bls spectrum which indicates the
*Address for correspondence: V. V. Nemoshkalenko, Institute of Metal Physics, Vernadskogo 36, Kiev-142, U.S.S.R. 37
V. V.
38
et ai.
NEM~.~~HI~ALENK~
Table 1. Electronic core levels of manganese and boron atoms in borides and binding energy shifts in respect to the pure metals
M&B Level
Eb of pure element
Mn 2~,,~ Mn 2p3, Mn 3p B1.S
652.0 640.4 47.4 186.8
formation
of boric
MnzB
(2)
tt?)
(2)
654.7 642.6 49.7 188-4
2.7 2.2 2.3 I-6
653.8 641.7 49-l 188-I
oxide
on the sample
MnB (Z\ 1.8 1.3 1.7 1.3
Eb (eV)
ft?)
653.2 641.2 485 187.7
0.8 1.1 0.9
surface.
The presence of the oxide peaks did not cause any complications, however, because the difference between the B 1s peaks in the oxide and in the borides is quite large (about 4 eV). In the electron spectra of manganese atoms the 3p, 2~3,~ and 2~~2 lines are quite broad and have structure which might be caused either by the presence of an oxide film or by the effect of multiple splitting[l3]. As is seen from Table 1, all measured Mn core-electron levels show the effect of higher binding in respect to the pure metal which is indicative of additional positive charges on manganese atoms in the borides. For the Bls level the shifts are of the order of 1.0-l ~5eV, but generally it has been found that depending on the electronegativity of compound constituents the range of Bl s shifts might be up to - 9.0 eV [II, 121. It is remarkable, however, that the shifts for both components are to the same direction with respect to the pure elements. To our knowledge, such a kind of trend has not been observed, either for the ionic compounds (which is natural), or for the compounds with mixed bonding. For example, in the reported data on boron compounds [ 121, II-VI [ 141 and III-V [ 151 semiconductors and even intermetallic compounds of carbon[4,5], oxygen and nitrogen[4] the inner level shifts for both components were to the opposite directions. Just recently, however, the data on the binding energies of the metal 2pra and Si 2p levels from transition metal silicides have been published [ 161: both component levels appeared also shifted to the same direction from the pure elements. Evidently, this fact might be taken as additional significance of considerable similarity between borides and silicides in respect to the nature of the chemical bonding [ 11. The most striking feature of the trend shown by the variation of core-level binding energies when going from one boride to another is that this variation is unidirectional for both the manganese and the boron levels, i.e. when the binding energies
MnB2
Mn,Ba
1.2
(2) 653.6 641.6 48.8 187.8
(Z$ 2.2 1.2 1.4 I.0
(2)
(“e?)
Error
654.0 642.1 49.4 188-4
2.0 1.7 2.0 I.6
-to.2 20.2 r?:0.1 -t 0.2-0.3
of manganese levels increase, that of B 1s level also increases and vice versa. Beisde being unidirectional, the dependence of core-level shifts on the boride composition is almost linear and shows a change of sign at the MnB-phase (Fig. 1). Thus, from a decrease of binding energies when going from lower manganese borides (Mn,B, MmB) to monoboride MnB it follows that both constituent atoms gain some electronic charge. Contrary to this, an increase in binding energies from the monoboride MnB to the diboride MnB2 suggests a loss of electronic charge in both the Mn and B atom spheres. At this point it is worth noting that in the region of the MnB phase we have observed[6] the same characteristic change of sign in the variation of peak positions of the Mn K and B K X-ray emission bands from these borides. It might be concluded therefore that the change in the energy position of core-electron levels follows the redistribution of outer levels in the valence band.
2> al
;, 2
I-
Fig. 1. Energy shifts of manganese (the average of the chemical shifts of 2~,,~, 2~~~~and 3p) and boron (2s) core levels as a function of boride composition.
On the charge distribution in manganese borides
Moreover, the change of sign at the composition corresponding to MnB is to be found also in the variation of other parameters of X-ray emission bands from manganese borides-such as halfwidths and asymmetry indices of metal I( and L spectra and boron K spectra[6,7]. Thus the position of the valence band, as well as its general shape, are influenced in the same manner and by the same factors of chemical bonding which determine a regular variation of core-electron binding energies. What can these factors be? The monoboride phase in the system of manganese borides is characterised by the minimum of interatomic distances Mn-B[l] and by the maximum of such strength characteristics as melting points and microhardness[l7]. For a series of carbon intermetallic compounds Ramqvist et al.[4] found a proportionality between the inner level shifts and the heat of formation, i.e. the bond strength, which enabled them to conclude that such correlation might be ascribed to the influence of a ionic contribution to the bond. This fact when applied to manganese borides along with the linear dependence of binding energy changes on the composition allow us to assume that the variation of the chemical shifts in these compounds is chiefly determined also by the change of ionic contributions. There are indications, however, in the present experimental results to the simultaneous influence of covalent contributions: such indications as the unidirectional vairation of A&, for both constituents and, possibly, the fact that the magnitudes of shifts are not small. From the crystal-structure considerations the covalent interactions in borides may exist both between the nonequivalent (metal and boron) atoms and the equivalent (boron) atoms: starting from the monoboride phase the formation of independent B-B bonds tends to confer the borides with such structural individuality which is a unique possession of elemental boron [ I]. Consequently, we can propose the following explanation: when the lower borides (MmB, Mn2B) are formed from the pure elements a portion of electrons is removed from the spheres of both sorts of atoms to build covalent bonds between the nonequivalent atoms; with the transition from MmB to MnB the covalent contribution to the bond does not change or, possibly, decreases, but the ionic contribution increases being the strongest in MnB (here the shifts are the smallest); when going from MnB to MnBz the ionic contribution decreases, but the covalent grows, probably, at the expense of B-B bonds.
39
In view of the contradictory opinions as to whether the boron, or the metal atoms, act as donors of electrons, and if so, what is the amount of charge transfer, it is interesting to try to extract a knowledge of such kind from the chargedetermined energy-shift scales for each element: for Mn 2~31,level such scale has been established by Carver and Schweitzer[lQ and for B 1s level-by Hendrickson et al.[ll]. In the former case atomic charges were calculated on the basis of Pauling electronegativities, in the latter-from the extended Htickel and CNDO molecular orbital eigenfunctions: the CNDO-calculated boron charges were found in fairly good correspondence with the measured B 1s binding energies [l 11. There is no doubt that the estimates of atomic charges obtained in that way for the transition metal borides will be only approximative, since as yet it is difficult to say to what extent the direct correlation between the binding energy shifts and the atomic charges will hold for the compounds with the intermediate bonding. Besides, the calculations based on molecular orbitals assume a sufficient separation of charges which is hardly to be expected in this type compounds. On the basis of such semiempirical plots[ll, 181 and the data from Table 1 we can calculate that on going from MnzB to MnB there is an increase by about 1 electron in the sphere of a manganese atom and only a negligible increase of charge (CNDO-charge scale) in the sphere of a boron atom. As the proportion of boron atoms is increased beyond the MnB concentration, the situation is reversed and with respect to MnB in MnBz the charge in the Mn atom sphere decreases by - 1.8 electrons and in the B atom sphere by - 0.1 electrons. Certainly, such estimates concern the net charge associated with atoms of each sort, and in these compounds the distribution of charge might be very complex since the metal-nonmetal bonds involve both the metal 3d, 4s, 4p orbitals and the nonmetal 2s,2p orbitals, so that the change of a net charge is difficult to attribute directly to the change of certain electron-type density. Such data might be taken however from other experimental evidence. In particular, for MmB-MnB-MnBz series we have obtained growing values for the relative intensities of the Mn L, bands which is indicative of increasing d-electron density 171. From the magnetic [3] and electronic specific heat measurements [ 191 follows a similar conclusion: with the transition MntB-MnB-MnB: the addition of each boron atom is accompanied by a successive involvment of 3d electrons into bonding with - 1.8 electrons, the
40
V. V. NEMOSHKALENKO et al.
density of 3d electrons being either not changed, or even increased. Thus, the variation of net charges as evidenced by our experimental data might be attributed rather to the different balance of 4spcharge transferred from the manganese atoms and 2sp-charge transferred from the boron atoms. It seems possible that up to the MnB phase the density of valence electrons removed from the spheres of both sort atoms is more drawn to the manganese atoms, while after the monoboride phase it is shifted more to the boron atoms because of an additional removal of 4sp electrons. The assumption about some partial removal of metal electrons to satisfy the requirements of bonding in borides with strong directional B-B bonds (MnB-MnB2) has been suggested to us by the study of the K emission spectra[b]. Because of the increased B 1s binding energy when going from MnB to MnBz it does not follow that the electrons taken from metal atoms are transferred to the boron atoms. It seems rather that the formation of independent B-B bonds involves a growing delocalization of boron 2sp electrons. The net charge changes associated with boron atoms as reflected by the B Is binding energy might be small due to the overlap of metal d-orbitals at the boron atom positions.
2. Samsonov
3. 4. 5. 8. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
REFERENCES B., Lundstrem T. and Rundquist S., Borides, Siiicides and Phosphides. Methuen, London; Wiley, New York (1%5).
1. Aronsson
19.
G. V., Gorjachev Ju. M., Kovenskaya B. A., and Telnikov E. Ja., Izv. t’uzov, fir. 6, 37 (1972). Cadeville M. C.. Thesis, Strasbourp. (l%S). Ramqvist L., Hamrin Kl, Johansson G., l&man A. and Nordling C., J. Phys. Chem. Solids 30, 1835 (1969). Ramqvist L., Ekstig R., Kallne E., Noreland E. and Manne R., .I. Phys. Chem. Solids 32, 149 (1971). Shashkina T. B., Phys. Status Solidi 44, 571 (1971). Shashkina T. B., Brytov I. A. and Romashchenki Ju. M., to be published. Bezruk E. T. and Markovskii L. Ja., Izu. Akad, Nauk SSSR, neorg. mat. 4, 447 (1965). Bearden J. A. and Burr A. F., Ben. Mod. Phys. 39,125 (1967). Sieebahn K. et al.. ESCA Almquist & Wiksells, Boktryckeri AB, Uppsala (1967). _ Hendrickson D. N.. Hollander J. M. and Jolly W. L., Inorg. Chem. 9, 6li (1970). Bremser W. and Linemann F., Chem. Ztg. 96, 36 (1972). Fadley C. S., Shirley D. A., Phys. Rev. 2A, 1109 (1970). Vesely C. J. and Langer D. W., Phys. Rev. 68 3770 (1972). Gudat W., Koch E. E., Yu P. Y., Cardona M. and Penchina C. M., Phys. Status. Solidi (b)52,505 (1972). Shabanova 1. N., Sergushin N. P., Kolobova K. M., Traneznikov V. A. and Nefedov V. I., Fiz. metail. 34, 1ISi (1972); 35, 947 (1973). Markovskii L. Ja. and Bezruk E. T., Izv. Akud. Nauk SSSR, neorg. mat. 3, 2165 (1967). Carver J. C. and Schweitzer G. K., .I. Chem. Phys. 57, 973 (1972). Kuentzler R., CR. Acad. Sci., Fr. 270, 197 (1970).
Chem.
Solids,
1975, Vol. 36, pp. 3740.
Pergamon Press. Printed
in Great Britain
ON THE CHARGE DISTRIBUTION IN MANGANESE BORIDES STUDIED BY X-RAY PHOTOELECTRON AND EMISSION SPECTROSCOPY V. V. NEMOSHKALENKO,*T. B. SHASHKINA, V. G. ALJESHIN and A. I. SENKEVICH Institute of Metal Physics, Kiev, U.S.S.R. (Received
3 July 1973; in revised form
3 April 1974)
Abstract-For the borides of the Mn-B system the values of binding energies of Mn 3p, 2p3,2, 2p,,, and Bls core levels were measured on an electron spectrometer. Depending on boride composition the variation of inner level energies is very similar, showing a change of sign in the region of the monoboride phase. The same variation has been observed for some characteristics of X-ray emission spectra from these borides. An attempt is made to understand which of the chemical bonding factors are chiefly responsible for a regular concentration dependence of the X-ray and photoelectron spectra.
INTRODUCTION As is known, both X-ray emission spectra and X-ray induced photoemission spectra can help obtain information concerning the charge distribution and the relative amounts of the covalent/ metallic/ionic bonds. In the former-positions of the higher levels containing the valence electrons are involved, in the latter-those of the inner levels. The energies of all electrons are subject to significant shifts depending on changes in the chemical environment and electronic structure. The question of charge distribution between the metal and the non-metal atoms in transition metal borides (for a survey, see [l]) still remains controversial and there are recent data some of which point to electron transfer from metal to boron atoms[2] while other support the idea that boron atoms act as electron donors [3]. It has been shown by Ramqvist et al.[4,5] that the experimental techniques of X-ray emission and electron spectroscopy can be very useful in elucidating the problem of charge transfer in transition metal-nonmetal compounds such as carbides, nitrides and borides. As an extension of previous X-ray spectroscopic studies of the compounds in the Mn-B system [6,7] we report in this paper on the core-level binding energies from the X-ray photoelectron spectra of M&B, MnzB, MnB, Mn3B4 and MnB2 borides. The
measurements were carried out with a Varian IEE-15 spectrometer using Al K, radiation for excitation. The operating vacuum was 1 x lo-’ torr. Since all compounds were powders, the samples (the same as in [6,7]) were deposited on adhesive tape. The energy calibration was based upon the position of the carbon Is-peak whose binding energy was taken as 284.0eV. All manganese borides are good metallic conductors (even better than manganese metal[S]) thus the binding energies of core electrons are determined with respect to the Fermi level. For the purpose of comparison similar measurements were run for metallic manganese and for pure boron. Prior to measurements the samples were cleaned by ion bombardment. The results are shown in Table 1 which contains the values for Mn 3p, 2~312,2p,,, and Bls electron-core level energies in the borides along with the experimental values for the pure elements. The latter values served as a reference for calculating binding energy shifts (AEb). It should be pointed out that our results for the 2~3,~and 2pln binding energies in pure manganese are in satisfactory agreement with the data of Bearden and Burr[9] and Siegbahn et al.[lO], but for the 3p level the discrepancy is somewhat larger. According to Bearden and Burr the Bls-energy is 188 eV[9], which does not agree with the values found by Hendrickson et al. [l l] (187.5 eV) and by Bremser and Linnemann[l2] (186.8 eV). Our result is exactly the same as received by the latter investigators. All boride samples used in this work showed two lines in the Bls spectrum which indicates the
*Address for correspondence: V. V. Nemoshkalenko, Institute of Metal Physics, Vernadskogo 36, Kiev-142, U.S.S.R. 37
V. V.
38
et ai.
NEM~.~~HI~ALENK~
Table 1. Electronic core levels of manganese and boron atoms in borides and binding energy shifts in respect to the pure metals
M&B Level
Eb of pure element
Mn 2~,,~ Mn 2p3, Mn 3p B1.S
652.0 640.4 47.4 186.8
formation
of boric
MnzB
(2)
tt?)
(2)
654.7 642.6 49.7 188-4
2.7 2.2 2.3 I-6
653.8 641.7 49-l 188-I
oxide
on the sample
MnB (Z\ 1.8 1.3 1.7 1.3
Eb (eV)
ft?)
653.2 641.2 485 187.7
0.8 1.1 0.9
surface.
The presence of the oxide peaks did not cause any complications, however, because the difference between the B 1s peaks in the oxide and in the borides is quite large (about 4 eV). In the electron spectra of manganese atoms the 3p, 2~3,~ and 2~~2 lines are quite broad and have structure which might be caused either by the presence of an oxide film or by the effect of multiple splitting[l3]. As is seen from Table 1, all measured Mn core-electron levels show the effect of higher binding in respect to the pure metal which is indicative of additional positive charges on manganese atoms in the borides. For the Bls level the shifts are of the order of 1.0-l ~5eV, but generally it has been found that depending on the electronegativity of compound constituents the range of Bl s shifts might be up to - 9.0 eV [II, 121. It is remarkable, however, that the shifts for both components are to the same direction with respect to the pure elements. To our knowledge, such a kind of trend has not been observed, either for the ionic compounds (which is natural), or for the compounds with mixed bonding. For example, in the reported data on boron compounds [ 121, II-VI [ 141 and III-V [ 151 semiconductors and even intermetallic compounds of carbon[4,5], oxygen and nitrogen[4] the inner level shifts for both components were to the opposite directions. Just recently, however, the data on the binding energies of the metal 2pra and Si 2p levels from transition metal silicides have been published [ 161: both component levels appeared also shifted to the same direction from the pure elements. Evidently, this fact might be taken as additional significance of considerable similarity between borides and silicides in respect to the nature of the chemical bonding [ 11. The most striking feature of the trend shown by the variation of core-level binding energies when going from one boride to another is that this variation is unidirectional for both the manganese and the boron levels, i.e. when the binding energies
MnB2
Mn,Ba
1.2
(2) 653.6 641.6 48.8 187.8
(Z$ 2.2 1.2 1.4 I.0
(2)
(“e?)
Error
654.0 642.1 49.4 188-4
2.0 1.7 2.0 I.6
-to.2 20.2 r?:0.1 -t 0.2-0.3
of manganese levels increase, that of B 1s level also increases and vice versa. Beisde being unidirectional, the dependence of core-level shifts on the boride composition is almost linear and shows a change of sign at the MnB-phase (Fig. 1). Thus, from a decrease of binding energies when going from lower manganese borides (Mn,B, MmB) to monoboride MnB it follows that both constituent atoms gain some electronic charge. Contrary to this, an increase in binding energies from the monoboride MnB to the diboride MnB2 suggests a loss of electronic charge in both the Mn and B atom spheres. At this point it is worth noting that in the region of the MnB phase we have observed[6] the same characteristic change of sign in the variation of peak positions of the Mn K and B K X-ray emission bands from these borides. It might be concluded therefore that the change in the energy position of core-electron levels follows the redistribution of outer levels in the valence band.
2> al
;, 2
I-
Fig. 1. Energy shifts of manganese (the average of the chemical shifts of 2~,,~, 2~~~~and 3p) and boron (2s) core levels as a function of boride composition.
On the charge distribution in manganese borides
Moreover, the change of sign at the composition corresponding to MnB is to be found also in the variation of other parameters of X-ray emission bands from manganese borides-such as halfwidths and asymmetry indices of metal I( and L spectra and boron K spectra[6,7]. Thus the position of the valence band, as well as its general shape, are influenced in the same manner and by the same factors of chemical bonding which determine a regular variation of core-electron binding energies. What can these factors be? The monoboride phase in the system of manganese borides is characterised by the minimum of interatomic distances Mn-B[l] and by the maximum of such strength characteristics as melting points and microhardness[l7]. For a series of carbon intermetallic compounds Ramqvist et al.[4] found a proportionality between the inner level shifts and the heat of formation, i.e. the bond strength, which enabled them to conclude that such correlation might be ascribed to the influence of a ionic contribution to the bond. This fact when applied to manganese borides along with the linear dependence of binding energy changes on the composition allow us to assume that the variation of the chemical shifts in these compounds is chiefly determined also by the change of ionic contributions. There are indications, however, in the present experimental results to the simultaneous influence of covalent contributions: such indications as the unidirectional vairation of A&, for both constituents and, possibly, the fact that the magnitudes of shifts are not small. From the crystal-structure considerations the covalent interactions in borides may exist both between the nonequivalent (metal and boron) atoms and the equivalent (boron) atoms: starting from the monoboride phase the formation of independent B-B bonds tends to confer the borides with such structural individuality which is a unique possession of elemental boron [ I]. Consequently, we can propose the following explanation: when the lower borides (MmB, Mn2B) are formed from the pure elements a portion of electrons is removed from the spheres of both sorts of atoms to build covalent bonds between the nonequivalent atoms; with the transition from MmB to MnB the covalent contribution to the bond does not change or, possibly, decreases, but the ionic contribution increases being the strongest in MnB (here the shifts are the smallest); when going from MnB to MnBz the ionic contribution decreases, but the covalent grows, probably, at the expense of B-B bonds.
39
In view of the contradictory opinions as to whether the boron, or the metal atoms, act as donors of electrons, and if so, what is the amount of charge transfer, it is interesting to try to extract a knowledge of such kind from the chargedetermined energy-shift scales for each element: for Mn 2~31,level such scale has been established by Carver and Schweitzer[lQ and for B 1s level-by Hendrickson et al.[ll]. In the former case atomic charges were calculated on the basis of Pauling electronegativities, in the latter-from the extended Htickel and CNDO molecular orbital eigenfunctions: the CNDO-calculated boron charges were found in fairly good correspondence with the measured B 1s binding energies [l 11. There is no doubt that the estimates of atomic charges obtained in that way for the transition metal borides will be only approximative, since as yet it is difficult to say to what extent the direct correlation between the binding energy shifts and the atomic charges will hold for the compounds with the intermediate bonding. Besides, the calculations based on molecular orbitals assume a sufficient separation of charges which is hardly to be expected in this type compounds. On the basis of such semiempirical plots[ll, 181 and the data from Table 1 we can calculate that on going from MnzB to MnB there is an increase by about 1 electron in the sphere of a manganese atom and only a negligible increase of charge (CNDO-charge scale) in the sphere of a boron atom. As the proportion of boron atoms is increased beyond the MnB concentration, the situation is reversed and with respect to MnB in MnBz the charge in the Mn atom sphere decreases by - 1.8 electrons and in the B atom sphere by - 0.1 electrons. Certainly, such estimates concern the net charge associated with atoms of each sort, and in these compounds the distribution of charge might be very complex since the metal-nonmetal bonds involve both the metal 3d, 4s, 4p orbitals and the nonmetal 2s,2p orbitals, so that the change of a net charge is difficult to attribute directly to the change of certain electron-type density. Such data might be taken however from other experimental evidence. In particular, for MmB-MnB-MnBz series we have obtained growing values for the relative intensities of the Mn L, bands which is indicative of increasing d-electron density 171. From the magnetic [3] and electronic specific heat measurements [ 191 follows a similar conclusion: with the transition MntB-MnB-MnB: the addition of each boron atom is accompanied by a successive involvment of 3d electrons into bonding with - 1.8 electrons, the
40
V. V. NEMOSHKALENKO et al.
density of 3d electrons being either not changed, or even increased. Thus, the variation of net charges as evidenced by our experimental data might be attributed rather to the different balance of 4spcharge transferred from the manganese atoms and 2sp-charge transferred from the boron atoms. It seems possible that up to the MnB phase the density of valence electrons removed from the spheres of both sort atoms is more drawn to the manganese atoms, while after the monoboride phase it is shifted more to the boron atoms because of an additional removal of 4sp electrons. The assumption about some partial removal of metal electrons to satisfy the requirements of bonding in borides with strong directional B-B bonds (MnB-MnB2) has been suggested to us by the study of the K emission spectra[b]. Because of the increased B 1s binding energy when going from MnB to MnBz it does not follow that the electrons taken from metal atoms are transferred to the boron atoms. It seems rather that the formation of independent B-B bonds involves a growing delocalization of boron 2sp electrons. The net charge changes associated with boron atoms as reflected by the B Is binding energy might be small due to the overlap of metal d-orbitals at the boron atom positions.
2. Samsonov
3. 4. 5. 8. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
REFERENCES B., Lundstrem T. and Rundquist S., Borides, Siiicides and Phosphides. Methuen, London; Wiley, New York (1%5).
1. Aronsson
19.
G. V., Gorjachev Ju. M., Kovenskaya B. A., and Telnikov E. Ja., Izv. t’uzov, fir. 6, 37 (1972). Cadeville M. C.. Thesis, Strasbourp. (l%S). Ramqvist L., Hamrin Kl, Johansson G., l&man A. and Nordling C., J. Phys. Chem. Solids 30, 1835 (1969). Ramqvist L., Ekstig R., Kallne E., Noreland E. and Manne R., .I. Phys. Chem. Solids 32, 149 (1971). Shashkina T. B., Phys. Status Solidi 44, 571 (1971). Shashkina T. B., Brytov I. A. and Romashchenki Ju. M., to be published. Bezruk E. T. and Markovskii L. Ja., Izu. Akad, Nauk SSSR, neorg. mat. 4, 447 (1965). Bearden J. A. and Burr A. F., Ben. Mod. Phys. 39,125 (1967). Sieebahn K. et al.. ESCA Almquist & Wiksells, Boktryckeri AB, Uppsala (1967). _ Hendrickson D. N.. Hollander J. M. and Jolly W. L., Inorg. Chem. 9, 6li (1970). Bremser W. and Linemann F., Chem. Ztg. 96, 36 (1972). Fadley C. S., Shirley D. A., Phys. Rev. 2A, 1109 (1970). Vesely C. J. and Langer D. W., Phys. Rev. 68 3770 (1972). Gudat W., Koch E. E., Yu P. Y., Cardona M. and Penchina C. M., Phys. Status. Solidi (b)52,505 (1972). Shabanova 1. N., Sergushin N. P., Kolobova K. M., Traneznikov V. A. and Nefedov V. I., Fiz. metail. 34, 1ISi (1972); 35, 947 (1973). Markovskii L. Ja. and Bezruk E. T., Izv. Akud. Nauk SSSR, neorg. mat. 3, 2165 (1967). Carver J. C. and Schweitzer G. K., .I. Chem. Phys. 57, 973 (1972). Kuentzler R., CR. Acad. Sci., Fr. 270, 197 (1970).