X-ray band spectra and electronic structure of carbides and nitrides of zirconium, niobium and molybdenium

I Phyr. Chtm. Solids. 195. Vol. 36, pp. 277-281.

Pergunon Press.

F’rinlcd in Great Main

X-RAY BAND SPECTRA AND ELEC~ONIC STRUCTURE OF CARBIDES AND NITRIDES OF ZIRCONIUM, NIOBIUM AND MOLYBDENIUM V. V. NEMOSHKALENKO, V. P. KRIVITSKII, A. P. NESENJUK, L. I. NIKOLAJEVand A. P. SHPAK Institute of Metal Physics, Kiev

(Receiced3 h/y 1912) Abstract-The L- and M-emission spectra of Zr, Nb and MOcarbidesand nitrides have been studied. The spectra were excited by primary techniques. it is concluded that the valence bands of these carbides and nitrides consist of two sub-bands: a sub-band of states which participate in covalent bonds and a sub-band of states which share in the formation of metallic bonding and account for the metallic properties of these phases.

The transition metal carbides and nitrides possess an unusual combination of properties, some of which are akin to these of covalent crystals (high melting point, high brittleness and hardness), while some of the others (electrical conductivity, weak paramagnetism) are comparable with those of the corresponding metals. Taken altogether these properties make these materials of great technical importance. Because of such an unusual combination of features these compounds are also of considerable scientific interest, especially from the point of view of their energy-band structure and the nature of interatomic interactions. However, results reported so far are somewhat contradictory. It has been suggested that the bonding in the interstitial phases is metallic [ 11,or metallic with a certain contribution from ionic and covalent components{Z], or covalent with a substantial portion of a metallic component which appears when the number of valence electrons is increased[3], or covalent between the metallic ion (p6-configuration) and the negative ion of a metalloid. The latter is formed as a result of the filling of metalloid p-states (up to the p6-configuration) by the valence electrons of the metal atom, with the excess of valence electrons, if available, accounting for the metallic properties [4]. The energy-band model proposed by Bilz[S] for the interstitial compounds of transition elements contains (in the order of sub-band energy) an antibonding sub-band of metal and non-mete interacting states (Me-X), a sub-band of metallic atom interactions (Me-Me), a bonding Me-X band and, finally, a sub-band of non/metal 2s-states. Together with the opinion that with the formation of interstitial phases the electrons are transferred from the metalloid to the metal atoms[l, 2,5,6], there are also ideas that the metal atoms give up some of their valence

electrons to the non-metal atoms, i.e. that the transfer of electrons occurs in just the opposite direction [4,7-91. The present work has been undertaken to study the X-ray emission spectra from nitrides and carbides of niobium, zirconium and molybdenum with compositions close to stoichiometry. The L-emission spectra of Zr, Nb and MO were obtained with an X-ray s~c~o~aph by the primary excitation technique. The resolution in the region of L-bands was 0.22-0.26eV. The tube was operated at a voltage of 10kV and a current of 2 mA under a vacuum of 2 x lo-’ Torr. The M-emission spectra were also excited using the primary technique at the soft X-ray spectrometermonochromator with a 2-m diffraction grating with a gold coating. The vacuum in the tube (1 x 10”Torr) was accomplished with a titanium sorbtion pump. The resolution of the inst~ment was 0.16 A. It is important to clean all targets in order to avoid distortion of the emission bands by chemical combination of the target with surface contaminants. Cleaning of the sample surface was achieved by argon ion sputtering at a pressure of lo-‘Torr. We consider the sample surface as clean when &-lines of carbon, nitrogen and oxygen vanish after cleaning the target surface. We shall start the discussion by considering experimental results for the niobium nitride and carbide (Fig. 1 and Table I). K-bands of pure zirconium, niobium and molybdenum are relatively narrow and have a weak hump on the low energy side. This hump is separated from the main peak by about 1.4, 1.3 and 2.1 eV for Nb. Zr and MO respectively. The intensity and clearness of this hump are higher if the sample surfaces are contaminated. The intensity distributions of the K-bands of pure metals obtained here are analogous to those presented in 277

V. V. NEMOSH~MENKO

-20

,

v,

-15

-IO

I

-5

0

E(eVj Fig. I. Nb I!,,- and Nb M+emission bands from pure niobium, niobium carbide and niobium nitride. C &-emission band from niobium carbide1 IO].

[IO-12). As is seen from Fig. 1, in the Nb M-band from the carbide, a longwavelength satellite is present analogous to those in [lo, 1l] but in the L-band of niobium there is no distinct structure at the low-energy side contrary to [1I]. The intensity distribution of the Nb &-band of NbN obtained here is analogous to that in ref. [I I]. For the pure elements the Lh- and Mv-bands were correlated at their inflexion points or the high-energy side which correspond approximately to the Fermi level positions. Both the Ln- and the Mv-emission bands of Nb from its nitride and carbide are characterised by their two-peak structure analogous to that in [ 1I], though in the carbide spectrum the short-wave sub-band is rather weak and

et (11.

shows up just as a hump. For the carbide with the composition C/Nb = 0.95 the Nb M-band has been obtained by Holliday [ IO] and% of a similar shape. To determine the energy position of niobium L+- and Mv-bands from the carbide with respect to those of pure niobium we used our measured values of the emissionband shifts, as well as the shift magnitudes of the Nb L,:-line (0.3 2 0.2 eV) and the Mv-energy sublevel [8] of niobium in the carbide phase. As a result, it has been found that the energy position of the short-wave sub-bands in the Lti- and Mv-spectra for the carbide is practically the same as that for the corresponding bands of the pure metal. Especially to be noted is a sharp falloff of intensity of the high-energy limit of the short-wave sub-bands, which is a typical situation for metals. The long-wavelength sub-bands of the Nb Mv-emission spectra from the carbide and nitride are several eV below the Fermi level. The shapes of these sub-bands are symmetrical. The long-wavelength sub-band of the &-bands also have a symmetrical shape. For example, the Zr L&-band from carbide (see Fig. 2) is symmetrical and might be regarded as a initial stage of the change into the Nb &-band for the carbide of niobium as the number of valence electrons are increased by one. The energy positions of the long-wave sub-bands in the Mv- and L,-spectra are the same. When the carbon concentration in the niobium carbide is decreased within the homogeneity region, the relative intensity of the long-wave sub-band falls, while that of the short-wave sub-band grows. This is observed both in the Nb L,,,-band[ll] and the Mv-band[lO]. Figure I shows also the carbon &-emission band from niobium carbide which has been reported by Holliday]lOl. The carbon-metal ratio in the specimen used by Holliday did not differ greatly from the composition of our sample.

Table I

Subject

Relative integrated intensity L, MvO,,,,,,

Relative peak intensity L-0,

Base width

(eV)

L,:

Mv0,t.m

L,:

Zr

I.00

I.00

1JXl

I ,oo

ZK

1.69

I.47

1.50

2.14

5.4 -

4.2 5.4

0.0 -1.5

ZrN Nb NbC

I.71 ltN3 1.02

I .97 1.00 1.23

I.00 I .Oo 0.87’

2.14 I.00 148

II.3 6.3 8.7

NbN MO Mo>C MozN

I.06 IXIO 0.37 0.23

I.58 I.00 0.94 I.17

0.69t I@0 0.40 0.29

I.12 I.00 0,85+ 0,73+

IO.7 8.4 8.8 8.2

6.0 4.0 5.4 7.4 6.3 7.4 IO.0

-3.3 0.0 -1.5 _,.4$

Error

*IO%

*Long-wavelength peak. Short-wavelength peak. *Distance between band maximum

*7%

fO.5

of pure metal and short-wavelength

Shift

Peak shift (eV)

MvOr,.,,,

1U”OII.lll

-

0.0 -1.2 -2.7 0.0 -1.7 -3.0 0.0 0.0: O.O$

0.0 to.1 -0.4 -0.3

maximum

of compound

(eV]

L‘,,

band.

0.0 -0.3 -0.1 0.0 -0, I 0.0 to.2

X-ray band spectra and

electronicstructureof carbidesand nitrides

The C K-band and the Llh-and Mv-emission bands of niobium can be joined in energy when correlating definitely fixed inflexion points at the short-wave sides of the L&-band and the hi~~nergy sub-band of carbon spectrum.* With this procedure the long-wave peak in the carbon K-band and the peaks of the niobium Lh- and Mv-bands appeared coincident in energy. On the basis of these considerations taken together we assume that the valence band of niobium carbide consists of two separable sub-bands-a sub-band of states which participate in the directional covalent bonds (longwavelength peaks in the spectra) and a sub-band of states participating in “metallic” bonding and responsible for the metallic behaviour of NbC, both the covalent and “metallic” bonds being formed by the 4d- and Spelectrons of niobium atoms and Zp-metalloid atom electrons. However, the term “metallic bonding” is not referred to here in the sense of the classical concept of metallic bonding, but, rather, to that type of bonding which exists in pure metallic niobium, in other words, the covalent component does not exceed that in the pure transition element. Such an interrelation of the emission bands from the investigated compounds makes understandable the above-mentioned trend in the short-wavelength and long-wavelength sub-band intensities when the composition of the NbC-phase is varied within the homogenity region; namely; that as the carbon content of the carbide increases, the number of metal-nonmetal bonds is also increased and the statistical weight of electrons which participate in the formation of directional covalent bonds becomes higher. It becomes clear also why in the L&- and MV-bands from niobium nitride the short-wavelength sub-bands are more pronounced than in the co~esponding bands from the carbide (Fig. 1). Since the carbide and nitride under consideration are of practically the same composition, the statistical weight of electrons participating in covalent bonds is also the same. An additional electron in the nitride as compared with the carbide enters the sub-band of “metailic” bonding. Unfortunately, for the nitride the inner level shifts of the Nb atoms are not known, which does not allow us to find the true energy position of the Nb emission bands in the nitride with respect to the pure metal bands. Since, however, this compound is of the same type as the carbide, the emission bands of Nb from the nitride should have been shifted by approx. I eV to the low energies. Consequently, all conclusions drawn when *Such a procedure of joining the spectra of different components is justified in the present case by the knowledge that NbC presents a typically metallic behaviour which means that the occupied and unoccupied states overlap (or are adjacent). Thus the inflexion point of the short-wavelength slope of the band is adequate to the limit of filled states.

JPCS Vol. 36. No. 4-E

279

considering the data on carbide can be equally extended to nitride. As can be seen from Table I, the relative integrated intensities of Nb L&-bands from the nitride and carbide and from the pure metal remain unchanged within experimental accuracy, but the relative integrated intensities of the Mv-bands are appreciably higher (particularly for the nitride). These facts allow us to suggest that the statistical weight of the occupied d-states in the vicinity of Nb atoms in the nitride and carbide is essentially the same as in the pure metal, while the weight of the p-symmetry states is increased thus possibly causing the binding energy of the inner electrons of Nb atoms to decrease. However, according to the results of several investigations[7-91, the binding energy of Nb 3d-states in the carbide phase is higher than in the pure metal, while that of carbon Is-states has a smaller value than in graphite. This clearly provides evidence for the appearance of excess positive charge in the atomic volume of Nb atoms and excess negative charge in the atomic volume of C atoms. (The same situation has been found for Ti diboride, nitride and oxides). The amount of negative charge being transferred from the transition metal to the non-metal atoms, as estimated from the magnitudes of inner level shifts, is not great[l3]. For the interstitual compounds of Ti it has been found that in free ion approximation the electron transfer is not high in TiC (0.3 + O-53d-electrons per Ti atom), and still less in TIN and TiO (about 0*1-0+23delectrons in TiO)[l3]. The data on the relative integrated intensities of the Lfiand Mv-emission bands suggest an electron transfer to niobium atoms in the carbide and nitride under consideration if the transition probability for these bands is assumed to be the same as in pure niobium. This assumption, however, does not seem to be valid in the present case. To remove the inconsistency with electron spectroscopy data[7-91 we can draw the tentative conclusion that the electrons participating in the directional covalent bonds and localized in this case near the metal atoms (the corresponding states lie near the bottom of the valence band) might be of more “atomic” character. For these electrons when filling the inner level vacancy in the metal atom, the transition probability is, probably, higher than for electrons which take part in the formation of “metallic” bonding. An increase in the integrated intensity of the Nb Mv-band from nitride compared to that from the carbide might be caused by the in~uence of additional electrons provided by nitrogen atoms. These electrons are supposed to fill1 the states which we correlate with the band of “metallic” binding. Thus an increase in the intensity of the whole band might be produced at the expense of its short-wavelength portion. Consider now the Lfh- and A&-emission spectra from

280

V. V.

N~~WSHKALFNKO

ef d.

sub-bands of the Nb L&-band from carbide and nitride. The C K,. -band in ZrC [ IO] also is of a symmetrical and contains a small hump on the low-energy

shape

side.

The peak of the Zr i~~-band from ZrC is shifted as well to the low-energy

side and. similarly

from

band is characterised

NbC,

this

to the Nb MY-band by

its intense

high-energy hump. Since the number of valence electrons in ZrC is less by one than in NbC. all electrons which are described by a wavefunction to

the

zirconium

p-symmetry

of d+ymmetry

atom<

and

by

with respect

a wavefunction

of

with respect to the carbon atoms in the ZrC

phase are taken to form covalent bonds. These electrons do not play a practical

role in the conductivity.

electrons with p-symmetry the formation density

of

Therefore expected the

zirconium carbide and zirconium nitride.

of ZrC obtained here are similar to those presented in [9] and

[ 141 respectively.

the &-band shifted

It has been mentioned

of zirconium

to the low-energy

earlier that

from ZrC is symmetrical

and

side as are the low-energy

is the

relatively

in ZrC

of

high.

might

As for symmetry

It is, probably,

zirconium

close to the stoichiometry

A “metallic” significant

of the Zr Lb- and Zr Mv-bands

level

density

from d-electrons.

component

formed by the ~-symmetry

Zr and MO in their carbides and nitrides (Figs. 2 and 3). The intensity distributions

states

diamagnetism

composition Fig. 2. Zr LGz-and Zr &-emmission bands from pure zirconium,

total

Fermi

be

to be less than in pure Zr, where there is no

contribution of

bond and only for these electrons

states at the the

The

of Zr atoms also take part in

the cause

carbide

with

a

(C/Zr = 0.97)(15].

of bonding in ZrC which is electrons of Zr atoms is less

that in the carbide of Nb. the zirconium

nitride.

also contribute

the electrons

here to forming

of

d-

“metallic”

bonds. Therefore the density of states at the Fermi level is higher in the nitride than in the carbide. The variation Mv-bands

of relative intensities

of the Zr LPz- and

when going from carbide to nitride shows the

same pattern as in the case of the Nb compounds. suppose that here the same explanation increased intensity

in the compounds

We

holds for the

relative to the pure

metal. It should be noted that the weak humps which observed at the long-wavelength from

carbide

and nitride

corrected for distortions

are

sides of the Zr &-bands

appear after these bands are

owing to the superposition

of the

L,,,-line. The profile of this line has been computed by the dispersion

formula

and then subtracted

from

the total

spectrum protile. There is a line between the ~,,~,~.-line in the M-spectra

and Miv,-band

of Zr (see Fig. 2). Its energy does not fit

any of the diagram

lines of Zr. The same line is also

present in the M-spectra more nondiagram

of Nb and MO. In addition,

ZrC as occurs in the Nb and MO M-spectra carbides. The Nb M-line

J/q.___ -2’5

/ -20

I -15

I -10

one

line appears in Zr M-spectrum

from

from their

appears in the Zr ~-spectrum

from ZrC’ because of a small amount of niobium impurity I -5

I 0

EteV) Fig. 3. MO L&- and MO &-emission bands from pure moly~enum, moly~enum carbide and molybdenum nitride.

in the sample of ZrC. An analysis carbide

contribution greater

of the MO La2- and Mv-bands

and nitride

leads to the conclusion

of d-electrons

in the compounds

to covalent

from

the

that

the

bonding

is no

than in the pure metal. The

X-ray band spectra andelectronic structure of carbides and nitrides probability thought

for

the

electron

to be influenced

of electrons

near character.

d-electrons

are transferred

itself

in the decreased

However,

the p-electrons

nitride

the nitride

more

atoms shows

intensity

and nitride

of the

relative

to the

MO

and “metallic”

in

fof the carbon

their

contribution

atoms to

in the former

To summarize,

of the C IL-line Mo2C

and

behave

“metallic”

very

the

p-

much alike,

bonding

is more

that the valence

bands of

case.

we conclude

the carbides

and nitrides

of Zr, Nb and MO consist of two

sub-bands:

a sub-band

of

covalent

bonds between states

states

which

participate

metal and non-metal

sub-band

of

which

share

“metallic”

binding and account

in the

in

atoms, and a formation

for the metallic

of

properties

lic” bonding

of states which

is higher in the nitrides

this being related valence series

to the increase

electrons.

ZrC + NbC --) Mo2C is not a decisive

of a compound: of

However

outer

e.g. NbC

electrons

but

to the “metal-

than in the carbides,

of one in the number

A corresponding

the same reason. trons

contribute

increase

and ZrN + NbN the number

factor their

constituent

formation

of

two

of bonding

in the carbides

and

is not significant

cause of their

physical

and cannot

properties.

REFERENCES 1. Dempsey E., Phi. msg. 8 (86), 285 (1%3). 2. Surovoj Ju. N., Shvartsman L. A. and Alekseev V. I., Doclodi soveshchaniia, Priroda metallicheskih fas i harakter himicheskoj sujazi v nib (l-3 June 1%5), M. 4il. Rotaprintnaja IYET

(I%5). 3. Randle R. E., Acta Crystal/. 1. 180 (1948). 4. Grigorovich V. K., sbomik, Visokotemperutumie neorganicheskijesojedinenija, Naukoca Dumka, p. 5. Kiev (1%5). 5. Bilz H., Zs. Phys. 153, 338 (1958). 6. Umanskij Ja. S., Karbidi tverdih splavou.. Gosudarstrennoje nauchno-tehnicheskoje tsuetnoj metallurgii.,

izdatelsluo M. (1947).

liferafuri

po

chemoj

i

7. Ramqvist 8.

IO.

of

L., Hamrin K., Johansson G., Fahlman A. and Nordling C., J. Phys. Chem. So/ids 30, 1835 (1%9). Ramqvist L., Hamrin K., Johansson G., Gelius U. and Nordling C., 1. Phys. Chem. Solids 31, 2669 (1970). Ramqvist L., Ekstig B., Kgllne Elisabeth. Noreland E. and Manne R., 1. Phys. Chem. So/ids 32, 149 (1971). Holliday J. E., Soft X-Ray Band Spectra and rhe Elecrronic Structure ofMefa/s and Materials (Edited by D. J. Fabian), p. 121. Academic Press, New York (1968). Nemnonov S. A. et a/., Fiziku meralloc i metallouedenije 28.

I I.

+ MeN

371 (1969). 12. Lukirskii A. P. and Zimkina T. M., fzu. AN SSSR. ser. fiz., 27, 330 (1963); Transl. Bull. Acad. Sri., USSR 27, 339 (1%3). 13. Ramqvist L. ef a/., 1. Phys. Chem. Solids 38, 1849 (1969). 14. Holliday J. E., Electron Microprobe. Wiley, New York (1966). IS. Borukhovich A. S. et al., Phys. Status Solidi, 36, 97 (l%9).

of valence

for elec-

structure

have the same number

electronic

to which

is seen in the

in the electronic

and ZrN

component

under consideration

be the dominant

9.

of these phases. The fraction

the

and

from

NbC

in

are different.

bonds,

bonding being larger in

an inspection

nitrides

and the degree participate

metal.

of the MO atoms in carbide

carbide[lO],

important

interaction electrons

‘The ionic

the fact that some of MO

to the “metallic”

from

molybdenum but

their

than in the carbide.

As follows electrons

and

to the metalloid

take part both in covalent

their contribution

atomic

sub-bands

atoms

integrated

in carbide

is not

“own”

Therefore

L&-bands

d +2p

atom

their

atomic-like

transition

(in this case) by the localisation

281

structure,

the