Evidence for localized character of electronic structure in vanadium borides from 51V nuclear magnetic resonance

Solid State Communications,

vol. 8, pp. 1411—1414, 1970.

Pergamon Press.

Printed in Great Britain

EVIDENCE FOR LOCALIZED CHARACTER OF ELECTRONIC STRUCTURE IN VANADIUM BORIDES FROM ~‘ V NUCLEAR MAGNETIC RESONANCE* R.G. Barnes, RB. Creelt and D.R. Torgeson Institute for Atomic Research and Department of Physics, Iowa State University, Ames, Iowa 50010

(Received 25 June 1970 by M.F. Collins)

Measurements 51V are reported of theintermetallic Knight shiftcompounds of the nuclear magnetic in the known of vanadium resonance of and boron, V 3B2, VB, V5B6, V3B4, V2B3, and VB2. The shifts ~are found to occur with four well-resolved distinct values which may be correlated with the four types of boron environment of vanadium which arises in these intermetallic compounds. This correlation suggests the possibility of treating the electronic structure of the borides in terms of a synthesis of the band structures of the basic structural units, i.e., those of vanadium metal, VB, and VB2.

THE FACT that the magnetic (Knight) shift parameters of the nuclear magnetic resonance (NMR) of a particular nucleus situated in inequivalent crystallographic sites in intermetallic compounds (and even in some metals) may differ significantly has lead naturally to the concept of local densities of states and conduc1 tion electron in such materials. We report herewavefunctions strong evidence for the existence of such localized electronic properties in the intermetallic compounds of vanadium and boron, based on a correlation of the Knight shift of 51V in these compounds with known structural features.24 The near-100 per cent abundance of the 51V nuclear species, as well as its small electric quadrupole moment, makes this isotope especially appropriate to the study of Knight shift variations in intermetallic compounds.

vanadium and boron were prepared by arc-melting the constituents in an inert gas atmosphere. The character of the specimens was checked by X-ray analysis, metallographic analysis, and by electron-microprobe study. In most cases, some second phase was present, and in several cases the NMR and signal from the impurityFor phase detected its shift measured. the was NMRalso study the arc-melted buttons were crushed to — 400 mesh. The NMR measurements were made with an induction spectrometer,6 utilizing a modified 400 channel multichannel analyzer for signal averaging,6 and a Varian Associates 15 in. electromagnet. In general, the 5tV NMR spectra were studied as a function of magnetic field strength to determine if second order quadrupole effects and/or Knight shift anistropy effects were present.7 Small shift anistropies were noted in some spectra, and in those cases the average shift value is reported. The shift measurements were made at the highest resonance frequency that could be conveniently used (nominally 26 26MHz)

All of the known intermetallic compounds of *Work performed in the Ames Laboratory of the U.S. Atomic Energy Commission. Contribution No. 2778. ~ Present Address : Queens University, Kingston, Ontario, Canada.

The measured Knight shift values (at 300K) are tabulated in Table 1. Experimentally, the 1411

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V NUCLEAR MAGNETIC RESONANCE

Vol. 8, No. 17

51V Knight shift values (at 300 K)and structural features of V—B compounds. The Table 1. Summary of uppermost of the two numbers given in each listing is the Knight shift (in per cent), with a maximum uncertainty of ± 0.01 per cent. The lower figure in parentheses is the fraction of V atoms occupying the specified structural site in the co:~pound.* Structural

V (metal)

V 3B2

VB

V5B6

V3B4

V2B3

VB2

feature metal-like (no adjacent B)

-1-

0.58

0.56 (0.33)

+

adjacent to linear B chains

0.34 (0.67)

+

0.32 (1.00)~

+

adjacent to hex. B network

0.15 (0.40)

+

within hex. B network *

0.31 (0.40)

+

0.28 (0.20)

—

0.13 (0.67)

+

—

0.24

0.09 (0.50)

+

0.25 (0.50) —

—

0.33

(1.00)

Reference 2. Reference 7. In V3B2 the boron chains are just pairs of boron atoms.

Knight shift is always given by K

=

occurs in each compound. In addition, in the

— Yr) /7. where ~c and Ym are the effective nuclear gyromagnetic ratios in the reference compound and in the metal, respectively. Since 7 = (v/2i~H), the conveniently determined effective frequency/field ratio (i-YH) may be substituted for 7 in the expression for K. The reference for the values is taken to be the 51V resonance in shift NaVO 3 (saturated 8 For comparison, the shift aqueous in vanadium solution). metal ~ is also included in Table 1. Our value for the shift in VB 2 is in good agreement with 1° that previously reported by Silver andfeatures Kushida. Table 1 also lists the environmental of the structurally different vanadium sites in each compound. Only four types of vanadium site occur in the borides.24 These are: (a) a vanadium metal-like site of almost cubic symmetry, (b) a site adjacent to a linear boron chain, (c) a site adjacent to a hexagonal boron network, and (d) a site within the hexagonal boron network.

metal-rich compound V3B2, the NMR of the ‘cubic’ site shows no quadrupole interaction. The third shift value occurs only for the compounds V5B~, V3B4, and V2 B3, in which vanadium sites occur adjacent to hexagonal boron networks. In V5B5 and in V3B4, the boron network may be characterized as a ‘double chain’ (type f of reference andchain’ in V2B3 it may characterized as a 3), ‘triple (type g ofbereference 3). The measurements indicate that there is negligible

As is evident from the data of Table 1, a continuous progression of Knight shift values from that of vanadium metal to that of VB 2 is not obtained. Instead, the shifts are closely grouped into four distinct sets. Three of these distinct values may be immediately identified with vanadium metal, VB, and VB2. This is because in VB and VB2 only one type of vanadium site

form.” The essential features of the correlation are not affected by the details of this choice, in

(Yrn

difference between the Knight shifts of the vanadium sites in these two cases. In Fig. 1, we have plotted the observed Knight shifts in these compounds (and in vanadium metal) as a function of the ‘conduction electrons/V atom’ ratio. This ratio has been calculated on the assumption that each boron atom contributes one electron to the metallic conduction band. This represents the electron transfer theory of boride structure in its simplest

any event. As is evident from the figure, the striking aspects of the data are that the shift values are nearly constant for each type of site, and that well-resolved changes occur between the steps in passing from one extreme of the Knight shift range to the other.

Vol.8, No. 17

~‘

V NUCLEAR MAGNETIC RESONANCE

+0.E

I

I

1413 I

I

VANADIUM - LIKE SITE

IV METALj I

+0.4 V ADJACENT TOLINEARB CHAIN

_________________

~

0 -

.0.2 V ADJACENT TO HEXAGONAL B NETWORK

I(0

III I

z

[~~] I

0 INDICATES IMPURITY PHASE RESONANCE

>

—0.2-

—0.4

V WITHIN HEXAGONAL B NETWORK

I

I

5.0

-

I

I

I

I

I

I

7.0

CONDUCTION ELECTRONS/V ATOM

51V resonances in vanadium borides plotted against the number of conduction FIG. 1. Knight shifts of the electrons per vanadium atom calculated on the basis of the simplified electron transfer theory. The lines drawn through the data points serve only to associate the points and have no theoretical significance.

The character of the vanadium sites may also

be described in terms of appropriate structural units, which are a trigonal prism and a rectanguJar pyramid. As emphasized by Spear and Gilles,2 e.g., the structures of all the vanadium borides can be accounted for in terms of appropriate sequences of these units. Those vanadium atoms which are located at interior prism corners lie within a hexagonal boron network, and in this case there is a one-to-one correspondence between the two descriptions. However, in the case of vanadium atoms located at the pyramid apices, the NMR results show clearly that a significant difference exists in the electronic environment, depending on whether the pyramid apex is located adjacent to a linear boron chain or a hexagonal boron network. In VB all vanadium atoms are situated adjacent to linear boron chains, and only one NMR signal occurs. But with the onset of hexagonal boron networks in the boride structure at V 5B6, vanadium atoms occur both adjacent to and within the hexagonal network. In VB2, all vanadium atoms are finally located with the hexagonal network, and again only one Knight shift value is found. It is interesting to note, also, that in the case of V3B2 in which the linear boron chain consists of

just two boron atoms (a single link), the Knight shift of this site is nonetheless that of the VB-like environment. To the extent that the metallic vanac~ium borides may be regarded as typical metals, the Knight shift of 51V must be considered as arising from a combination of hyperfine interactions reflecting the s-band Pauli paramagnetism, the d-band orbital paramagnetism, and d-band spin-paramagnetism (via core-polarization).1 A complete resolution of the observed shifts into these components requires careful study of the temperature dependence of both the shifts and the paramagnetic susceptibility of these compounds. In more general terms, the Knight shift samples the product of the density-of-states at the Fermi level with the average value of the electronic density at the nucleus for electrons at the Fermi surface, E~in terms of a synthesis of the band structures of the basic structural~jnits, i.e., those of vanadium metal, VB, and VB2. In this connection it would also be interesting to determine if macroscopic properties of the borides, in particular the magnetic susceptibility and electronic heat capacity, could be rationalized as appropriate averages of the properties of the

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V NUCLEAR MAGNETIC RESONANCE

basic structural units. Acknowledgements

—

Vol. 8, No. 17

to Mr. F.A. Schmidt of the Metallury Division of the Ames Laboratory for the preparation of the The authors are indebted

boride specimens.

REFERENCES 1.

See for example, BENNET L.H., WATSON R.E. and CARTER G.C., Proc. 3rd Materials Research Symposium, Electronic Density of States, Nov. 3—6, ~Jaithersburg,

Md. 1969.

2.

SPEAR K.E. and GILLES P. W.,J. High Temp. Science, 1, 86 (1969).

3.

LUNDSTROM T., Arkiv Kemi, 31, 227 (1969).

4.

ARONSSON B., LUNDSTROM T. and RUNDQVIST S., Borides, Suicides and Phosphides, Methuen,

London, 1965. 5.

TORGESON DR., Rev. scient. Instrum. 38, 612 (1967).

6.

TORGESON D.R., USAEC Report IS—1312 (1965).

7.

JONES W.H. Jr., GRAHAM T.P. and BARNES R.G., Phys. Rev. 132, 1898 (1963).

8.

WALCHLI H.E., LIVINGSTONE R. and HERBERT G., Phys. Rev. 87, 541 (1952).

9.

DRAIN L.E., Proc. Phys. Soc. 83, 755(1964).

10.

SILVER A.H. and KUSHIDA T., J. Chem. Phys. 38, 865 (1963).

11.

TYAN Y.S., TOTE! L.E. and CHANG TA., J. Phys. Chem. Solids, 30, 785 (1969).

51V Es den wurde die Knight-shift der kernmagnetische Resonanz von in bekannten intermetallischen Verbindungen Bor-Vanadin gemessen. Die shift-Werte lässen sich in vier verschiedenen Gruppen -verteilen, die die vier Bor-Umgebungstypen der V-atome in diesen Verbindungen entsprechen. Daraus entsteht die MOglichkeit das die Elektronenzustánde dieser Boriden durch eine Synthese der Bänderstrukturen der grundsatzlichen Struktureinheiten, d.h., die von Vanadin, VB und VB 2, ermittelt werden konnen. -