ELECTRICAL AND MAGNETIC PROPERTIES OP VANADIUM SUBSTITUTED NIOBIUM DISELENIDES

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ELECTRICAL AND MAGNETIC PROPERTIES OP

VANADIUM SUBSTITUTED NIOBIUM

DISELENIDES

M. Bayard, M. Sienko

To cite this version:

JOURNAL DE PHYSIQUE Colloque C4, suppl6ment au no 10, Tome 37, Octobre 1976, page C4-169

ELECTRICAL AND MAGNETIC PROPERTIES OF' VANADIUM-

SUBSTITUTED NIOBIUM DISELENIDES

(*)

M. BAYARD (**) and M. J. SIENKO (***)

Baker Laboratory of Chemistry Cornell University Ithaca, New York 14853, U. S. A.

R6sum6. - La conductivitt klectrique, la susceptibilite magnetique statique, et la tension de Hall ont 6te mesurks entre la tempkrature ambiante et celle de I'helium Iiquide sur une skrie de disklk- niures de niobium dopks au vanadium ( N b ~ - ~ v ~ S e z , 0 x < 0,30) bien caracterists. Un fort paramagnktisme de Pauli dependant de la tempkrature, une variation quadratique de la rksistivitk en fonction de la tempkature, sugg6rent l'existence d'un gaz d'klectrons fortement corre16s et certaines anomalies de la tension de Hall et de la rksistivitk rksultent d'une transition associk a une onde de densite de charges (CDW). L'effet principal de la substitution du vanadium est un changement de la nature de la coordination d'ou i1 rksulte un changement dans la composition des couches du polytype. La tempkrature critique de supraconductivite diminue rapidement dans la phase 2 H puis se stabilise lorsque des quantitks croissantes de vanadium entrent dans les couches

a

coordination octakdrique.

Abstract. - The electrical conductivity, static magnetic susceptibility, and Hall voltage have lieen measured from room temperature to liquid helium on a series of carefully characterized vanadium-substituted niobium diselenides ( N ~ I - ~ V ~ S ~ ~ , 0 < x c 0.30). Relatively large tempera- ture-dependent Pauli paramagnetism and quadratic dependence of resistivity on temperature suggest a strongly correlated electron gas. Anomalies in the Hall voltage and resistivity are consistent with a charge-density-wave transition. The effect of vanadium substitution is mainly to change the nature of the coordination and, thereby, the component layers of the polytype struc- ture. The superconducting critical temperature was observed to drop strongly in the 2 H phase and then flatten out as progressive vanadium substitution is accomodated in layers with octahedral coordination.

1. Introduction. - Bayard et al. [l] have recently shown that on progressive replacement of niobium by vanadium the layer compound niobium diselenide changes polytype stacking from 2 H (trigonal prisma- tic) to 4 Hb (alternating trigonal prismatic-octahedral) to 1 T (octahedral). The change is in line with the well-known preference of vanadium for octahedral rather than trigonal prismatic coordination [2, 31. The results make clear that this is primarily a size effect ;

the bond ionicity of V-Se and Nb-Se being the same, only the radius ratio RM/Rs, varies in the sequence Nb,-,V;Se,. Once the evolution of structure with composition had been established, it was of interest to investigate the electron transport properties. These are expected to be more dramatically responsive t o compo- sition dependence than are gross structural features. The first ionization potentials of V and Nb are almost identical (6.74 and 6.77 eV, respectively), but successive ionizations are clearly easier for N b than for V. One might expect, therefore, at least partial electron localization on a vanadium atom placed in a conducting NbSe, matrix. Indeed, preliminary work in this laboratory on Nb,-,V,& suggested that the vanadium developes a localized moment [4]. Thus, it

(*) This research was sponsored by the Air Force Office of Scientific Research (AFSC) under grant No. AFOSR-74-2583 and was supported in part by the National Science Foundation and the Materials Science Center at Cornell University.

(**) Present address : Texas Instruments, Inc., Dallas, Texas.

( * * *) To whom correspondence should be addressed.

was of interest also to investigate the effect of magnetic moment on superconductivity.

The structure of 2 H-NbSe2 has been determined by Marezio et al. [5]. It is a metal from room tempera- ture to 7.2 K, where it becomes superconducting [6]. Evidence for a low-temperature phase transition has been suggested. Lee et al. 171 observed a peak in the magnetic susceptibility and a sign reversal in the Hall coefficient near T z 40 K which they attributed to antiferromagnetic ordering. Nuclear magnetic reso- nance studies by Ehrenfreund et al. [8] excluded any magnetic ordering below 40 K but suggested a struc- tural distortion giving two inequivalent sites for the niobium. The results of X-ray studies at 15 K7 are compatible with such a distortion but are also consis- tent 'with the room-temperature form of 2 H-NbSe2. Measurements of the magnetic susceptibility [5] failed t o reveal the peak reported earlier by Lee et al. Several energy band diagrams for NbSez have been reported [9, 10, 11, 121. Their common feature is the existence of a half-filled band formed from the niobium dZz orbitals. Whether overlap exists between this d band and the valence band (formed from selenium s and p orbitals) is still unresolved: However, measurements of the Hall coefficient and of the spedfic heat seem to support such an overlap [lO, 1 l].

A charge-density-wave (CDW) transition has been observed in 2 H-NbSe, by neutron scattering [12aJ and by electron diffraction [12b]. Correlation between pressure enhancement of T, and the presence of CDW

C4-170 M. BAYARD AND M. J. SIENKO

Sample designation

Composition of Nb, -,VxSe2 samples

Based on Based on Based on

starting materials oxidation analysis neutron activation

-

-

-

U

NbSe, NbSe, N b S e ~ .g96

A Nb0.995V~.0~5Se2 Nb0.995V0.005Sel.995 Nb0.995 V0.005Se2

B Nb0.990V0.010Se2 Nb0.990V0.010Se1 .995 Nb0.9924V0.0076Se2

D Nb0.890V0.110Se2 Nb0.890V0.110Sel.994 Nb0.897 V0.103Se2

E Nb0.850V~.150Se2 Nb0.850V0.105Sel.996 Nb0.858 V0.142Se2

F Nb0.800V0.200Se2 Nb0.8~OV0.20~Se1.997 Nb0.828 V0.172Se2

c

N b ~ . 7 ~ O V ~ . 3 ~ ~ S e 2 N b ~ . 7 ~ ~ V ~ . 3 ~ ~ S e 1 . 9 9 3 Nb0.752 V0.248Se2

in the crystal has been noted by Berthier, MoliniC, and JCrome [12c]. Single-crystal pulsed NMR studies

are consistent with the onset of incommensurate CDW at 33 K [12d].

2. Experimental.

-.

2.1 MATERIALS. - Preparation, crystal growth, chemical analysis, and X-ray characte- rization of the phases in question have been described elsewhere [ l ] . In summary, the Nb,-,VXSe2 samples

were prepared by heating the high purity elements in evacuated silica tubes at 750 OC for 7 days, grinding the

well-crystallized powders obtained in an agate mortar under nitrogen, and refiring at 750 OC for 7 days in

evacuated silica. Large crystals were grown from the powder charge in evacuated silica tubes with 1 mg/ml

I, at 800 OC for 21 days. Stoichiometry was monitored by weight analysis, neutron activation, and spectros- copic analysis. Powder compositions are believed to be known to 0.1

%

; crystal compositions, to 5

%.

2.2 MAGNETIC MEASUREMENTS. - Magnetic sus- ceptibilities were measured over the range 1.6-300 K using the Faraday method with Spectrosil quartz buckets and Cahn Electrobalance force recording as described elsewhere [13]. The temperature was moni-

tored by a calibrated germanium resistance thermome- ter (1.5-100 K) and a copper-constantan thermo-

couple calibrated against a gallium arsenide diode

(100-300 K). Susceptibility measurements, which were

field independent, were corrected for the diamagnetism of the quartz bucket and are believed to be accurate within I

%.

2.3 ELECTRICAL MEASUREMENTS.

-

Resistivity per- pendicular to the c axis was measured with a dc appa- ratus by the conventional van der Pauw technique [14].

Copper probes were mounted on the crystals with silver paint. The Hall voltage, measured perpendicular to c with the magnetic field parallel to c, was determi- ned with an ac set-up using a mis-alignment resistor and lock-in amplifier. Linear proportionality of the Hall voltage to magnetic field (0-6 kG) and to ac-

current (0-50 mA) was verified. Ohmic behaviour was

observed throughout. An uncertainty of about 5

%

can be attributed to the results.

Superconductivity was investigated by the Meissner

effect. ,The apparatus is described elsewhere [15].

An ac voltage (400-4 000 Hz) was impressed on a

primary coil wound around a silica tube containing the sample, while the output of the concentric secon- dary coil was measured. All the samples showed critical fields greater than 750 G, the limiting value available. The temperature sensor for the superconductivity was identical to that used for the low-temperature magnetic susceptibility measurements.

3. Results and discussion. - The compositions of the various samples studied are presented in table I. Samples A and B have the 2 H. structure ; samples D, E, and F have the 4 Hb structire ; sample C has the

1 T structure. The interrelation of the polytypes is

shown in figure 1. X X X " V " m Niobium, Vanadium 0 Selenium X Octahedral hole

FIG. 1.

-

Idealized structural arrangements in 2 H, 4 Hb, and 1 T polytypes of Nbl-zVzSez. In the 2 H-NbSez structure, which is observed for 0 G X G 0.01, the niobium or vanadium atoms occupy trigonal prismatic holes between hexagonal-packed selenium layers that are stacked A over A or B over B. In the 1 T-VSe2 structure, which is observed for 0.30 S x 6 1.00,

the Nb or V atoms occupy octahedral holes between Se layers A and B. In the 4 Hb structure, 0.11 X S 0.20, trigonal and

ELECTRICAL AND MAGNETIC PROPERTIES OF VANADIUM-SUBSTITUTED NIOBIUM DISELENIDES C4-171

Figures 2, 3, and 4 show the observed resistivity, magnetic susceptibility, and Hall constant, respectively, as functions of temperature for pure NbSez and the 2 H-structure samples A and B. The resistivity beha- viour (Fig. 2) is generally metallic. As showninfigure2a, resistivity is linear with T Z up to 35 K for NbSe,, to

63 K for sample A, and to 80 K for sample B. Logically, one would expect the resistivity of sample B to be higher than that of sample A, but figures 2 and 2a show that the opposite is true. This is believed to be a real effect, not an artifact of the composition. It may

be due to a small admixture of polytype 4 Hb, not

detectable by X-ray analysis. The magnetic suscepti- bility (Fig. 3) is not temperature-independeilt as would be expected for pure Pauli paramagnetism but shows a

FIG. 3. -Observed per-gram susceptibility as a function of temperature for 2 H polytype of N ~ I - ~ V X S ~ Z . Sample A has

X = 0.005 ; sample B, X = 0.010. Core diamagnetism is not corrected for.

FIG. 2. - Electrical resistivity as a function of temperature for

2 H polytype of Nbl-xVsSez. Sample A has X = 0.005 ; sample B, X = 0.010.

FIG. 2a.

-

Electrical resistivity as a function of squared ternpe- rature for 2 H - N ~ I - ~ V ~ S ~ ~ .

definite increase with falling temperature, even in the range 30 < T < 300 K. Below 30

K,

the pronounced rise in X, can be fitted with a Curie law (molar Curie constant 9 X 10-S for sample A and 35 X lO-' for

sample B), suggesting presence of a small amount

of M4+ (2.40 X 10-4 mole per formula unit of

sample A and 9.33 X 10F4 mole per formula unit of

sample B) in the octahedral holes between layers. It

should be noted, however, that this amount is an order of magnitude less than the amount of vanadium added. The observed Hall constant (Fig. 4) shows the

FIG. 4. - HaI1-voltage constant as a function of temperature for

C4- 172 M. BAYARD AND M. J. SIENKO same sign reversal as that previously reported by

Lee et al. [7J for pure NbSe, and the wiping out of the sign reversal by impurities as noted by Huntley and Frindt [l61 and Morris 1171. No Hall reversal was observed in sample B.

Figures 5, 6, and 7 show t&.observed resistivity, magnetic susceptibility, and Hall constant, respectively, as functions of temperature for the 4 Hb samples of Nb, -,V,Se,. As is evident from figure 1 thece samples contain interleaved laycrs of trigonal prismatic type, characteristic of pure NbSe,, and octahedral, charac- teristic of pure VSe,. Not surprisingly, the properties presented in figures 5, 6, and 7 reflect features of both NbSe, and VSe,. In particular the resistivity curves of figure 5 show quadratic behaviour at very low tempera-

FIG. 5. - Electrical resistivity as a function of temperature for the 4 Hb polytype of Nbl-,V,Sez. Sample D has x = 0.11 ;

sample E, X = 0.15.

tures and an anomaly around 100 K. The fact that the resistivities of NbSe2 and VSe, are of equal order of magnitude suggests that neither type of layer should dominate the electric transport parallel to the layers. The fact that the VSe, anomaly at

--

100 K shows up so definitely in Nbo,,,Vo.,lSe2 (and probably also in Nb,

.,

5V0. 5Se2) suggests that whatever produces the bump in VSe2, a charge-density-wave transition, impresses itself on contiguous NbSe2-type layers. Support for such a view comes from the magnetic results of figure 6. The overall appearance of the curves (viz., flat Pauli-type susceptibility at high temperature giving way with a discontinuity in slope around 200 K to a minimum in susceptibility at lower temperature) is

FIG. 6 .

-

Observed per-gram susceptibility as a function of temperature for the 4 Hb polytype of Nbl-,VXSez. Sample D

has x = 0.11 ; sample E, X = 0.15 ; sample F, x = 0.20. Core diamagnetism is not corrected for.

very reminiscent of what is observed in pure VSe,. In VSe, the slope discontinuity has been associated with a charge-density-wave transition. [IS]. However, there is a problem in Nb, -,V,Se,. The Hall-effect data of figure 7 suggest that the resistivity anomaly at

FIG. 7. - Hall-voltage constant as a function of temperature for the 4 Hb polytype of Nbl-,VzSe2. Sample D has X = 0.11 ;

sample E, x = 0.1 5.

ELECTRICAL AND MAGNETIC PROPERTIES OF VANADIUM-SUBSTITUTED NIOBIUM DISELENIDES C4-173

significantly lower temperature. A similar lack of correspondence has also been observed in pure VSe, [19].

Figures 8,' 9, and 10 show observed resistivity, magnetic susceptibility and Hall constant, respectively, as functions of temperatures for a I T-structure sample of Nb,-,V,Se,. The resistivity behaviour (Fig. 8) is

I

I I

I

0 100 200 300

T (K)

FIG. 9. - Observed per-gram susceptibility as a function of temperature for the 1 T polytype of N b ~ - ~ v ~ S e z . Sample C

has X = 0.30. Core diamagnetism is not corrected for.

FIG. 8. -Electrical resistivity as a function of temperature for the l T polytype of Nbl-zVxSe2. Sample C has x = 0.30.

most peculiar, resembling neither pure VSez nor NbSe,,. There is a three-fold increase going below 140 K and a reproducible hysteresis of

-

100 between cooling and heating. At T w 230 K, there seems to be a slight minimum in resistivity ; a t T z 40 K, a definite maximum. The minimum at

--

230 K matches, a slope discontinuity in magnetic susceptibility (see inset of Fig. 9) that sets in at the same temperature. (Again, the Curie-type behavior at very low tempera- ture can be ignored since it can be fitted by

-

8 X 10-6 mole of interlayer M4' per formula unit

of Nb,-,V,Se,.) The increase in resistivity below 140 K matches the gradual increase in Hall coefficient (Fig. 10) below this temperature. The combined results may be due again to a CDW below 230 K and are consistent with McMillan's phenomenological Landau theory 1201 as well as the model of Rice and Scott [21]. In the latter, instability in 1 T polymorphs is by nesting sections in the band structure in contrast with saddle points for 2 H polymorphs. Saddle points have much smaller effects on the Fermi surface at the transitions to CDW states.

It is not yet clear whether CDW helps or hinders superconductivity. In VSe,, where CDW is supposed to

FIG. 10.

-

Hall-voltage constant as a function of temperature for the 1 T polytype of Nbl-xVzSe2. Sample C has X = 0.30.

C4-174 M. BAYARD AND M. 3. SIENKO

F1 2 1 m~nimum experimental temperature

-4,

Vleck paramagnetism (estimated by comparison with MoS, and P-MoTe, to be about 40 X 10-6 emu/mole),

this leaves about 310 X 10-6 emulmole for the Pauli

paramagnetism. Because it is linear with Tit cannot be due to magnetic impurities. The band calculation of Mattheiss [l21 suggests that the conduction takes place in a narrow d,z band that is about 0.075 Ry wide. This

U

corresponds to N(E) w 2 eV-l, leading to

o o 10 o 20

X,

= 130 X 10d6 emu/mole. According to the theory X in ~ b , - , V , Se, _{of Brinkman and Rice [22], high, temperature-}

Superconducting temperatures _{dependent, Pauli paramagnetic susceptibility may be a}

FIG. 11.

-

Superconducting critical temperatures as a function result of spin fluctuations in a strongly correlated

of composition in

N ~ ~ - ~ v ~ s ~ ~ .

A and B are 2 H polytypes ; electron gas. In their model, at a critical value of the

D, E, and F, 4 Hb. Sample C, which has x = 0.30 and 1 T intra-atomic Coulomb repulsion C = C, the number

structure, did not superconduct down to 1.5 K.

attributed to a decrease in the electron-phonon coupling constant V.

In the 4 Hb phase, superconductivity probably passes only through the trigonal prismatic layers, containing mainly niobium atoms. As discussed elsewhere [l], in 4 Hb-Nb,-xVxSe, only one layer out of two contains niobium atoms in trigonal pris- matic coordination. The other layer is forced into octahedral symmetry by the presence of an appreciable amount of vanadium. The results of figure 1 1 suggest that once a trigonal prismatic NbSe, layer has acco- modated its maximum uptake of substitutional vana- dium the superconducting character of the layer is not much modified by what happens in an adjacent layer. Two features of the above study that are most difficult to account for are the temperature-dependent Pauli susceptibility in the 2 H phase (Fig. 3) and the slope discontinuity at the apparent CDW transition in the 4 Hb and 1 T phases (Fig. 6 and 9). At T = 20 K, NbSe, shows a magnetic susceptibility of

238 X 10-6 emu/mole ; after correction for core

diamagnetism (- 112 X 10-'j emu/mole) and Van

of doubly occupied sites and the discontinuity in the single-particle occupation number at the Fermi surface go to zero. As C approaches C, both the susceptibility and the effective mass diverge as [l

-

(C/Co)2]

-

l. The density of states N(E) calculated

from the experimental

X,

and from specific heat should be equally enhanced. Indeed, using specific heat results from Van Maaren and Harland [l01 we calculate N(E),/N(E), z 0.6. Additional evidence for a highly correlated electron gas can be obtained from the resis- tivity measurements (Fig. 2a). ~t low temperature, the resistivity turns out to be a function of T2 and there is some tendency for p to saturate at higher tempera- tures. This is exactly the behaviour originally predicted by Baber and Wills [23] for a multivalent metal in which strong scattering comes from collisions between electrons. Writing p = p,

+

RT' we find large values for R(6.6 X 10L3 ~ C l c m K - ~ for NbSe,, 5.6 X 10-3

for sample A, and 3.9 X 10-3 for sample B), which are

about 100 times the typical values for transition metals. As indicated by Baber and Wills, the condition for large contribution of p = f(T2) is that the effective mass of some of the conduction electrons be large, leading to high paramagnetism and large electronic specific heat. This is the situation found in these materials.

References

[l] BAYARD, M., MENTZEN, B. and SIENKO, M. J., I m g . Chem.

15 (1976) 1763.

121 GAMBLE, F. R., J. Sotid State Chem. 9 (1 974) 358. [3] MADHUKAR, A., Solid State Commun. 16 (1975) 383. [4] PIVNICHNY, J., private communication.

[5] MAREZIO, M., DERNIER, P. D., MENTH, A. and HULL, G. W. Jr, J. Solid State Chem. 4 (1972) 425.

[6] REVOLINSKY, E., SPIERING, G. A. and BEERNTSEN, D. J., J.

Phys. Chem. Solids 26 (1965) 1029.

[7] LEE, H. N. S., GARCIA, M., MCKINZIE, H. and WOLD, A.,

.K Solid State Chem. 1 (1970) 190.

[8] EHRENFREUND, E., GROSSARD, A. C., GAMBLE, F. R. and GEBALLE, T. H., J. AppZ. Phys. 42 (1971) 1491. [9] WILSON, J. A. and YOFFE, A. D., Adv. Phys. 18 (1969) 193. [l01 VAN MAAREN, M. H. and HARLAND, H. B., Phys. Lett. 29A

(1969) 571.

[l11 HUISMAN, R., DE JONGE, R., HAAS, C. and JELLINEK, F., J.

Solid State Chem. 3 (1971) 55.

[l21 MATTHEISS, L. F., Phys. Rev. B 8 (1973) 3719.

[l&] MONCTON, D. E., AXE, J. D. and DI SALVO, F. J., Phys. Rev. Lett. 34 (1975) 734.

[l261 WILLIAMS, P. M., Solid State Commun. 17 (1975) 1197. [12c] BERTHIER, C., MOLINU?, P. and J~ROME, D., Solid State

Commun. 18 (1976) 1393.

[12d] BERTHIER, C., JEROME, D., MOLIN&, P. and ROUXEL, J.,

Solid State Commun. 19 (1976) 131.

1131 YOUNG, 3. E. Jr., Ph. D. thesis, Cornell University, 1971. (141 VAN DER PAUW, L. J., PhiZQs Res. Repts. 16 (1961) 187. [l51 WANLASS, D., Ph. D. Thesis, Cornell University, 1973. [l61 HUNTLEY, D. J. and FRINDT, R. F., Can. J. Plrys. 52 (1974)

861.

[l71 MORRIS, R. C., Phys. Rev. Lett. 34 (1975) 1164.

[l81 THOMPSON, A. H. a n d SILBERNAGEL, B. G., Bull. Am. Phys.

Soc. 21 (1976) 260.

[l91 BAYARD, M. and SIENKO, M. J., J. Solid State Chem. (in press).

[20] MCMILLAN, W. L., Phys. Rev. B 12 (1975) 1197. [21] RICE, T. M. and SCOTT, G. K., to be published.

[22] BRINKMAN, W. F. and RICE, T. M., Phys. Rev. B 2 (1970) 4302.