ELECTRIC AND MAGNETIC PROPERTIES OF Vx Ti1-xS2

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ELECTRIC AND MAGNETIC PROPERTIES OF Vx Ti1-xS2

A. Chang, P. Molinié, M. Sienko

To cite this version:

A. Chang, P. Molinié, M. Sienko. ELECTRIC AND MAGNETIC PROPERTIES OF Vx Ti1-xS2.

Journal de Physique Colloques, 1978, 39 (C6), pp.C6-1070-C6-1071. �10.1051/jphyscol:19786474�.

�jpa-00217955�

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JOURNAL DE PHYSIQUE Colloque C6, supplPment au no 8, Tome 39, aotit 1978, page C6-1070

ELECTRIC AND MAGNETIC PROPERTIES OF V

T i

S *

X l-X 2 A.T. Chang, P. Molinid and M.J. Sienko

Come22 University, Ithaca, New York 24853, U . S . A .

R6sumd.- Les propridtds de transport (rdsistivitd et effet Hall), et les propri6tL.s magndtiques (susceptibilitd) ont 6tB mesurses dans le systsme VxTi,-xS, (0

5

^X< 0,IO) entre 4 et300K. Pour X

5

^0,02

la rgsistivitd varie c o m e T~ avec apparition de mlnlmum vers 20 K. Les cristaux fortement dopds (0,04 _< X < 0,10) ont un comportement semicor.ducteur avec un "gap" dependant de la tempdrature. La susceptibilitd magnLtique obdit i3 une loi dp type Curie-Weiss correspondant B des ions v3+, avec apparition d'un ordre ferrimagndtique B basse tempgrature associd a w anomalies observLes dans la rdsis tivitd. L'dtat semiconducteur peut Stre expliquL par une localisation du type Anderson des dlectrons de conduction.

Abstract.- Electrical conductivity, magnetic susceptibility, and Hall voltage have been measured from 4 to 300 K on VxTi S (0 _< X < 0.10). T~ dependence in resistivity was observed for X

5

0.02 with resistivity minlmals% ;P 20 K suggesting a local moment in agreement with magnetic susceptibility measurements. Crystals with 0.04 _< X < 0.10 exhibit semiconducting behavior with temperature dependent "band gap", we suggest Anderson localization of conduction electrons. Magnetic susceptibility data show Curie-type behavior corresponding to with ferrimagnetic ordering at low temperature..

Anomalies in resistivity were observed at temperatures corresponding to ordering in the susceptibility.

Several studies /1,2/ have been carried out on Til+XSP to determine conditions for obtaining stoi- chiometric TiS,. TiS, is complicated by possible ex- istence of vacancies at titanium sites and intersti- tial titanium. All these studies have attempted to explain the discrepancy between the resistivity data

(metallic behavior with ~"e~endence) /l/, the pho- toemission work /3/, and the band structure calcula- tion /4/ which gives an indirect gap of 0.2

-

^0.3eV.

The present work was undertaken to investigate the effects of vanadium substitution.

Vanadium concentration was determined by neu- tron activation. Electrical conductivities and Hall effect were measured on single crystals by conventional methods. Magnetic susceptibility measurements were made using the Faraday method.

Room temperature values of

&

and p are linear- ly dependent on composition. Resistivity as a function of temperature is shown in figure I. At low va- nadium concentration, T' dependence is observed with resistivity minima at 18 K and 22 K, respectively.

At 3.7 % V there is practically no temperature dependence of p except for the anomaly at 30 K. The more heavily substituted crystals behave like semi-

conductors. These are not conventional semiconduc- tors ; the "band gap" diminishes with decrease in temperature. Anomalies were also observed at 30K.

--p-

*supported by AFOSR 74-2583 and NSF.

Fig. 1 : Resistivity as a function of temperature of VxTi S .

1-X 2

The Hall constants in figure 2 show that the metallic samples are insensitive to variation intem- perature. 8.0 % V and 6.2 % V show an increase with falling temperature, indicating decrease of the conduction electron density in the semiconducting crystals. For 3.7 % V, the anomaly in the resistivity can be seen as partially due to sudden increase in the conduction electron concentration.

In figure 3, the samples show Curie-type behavior with 2.8 11 per mole of vanadium suggestingvta

B

(d2). At low temperature (< 30 K), ferrimagnetic ordering sets in. It decreases abruptly the electron phonon scattering and hence causes the observed anomalies in resistivity.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786474

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Fig. 2 : H a l l c o n s t a n t a s a f u n c t i o n of temperature of VxTi l-xS

,.

Fig. 3 : Reciprocal molar s u s c e p t i b i l i t y (diamagne- t i c a l l y c o r r e c t e d -70 X 10- c. g. S. mole

-

^l)^{a s a}

f u n c t i o n of temperature of VxTil-xS2.

There a r e two e x i s t i n g band models f o r TiS

.

2

The s e m i m e t a l l i c model of Thompson / l / , Kukkonen and Maldague 151, a t t r i b u t e s t h e T~ dependence of r e s i s - t i v i t y t o e l e c t r o n h o l e s c a t t e r i n g . I n t h i s model, t h e p and b bands a r e , v e r y c l o s e t o each. o t h e r o r even overlap. According t o Kukkonen and Maldague, t h e presence of 0.39 X excess Ti (Q 1oZ2e-1 t h e r e i s a s u b s t a n t i a l d e v i a t i o n from T~ dependence. With 1.4 X V t h e change i n e l e c t r o n c o n c e n t r a t i o n (%2 X

1022, assuming i s g r e a t enough t o cause devia- t i o n from t h e T~ dependence. Wilson 161, i n h i s d i r - t y semiconductor model, a t t r i b u t e s t h e T~ r e s i s t i v i - t y dependence t o e l e c t r o n s c a t t e r i n g by "Fivaz mode"

/7/ o p t i c homopolar phonons, an A v i b r a t i o n i n t h e 1g

c a s e of T i S . . With t h e "Fivaz mode", p r i m a r l y due t o l a t t i c e d i s o r d e r , t h e c a r r i e r m o b i l i t y was derived a s y a ~ n where n depends s o l e l y on t h e energy of t h e A mode ( n = 2 f o r TiS ). I n a Raman study of HfS

l g

ZrS and TiS

,

Smith e t a l . 181 observed t h a t t h e

energy of t h e A mode i s independent of t h e metal l g

i o n . Hence, t h e replacement of Ti by V should n o t a l t e r t h e energy of t h e A mode, and consequently,

r g

t h e temperature dependence of t h e m o b i l i t y . With t h e observed temperature-independent H a l l c o n s t a n t f o r t h e s l i g h t l y doped samples, t h e r e s i s t i v i t y should obey t h e same T~ laws a s i n TiS which was indeed

2'

t h e observed case. Recent work by Friend e t a l . /g/

on t h e p r e s s u r e dependence of r e s i s t i v i t y and H a l l c o n s t a n t of TiS and TiSe c r y s t a l s i s i n agreement

2 2

with the e x t r i n s i c semiconductor model.

For h i g h e r vanadium c o n c e n t r a t i o n s (X > 0.04) t h e c r y s t a l e x h i b i t semiconducting behavior w i t h a varying "band gap". We suggest Anderson l o c a l i z a t i o n of conduction e l e c t r o n s due t o p o t e n t i a l f l u c t u a t i o n s c r e a t e d by randomly d i s t r i b u t e d vanadium ions. The e f f e c t , however, i s complicated by f e r r i m a g n e t i c in- t e r a c t i o n between t h e vanadium i o n s .

References

/ l / Thompson, A.H., Phys. Rev. L e t t .

35

(1975) 1786 and r e f e r e n c e s t h e r e i n .

/ 2 / Takeuchi, S. and Katsuta, H., J. Japanese I n s t i - t u t e of Metals

36

(1970), 758, 1041 ; Saeki, M., J. Cryst. Growth

36

(1976) 77.

/ 3 / Shepherd, F.R. and Williams, P.M., J . Phys. C.

7 (1974) 4416.

-

141 Zunger, A. and Freeman, A . J . , Phys. Rev.

B16

(1977) 906.

/ S / Kukkonen, C.A. and Maldague, P.F., Phys. Rev.

L e t t .

37

(1976) 782.

/ 6 / Wilson, J.A.,Solid S t a t e Commun.

22

(1977) 551.

/ 7 / Fivaz, R.C., Nuovo Cimento

B63

(1969) 10.

/8/ Smith, T.E., Nathan, M . I . , Shafer, M.W., and Torrance, B.J., Proc. 11th Conference on Semi- conductors (1972) 1306.

/ g / F r i e n d , R.H., Jerome, D., Liang, W.Y., Mikkelsen J . C . , and Yoffe, A.D., J. Phys. C.

19

(1 977) L705.