Transport properties of iodine-free TiSe2

Physica 105B (1981) 146-150 North-Holland Publishing Company

TRANSPORT PROPERTIES OF IODINE-FREE TiSe2 I. T A G U C H I , M. A S A I Department of Physics, Shimane Medical University, Izumo, Shimane 693, Japan

Y. W A T A N A B E and M. O K A Department of Physics, Shimane University, Matsue, Shimane 690, Japan

Iodine-free crystals of T i l e 2 have been grown at growth temperatures T~ between 650 and 700°C by the selenium vapor transport method. Studies of the electrical properties show that they are of higher quality than the crystals grown for 500 --
1. Introduction The layered c o m p o u n d TiSe2, a dichalcogenide of a group IVB metal, exhibits a c o m m e n s u r a t e superlattice (200 × 2a0 × 2c0) below a second-order transition (To) near 200 K [1]. In contrast to the case of dichalcogenides of group VB metals [2], an incommensurate phase is lacking just below To. Band-structure calculations [3] and photoemission m e a s u r e m e n t s [4] showed that TiSe2 is a semimetal with carriers of no-- nh 5 × 102o cm -3 [5] which result from a slight p - d band overlap. The resistivity exhibits an anomalous peak associated with the superlattice formation near 165 K [1, 6] and the t e m p e r a t u r e gradient of the resistivity shows a rapid change at the onset t e m p e r a t u r e of the superlattice [1]. Crystals grown by the iodine vapor transport method at various growth temperatures were systematically studied by DiSalvo et al. [1]. The results showed that strong stoichiometry effects exist and that less deviation from stoichiometry favors lower t e m p e r a t u r e growth. Furthermore, iodine, used in the crystal growth, is included in the crystals. It is possible that a relatively large n u m b e r of extra carriers are produced from these imperfections and easily upset the balance between the n u m b e r of electrons and holes. This leads to suppression of the superlattice. Con-

sidering that no generally accepted mechanism for the lattice instability exists at present, studies of iodine-free crystals are helpful for understanding the origin of the instability. The only attempt to avoid iodine contamination was direct sublimation by DiSalvo et al. [1]. While their samples were too thin (5-10/~m) for neutronscattering studies, they showed substantially larger anomaly in the resistivity. In this p a p e r we give results of electrical properties of thick crystals which are of high quality and free from iodine contamination. T h e crystals were grown by the selenium v a p o r transport procedure. A report of microwave conductivity of TiSe2 is also presented.

2. Experimental Powders of TiSe2 were p r e p a r e d at 600°C by direct reactions between the elements (Ti: purity 99.99%, Se: purity 99.999%) in an excess of Se. Single crystals were grown by chemical v a p o r transport using selenium as the transport agent ( - 4 m g c m - 3 ) . The t e m p e r a t u r e difference between the charge and the growth zone was 5060°C. T h e t e m p e r a t u r e of the growth zone (Tg) was 650-700°C. The transport was usually carried out for 21 days. The crystals grown at Tg > 655°C were thick (50-250/~m) and had regular shapes

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L Taguchi et al./Transport properties of iodine-free TiSe2

of about 10 mm 2. When Tj --<655°C, the crystals were of low quality in appearance although the sizes of them were large. No systematic investigation of the Ti/Se ratio was carried out. The dc electrical resistivity (p) parallel to the layers and Hall coefficient (R.) were measured by the van der Pauw technique [7]. The absolute accuracy of p and RH was within 20% which is limited by nonuniform sample thickness. Microwave resistivity at 9.0 GHz was measured by the eddy current loss method [8]. Cooling of the sample was achieved by the flow of nitrogen gas. At room temperature the sample could be moved in and out of the cavity to determine the change in Q of the cavity.

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3. Experimental results Some parameters characterizing the growth conditions and the properties of the crystals are summarized in table I. The temperature dependence of the dc resistivity parallel to the layers is shown in fig. 1 for several samples. The curves exhibit an anomalous peak close to 157 K. If the resistivity ratio is defined by pmJp(3OOK), it gives a measure of sample purity and/or stoichiometry. For all the samples grown by selenium vapor transport, we find 3.4-< pm~/p(300K) - 3.8. The unexpectedly small value of the ratio for the sample grown at 655°C is attributed to low quality. As reference standards, the values of the

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Fig. 1. Temperature dependence o f the normalized dc resistivity o f TiSe2 single crystals grown at various growth temperatures by the selenium v a p o r transport method.

resistivity ratio are shown in table I for a number of crystals in published works [1, 5,9, 10]. It should be noted that the iodine-free crystal grown at 700°C has a larger value of the ratio (= 3.6) than those (--3.3) of DiSalvo's [1, 5] and Lrvy's [9] best samples that were grown at 575°C and 500°C, respectively. Fig. 1 also shows that

Table I M e t h o d s o f g r o w t h a n d e l e c t r i c a l p r o p e r t i e s o f TiSe2 c r y s t a l s

Crystal

Growth temperature (*C)

Transport agent

0(300 K) (10 -3 D,c m )

Resistive peak (K)

p(300 K)

(10 -2 c m 3 C -l)

R~300 K)

ReL

1 2 3 4 5 6a 7 8 9

670 670 700 655 630 590 500 600

Se2 Se2 Se2 Se2 Se2 12 IC13 I2

1.11 0.829 0.983 1.46 1.07 0.95 - 3.6 -0.9

157 158 159 156 159 ~ 150 165 165 165

3.80 3.63 3.58 3.35 3.31 4.85 -3.3 - 3.3 -2.9

1.80 1.26 2.30 - 1.6 - 58 - 1.5

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[1] [1, 5] [91 [101

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R.(300K)

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148

I. Taguchi et al.I Transpon properties of iodine-free TiSe2

the removal of iodine shifts the peak of the resistive anomaly toward lower temperatures and increases the resistivity ratio p(22 K)/p(300 K). The temperature gradient of the resistivity normalized to p(300 K) is plotted in fig. 2, where each point was determined as follows. The experimental equation for the resistivity was first calculated by the least-squares method for every six adjacent temperatures. Then, its first derivative with respect to temperature was obtained at the center of the set of temperatures. It is apparent that the minimum value of d[p/p(3OOK)]/dT decreases as the resistivity ratio pm,x/p(300 K) is increased. As first pointed out by DiSalvo et al. [1], all the curves show a break near 200 K. We analyze the discontinuous variation of dp/dT in more detail in fig. 3, where the second derivative of the resistivity with respect to temperature, d2Lo/p(300 K)]/dT 2, is plotted. With decreasing temperature, the curves change upward slowly. They suddenly start to rise at about 210 K and peak at 202-205 K, after which they rapidly drop. When the resistivity ratio pmJp(300K) is increased from 3.4 to 3.8, the absolute value of the slope in the d2Lo/p(300 K)]/dT 2 vs. T curve just below 210 K

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Fig. 4. T e m p e r a t u r e dependence of the normalized Hall coefficient of TiSe2 single crystals grown by the selenium vapor transport method.

I. Taguchi et al./Transport properties of iodine-free TiSe2 decreases and the temperature corresponding to the peak decreases. The temperature where the second derivative curve begins to abruptly go up does not shift from 210 K. The temperature dependence of the Hall coefficient is shown in fig. 4 for the same crystals that the resistivity was measured on. The value of RH was normalized to Rn(300 K) which is ineluded in table I. While the Hall coefficient goes through zero close to 181 K for all the samples, the minimum value (RH~,) of RH at low temperatures decreases when the resistivity ratio pmaJp(3OOK) is improved. We find IR.mi, l/ RH(300 K) = 9.4 for the best sample. The preliminary results of the microwave resistivity at 9.0 GHz are represented in fig. 5 as a function of temperature between 100 and 300K. The measured sample was grown by direct sublimation at 800°C. The room-temperature value is 1.14x 10-3flcm, which almost agrees with that of the dc resistivity. The profile of the curve in fig. 5 resembles the temperature dependence of the dc resistivity and the resistive anomaly appears at about 155 K.

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100

=

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200 T (K)

,

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Fig. 5. Temperature dependence of the microwaveresistivity (at 9.0 GHz) of a TiSe2 single crystal grown by direct sublimation at 800°C.

149

4. Discussion

It is known for crystals grown by iodine vapor transport that stoichiometry effects depending on growth temperature (T o exist and iodine of 0.3 at.% is included for 600 < Tg < 900°C [1]. The iodine-free crystal grown at 700°C by selenium vapor transport has a larger value of pm~/p(300K) than the best crystals grown at 500-600°C by vapor transport using iodine [1, 5] or IC13 [9] as the transport agent. Considering that stoichiometry effects may also exist in the selenium vapor transport and that the deviation from stoichiometry is much reduced at Tg = 500600°C [1], we find that iodine contamination strongly influences the quality of ZiSe2 crystals. The inclusion of 0.3 at.% iodine corresponds to that of - 4 x 1019 iodine atoms/era 3. Because, in a rigid band scheme, substitution of iodine for selenium should add one electron per iodine to the bands, ne is increased and nh decreased. If compensating vacancies are not present, the donation implies that (ne-nh)/ne<--0.1. The removal of this small amount of donation greatly increases the size of the resistive anomaly, as mentioned above. Such high sensitivity to the presence of iodine supports models which attribute the lattice instability to an electron-hole coupling [1, 11]. The resistivity and neutron-scattering measurements [1] on crystals grown by iodine vapor transport showed that the second-order phase transition occurs at the temperature (To) where the dp/dT vs. T curve breaks or reaches the minimum. This temperature almost agrees with the temperature (Tp) where d2[p/ p(300K)]dT 2 peaks. The temperature Tc increased when the resistive anomaly was enlarged [1]. Fig. 3, however, does not confirm the increase of Tp when the sample quality is improved. Another feature of fig. 3 is the onset of a sudden rise in d2[p/p(300 K)]/dT 2 at about 210 K. When temperature is varied, the increment of p and dp/dT is produced by dp/dT and d2p/dT 2, respectively. The quantity of d2p/dT 2 can thus be regarded as "acceleration", similar to that in the linear motion in mechanics. Sudden increase of

150

I. Taguchi et al./Transport properties of iodine-free TiSe2

"acceleration" must be induced by the action of "force", which is connected with the establishment of the long-range-order condensation. This gives us the possibility that the phase transition temperature corresponds to the onset of a rapid rise in the second-derivative curve of p. The specific heat data [12] also suggest that an anomaly starts at about 210K. In addition, Raman spectra [13] show a trace of the superlattice at 214 K, although it may be correlated with short-range-order effects existing even above 300 K in the resistivity [1] or electron-diffraction patterns [14]. It would be valuable to make neutron-scattering experiments on iodine-free crystals. A general characteristic of two- and onedimensional materials appears in their instabilities toward the formation of charge-density-wave (CDW) states. In NbSe3, which is a linear-chain metal, anomalies in the dc resistivity appear at 145 and 59 K [15]. It was found that NbSe3 shows non-Ohmic conductivity below these temperatures and that the anomalies are strongly suppressed at microwave frequencies [16]. The results were interpreted with a model based on the depinning and the oscillation of the CDW. Neither strong absorption nor nonlinear conduction is observed for TiSe2. It seems that

the difference is connected with the dimensionality. The CDW may become more immovable with increasing the dimension of the system. References [1] F.J. DiSalvo, D.E. Moncton and J.V. Waszczak, Phys. Rev. B14 (1976) 4321. [2] J.A. Wilson, F.J. DiSalvo and S. Mahajan, Advan. Phys. 24 (1975) 117. [3] A. Zunger and A.J. Freeman, Phys. Rev. B17 (1978) 1839. [4] M.M. Traum, G. Margaritondo, N.V. Smith, J.E. Rowe and F.J. DiSalvo, Phys. Rev. B17 (1978) 1836. [5] F.J. DiSalvo and J.V. Waszczak, Phys. Rev. B17 (1978) 3801. [6] J.A. Benda, Phys. Rev. B10 (1974) 1409. [7] L.J. van der Pauw, Philips Tech. Rev. 20 (1958) 220. [8] Y. Watanabe, K. Maeda, S. Saito and K. Uda, Japan J. Appl. Phys. 16 (1977) 2007. [9] F. L6vy and Y. Froidevaux, J. Phys. C12 (1979) 473. [10] N. Ogasawara, K. Nakamura and S. Tanaka, Solid State Commun. 31 (1979) 873. [11] J.A. Wilson and S. Mahajan, Commun. Phys. 2 (1977) 23. [12] R.A. Craven, F.J. DiSalvo and F.S.L. Hsu, Solid State Commun. 25 (1978) 39. [13] J.A. Holy, K.C. Woo, M.V. Klein and F.C. Brown, Phys. Rev. B16 (1977) 3628. [14] F.C. Brown, Physica 99B (1980) 264. [15] P. Monceau, N.P. Ong, A.M. Portis, A. Meerschaut and J. Rouxel, Phys. Rev. Lett. 37 (1976) 602. [16] N.P. Ong and P. Monceau, Phys. Rev. B16 (1977) 3443.