METAL-INSULATOR TRANSITION IN PYRITE TYPE NiS2-xSex SYSTEM

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METAL-INSULATOR TRANSITION IN PYRITE

TYPE NiS2-xSex SYSTEM

T. Miyadai, Y. Tazuke, S. Kinouchi, T. Nishioka, S. Sudo, Y. Miyako, K.

Watanabe, K. Inoue

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, SuppMment au no 12, Tome 49, dhcembre 1988

METAL-INSULATOR TRANSITION IN PYRITE TYPE

NiS2-,Sea

SYSTEM

T. Miyadai, Y. Tazuke, S. Kinouchi, T. Nishioka, S. Sudo, Y. Miyako, K. Watanabe and K. Inoue

'

Department of Physics, Faculty of Science, Hokkaido University, Sapporo 060, Japan

Abstract. - Specific heat and Raman scattering were measured on sintered samples with x around 0.5, which show a

sharp metal-to-insulator transition (MIT) with increasing temperature. Specific heat below MIT can be well expressed by C = yT

+

P T ~

+

6 ~ ~ In T with parameters comparable to those of TiBez. Spectrum of Raman scattering shows no anomaly through MIT.

Since the first suggestion by Mott, the phe- nomenon of the metal-insulator transition (MIT) has been extensively studied from both experimental and theoretical points of view. MIT's in various compounds were reported so far. However, many of these compounds ehibit simultaneously some crystal distortion through MIT. So, the theoretical interpretation of them is not so simple. NiSz-,Se, system also shows MIT around x = 0.5, and the absence of crystal distortion through MIT was reported from X-ray diffraction measurements [I]. Here, we report specific heat data and Raman scattering data which give the information on the local crystal distortion.

The samples used here were prepared by sintering the pressed mixture of Nil S, Se-powders in an evacu-

Fig.

-

Temperature dependence of the specific

ated silica tube at about 700 "C for 7 days. The electri- _heat_for_{N ~ s ~ . ~ ~ s ~ ~ . ~ ~ .}_{~h~ solid}curve is calculated with

cal resistivity p and the magnetic susceptibility

x were

OD= 365 K. also measured. For 0.48

<

x

<

0.53, as temperature

increases through MIT, p increases discontinuously by three orders of magnitude (from to 10-I 0-cm), and

x

also does but by a few tens percents. Specific heat measurements were made, by means of a heat pulse technique, on two samples with MIT temperature (Tt) of 35 K and 38 K, respectively. Figure 1 shows, by the dots, the temperature dependence of the observed specific heat for x = 0.51 and Tt = 35 K sample. A clear latent heat of 1 J/mol.K was observed at Tt. In figure 1 is also shown, by the solid curve, the calculated curve with Debye temperature

(OD)

of 365 K. From the observed and the calculated curves, TN is estimated as about 70 K, MIT occuring in the antiferromagnetic phase. The difference between the dots and the solid curve is considered due to the antiferromagnetic ordering and the itinerant electron contribution. Figure 2 shows, by the dots, the observed C/T us. T~ curve at low temperatures below

Tt,

re- plotted from figure 1. As seen,

C

cannot be expressed by T- and

terms

only. The solid curve in figure 2 is calculated by a function

Fig. 2. - C / T vs. T~ plot from figure 1. The solid curve is calculated by equation (1) (see Tab. I for the parameters).

with y, ,8 and

S

given in table I. The agreement between the observed and the calculated curves is very good. For x = 0.52 and Tt= 38 K sample, a similar data was obtained with y,

P

and 6 somewhat larger (by about ten percents) than those for

x

= 0.51 sample. It is noted that these values are comparable to those of a nearly ferromagnetic metal TiBez (see Tab. I). If

'Fiesearch Institute of Applied Electricity, Hokkaido Univ., Japan.

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C8 - 188 JOURNAL DE PHYSIQUE

Table I. - Experimentally obtained three parameters in equation (1) for several compounds. The units are cho-

sen so that C is measured i n mJ/mol.K.

(*) Present work. Reference [2].

(b) J. J. M. Fyanse et at., Physica 126B (1984) 116. ( C ) R. J. Trainor et al., Phys. Rev. Lett. 34 (1975)

1019.

(d) S. K. Dhar et al., Phys. Rev. B 36 (1987) 341. Compounds NiSl.rgSeo.s~ (*) TiBe2 (") uPt3 (b) UAlz (") CeSil.90 (d) CeSil.83 (d)

equation (1) is formally rewritten as C = y T

+

@ o

+

~ ~ 6~~ In (TITSF) in which

Po

is the lattice contribution, we have TsF=28 K using O ~ = 3 6 5 K. This vdue of T S ~

is again comparable t o that of ~ i ~ e 2 ( T s ~ = 3 3 K for O ~ = 6 0 0 K [21]).

y is considered as the itinerant electron contribution, and is obviously enhanced over normal metals and also NiSe2 (8.5 m ~ / m o l . ~ ~ [3]).

T~ In T term due to spin fluctuations is expected y 32.5 51.1 422 143 63.6 90.3

from the paramagnon theory. But, as far as we know, there is no theory that predicts such a term for antiferromagnetic metals. Therefore, the here obtained

P

-0.0425 -0.207 -4.18 -4.38 -0.475 -1.09

parameters,

p

and 6, cannot be compared with mi- croscopic parameters such as the spin fluctuation temperature

TSF,

the intraatomic Coulomb interaction en-

6 0.049 0.0656 1.54 1.94 0.153 0.339

ergy U, etc. However, it is safely concluded that the 6 / r 0.0015 0.0012 0.0036 0.0136 0.0024 0.0037

present compound has excitations other than antiferromagnetic spin waves which would give rise to

term

in C. In table I are given the specific heat data on sevral compounds known as spin fluctuation systems. It is noted that the larger y, the larger 6; the ratio 6/y is roughly the same for all compounds in spite of quite different 6 and y values from each other. Considering a similarity (same 617) of NiS1.49Seo.51 to spin fluctuation compounds, it might be possible that the here observed excitations below Tt be due to spin fluctuations other than spin waves, and that spin fluctuations play some role in MIT. Of course, there remain other possibilities.

The temperature variation of Raman scattering was carefully measured on two samples with Tt=35 K and 30 K from 12 K up to 280 K, using ~ r + laser (5145

A

30 mW, CW, unpolarized). Two Ag modes (- 470 cm-I and N 380 ern-') and one Eg mode

(- 270 cm-l) were observed at all temperatures from

150 cm-I to 700 cm-l. The width of Ag mode is about 35 cm-l. The observed spectra are almost the same for two samples, and change scarecely with temperature. Figure 3 shows the temperature dependence of the Raman shift for two Ag modes. In the case of the pressure-induced MIT in NiS2, a discontinuous change of about 8 cm-I in the Raman shift of Ag mode was observed [4]. However, in our case, at least such a change was not observed. (Although the width is larger than 8 cm-l, such a change in the temperature variation could be well detected). The absence of any anomaly through MIT indicates the absence of the local distortion in the crystal structure, and also the suitability of NiS2-,Sex system for the investigation of MIT.

Fig. 3. - Temperature dependence of Raman shift for two Ag modes. The vertical line indicates the width of each mode. I and M on the ordinate indicate Raman Shift in NiS2 corresponding to the insulating and the pressure- induced metallic phase, respectively [4].

Acknowledgment

This work is partly supported by the Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture.

[I] Jarrett, H. S., Bouchard, R. J., Gillson, J. L., Jones, G. M., Marcus, S. M. and Weiher, J. F.,

Mater. Res. Bull. 8 (1973) 877.

[2] Stewart, G. R., Smith, J. L., Giorgi, A. L. and Fisk, Z., Phys. Rev. B 25 (1982) 5907.

[3] Inoue, N., Yasuoka, H. and Ogawa, S., J. Phys.

Soc. J p n 48 (1980) 850.