*Corresponding author. Tel.: 81-298-59-2603; fax: 81-298-59-2601; e-mail: mzmt@nrim.go.jp.

qSubmitted to the International Conference on Strongly Cor-related Electron Systems (Paris, 15—18 July 1998).

Physica B 259—261 (1999) 857—859

Instability of metallic phase in CuIr2Se

4at high pressureq

J. Tang!, T. Matsumoto!,*, T. Naka!, T. Furubayashi!, S. Nagata", N. Matsumoto"

!National Research Institute for Metals, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan"Department of Materials Science and Engineering, Muroran Institute of Technology, Muroran, 050-8585, Japan

Abstract

We report the temperature and pressure dependence of the electrical resistivity in CuIr2Se

4. The measurement was

performed from room temperature to 2 K at different pressures up to 6 GPa. The electrical resistivity at the lowtemperature (4.2 K) shows pressure-insensitive up to 1.5 GPa and increases rapidly at higher pressures. Above 1.5 GPa,an anomalous hump appears in the temperature dependence of the electrical resistivity around 100 K. In the pressurerange, 2.6(P(4 GPa, the resistance minimum appears around 15 K. The pressure effect on the electrical resistivity inCuIr

2Se

4might be interpreted in terms of the pressure-induced band gap opening by electron correlation among Ir-5d

electrons. ( 1999 Elsevier Science B.V. All rights reserved.

Keywords: Pressure effect; CuIr2Se

4; Electrical resistivity

The sulphospinel and selenospinel of CuIr2X

4(X"S,

Se) show interesting structural and electrical properties.It was found that there is a transition from metallic toinsulating at 230 K for CuIr

2S4

[1]. The resistivity ofmetallic phase in higher temperatures discontinuouslyincreases at the metal—insulator transition (MIT). Ac-companied with the transition, a structural transitionfrom the cubic to the tetragonal was also observed [2].The volume of the metallic cubic phase becomes smallerby 0.7% at the transition to the insulating tetragonalphase. In fact, the insulating phase becomes stable at highpressure, which is quite unusual. Also, from a recent workof photoemission, the MIT is insisted to be due to elec-tron correlation among Ir5d electrons [3]. On the otherhand, the MIT temperature decreases with the substitu-tion of Se for S and the terminal compound, CuIr

2Se

4becomes metallic above 0.5 K. It is very interesting toknow a response of CuIr

2Se

4for the volume contraction.

Here we report, the temperature dependence of the elec-trical resistivity at pressure in CuIr

2Se

4, and partially

substituted sample by S.Polycrystalline samples were prepared by a solid-state

reaction using the mixtures of Cu, Ir, S and Se elementswith their stoichiometric ratio. The mixture sealed ina quartz ample was heated at 850°C for 10 days. Asample for the resistivity measurements was prepared bysintering of synthesized powder at 850°C.

The electrical resistivity of CuIr2(S

x, Se

1~x)4

(x"0,0.1) was measured with the DC four-probe method usingthe cubic anvil apparatus operated up to 6 GPa fromroom temperature to 10 K and piston cylinder up to2.8 GPa from room temperature to 2 K. Fluorinert liquidwas used as the pressure medium. The gasket with a Tef-lon inner cell is made of the mixture of amorphous boronand epoxy.

Fig. 1 shows the temperature dependence of electricalresistivity of CuIr

2Se

4at various pressures up to

2.8 GPa. When the pressure is below 1.5 GPa, the samplestill behaves as metallic. Above 1.5 GPa, an anomaloushump appears in the temperature dependence of theelectrical resistivity at about 100 K: The resistivity showsa metallic behavior on the low-temperature side of thehump, while a semiconducting behavior emerges on the

0921-4526/99/$ — see front matter ( 1999 Elsevier Science B.V. All rights reserved.PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 0 6 1 - 8

Fig. 1. Temperature dependence of electrical resistivity at vari-ous pressures for CuIr

2Se

4.

Fig. 2. Temperature dependence of electrical resistivity at vari-ous pressures for CuIr

2(S

0.1Se

0.9)4.

Fig. 3. Pressure dependence of the electrical resistivity at 4.2 Kfor CuIr

2Se

4and CuIr

2(S

0.1Se

0.9)4.

high-temperature side. Further, in the pressure range,2.6(P(2.8 GPa, the resistance minimum appearsaround 15 K. That is, an obvious enhancement was ob-served in the electrical resistivity at the lower temper-atures. As it was reported previously [4], above 4 GPa,the temperature coefficients of electrical resistivity areobtained to be negative in the measured whole temper-ature region. This result suggests that the insulatingphase becomes stable with increasing pressure forCuIr

2Se

4. The temperature dependence of electrical resis-

tivity of CuIr2(S

0.1Se

0.9)4

at various pressures up to2.8 GPa is similar to that of CuIr

2Se

4. The result is

shown in Fig. 2. But at 2.3 GPa, the resistivity alreadyshows a negative temperature coefficient in the wholeregion. The pressure dependence of the resistivity at4.2 K for both compounds is shown in Fig. 3. The electri-cal resistivity shows pressure-insensitive up to 1.5 GPaand increases rapidly by applying pressure especiallyhigher than 2.5 GPa. In the case of S-substituted com-pound, the enhancement of the resistivity becomes largerwith an increase of pressure.

We also measured the Hall effect at ambient pressure.The result indicated a negative carrier with low concen-tration of 0.01/unit-cell [5], from which we can estimatethe net carrier density as an order of 1019 cm~3. ThusCuIr

2Se

4is regarded as a semimetal at ambient condi-

tion.Combining with these results, the semimetallic

CuIr2Se

4at ambient condition becomes unstable at high

pressure and finally results in a band gap opening.A spinel structure is formed by two kinds of polyhedrons,tetrahedron and octahedron. From the calculation of theelectronic structure [6], the edge shared network of theoctahedrons that consists of a B-site cation (Ir) and sixanions is believed to contribute dominantly to the elec-tric conduction. The rotation of octahedrons whichmight occur at higher pressure to diminish the volumechanges the network into more wavy and finally pro-duces the band gap due to the suppression of the inter-site transfer of electrons. Now, we are carrying outa structural study at high pressure and low temperatureto confirm our speculation. In the intermediate pressurerange, the temperature dependence of the resistivityshows the crossover from a metallic behavior to asemiconducting one for both samples. At present, thistransient phenomenon is an open question, but the crys-tallographic study at high pressure might also give some-thing to solve this problem.

858 J. Tang et al. / Physica B 259—261 (1999) 857—859

References

[1] S. Nagata, T. Hagino, Y. Seki, T. Bitoh, Physica B 194—196(1994) 1077.

[2] T. Furubayashi, T. Matsumoto, T. Hagino, S. Nagata,J. Phys. Soc. Japan 63 (1994) 3333.

[3] J. Matsuno, T. Mizokawa, A. Fujimori, D.A. Zatsepin, V.R.Galakhov, E.Z. Kurmaev, Y. Kato, S. Nagata, Phys. Rev.B 55 (1997) 979.

[4] J. Tang, T. Furubayashi, T. Kosaka, S. Nagata, Y. Kato,H. Asano, T. Matsumoto, Rev. High Pressure Sci. Technol.7 (1998) 496.

[5] J. Tang et al., to be submitted. Original data was presentedat the 53rd Annual Meeting of the Physical Society ofJapan, Meeting Abstracts, 1998, p. 632.

[6] T. Oda, M. Shirai, N. Suzuki, K. Motizuki, J. Phys.: Con-dens. Matter 7 (1995) 4433.

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