Physics Procedia 58 ( 2014 ) 90 – 93

Available online at www.sciencedirect.com

1875-3892 © 2014 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Peer-review under responsibility of the ISS 2013 Program Committeedoi: 10.1016/j.phpro.2014.09.030

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26th International Symposium on Superconductivity, ISS 2013

Crystal Structure of Pt-doped IrTe2 Superconductor

Takashi Mochikua*, Yoshitaka Matsushitab, Akira Satob, Yuita Fujisawac, Kei Igarashic, Tadashi Machidac, Hideaki Sakatac, Kazuto Hirataa

aSuperconducitng Properties Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan bMaterials Analysis Station, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

cDepartment of Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan

Abstract

IrTe2 has a trigonal CdI2-type structure which consists of stacked layers of triangular Ir lattices sandwiched between Te layers with octahedral coordination. Pt-doping prevents a first-order structural phase transition to a monoclinic structure at 250 K, and induces superconductivity at 2.8 K. When superconductivity is exhibited, it competes with the structural phase transition. We performed single-crystal X-ray diffraction to investigate the relationship between superconductivity and the structural change by Pt-doping. Pt was substituted at the Ir site, and was not likely to be intercalated between the Te layers. With increasing Pt-doping content, the degree of the distortion of the IrTe6 octahedron increased and the thickness of the IrTe2 layer decreased although the Ir-Te distance hardly changed. © 2014 The Authors. Published by Elsevier B.V. Selection and peer-review under responsibility of the ISS 2013 Program Committee.

Keywords: IrTe2; Pt-doping; crystal structure; X-ray diffraction

1. Introduction

Layered chalcogenides with a stacking polyhedron slab separated by van der Waals gaps exhibit low-dimensional physical properties, such as superconductivity and topological insulating phenomena. In particular, structural modulation is observed in layered transition-metal dichalcogenides with a trigonal CdI2-type structure (space group P3 ̄m1). In their compounds such as TaS2 and TiSe2, applying pressure prevents the structural modulation and

* Corresponding author. Tel.: +81-29-859-2320; fax: +81-29-859-2301.

E-mail address: mochiku.takashi@nims.go.jp

© 2014 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Peer-review under responsibility of the ISS 2013 Program Committee

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Takashi Mochiku et al. / Physics Procedia 58 ( 2014 ) 90 – 93 91

induces superconductivity by competition between the structural modulation and superconductivity [1,2]. IrTe2 is one of the layered transition-metal dichalcogenides with the CdI2-type structure [3]. IrTe2 exhibits a first-order structural phase transition from a trigonal structure to a monoclinic structure with the space group C2/m at 250 K, where distinct anomalies of electrical resistivity and magnetic susceptibility have been observed [4]. This transition is interpreted as the phenomena due to the structural phase transition, and the intercalation of Pd into the van der Waals gap, and Pd- or Pt-doping at the Ir site prevents the structural phase transition and induces superconductivity [5,6]. There is a correlation between the chemical doping and the structural phase transition because the structural phase transition temperature decreases with increasing content of the doped elements. It has also been also proposed that superconductivity in the Pt-doped IrTe2 was induced by the breaking of the Ir-Ir bonds due to the substitution of Pt at the Ir site [6]. Although PtTe2 has the same structure as IrTe2 [7], the structure of Ir1-xPtxTe2 is not clear. In this letter, we report the effect of the Pt-doping on the crystal structure of IrTe2.

2. Experimental

Single crystals of Ir1–xPtxTe2 (0 ≤ x ≤ 0.07) were grown by melting a mixture of Ir (3N), Pt (3N) and Te (6N) with a ratio of 0.18(1–x):0.18x:0.82. The mixtures were sealed in evacuated quartz tubes and placed in a vertical furnace. The temperature was raised to 1200 ˚C over 30 h, fixed at 1200 ˚C for 24 h, and slowly lowered to room temperature over 330 h. Electrical resistivity was measured by the standard DC four-terminal method to check whether or not the crystals exhibited superconductivity. The crystal with x = 0.07 exhibited superconductivity at 2.8 K. X-ray diffraction intensity data were collected at room temperature using a four-circle single-crystal diffractometer with graphite monochromatized Mo Kα radiation (λ = 0.071073 nm) to analyze the crystal structures. The raw intensity data were corrected for background, polarization, the Lorentz factor and absorption. Their structures were refined using the SHELXL-97 computer program.

Fig. 1. Crystal structure of IrTe2 with the trigonal CdI2-type structure. Black and gray balls show Ir and Te atoms, respectively. Ir and Te are connected by bonds.

3. Results and discussion

X-ray diffraction analysis indicates that all the crystals have the trigonal CdI2-type structure (space group P3 ̄m1) which consists of stacked layers of triangular Ir lattices sandwiched between Te layers with octahedral coordination

Ir Te

a b

c

interlayer distance between Te layers

thickness of IrTe2

layer

92 Takashi Mochiku et al. / Physics Procedia 58 ( 2014 ) 90 – 93

(Fig. 1). The atomic coordinates are as follows: Ir in the 1a site (0, 0, 0) and Te in the 2d site (1/3, 2/3, zTe). Difference-Fourier synthesis, which can locate atoms at the interstitial sites using the difference between the observed and calculated structure factors, was also performed to check the intercalation of Pt into the interlayer. The results indicate that there is no possibility of Pt occupation at the interlayer of the Te layers. Therefore, the occupation factors at the Ir site were fixed at the nominal content, Ir1–xPtx.

Depending on the Pt content, the x of lattice parameters a and c indicates that a increased and c decreased with increasing x monotonically, as shown in Fig. 1. The behavior of the lattice parameters is reasonable as compared with those of PtTe2 (a = 0.40259, c = 0.52209 nm) [7]. Although the Ir-Te distance in the IrTe6 octahedron hardly changes by x, the thickness of the IrTe2 layer and the interlayer distance between the Te layers, corresponding to the van der Waals gap (see Fig. 1), decreased with increasing x. They were also consistent with the dependence of the lattice parameter, c. Because the interlayer distance between the Te layers decreased by Pt-doping, it becomes difficult to intercalate Pt into the interlayer.

Fig. 3 (a) shows the dependence on x of the Te–Ir–Te bond angles. Whereas the bond angle (i) increased, the bond angle (ii) decreased with increasing x. Since these two kinds of bond angles have opposite change, there is a possibility that the degree of the distortion of the IrTe6 octahedron increased with increasing x. To estimate the degree of the distortion of the octahedron quantitatively, we calculated the quadratic elongation, <λ>, and the bond angle variance, σ2, which are linearly correlated [8]. They are defined as

λ =1

n

lil0

⎛

⎝⎜

⎞

⎠⎟

2

i=1

n

∑, (1)

where li is the distance from the central atom to the ith coordinating atom and l0 is the center-to-vertex distance of a regular octahedron of the same volume, and

σ 2 =1

m−1φi −φ0( )2

i=1

m

∑, (2)

where m is number of bond angles (m = 12), φi is the ith bond angle and φ0 is the ideal bond angle for a regular octahedron (φ0 = 90˚).

Fig. 2. (a) Depending on the Pt content, x, of (a) lattice parameters, a and c. (b) Depending on the Pt content, x, of (i) Ir–Te distance in the IrTe6 octahedron, (ii) interlayer distance between Te layers (distance of van der Waals gap) and (iii) thickness of the IrTe2 layer.

0.396

0.395

0.394

0.393

0.392

a / n

m

7x10-26543210

Pt content, x

0.541

0.540

0.539

0.538

0.537

c / nm

0 0.01 0.02 0.03 0.04 0.05 0.06 0.53

0.07

a

c

0.267

0.266

0.265

0.264

0.263

l / n

m

7x10-26543210

Pt content, x

0.276

0.275

0.274

0.273

0.272

l / nm

00 0.01 0.02 30.03 0.04 0.05 60.06 20.07

(a) (b)

(i)

(ii)

(iii)

Takashi Mochiku et al. / Physics Procedia 58 ( 2014 ) 90 – 93 93

Fig. 3. (a) Depending on the Pt content, x, of (i) Te–Ir–Teα and (ii) Teα–Ir–Teβ bond angles. Symmetry codes: α) x, x–y, z, β) y, x, –z. (b) Depending on the Pt content, x, of quadratic elongation, <λ>, and bond angle variation, σ2.

Fig. 3 (b) shows the dependence on x of <λ> and σ2. Because both parameters increased monotonically with increasing x, we conclude that the substitution of Pt at the Ir site affects the distortion of the IrTe6 octahedron.

References

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85.0

84.8

84.6

84.4

84.2

84.0

83.8

φ / d

egre

e

7x10-26543210

Pt content, x

96.2

96.0

95.8

95.6

95.4

95.2

95.0

φ / deg

ree

1.015

1.014

1.013

1.012

1.011

1.010

<λ>

7x10-26543210

Pt content, x

40

39

38

37

36

35

σ2 / d

egree

2

(a) (b)

<λ>

σ2

0 0.01 0.02 0.03 0.04 0.05 0.06 95.

0.07 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07

(i)

(ii)