New superconductors with layered crystal structures:
Ba-Ir-Ge and A-P-X (A = Zr, Hf; X = S, Se) ternary compounds
Shigeyuki Ishida, Yousuke Yanagi+, Kunihiko Oka, Hiroshi Fujihisa, Yoshito Gotoh, Akira Iyo,
Hiroshi Eisaki, Yoshiyuki Yoshida, Izumi Hase, and Hijiri Kito
National Institute of Advanced Industrial Science and Technology (AIST), Japan
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IMRA Material R&D Co., LTD., Japan
s.ishida@aist.go.jp
Unconventional superconductivity (SC) is often associated with characteristic crystal
structures. For example, unconventional SC occurs in materials possessing low-dimensional structural
units, such as 2-D planes, 1-D chains, or ladder structures, which lead to enhanced spin, charge, or
orbital fluctuations, resulting in non-phononic SC. Especially, 2-D crystal structure has been
considered as a key requirement to achieve high transition temperature (Tc), as represented by copper
oxides and iron pnictides (chalcogenides). Accordingly, we have focused on materials with layered
crystal structures and succeeded to synthesize several new superconductors as shown below.
Example 1: Ba-Ir-Ge ternary compounds BaIr2Ge7 and Ba3Ir4Ge16 exhibit SC at 2.5 and
5.2 K, respectively [1]. Single-crystal structural analysis revealed that these compounds share unique
quasi-two-dimensional networks composed of crown-shaped Ge rings that accommodate Ba atoms at
the center, referred to as “edge-shared crown-shaped BaGe16 polyhedra”. The Ba-Ge structural unit is
similar to cage structures seen in various clathrates in which the anharmonic vibration of the central
atoms, the so-called “rattling” behavior, brings about strong-coupling SC.
Example 2: A series of PbFCl-type AP2-xXx (A = Zr, Hf; X = S, Se) compounds show
superconductivity with a maximum Tc of 6.5 K in ZrP2-xSex [2]. They possess the same structure type
as LiFeAs, contain two-dimensional pnictogen square nets, and are subject to chemical substitution.
The Tc shows a dome-like curve as a function of substitution level x, mimicking the phase diagram of
high-Tc copper oxides and iron pnictides.
In this presentation, I will talk about the crystal structure, physical properties, and
electronic structure of those compounds.
[1] S. Ishida, et al., J. Am. Chem. Soc. 136, 5245-5248, (2014) / J. Guo, et al., Phys. Rev. B 88, 140507(R) (2013)
[2] H. Kito, et al., J. Phys. Soc. Jpn. 83, 074713 (2014) / S. Ishida, et al., unpublished.