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Superconductivity in BaPtSb with the SrPtSb-type structure (space group P6m2, D 1 3h , No. 187) is reported. The structure consists of a PtSb ordered honeycomb network that stacks along the c-axis so that spatial inversion symmetry is broken globally. Electrical resistivity and specific-heat measurements revealed that the compound exhibited superconductivity at 1.64 K. The noncentrosymmetric structure and the strong spin-orbit coupling of Pt and Sb make BaPtSb an attractive compound for studying the exotic superconductivity predicted for a honeycomb network.

The specific heat of two polymorphs of BaPd 2 As 2 was measured. The ThCr 2 Si 2 -type polymorph (space group I4/mmm, D 17 4h , No. 139) is a previously reported superconductor with a transition temperature T c ≃ 3.5 K, while the CeMg 2 Si 2 -type polymorph (P4/mmm, D 1 4h , No. 123) is a normal metal and does not exhibit superconductivity down to 1.8 K. Our results revealed that the ThCr 2 Si 2 -type has an anomalously low Debye temperature, indicative of soft phonons, compared to the CeMg 2 Si 2 -type. Moreover, a large specific-heat jump at T c indicated that the superconductivity of ThCr 2 Si 2 -type is a strong-coupling type, which is likely derived from soft phonons.Energetically low-lying phonons often result in strongcoupling superconductivity with an enhanced superconducting transition temperature T c .1-10) Thus, engineering materials to produce low-lying phonons has become an important issue in superconductivity. A promising route to producing low-lying phonons is a local anharmonic vibration of the ion that is weakly bound in a cage-like structure. Such a vibration is called a rattling phonon and characterized by a lowfrequency Einstein mode. A remarkable example is the β-pyrochlore oxide KOs 2 O 6 with T c = 9.6 K, [1][2][3][4] as well as Ba 3 Ir 4 Ge 16 with T c ≃ 6 K. 5,6) Another route to producing low-lying phonons is structural instability that becomes evident from the occurrence of a structural phase transition due to applied pressure or chemical doping. Prominent examples of such superconductors include the 122-type pnictides BaNi 2 (As 1−x P x ) 2 with T c = 3.3 K, 7) BaNi 2 (Ge 1−x P x ) 2 with T c = 2.9 K, 8) and Ba(Ni 1−x Cu x ) 2 As 2 with T c = 3.2 K, 9) all of which exhibit strong-coupling superconductivity because of a structural phase transition and the subsequent phonon softening characterized by an anomalously low Debye frequency. The MnP-type compound IrGe with T c = 4.7 K is a rare example that exhibits strong-coupling superconductivity with lowlying phonons but does not exhibit a structural phase transition.10) Such superconductors suggest how low-lying phonons may be produced for enhancing superconductivity.The 12) The ThCr 2 Si 2 -type structure consists of PdAs 4 tetrahedra, while the CeMg 2 Si 2 -type consists of PdAs 4 planar squares, as shown in Figs. 1(a) and 1(b), respectively. The intergrowth structure, shown in Fig. 1(c), consists of Pd 4 As tetrahedra, which are characteristic of the CaBe 2 Ge 2 -type structure such as BaPt 2 As 2 , 13, 14) * E-mail: kudo@science.okayama-u.ac.jp † E-mail: nohara@science.okayama-u.ac.jp as well as PdAs 4 planar squares, which are characteristic of the CeMg 2 Si 2 -type structure. Guo et al. reported superconductivity at T c = 3.85 K for the ThCr 2 Si 2 -type structure. 15)This T c of BaPd 2 As 2 is anomalously high compared with T c = 1.27 and 0.92 K for CaPd 2 As 2 and SrPd 2 As 2 , respectively, 16) which are isoelectronic and isostructural with BaPd 2 As 2 of the ThCr 2 Si 2 -type structure. On the other hand, no bulk superconductiv...