On the possibility of MgB 2 -like superconductivity in potassium hexaboride Yukari Katsura a, * , Ayako Yamamoto b , Hiraku Ogino b , Shigeru Horii b , Jun-ichi Shimoyama b , Kohji Kishio b , Hidenori Takagi a a Magnetic Materials Laboratory, RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan b Department of Applied Chemistry, School of Engineering, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan article info Article history: Accepted 30 December 2009 Available online 6 January 2010 Keywords: KB 6 Hexaboride Boride superconductor Electronic structure Transmission electron microscopy abstract We discussed the possibility for occurrence of MgB 2 -like superconductivity in cubic hexaboride KB 6 , from experimental and theoretical points of view. We successfully synthesized KB 6 polycrystals with different K-site occupancies (84–98%). Transmission electron microscopy revealed that K-site vacancies are ran- domly but anisotropically distributed. Magnetization measurements showed that KB 6 is a paramagnetic metal, with a small hysteresis around 60 K. First-principles calculations indicated that KB 6 has conduc- tive covalent network of boron like in MgB 2 superconductor. We found a clear difference between the Fermi surfaces of stoichiometric and hole-doped KB 6 , implying a possibility for control of electronic struc- ture for superconductivity, by introduction of K-site deficiency. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction In MgB 2 superconductor, the presence of carriers within cova- lent network of boron leads to anomalously high transition tem- perature T c  39 K [1]. The E 2g phonon mode, which has a high frequency due to the light mass of boron, strongly couples with the B p x + p y bands in the honeycomb network of B. However, searches for similar superconductors have been difficult, since such electron-deficient borides are usually unstable. Potassium hexaboride KB 6 is a stable electron-deficient boride, which has a cubic covalent network of B [2].B 6 octahedral clusters are covalently bonded to the neighbouring six B 6 clusters, forming covalent ‘‘cages” containing K + ions. This structure is very rigid, so that unusually high concentration of K-site deficiency is allowed. Despite that KB 6 has been proposed to have metallic electronic structure with carriers in B p-band [2–4], experiments have shown that KB 6 is a poor conductor [2,4]. Etourneau et al. proposed that KB 6 has localized electrons in B 6 octahedra due to bipolarons inter- actions [5,6]. Since electron localization by bipolarons can be inter- preted as a strong electron–phonon interaction, slight changes in the electronic structure may lead to evolution of phonon-mediated superconductivity in KB 6 . Besides, superconductors are often discovered in pairs, with ‘‘hole-doped” and ‘‘electron-doped” conjugates of the same crystal structure. Yttrium hexaboride YB 6 is a strong-coupled BCS super- conductor with T c = 7.1 K, where the d-band of Y contributes to superconductivity [7–9]. By considering KB 6 as a hole-doped con- jugate for YB 6 , we can expect superconductivity in KB 6 . In this study, we investigated the physical properties, nanostruc- tures and electronic structures of KB 6 with varied K-site occupan- cies, and discussed on the possibility of superconductivity in KB 6 . 2. Experimental Polycrystalline KB 6 samples were prepared by reacting K metal and amorphous B powder in closed Nb tubes at 900 °C for 96 h. K- site occupancy was controlled by varying the starting composi- tions. K-site occupancy was determined by Rietveld refinement of powder XRD patterns [10]. Magnetization was measured by using a SQUID magnetometer from 1.8 K to 300 K. Pulverized sam- ples were observed by TEM. First-principles calculations were car- ried out by a full-potential LAPW code [11] with a visualization software [12]. 3. Results and discussion Powdery bulks of KB 6 with K-site occupancy from 84% to 98% were successfully obtained. Lattice parameter a was almost con- stant (4.2250–4.2276 Å), regardless of K-site occupancy. Samples near stoichiometry were reddish black, while K-deficient samples were bluish. Oxide layers were covering most of the grain bound- aries, resulting in mutually insulating behavior. The grain sizes of KB 6 were 100–300 nm. Although electron diffraction patterns indicated no superstructures, K-deficient samples contained large fraction of needle-like crystals, implying an anisotropic distribu- 0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.12.067 * Corresponding author. Tel.: +81 48 462 1111x8756; fax: +81 48 462 4649. E-mail address: ykatsura@riken.jp (Y. Katsura). Physica C 470 (2010) S633–S634 Contents lists available at ScienceDirect Physica C journal homepage: www.elsevier.com/locate/physc