Synthesis and electrical conductivity of Na3B20
Graphical abstract
Single-crystal and the bulk ceramics of Na3B20 were prepared, and the electrical conductivity of the bulk sample was measured.
Introduction
Boron networks significantly affect the electrical and mechanical properties [1] of boride compounds, and the structure of these networks depends on the B content. MgB2, having two-dimensional network layers, is known as a superconducting material. LaB6, used in thermionic electron-emitting materials [1], and the metal hexaborides MB6 (M = Ca, Sr, and Ba) are thermoelectric materials [2] containing octahedral clusters of B within the crystal structures. YB66, which has a high B content and a three-dimensional network structure composed of B icosahedral clusters, shows semiconducting behavior [1]. B4C, a typical heat-resistance material, is composed of covalently bonded icosahedral clusters and exhibits high hardness [1]. In the development of boride materials, it is essential to clarify the correlation between the B network structures and associated physical properties.
Our group has synthesized bulk ceramic NaB5C [3,4] and Na–B–Si compounds [5,6], and characterized them as thermoelectric and heat-resistance materials. In the synthesis of these compounds, Na–B binary compounds were found to precipitate as secondary phases, which significantly affected the electrical and mechanical properties of the bulk samples containing a ternary compound as a main phase. Among the Na–B binary systems, Naslain et al. first reported the structures of NaB6 [1] and NaB15 [1,7,8] in the 1960s, and Albert et al. analyzed the crystal structures in detail, and revealed these compounds to be Na3B20 [9] and Na2B29 [10]. Na3B20 powder was prepared by heating Na and B above 1323 K. The crystal structure of this compound was determined to be orthorhombic (space group; Cmmm) and had a boron network composed of distorted octahedra and pentagonal bipyramids, as determined by powder X-ray diffraction (XRD) [9]. The crystal structure of Na2B29 (monoclinic, space group I1m1) was studied by single-crystal XRD, low-temperature neutron diffraction, electron diffraction, high-resolution transmission electron microscopy (HRTEM), and NMR spectroscopy [10]. It has been reported that Na2B29 has B12 icosahedral clusters in its structure and exhibits semiconducting behavior [1]. Studies on Na–B binary compounds have been focused on crystal structure analysis; however, the detailed synthesis conditions of single-crystal and bulk Na3B20 samples have not been clarified. Additionally, Na3B20 has been previously reported to be an insulator [1]; however, the electrical properties have not been elucidated.
Herein, we synthesized Na–B binary compounds under various temperature conditions and ascertained the synthesis conditions for Na3B20. The stability of Na3B20 in air was also investigated. Bulk samples of Na3B20 were prepared by solid-vapor reaction sintering, and the electrical properties were evaluated.
Section snippets
Experimental
Metallic Na pieces (Nippon Soda Co., Ltd., purity; 99.95%), and amorphous B powder (PAVEZYUM, purity; 99%, particle diameter; 0.35 μm) or crystalline B (β-rhombohedral-boron (β-r-B)) powder (Hirano Seizaemon Shouten Co., Ltd., purity; 99.4%, particle size; <0.5 mm) were used as starting materials. The amorphous B powder was weighed in air, and was then placed in a crucible made of boron nitride (BN) (Showa Denko Co., Ltd., purity; 99.5%, outer diameter; 8.5 mm, inner diameter; 6.5 mm, depth;
Results and discussion
The XRD patterns of the samples prepared by heating Na and amorphous B (molar ratio Na:B = 1:1) after the removal of excess Na by alcohol-water-centrifuge treatment are shown in Fig. 1. In the XRD pattern of the sample synthesized at 1073 K (Fig. 1(a)), the sharp peaks were assigned to Na3B20, while a broad peak around 2θ = 20–30° corresponded to amorphous B. Most peaks in the XRD pattern of the sample prepared at 1223 K (Fig. 1(b)) could be indexed with the orthorhombic lattice parameters of Na
Conclusion
Fine-powdered Na3B20 was synthesized by heating amorphous B powder and Na at 1173–1373 K. Columnar black single crystals of Na2B29 were formed by heating above 1453 K. Black, square, and plate-like single-crystals Na3B20 were grown at 1223 K from crystalline B powder. Na3B20 gradually reacted with the water moisture, and H3BO3 was formed in the air. A bulk Na3B20 sample with a relative density of 73.4% was obtained by heating the compact of amorphous B powder in Na vapor at 1223 K. The
CRediT authorship contribution statement
Haruhiko Morito: Conceptualization, Investigation, Writing - original draft, Project administration. Syouta Shibano: Investigation. Takahiro Yamada: Investigation. Takuji Ikeda: Investigation. Masami Terauchi: Investigation. Rodion V. Belosludov: Investigation, Writing - review & editing. Hisanori Yamane: Investigation, Writing - review & editing.
Declaration of competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors would like to thank T. Kamaya for the EPMA. This work was supported by Japan Science and Technology Agency (JST) CREST, grant number JPMJCR19J1, Japan.
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