Preparation of sodium contained aluminum higher borides and a search of higher hardness material

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Single crystals of a new ternary boride NaAlB_<14> (orthorhombic, Imma) were obtained from the Na-Al-B system using a high-temperature Al self-flux under an Ar atmosphere. The reagents used to prepare the samples were NaF or Na_2B_4O_7 powder, crystalline boron powder and Al metal chips. The optimum conditions to obtain relatively large crystals were found to include soaking temperature 1573 K, soaking time 1 h, cooling rate 50 K/h and the atomic ratios (n=B/Na=1.0 to 4.0) of starting materials. The NaAlB_<14> crystal prepared had maximum sizes of approximately 7.3mm. The crystals were generally obtained in the form of plate-like crystals with well-developed {010} faces or trapezoidal-shape crystals. The values of Vickers microhardness are in the ranges of 23.3(1.0) and 28.4(0.6) GPa, respectively. Single crystals of quaternary compounds in the solid solution (Na_xRE_<1-x>)AlB_<14> (RE=Li, Mg, rare earths (Tb, Dy, Ho, Er, Tm, Yb, Lu)) were grown by the Na-RE-Al-B system using Al-self flux. The single crystals of LiAlB_<14>, NaAlB_<14>, MgAlB_<14> and (Na_xMg_<1-x>)AlB_<14> (0.21≦x≦0) were grown; the quaternary boride (Na_xLi_<1-x>)AlB_<14> and (Na_xRE_<1-x>)AlB_<14> (RE=rare earths) phases were not obtained at the single crystal growth experiment in the Na-Li-Al-B and Na-RE-Al-B systems. The homogeneity range for the solid solutions of (Na_xMg_<1-x>)AlB_<14> (0.21≦x≦0) crystals was studied by XRD and chemical analyses. The hardness values of (Na_xMg_<1-x>)AlB_<14> (0.21≦x≦ 0.08) crystals were in the range of 31.1(0.8) to 33.4(0.9) GPa. Magnetic susceptibility of as-grown (Na_xMg_<1-x>)AlB_<14> (x=1, 0.21≦x≦0) have been measured in the temperature range of 300 K to 2 K. The magnetic susceptibility of (Na_<0.21>Mg_<0.79>)AlB_<14> and (Na_<0.08>Mg_<0.92>)AlB_<14> crystals are diamagnetic and shown an increase at low temperatures indicative of small paramagnetic contributions which can normally be attributed to impurities or defects.