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

在Ar气氛下，用高温Al自熔剂从Na-Al-B体系中获得了新型三元硼化物NaAlB_<14>（正交，Imma）的单晶。制备样品所用的试剂为NaF或Na_2B_4O_7粉末、结晶硼粉末和Al金属片。得到较大晶粒的最佳条件为：浸泡温度1573 K，浸泡时间1 h，冷却速度50 K/h，起始材料的原子比（n=B/Na=1.0 ~ 4.0）。制备的NaAlB_<14>晶体最大尺寸约为7.3mm。晶体一般为片状晶体，具有发育良好的{010}面或梯形晶体。维氏显微硬度分别为23.3（1.0）和28.4(0.6)GPa。采用al自助熔剂在Na-RE-Al-B体系中生长出固溶体（Na_xRE_<1-x>） al_ <14> (RE=Li, Mg，稀土（Tb, Dy, Ho, Er, Tm, Yb, Lu）的季元化合物单晶。生长出LiAlB_<14>、NaAlB_<14>、MgAlB_<14>和（Na_xMg_<1-x>）AlB_<14>(0.21≦x≦0)的单晶；在Na-Li-Al-B和Na-RE-Al-B体系的单晶生长实验中未得到季硼化物（Na_xLi_<1-x>）AlB_<14>和（Na_xRE_<1-x>）AlB_<14> （RE=稀土）相。采用XRD和化学分析研究了（Na_xMg_<1-x>） al_ <14>(0.21≦x≦0)晶体固溶体的均匀性范围。（Na_xMg_<1-x>）AlB_<14>(0.21≦x≦0.08)晶体的硬度值在31.1(0.8)~ 33.4(0.9)GPa之间。在300 ~ 2 K的温度范围内测定了生长态（Na_xMg_<1-x>）AlB_<14> (x=1, 0.21≦x≦0)的磁化率。（Na_<0.21>Mg_<0.79>）AlB_<14>和（Na_<0.08>Mg_<0.92>）AlB_<14>晶体的磁化率是抗磁性的，并且在低温下呈增加趋势，表明顺磁性的贡献很小，通常归因于杂质或缺陷。

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.