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系中获得了新的三元硼化物NaAlB_&lt；14&gt；(正交晶系，IMMA)单晶。制备样品所用试剂为NaF或Na2B_4O_7粉末、晶态硼粉和铝金属屑。获得较大晶体的最佳工艺条件为：保温温度1573K，保温时间1h，冷却速度50K/h，原料原子比n=B/Na=1.0~4.0。所制备的NaAlB；14&gt；晶体的最大尺寸约为7.3 mm。晶体一般为具有发达的{010}面的片状晶体或梯形晶体。维氏硬度分别在23.3(1.0)和28.4(0.6)Gpa范围内。用Na-RE-Al-B系熔剂，在固溶体(Na_xRE_&lt；1-x&gt；)Alb_&lt；14&gt；(RE=Li，Mg，稀土(Tb，Dy，Ho，Er，Tm，Yb，Lu))中生长了四元化合物单晶。生长了四元硼化物(Na_xLi_&lt；1-x&gt；)Alb_&lt；14&gt；(Na_xRE_&lt；1-x&gt；)Alb_&lt；14&gt；(Na_xMg_&lt；1-x&gt；)AlB_&lt；14&gt；在Na-Li-Al-B和Na-RE-Al-B系统的单晶生长实验中没有发现(RE=稀土)相。用X射线衍射法和化学分析法研究了(NaxMG&lt；1-x&gt；)Alb&lt；14&gt；(0.21≦x≦0)晶体固溶体的均匀范围。(NaxMG&lt；1-x&gt；)Alb&lt；14&gt；(0.21≦x≦0.08)晶体的硬度值在31.1(0.8)~33.4(0.9)Gpa之间。在300K到2K的温度范围内测量了((Na_&lt；0.21&gt；Mg_&lt；0.79&gt；)AlB_&lt；14&gt；)Alb_(14)(x=1，0.21≦x≦_0)的磁化率。晶体是抗磁性的，在低温下表现出增加，这表明顺磁性的贡献很小，这通常可以归因于杂质或缺陷。

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.