Preparation and Properties of SmB₆ and SmB₄ Single Crystals

Preparation of single crystals of SmB₆ (cubic system) and SmB₄ (tetragonal system) using molten aluminum flux method in an argon atmosphere have been investigated. As-grown SmB₆ and SmB₄ single crystals were both used for measurements of lattice constant, density and Knoop-microhardness, and also for the study of oxidation reaction and kinetics in air at temperature range between 700°C and 1250°C and time between 10 and 120min. Results obtained are as follows. The condition to obtain single crystals of SmB₆ as the single phase was found, but it was not found for SmB₄; single crystals of SmB₄ were formed accompanied by the formation of those of SmB₆ in any condition. The optimum preparation conditions to obtain cubical SmB₆ and polyhedral SmB₄ single crystals are summarized as follows. The mixing atomic ratios of B, Sm and Al: for SmB₆; B/Sm=5.7, Al/Sm=87, and for SmB₄; B/Sm=3.0, Al/Sm=87. For both of SmB₆ and SmB₄, the heating temperature (the temperature of molten flux) and time held at the temperature: 1500°C for 10h. SmB₆ and SmB₄ single cryatals of blue SmB₆ and grayish brown SmB₄, both of metallic appearances, were obtained. Cubical single crystals of SmB₆ composed of {100} face, and needle and thick-plate single crystals, each of which has well-developed (100) face, were obtained. In case of SmB₄, polyhedral single crystals and thin-plate single crystals having well-developed (001) faces were obtained, Lattice constants and densities (D) determind on single crystals at room temperature are as follows;<br>SmB₆: a₀=4.1335±0.0001Å, D=4.97±0.04g/cm³<br>SmB₄: a₀=7.1781±0.0005Å, c₀=4.0694±0.0003Å, D=6.09±0.03g/cm³<br>Values of Knoop-microhardness determined on (100) faces of SmB₆ single crystals and that on (001) and (211) faces of SmB₄ single crystals are as follows;<br>SmB₆: (100) 1630-1930kg/mm²<br>SmB₄: (001) 1560-1920kg/mm²<br>(211) 1460-2130kg/mm²<br>It was observed that the oxidation reaction of both of SmB₆ and SmB₄ single crystals began to proceed at 740°-750°C, and that, in case of SmB₆, the oxidation products were Sm(BO₂)₃ (monoclinic system) and noncrystalline B₂O₃, however, in case of SmB₄ the final oxidation products were the same as in case of SmB₆, but SmB₆ and SmBO₃ (hexagonal system) appeared intermediately. The oxidation rate of SmB₆ single crystals were able to be expressed by the general oxidation rate equation, (dw)ⁿ=kt, where n was 2.00±0.10. Based on the relation mentioned above, the calculated apparent activation energy of SmB₆ single crystal is 201.3±6.4kcal/mol. In case of SmB₄, it was unable for the oxidation rate to be expressed by the equation described above. The investigation into oxidation kinetics is left in future.