Observations on Boron and Some Borides

Abstract

Single crystals of beta-rhombohedral boron have been grown in both needle and plate habit from the vapor phase. Maximum size of needle crystals was approximately 5 mm × 0.1 mm × 0.1 mm. Penetration twinning and growth steps at one end of a crystal were detected in some cases. These boron needles were characterized by smooth lateral faces of high reflectivity. Thin lathlike red crystals of AlB₁₂ were prepared from the elements at reduced pressure.

Dense polycrystalline structures of boron, silicon hexa-boride, and strontium hexaboride were prepared by vacuum hot-pressing finely divided powders. Microstructure was studied microscopically with reflected light after diamond polishing and etching.

Change in resistance with temperature was measured for both the mono- and polycrystalline material. Energy gap and intrinsic conductivity determinations from log R vs 1/T curves indicate that boron prepared by the vacuum process is of higher purity than commercially available boron and of as high a purity as that prepared experimentally by other workers.