Crystal Structure of Vapor-Deposited Boron Filaments

Abstract

The structure of boron filaments, vapor-deposited on a tungsten-wire substrate in the temperature range 870°–1260°C, is studied by x-ray diffraction and transmission-electron microscopy. The typical diffraction pattern observed with both x rays and electrons consists of six or seven broad maxima, or halos. This pattern is interpreted as arising from a microcrystalline (∼30 Å diam) material having an average unit-cell structure intermediate between the α-rhombohedral and ``simple'' tetragonal polymorphs of boron. This interpretation appears to be more satisfactory than others based on an ``amorphous'' or ``faulted fcc'' description, although several discrepancies remain to be explained. Single-crystal spot patterns are also observed in the electron microscope. These arise from inclusions of α- and β-rhombohedral boron, less than 1 μ in diameter, which are distributed throughout the microcrystalline matrix. These inclusions constitute a very small volume fraction of the vapor-deposited boron and are not detected by x-ray diffraction.