SOLUTION GROWTH, STRUCTURE AND COMPOSITION OF BORON-CARBIDE CRYSTALS

Boron carbide crystals, with dimensions up to several millimeters on a side and typically twinned, have been prepared by slow cooling in molten copper, palladium and platinum solutions. Despite the wide single-phase region of boron carbides prepared by solid state reactions, the lattice parameters and Raman spectra of these crystals indicate that their composition is always near 20 at.% C ("B4C"), independent of B:C ratio in solution or the solvent employed. Thermodynamic analysis shows that the relative instability of pure β-rhombohedral boron allows the formation of low carbon boron carbides prepared by solid state techniques. However, the reduced chemical potential of boron in these metal solutions limits the composition of the solution-grown boron carbide crystals to near B4C. X-ray structural refinements of these B4C crystals are consistent with a model of B11C icosahedra and CBC chains. The lattice constants, bond lengths and thermal parameters of the solution-grown B4C crystals show remarkably little variation between samples. Crystals utilized in a previous study have larger unit cells, suggesting a reduced carbon concentration of 15-16 at.%, and thermal factors consistent with expectations of a mixture of CBB and CBC chains at this composition. © 1990.