ROLE OF BONDING AND COORDINATION IN THE ATOMIC-STRUCTURE AND ENERGY OF DIAMOND AND SILICON GRAIN-BOUNDARIES

The high-temperature equilibrated atomic structures and energies of la rge-unit-cell grain boundaries (GB's) in diamond and silicon are deter mined by means of Monte-Carlo simulations using Tersoff's potentials f or the two materials, Silicon provides a relatively simple basis for u nderstanding GB structural disorder in a purely sp(3) bonded material against which the greater bond stiffness in diamond combined with its ability to change hybridization in a defected environment from sp(3) t o sp(2) can be elucidated. We find that due to the purely sp(3)-type b onding in Si, even in highly disordered, high-energy GB's at least 80% of the atoms are fourfold coordinated in a rather dense confined amor phous structure. By contrast, in diamond even relatively small bond di stortions exact a considerable price in energy that favors a change to sp(2)-type local bonding; these competing effects translate into cons iderably more ordered diamond GB's; however, at the price of as many a s 80% of the atoms being only threefold coordinated. Structural disord er in the Si GB's is therefore partially replaced by coordination diso rder in the diamond GB's. In spite of these large fractions of three-c oordinated GB carbon atoms, however, the three-coordinated atoms are r ather poorly connected amongst themselves, thus likely preventing any type of graphite-like electrical conduction through the GB's.