An atomistic model to calculate the core structure and energy of dislocations in ionic crystals. Part 2: Flexible boundary models

A discussion is given of various flexible boundary models that can be used to improve upon the atomistic rigid boundary calculation of dislocation energetics in ionic crystals. A flexible boundary method due to Sinclair (Flex-PT) and a method due to Hoagland, Gehlen and Hirth (Flex-II) are described in detail. These have been used to calculate the core structure, volume of expansion and strain energy of the a/2[110] edge dislocaton in MgO. Comparison between these two models and the rigid boundary model shows that Flex-II is superior both in terms of efficiency and accuracy. Thus Flex-II uses only 20% of the computing time required by the rigid boundary model. Essential findings are as follows. The configuration of the region closest to the core (within approximately 4b from the dislocation line) is essentially the same as that obtained previously from the rigid boundary model. The strain field in regions far from the core can be expressed as a sum of the Volterra field and two linear dipole expansion fields. The presence of the dislocation causes a volume expansion of the entire crystal of approximately 1 atomic volumes/plane. (author)