Role of silicon vacancies in yttrium-disilicide compounds from ab initio calculations

The structural properties of the compound ${\mathrm{YSi}}_{2}$ are investigated by means of ab initio simulations based on density-functional theory. More particularly we emphasize the role played by Si vacancies, show that the ${\mathrm{Th}}_{3}$${\mathrm{Pd}}_{5}$ structure can be deduced from the ${\mathrm{AlB}}_{2}$ structure by a relaxation process around the Si vacancies within the (0001) plane. A specific ordered arrangement of the Si vacancies along the [0001] direction is found to be energetically the most favorable. Geometries obtained from our theoretical calculations are in good agreement with experimental results.