Vacancy ordering in Sc 1- xS

This is a study of scandium vacancy ordering in slowly cooled Sc _{1- x}S using high-resolution electron microscopy. Previous X-ray analysis had shown vacancy segregation into alternate (111) planes of the rocksalt substructure. The present study reveals a further incommensurate ordering of the vacancies within the (111) _c planes in approximately every third ( overline110) _rhombohedral plane. The incommensurate order was easily destroyed by the irradiation and heating of the electron beam. These observations are discussed in terms of the termination of the wave vector within the Brillouin zone and described in terms of a possible continuous transition to an incommensurate Sc atom population wave.