VACANCY-ORDERED SUPERSTRUCTURES IN CU2SE

The superionic Cu2Se is studied by means of electron diffraction and high resolution electron microscopy. It is found that in beta-Cu2Se (room temperature phase), the ordering of Cu-atoms and vacancies, in the predominantly observed phase, occurs between eight Se layers. The superstructure is the result of the stacking of four double layer monoclinic blocks (existing in three types) along the [111]c direction. Evidence is found for the existence of a phase with ordering between four Se layers, as well for an incommensurate one. There are also strong indications that phases presenting an ordering of copper vacancies exist, leading to overall compositions Cu2-xSe. Heating above 140-degrees-C, the f.c.c.-alpha-Cu2Se is obtained and after cooling down to liquid nitrogen temperature, the presence of a new phase (gamma-Cu2Se) is detected.