Eu₃Si_15 − x Al_1 + x O _x N_23 − x (x ≃ 5/3) as a commensurate composite crystal

Abstract

A new Eu-SiAlON crystal, Eu 3 Si 15 − x Al 1 + x O x N 23 − x ( x ≃ 5/3), was found and the structure was determined by an X-ray diffraction technique using a twinned sample. The structure consists of a host framework, which is constructed by the connection of MX 4 tetrahedra ( M : Si or Al; X : O or N), and Eu ions as the guest ions. The structure is considered to be a commensurate composite crystal. The basic vectors are a 1 = a /3, b and c for the first substructure, and a 2 = a /5, b and c for the second substructure. The first substructure consists of part of the host framework and the Eu ions, while the remainder of the host structure is taken as the second substructure. Possible phases belonging to the series are proposed using the composite crystal model in (3 + 1)-dimensional superspace. Chemical composition, possible space groups, cell parameters, and the basic model for those phases are presented.