STUDY ON THE STRUCTURES OF N-CONTAINING MELILITE Y2SI3O3N4 AND ND2SI2.5AL0.5O3.5N3.5

Rietveld refinements have been used to determine the structure of Y2Si 3O3N4 from X-ray data and Nd2Si2.5Al0.5O3.5N3.5 from neutron powder di ffraction data. The refinements show that in the melilite phase Y2Si3O 3N4 and melilite solid solution Nd2Si2.5Al0.5O3.5N3.5 the distribution s of cations and anions are almost identical. They are analogous to th e akermanite (Ca2MgSi2O7) structure, with Si/Si,Al atoms at the origin and centre of the unit cell and with four N/N,O atoms forming the SiN 4/(Si,Al)(N3.5O0.5) tetrahedra which share corners with SiO2N2/(Si,Al) O2.25N1.75 tetrahedra to form a continuous sheet structure. Each Y3+ o r Nd3+ ion is surrounded by eight N/O atoms forming the coordination p olyhedron in Y2Si3O3N4 and Nd2Si2.5Al0.5O3.5N3.5 respectively. The arr angement of Al, Si atoms in the tetrahedra in Nd2Si2.5Al0.5O3.5N3.5 st ructure is also discussed.