Crystal structure of Ca2Ln3Sb3O14 (Ln=La, Pr, Nd and Y): A novel variant of weberite

The crystal structures of Ca2Ln3Sb3O14 (Ln=La, Pr, Nd and Y) and Ca2Sb2O7 at room temperature were refined by the Rietveld method using combined X-ray and neutron powder diffraction data. Ca2Sb2O7 adopts the weberite structure having the space group Imma. The structures of Ca2Ln3Sb3O14 are, however, neither the orthorhombic nor the tetragonal chiolite as has been suggested previously. They crystallize in the monoclinic space group I2/m11 belonging to a hitherto unknown type of deformation of the parent (orthorhombic) weberite structure.