The disordered cubic structure of Ca7Co3Ga5O18

The new mixed oxide having composition close to Ca7Co3Ga5O18 was synthesized from CaCO3, Co3O4 and Ga2O3 at 1150 °C in air and studied by neutron and synchrotron X-ray powder diffraction, selected-area electron diffraction and high-resolution electron microscopy. The structure was refined, using time-of-flight (TOF) neutron powder diffraction data, in space group F  432, with a=15.0487(4)Å and Z=8Z=8, to RF=0.7%RF=0.7%. It is considerably disordered, with four different tetrahedral sites randomly occupied by Co and Ga atoms at a ratio of 1:2. The tetrahedra form a disordered (Co1/3Ga2/3)O2 3D-framework inside which isolated CoO6 octahedra, surrounded by 8 Ca atoms, are located. The structure is related to the ordered structure of Ca14Al10Zn6O35. Electron diffraction patterns confirmed the symmetry and unit cell and revealed no diffuse scattering. High-resolution electron microscopy images showed the absence of extended structural defects.

Graphical AbstractThe crystal structure of Ca7Co3Ga5O18 is considerably disordered, with four different tetrahedral sites randomly occupied by Co and Ga atoms at a ratio of 1:2. The tetrahedra form a disordered (Co1/3Ga2/3)O2 3D-framework inside which isolated CoO6 octahedra, surrounded by 8 Ca atoms, are located. Cube represents statistical tetrahedron around position (M1) occupied by Co and Ga.Figure optionsDownload full-size imageDownload as PowerPoint slide